usps nautical glossary · 2006. 6. 7. · with a sextant. aspect ratio 1. square of the luff length...

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USPS® Nautical Glossary© 1999 United States Power Squadrons

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USPS® Nautical Glossary

Aaback

To have the wind on the forward side of the sails.

abaftBehind, aft of.

abandonmentAn abandoned race is one that is declared void by the racecommittee at any time; can be resailed at the race committee’sdiscretion.

abeamOff the boat at right angles to centerline.

aboardUpon or in the boat.

aboutSee come about, ready about.

absolute humidityThe mass of water vapor per unit volume of air. Generallyexpressed in grams per cubic meter.

absolute temperatureScientific temperature scale with 0° as the point at which mol-ecules cease their motion. See Kelvin temperature.

absorption (of radiation)The process by which radiant energy is retained by (i.e., notreflected from or transmitted through) a substance.


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accelerator pumpA small pump in the carburetor, operated by a throttle linkagethat supplies additional fuel needed for acceleration of the boat.

active deviceA device that requires and controls electrical power to per-form its function, such as a transistor.

adapterA device for connecting two dissimilar connectors.

adiabatic processA process in which no heat is transferred between the sur-roundings and the system being changed. The temperature,pressure, and volume of a parcel of air (or any gas) are inter-related. A change in volume or pressure can thus produce achange in temperature without the addition or removal of heat.See general gas law, temperature.

adriftUnattached to shore or bottom, floating out of control.

advectionTransport of atmospheric conditions such as temperature, pres-sure, and humidity, from one place to another by the motion ofair (wind).

advection fogFog formed when warm, moist air is transported over a coldsurface that lowers the temperature of the air below the dewpoint.

aftAfter; at, near, or toward the stern.


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agroundTouching or stuck on the bottom.

ahead1. The direction toward the bow or a boat (relative to theposition of the observer). 2. In front of a boat. 3. Moving in aforward direction. Compare astern.

ahoyTerm used in hailing a boat.

ahullSee lie ahull.

aids to navigationCharted objects available to assist in determination of positionor safe course, or to warn of danger (e.g., buoys, beacons,fog signals, lights, radio beacons, range marks). Also, any elec-tronic devices used for navigation.

airfoilA device designed to deflect an air stream, thereby derivingenergy from it. Examples: an aircraft wing, elevator, or rud-der; a kite; a sail.

air-fuel ratioRatio by weight of fuel compared to air in the carburetor mix-ture.

air-gapSpace between spark plug electrodes.


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air massAir with uniform properties over a large surface area. Theseproperties will vary vertically within the air mass, but be nearlyuniform horizontally.

air-mass thunderstormA thunderstorm that is not associated with a weather frontthat develops locally owing to heating of humid air near thesurface (such as by hot asphalt in a parking lot), causing theair to rise, cool, and form storm clouds.

aleeTo leeward, away from the wind.

alignmentThe bringing together of two parts of a mechanical coupling innear-perfect horizontal and vertical agreement.

aloftAbove deck, usually in the rigging.

alongsideAlong or close to another boat or a pier.

alternating current (AC)An electric current that reverses direction of flow at regularlyrecurring intervals of time.

alternatorA device that generates an alternating voltage by rotating acoil through a magnetic field, or by a rotating magnetic fieldcutting the turns of a stationary coil. Diodes may be attachedto convert the AC voltage to DC voltage.


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altitudeAngular distance above the horizon, measured along a verti-cal circle from the horizon to a celestial body.

altitude differenceSee intercept.

altocumulusA form of middle cloud with many dome-shaped orpillow-shaped elements, indicating some vertical development.

altostratusA form of middle cloud that is sheet-like, or layered, in shape.

ambientGeneral current conditions of the atmosphere such as tem-perature and pressure in a region.

amidshipsIn or near the middle of the boat.

ammeterA meter connected in series with a circuit to measure thecurrent flowing through the circuit. It is also possible to havea clamp-on meter.

ampacityThe current-carrying capacity of a conductor, expressed inamperes.

ampere (I)The unit of measurement of current flow. The symbol for cur-rent in amperes in equations is (I).


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ampere-hour (Ah)One ampere flowing in a circuit for one hour, or 0.1 amperefor 10 hours. Also used to rate the capacity of a storage bat-tery, stating the number of hours the battery will supply a givennumber of amperes before discharge.

amplificationIncreasing the output voltage, current, or power of a deviceover that which is supplied to its input terminals.

amplifierAny device that produces amplification.

amplitude1. The magnitude of a voltage, current, or power. 2. The arc,measured along the observer’s horizon, between an observed(celestial) body on the horizon and the observer’s prime vertical.

amplitude modulation (AM)The process of impressing intelligence on a radio-frequencysignal by varying its amplitude in accordance with the ampli-tude of the information to be transmitted.

analemma curveA graduated scale of the declination of the sun and the equa-tion of time for each day of the year, located in the TorridZone on the terrestrial globe.

analogValue of a measurement displayed graphically or by move-ment of a meter pointer; pertains to data in the form of con-tinuously variable physical quantities.


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analog displayMethod of presenting physical variables such as speed or di-rection, indicated by a pointer on a scale, usually a graduateddial.

anchorDevice used to secure a boat to the bottom of a body of wa-ter.

anchorageSuitable or designated place where boats anchor.

anchor watchA security lookout maintained while a vessel is anchored ordocked.

anemometerAn instrument for measuring wind speed.

aneroid barometerAn instrument that measures atmospheric pressure via the riseor fall of a flexible metal disk that is part of a sealed chambercontaining a partial vacuum.

angle of attackThe angle between the apparent wind and the sail chord, orthe water flow and the rudder.

anodeThe positive electrode, toward which the negative ions aredrawn.

antennaA wire, rod, or other assembly designed to transmit or receiveradio signals.


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antenna gainRatio of power radiated from a complex antenna system orarray to that radiated by a simple reference antenna, such asa dipole.

anticycloneA high-pressure area (highest pressure at the center) in whichthe wind is spiraling outward—clockwise in the Northern Hemi-sphere. Referred to as a high. See cyclone.

antifreezeA chemical that lowers the freezing point of water. It mayalso contain anti-rust and lubricating compounds.

anti-friction bearingA bearing constructed with balls or rollers between journaland bearing surfaces to provide rolling instead of sliding fric-tion.

anti-siphon valveA valve that admits air to a line and prevents siphonic action.

anvil topCommon appearance of the cloud tops of well-developed thun-derstorms. This portion of the cloud is often blown downwindby strong air currents at high altitudes, causing the top to looklike an anvil.

apparent altitude (ha)Resulting value after sextant altitude (hs) is corrected for in-dex error and dip.


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apparent timeTime based on the rotation of the earth relative to the appar-ent (true) sun.

apparent windThe vector (speed and direction) sum of the actual wind andthe wind created by the movement of the boat. The wind di-rection and speed experienced by an observer on the movingboat.

aqualiftAn engine exhaust system and silencing system in which cool-ing water is injected into the exhaust and carried out by theexhaust gases.

arc1. The discharge of electrical current across a gap. 2. Gradu-ated scale of a marine sextant. 3. Part of a curved line, as ofa circle.

arc to timeThe converting of arc in degrees to time in hours (hh), min-utes (mm), and seconds (ss).

arctic air massThe coldest of air masses originating in high latitudes.

arctic smokeSee steam fog.


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AriesSee first point of Aries.

armaturePart of an electric motor or generator that includes the maincurrent-carrying winding.

artificial horizonA device, such as a pan of dark oil, used to indicate a horizon-tal reference when a true horizon is not available to take sightswith a sextant.

aspect ratio1. Square of the luff length of a sail divided by the sail area. 2.Square of the depth of a keel divided by the keel area.

assumed latitude (Asm L)The latitude at which an observer is assumed to be for a cal-culation of computed altitude (Hc) in some methods of sightreduction.

assumed longitude (Asm Lo)The longitude at which an observer is assumed to be for cal-culation of computed altitude (Hc) in some methods of sightreduction.

assumed position (AP)A point at which a vessel is assumed to be located, particu-larly one used as a preliminary to establishing certain naviga-tional data, as a point on the surface of the earth for which thecomputed altitude (Hc) is determined in the solution of a ce-lestial observation. An arbitrarily chosen position near the DR(See dead reckoning), used for computation of Hc in somemethods of sight reduction.


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astern1. The direction toward the bow of a boat (relative to theposition of the observer). 2. In back of or behind a boat. 3.Moving in reverse (backwards). Compare ahead.

athwartAt right angles to the fore-and-aft line of a vessel.

athwartshipDirection at right angles to the centerline of a boat.

atmosphereThe envelope of air surrounding the earth.

atmospheric pressureThe force per unit area exerted by the weight of the atmo-sphere. Expressed in several units such as millibars, inches ofmercury, and pounds per square inch.

atoll cloudA tall pillar of cloud caused by the sea breeze flowing towardan isolated island.

atomThe smallest part into which an element can be divided andstill retain the characteristics of that element.

atomizerSee injector.

attached flowThe movement of a fluid (i.e., water or air) along a surface(i.e., a hull, keel, or sail) in a smooth, sheet-like fashion with-out turbulence.


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attenuateTo make thin or to reduce in force.

attenuationThe reduction in magnitude during transmission from one pointto another.

augmentation (A)The apparent increase in semidiameter of a celestial body asits altitude increases, due to reduced distance from the ob-server.

aurora (borealis and australis)The northern and southern lights. Created by reactions be-tween charged particles emitted by solar storms and the at-mosphere at high altitudes near the magnetic poles. Much ofthe energy released by the reaction is visible radiation (light).

autotransformerA type of transformer with a single winding having one ormore taps. Part of the winding is common to both primary andsecondary circuits.

auxiliaryThe engine of a sailboat; a sailboat with an auxiliary engine.

awashMostly under water, but not completely submerged.

aweighPosition of anchor when raised off the bottom of a body ofwater.


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azimuth (Zn)The true direction of an observed body, measured clockwisealong the visible horizon from 0° to 360°.

azimuth angle (Z)The computed direction of an observed body from a point onthe horizon toward the elevated pole (north or south), mea-sured along the visible horizon (eastward or westward) to-ward the geographic position (GP) of the body.

Bbabbitt

A soft, white metal alloy frequently used to line replaceableshell-type engine bearings.

back1. Change in direction of the wind in a counterclockwise di-rection in the Northern Hemisphere, and a clockwise direc-tion in the Southern Hemisphere. 2. To cause to movebackwards. See veer.

backfireIgnition of the air-fuel mixture in the intake manifold by a flamefrom a cylinder.

backing plateA plate used inside the hull to assure that fittings attached tothe outside with bolts will not pull loose.

backing windA wind whose direction changes progressively in a counter-clockwise direction (e.g., W-SW-S-SE). Opposite of veeringwind.


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backstayPart of standing rigging, usually cable, that supports a mastfrom aft.

backwindedA sail is backwinded when another sail to leeward of it istrimmed so closely as to cause increased pressure on the leeside of the windward sail, thus causing the windward sail tobulge to windward.

baffle (baffleplate)An obstruction for checking or deflecting the flow of gases,liquids, or sound.

baggywrinkleAnti-chafe material made from old rope yarns woven betweenstrands of marline.

Bahamian moorA way of anchoring using two anchors to allow for reversingtidal currents.

bail1. To remove water by scooping it out with a bucket. 2. Acurved fitting for securing a line or a block to a spar.

balanceThe ability of a sailboat to stay on a steady course withouthaving to apply helm.

ballastWeight carried low in a boat to improve trim or stability.


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ball bearingAn anti-friction bearing consisting of a hardened inner andouter face with hardened steel balls set between the two faces.

ball lightningA visible electrical discharge in the form of a concentratedball. May appear to roll along for a short time before disap-pearing (sometimes explosively).

band width1. Frequency limits between which electrical signals are con-fined. 2. The part of the spectrum occupied by the carrier andside bands.

barber haulerA trimming line for positioning the athwartship lead of the jib.

bare polesUnder way with no sail set; usually a heavy-weather precau-tion.

bargeAn unpowered vessel used to carry cargo.

barnacleA crustacean often found attached to the bottom of a boat,especially in salt water.

barographAn aneroid barometer that records pressure versus time.

barometerAn instrument used to measure atmospheric pressure. Seeaneroid barometer and mercurial barometer.


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base metalA metal chemically active in an electrolyte. Anodic metals asopposed to cathodic or passive metals.

bass boatAn open-cockpit, outboard-powered fishing boat used prima-rily on inland waters.

battcars®A system of attaching battens to the mast slide to reduce fric-tion while raising and lowering the sail, and to improve sailshapes compared with traditional sail slugs and slides.

batten1. A thin, narrow strip of wood, plastic, or fiberglass used tostiffen the leech of a sail. 2. A wooden strip fastened over aseam to stop leakage.

batten downTo secure all hatches and ports in preparation for heavyweather.

batten pocketAn envelope-like enclosure stitched into the trailing edge of asail to hold a batten.

batteryA group of cells that produce an electrical potential by chemi-cal reaction.

beachTo run the bow of a boat up on a beach.


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beaconAnything that serves as a signal or indication for guidance orwarning. A fixed (non-floating) aid to navigation.

beam1. Maximum width of a boat. 2. A horizontal athwartship sup-port for the deck.

beam endsA boat heeled completely on her side.

beam reachSailing with the wind at a right angle to the boat.

beam seaSea that is abeam.

beam widthApplied to radar or other directional antennas. The width orsharpness of the emitted radiation in degrees of angle, hori-zontally or vertically. Usually measured at the halfpower points.

bearing1. Horizontal direction of an object from an observer, ex-pressed as an angle from a reference direction, e.g., compassbearing, true bearing, relative bearing. 2. A device for sup-porting a rotating shaft with minimum friction. May take theform of a metal sleeve (a bushing), a set of balls (a ball bear-ing), or a set of pins around a shaft (a roller or needle bear-ing).

bear off1. Steer away from the wind, shore, or any object. 2. To steeron a new course farther off the wind.


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beatA close-hauled course.

Beaufort scaleAn arbitrary set of numbers assigned to describe various esti-mated wind velocities from calm to hurricane force. TheWeather course Student Manual includes a modified Beaufortscale. See pilot chart.

becketAn eye in the end of a block, used for securing an end of aline; an eye in the end of a line.

before the windSailing the same direction the wind blows; sailing downwind.

belay1. Secure a line without a knot or hitch. 2. Make fast by wind-ing a line around a cleat or belaying pin. 3. An order to ceaseaction.

belowBeneath the deck.

bend1. To make fast a sail to a spar or stay. 2. A knot to secure aline to another line or object.

bendix gear or driveA gear mounted on a screw shaft attached to the starting-motor armature; automatically engages and disengages theelectric starting motor.

Bermuda (Bermudan or Bermudian) rigSame as Marconi rig.


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Berth1. A place to sleep on a boat. 2. A boat’s position at a pier orfloat. 3. A margin of safety, as in “give it a wide berth.”

BHP (brake horsepower)The actual power output of an engine at the flywheel.

bight1. Open or closed loop in a line. 2. A small bay off a sea-coast.

bilgeLowest part of a boat’s interior.

bilgeboardOne of a pair of centerboards on either side of the centerline.

bilge keelTwin keels, similar to bilgeboards.

bilge strainersScreens to keep debris out of the bilge-pump suction hoses.

binnaclePedestal in which a compass is mounted.

bittA perpendicular post through the deck, used for securing linesand cables.

bitter endThe inboard end of a line, chain, or cable; the end made fastto the vessel, as opposed to the “working end,” which may beattached to an anchor, cleat, or other vessel. Compare work-ing end.


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blanketTo come between the wind and a sail so the sail is not full.

bleedingThe process of purging air from a fuel or hydraulic system.

blockA sheave that rotates on a pin; a nautical pulley.

blockingThe effect of a stalled high in slowing or deflecting aneastward-moving weather system.

blooperA full sail set opposite a spinnaker.

blow-byThe escape of combustion gases past piston rings or closedvalves.

board boatSmall (car-top) centerboard sailing dinghy with very lowtopsides and virtually no cockpit.

boat hookA pole with a blunt hook on the end, used to pull in lines orpush things away from the boat.

boatswainA senior seaman in charge of a ship’s boat, and skilled in ropeand maintenance work. (Pronounced “bo’sun.”)

boatswain’s chairA seat, often including a harness, hauled aloft by a halyard tofacilitate work on a mast. (Pronounced “bo’sun’s” chair.)


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boat windThe so-called wind produced by the movement of the boatthrough the water.

bobstayA stay from the stem to the outer end of the bowsprit.

bollardStout post on a deck, wharf, or pier for securing mooring lines.

boltropeA rope sewn along the edge of a sail to reinforce, to limitstretch, or to allow the edge to slide into a cove in the boom ormast to eliminate slides.

bondingThe process of electrically tying together all major fixed metalitems on a boat.

bonding groundA conductor connecting all metallic structures, tanks, and otherequipment on board to maintain a common local (earth) ground.Used to control galvanic corrosion and induced lightning cur-rents. It should not carry current for equipment operation.

boomSpar used to extend and control the foot of a fore-and-aft sail.

boom crutchA temporary support for the boom when the sail is not hoisted.

boomkinA spar projecting aft of the stern with a boomkin stay to sup-port a backstay.


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boom vangA tackle or hydraulic system that removes twist from the sailby restraining the boom from lifting.

boot topDistinctive stripe on exterior of hull just above the waterline.

boreThe diameter of a cylindrical hole such as an engine cylinder.

bossAn extension or strengthened section, such as projectionswithin a piston, that supports the piston pin or piston pin bush-ings.

bottom1. Land beneath a body of water. 2. A boat's underbody orthe portion of the hull under the water.

bottom dead center (BDC)The position of a crankshaft when the piston is at the verybottom of its stroke.

bowForward end of a boat.

bow chocksFittings (usually metal) on deck at the bow, through whichmooring and anchor lines are led.

bow lineA line used at the bow of a boat.


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bowlineKnot used to form an eye or loop in the end of a line. (Pro-nounced “bo-lin.”)

bowspritA spar that extends forward from the bow. Sails and riggingare attached to the bowsprit.

braceA line used to set the angle of a yard on a square-rigged ves-sel.

brake horsepowerActual horsepower delivered by the crankshaft, measured bymeans of a dynamometer or prony brake.

branchOne of several paths for current flow in a circuit.

breakerA sea-surface wave that has become too steep to be stable.

breaker armMovable part of a pair of contact points in a distributor ormagneto.

breaker pointsTwo separable points, usually faced with silver, platinum, ortungsten, that interrupt the primary circuit in the distributor ormagneto for the purpose of inducing a high-tension current inthe ignition system.

bridge rectifierAn arrangement of diodes for converting alternating current(AC) to direct current (DC).


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bridleA length of line or wire rope with both ends secure (or a se-cured loop), to the middle of which another line is attached.

brightwork1. Polished metal fittings. 2. Varnished woodwork.

Bristol fashionIn good sea-keeping order.

British thermal unit (BTU)The quantity of heat required to raise the temperature of 1pound of water 1°F.

broachTurn a boat broadside to wind or waves, subjecting it to pos-sible capsizing.

broad reachA point of sail with the wind more or less over either quarter.

Bruce® anchorA plow anchor designed to right itself no matter how it landson the bottom.

brummel hooksPatented type of rigging devices that attach to each other easilyand quickly and remain secure even when not under load.

brushesBars of carbon or other conducting material that contact thecommutator of an electric motor, generator, or alternator.

BTUSee British thermal unit.


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bulkheadVertical partition separating compartments in a boat.

bulwarkPortion of hull extending above the deck.

bunk boardsCanvas cloths installed to keep a person from falling out of abunk.

buoyAnchored floating device used as an aid to navigation. Maycarry a light, horn, whistle, bell, gong, or combination for iden-tification. Also may be used to mark a mooring (e.g., anchorbuoy).

buoyancyThe upward force exerted by a fluid on a body or object inthat fluid. The upward force that keeps a boat floating.

burgeeA tapered flag with a device denoting a yachting organization.

bus barA heavy copper strap used in breaker boxes and circuit panelboxes for carrying high current and multiple connections.

bushingA removable liner that forms a bearing surface.

butterfly valveA hinged flap that serves to control the air entering a carbure-tor or the air entering the air inlet manifold of an engine. Mayalso be used to shut down a diesel engine in an emergency.


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butt jointJoint formed by squared ends or edges of two planks or plates.

buttockUnderside of boat’s stern above waterline. See also counter.

Buys-Ballot’s LawA method for approximating the location of a storm center.When an observer faces the wind, the storm (low-pressure)center is roughly 135° to the observer’s right in northern lati-tudes; to the observer’s left in southern latitudes. (Pronouncedlike “Bice BaLots.”)

by the leeSailing on a run with the wind on the same side as the mainboom, making a sudden jibe possible.

Ccabin

The enclosed or decked-over living space of a boat.

cable1. A number of conductors individually insulated and boundtogether as a group. 2. A heavy-stranded conductor such asis used for battery connections. 3. A strong, heavy rope orchain used for mooring. 4. A unit of length equal to 720 feet.

calorieThe quantity of heat required to raise the temperature of 1gram of water 1°C.


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cam1. A moving piece of machinery that imparts motion to anadjacent component. In engines, used to refer to a camshaftor one of the cams on a camshaft. 2. An elliptical protrusionon a camshaft. See also lobe.

camber1. Draft or fullness in a sail. 2. Athwartship arching of deckor cabin top.

cam cleatA fitting that grips a rope in the teeth of two rotating cams.

cam or breaker camMulti-lobed cam rotating in the ignition distributor; serves tointerrupt the primary circuit to induce a high-tension spark forignition.

camshaftA shaft with cams, used to operate the valve mechanism onan engine.

capacitanceThe property that permits the storage of electrically separatedcharges when potential differences exist between two con-ducting materials.

capacitive reactanceThe amount of opposition to the flow of an alternating currentdue to the presence of capacitance in a circuit. Symbol XC;measured in ohms.


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capacitorAlso called a condenser. A device that can store an electricalcharge on conducting plates separated by an insulating me-dium.

capsizeTo turn over, upset.

carA sliding bracket to which a block may be attached. The carslides along a track to provide a range of positions for theblock.

carburetorA device for automatically mixing fuel in proper proportionwith air to produce a combustible mixture.

carrierThe center or reference radio frequency upon which informa-tion is impressed in radio transmissions.

carry awayTo break free and become lost; said of gear that is stressedbeyond its strength or its fastenings.

carry her way, toThe movement of a boat through the water as a result of mo-mentum.

carvelSmooth-planked hull construction.

cast offTo let go a line; to set loose, unfasten; to undo all mooringlines in preparation for departure.


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cat’s pawA small, isolated puff of wind that ripples the water surface ina small area.

catamaranBoat with twin, narrow hulls connected by a deck orcrossbeams, resulting in a wide beam and good stability.

catboatSailboat with a mast stepped near the bow, and no jib.

cathodeThe negative pole or electrode of an electrolytic cell. The el-ement from which the current leaves the electrolyte.

cathodic protectionReduction or prevention of corrosion by the use of sacrificialanodes.

caulkFill seam with a material to prevent leakage.

cavitationA partial vacuum created in the water around a rapidly re-volving propeller that loses contact with the water in which itis turning.

CB radioSee citizens band radio.

ceiling1. The height above the ground at which there appears to besix-tenths or more cloud cover. 2. Inner sheathing of a boat.


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celestial equatorA great circle located at the intersection of the extended planeof the earth’s equator and the celestial sphere, thus 90° fromthe celestial poles. Declination of celestial bodies is measuredfrom the celestial equator.

celestial horizonA great circle at the intersection of an extended plane throughthe center of the earth and the celestial sphere, parallel to theobserver’s horizon and perpendicular to the zenith-nadir line.

celestial polesThe intersections of the extended axis of the earth with thecelestial sphere; labeled Pn and Ps.

celestial sphereAn imaginary globe of infinite radius, concentric with the earth,on which the stars seem to be fixed and the sun, moon, andplanets appear to wander.

celestial triangleA spherical triangle on the celestial sphere, especially the navi-gational triangle.

cellA single unit that either converts chemical energy to electri-cal energy or the reverse of this process.

Celsius (C)A temperature scale (formerly called centigrade) on which 0°represents the freezing point of water and 100° the boilingpoint at standard pressure (1,013 millibars). Formula for con-verting to Fahrenheit: F° = 9/5 C° + 32


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centerboardA pivoted board that can be lowered through a slot in the keelto reduce leeway.

centerlineFore-and-aft line that runs along the exact center of a boat.

center of buoyancyThe center of gravity of the water displaced by a floating boat.

center of effortStatic: the geometric center of a single sail or the combinedgeometric centers of several sails. Dynamic: the point whereall the lateral forces of the sails of a boat under sail could beconcentrated without changing their effect.

center of gravityThe point from which a body could be freely suspended with-out rotating in any direction.

center of lateral resistanceStatic: the exact middle of the lateral plane. Dynamic: the pointwhere all lateral forces acting on a hull moving through thewater could be concentrated without changing their effect.

Centigrade temperature scaleSee Celsius.

centrifugal forceA force that tends to move a body away from its center ofrotation. Also, the apparent force that deflects wind outwardin a rotating system.

cetaneA measure of the ignition quality of diesel engine fuels.


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chafeWear on a sail, spar, or line. Prevented by chafing gear—sacrificial coverings, such as leather, cloth, or rubber.

chafing gearSacrificial wrapping around lines, rigging, or spars to preventwear.

chain plateMetal strap on a sailboat, usually secured to the hull or bulk-head, to which a shroud or stay is attached.

change of stateThe process causing a material to change form from one state(solid, liquid, gas) to another (e.g., melting or boiling).

channelPortion of a waterway that is navigable, usually marked, andhaving a known depth of water.

chartA map intended primarily for navigational use by aircraft orvessels.

charterTo rent a boat for a predetermined price and time.

cheaterA low-cut sail set below a high-flying spinnaker.

check valveA gate or valve that allows passage of gas or fluid in onedirection only.


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cheek blockA pulley with one flat side so that it can be mounted on a sparor structure.

chineLine formed by intersection of sides and bottom of a flat- orV-bottomed boat.

Chinook windName given to a warm, dry wind on the east side of the RockyMountains, caused by a rush of cold air down the mountainside;the air warms due to an increase in pressure produced bydecreasing altitude. See foehn wind, Santa Ana wind.

chock1. Fitting to guide a line or cable. 2. Wedge or block to keepan object from moving.

choke1. A reduced passage. 2. In an engine, the valve used in thecarburetor air inlet to reduce the volume of air admitted.

choke coilA coil of wire or inductor used to oppose changes in currentflow. Normally used to impede an unwanted alternating cur-rent from a circuit for interference suppression.

chordThe straight line between the luff and leech of a sail.

chronometerA timepiece with a nearly constant rate of gain or loss, madeespecially to keep accurate time.


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chuteSlang for spinnaker (because it resembles a parachute).

circle of equal altitudeA circle on the surface of the earth from any point of whichthe altitude of a celestial body is the same at a given instant.

circle of positionCircular line of position determined by a distance from an ob-ject or point or celestial body.

circuitPath of electric current, fluids, or gases.

circuit breakerAn automatic device that, under an overload condition, willopen a current-carrying circuit to prevent damage. It can bere-set to reestablish the circuit.

circulationPrimary wind circulation at various latitudes when unaffectedby storms and local phenomena (e.g., northeast trade winds).

circumpolarRevolving about the elevated pole without setting.

cirrocumulusA form of high cloud made up of small, rippled, scale-like orpad-like elements. It does not show gray shadows at the basesof thicker spots as altocumulus does.

cirrostratusA form of high cloud appearing as a whitish veil.


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cirrusA form of high cloud composed of detached white filaments,patches, or narrow bands. Sometimes called mares’ tails.

citizens band radioAn unlicensed (non-marine) type of 2-way radio.

civil twilightThe period of incomplete darkness when the upper limb of thesun is below the visible horizon, and the center of the sun isnot more than 6° below the celestial horizon. Useful in takingsights on stars or planets.

claw offTo beat off of a lee shore under difficult conditions.

clear astern; clear ahead; overlapSailing terms describing the relative positions of two boats closeenough to each other to possibly restrict their maneuverabil-ity. A boat is clear astern of another when her hull and equip-ment in normal position are behind an imaginary line abeamfrom the aftermost point of the other’s hull and equipment innormal position. The other boat is clear ahead. They over-lap when neither is clear astern of the other or when a thirdboat between them overlaps both.

cleatFitting, usually with two projecting horns, to which lines aremade fast.

cleat hitchA figure-eight hitch used to attach a line to a cleat.


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clevis pinAn unthreaded locking pin, with a head at one end and a holeat the other end, through which a cotter pin or split ring maybe inserted to prevent the clevis pin from being withdrawn.

clewAfter lower corner of a triangular sail. Of a spinnaker, thelower corner attached to the sheet.

climateAverage weather conditions of a place or region based onyears of observation.

clinker-builtType of hull construction where the lower edge of each plankoverlaps and is fastened to the upper edge of the plank belowit. Also called lapstrake.

clinometerAn instrument, usually a bubble in a curved tube filled withwater, that indicates the degree of heel angle.

close aboardClose enough, in the judgment of the skipper, to a charted ob-ject to consider the object's position a fix.

close-hauledSailing as close to the wind, or as directly into the wind, aspossible. Also on-the-wind or by-the-wind, beating.

close reachThe point of sail between close-hauled and beam reaching.

close-reefedWhen all possible reefs have been taken in.


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cloudA visible cluster of minute water droplets or ice crystals in theatmosphere above the earth’s surface.

clove hitchA knot used to secure a line temporarily to a bitt, spar, orrope.

club-footed jibA jib with a boom.

coalescenceThe process by which raindrops merge and grow in size.

co-altitude (co-alt)Ninety degrees minus the altitude of a body above the celes-tial horizon. Same as zenith distance. The side of the naviga-tional triangle between the AP or DR and the GP of the body.

coamingVertical structure around a hatch, cockpit, or skylight to repelwater.

coaxial cableA transmission line in which one conductor completely sur-rounds the other, the two being coaxial and separated by adielectric. There is no external radiated field and the line isnot susceptible to other external fields.

cockpitWell or sunken space in the deck.


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co-declination (co-Dec)Ninety degrees minus the declination of a body; when decli-nation and latitude have the same name; co-declination is thesame as polar distance measured from the elevated pole; theside of the navigational triangle between the GP and the el-evated pole.

coil1. To arrange a line in loops. 2. An electrical device com-posed of a number of turns of wire in spiral form for electro-magnetic effect or for providing electrical resistance.

colThe intersection of a ridge of high pressure and a trough oflow pressure. An area of little pressure gradient and light wind.

co-latitude (co-L)Ninety degrees minus the latitude of the observer. The side ofthe navigational triangle between the AP or DR and the el-evated pole.

cold cranking amperes (CCA)The number of amperes a battery will supply to a load for 30seconds at 0°F with the terminal voltage not dropping below7.2 volts in a 12-volt system.

cold frontThe boundary between the forward edge of an advancing coldair mass and the rear edge of a retreating warm air mass.

cold sectorLocation of the area surrounding a low-pressure frontal sys-tem (usually behind the cold front), where the colder air massis in contact with the earth’s surface.


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collision matA tarpaulin with lines leading from its corners to cover, fromthe outside, a breach in the hull below the waterline. Also,anything usable as a substitute for this.

color codeDifferent colors of insulation used on conductors to identifytheir use in wiring circuits. Also used to identify the value ofcertain types of components.

COLREGSInternational Regulations for Preventing Collisions at Sea, in-cluded as the International Rules in the Navigation Rules pub-lished by the United States Coast Guard and distributed by theGovernment Printing Office. These Rules are applicable onwaters outside of established navigational lines of demarca-tion. The lines are called COLREGS Demarcation Lines anddelineate those waters upon which mariners shall comply withthe Inland and International Rules.

combustion chamberThe space left in a cylinder (and cylinder head) when a pistonis at the top of its stroke.

come aboutSignificant change in direction; in sailing, changing course forthe bow to swing through the eye of the wind and pay off onthe other tack.

common railA type of fuel injection in which fuel circulates to all injectorsall of the time. Each injector contains its own injection pumpwith this system.


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commutatorCopper segments on the rotor of a DC motor or generatorused to conduct current through brushes to or from the rotat-ing coils.

companionwayStairway or ladder leading below from the deck.

compass conversionA standard system employed to convert True values to Com-pass values, and vice versa, by the addition or subtraction ofvariation and deviation.

compass courseThe course by boat’s compass. The angle between the boat’skeel and the north point of the compass card when the boat ison course.

compass errorCombined effect of variation and deviation.

compass roseA circular diagram on a chart showing directions, usually tothe nearest degree. The outer ring (rose) shows directionsrelative to the geographic north pole (true direction). A con-centric inner rose shows directions relative to the magneticnorth pole at that location. The offset between the two rosesis the variation at that location. The numerical value of thevariation and its rate of change are often indicated in the cen-ter of the rose.


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compressibleCapable of being reduced in volume without loss of mass. Thisproperty is, for all practical purposes, limited to gases as op-posed to liquids and solids, and is due to the lack of any mo-lecular structure in gases.

compression postThe post between the keel and cabin top supporting a deck-stepped mast.

compression ratioThe volume of a cylinder with the piston at the bottom of itsstroke, compared to the volume of the cylinder when the pis-ton is at the top of its stroke.

compulsorily equippedA vessel required by FCC rules or International Law to haveradio transmitting and receiving equipment on board.

computed altitude (Hc)Altitude of a body above the celestial horizon from a deadreckoning position (DR), an assumed position (AP), or a knownposition (KP) at a given time, as determined by computation,usually with the aid of a sight-reduction table or calculator.

computed course (Cn)Course angle computed from true north.

concentricHaving a common center, as circles or spheres.

condensationThe process by which water vapor is changed to liquid water.


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condensation nucleiMinute, usually water-absorbent particles on which water va-por condenses or sublimes.

condenserSee capacitor.

conditional instabilityState of the atmosphere in which rising air will stop rising (sta-bility) if it is not saturated, and will continue to rise if it is(instability). A condition under which air-mass thunderstormsmight form.

conductionA process of heat transfer involving heat flow through a bodyor from one body to another in physical contact with it. This isa mechanical transfer of molecular thermal energy.

conductorA material that has many free electrons and will readily carryan electrical current.

cone of protectionThe area surrounding a grounded conducting mast that is thecenter of a cone whose inner surface is at an angle of 60° tothe mast. This area will be more or less free from lightningstrikes, protected by the mast.

conformal projectionA projection in which all angles around any point are correctlyrepresented.

connecting rodThe rod connecting the piston to the crankshaft.


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connecting rod bearingThe bearing at the crankshaft end of the connecting rod.

connecting rod capThe housing that bolts to the end of a connecting rod, holdingit to a crankshaft.

conservation of angular momentumA body moving in a circular path has a mass (m), a speed (v),and a radius or distance from the center of rotation (r). Oncethe mass is set in motion, its angular momentum (mvr) re-mains constant unless some unbalanced outside torque exists.

conservative propertyA property that remains unchanged while certain other prop-erties vary (e.g., dew point, which remains virtually unchangedeven though the temperature of the air changes).

constellationOne of the 88 recognized groups of fixed stars, or the regionof the sky in which the group appears.

contact pointsSee breaker points.

continental air massAn air mass that originated over land.

continuous refractionThe refraction due to increasing density of air with decreas-ing altitude.

convectionThe transfer of heat by fluid motion; in a meteorological sense,generally by vertical air currents. See advection.


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convergenceThe phenomenon of air flowing to a point within a layer of theatmosphere. Since this air cannot be collected at the point ofconvergence, a vertical exhaust current develops.

Coordinated Universal Time (UTC)For purposes of marine navigation, equivalent to Greenwichmean time.

cordageA general term for all types of rope.

Coriolis effectApparent deflection of a moving object due to rotation of theobserver’s frame of reference. In meteorology, the Corioliseffect is important in that it explains the apparent deflectionof wind relative to the observer, whose reference, the earth,is rotating beneath the moving air.

coronaA narrow, luminous ring around the sun or moon resulting fromdiffraction of light through thin, water-droplet clouds. Bandsof colors can sometimes be distinguished, like those of a pri-mary rainbow, but fainter. See halo.

corrosionChemical change or displacement of a metal at an anodic polefrom which current flows to enter an electrolyte.

cotter pinA split pin, the ends of which can be bent after insertion througha hole.


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counterAbove-water stern extending back from the after end of thewaterline. See also buttock.

couplingA connecting means for transferring movement from one partto another.

course (C)The direction in which a boat is steered through the water.

course made good (CMG)The resultant direction from a point of departure to a point ofarrival. (This is a term in common usage, although course is amisnomer. The path of a boat with respect to the bottom is itstrack.)

course over groundActual direction of travel of a boat over the bottom. (This is aterm in common usage although course is a misnomer. Thepath of a boat with respect to the bottom is track.)

course steeredThe direction in which the bow of the boat is pointed whenunderway.

coveA groove or slot built into the boom, the roller furling airfoil, orafter edge of a mast.

cowlA hooded opening in the deck for purposes of ventilation.

CPRCardio-pulmonary resuscitation.


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CQR® anchorA plow anchor with a single fluke shaped like a plowshare,which digs itself deeper under heavy horizontal strain.

cradleA fitted frame by which a boat is supported during storage orshipping.

crankAn offset section of a crankshaft to which a connecting rod isattached.

crankshaftThe main part of an engine that changes the reciprocatingmotion of the pistons into rotary motion.

crazeTo develop small cracks emanating outward from a centralpoint.

crimp terminalsMetallic terminals that are designed to clamp on a conductorby compression.

cringleA ring sewn into a sail through which a line can be passed.See grommet.

critical tableA table in which values of the quantity to be found are tabu-lated for limiting values of the entering argument. No interpo-lation is required.


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crownThe point on an anchor where the shank joins the arms. Also,deck camber.

crystalA device to establish an electrical signal of precise frequency,in radio, radar, etc. Usually a precisely ground quartz chip thatwill vibrate at a constant frequency when subjected to a volt-age. Crystals of other materials are sometimes used for spe-cial applications, such as the barium titanate crystals used indepth sounders.

cuddyA shelter cabin in a small boat.

cumulonimbusA form of cloud with extensive vertical development, usuallywith a visible anvil top and showers falling from the base. Thethunderstorm cloud.

cumulusA form of cloud showing vertical development, generallydome-shaped on top and flat on the bottom.

cunningham (rig)An arrangement of gear used to adjust the luff tension in asail without using a halyard or moving the boom.

current1. The horizontal movement of water; a horizontal motion ofsuch set and drift as to account for the difference between adead reckoning position and a fix at the same time. 2. Themovement of electrons through a conductor.


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cutThe angle of intersection of the lines of position in a fix.

cutless bearingA rubber-sleeved bearing that supports the propeller shaft inthe stern of a boat.

cutterA single-masted boat with mainsail and usually two headsails,with the mast stepped close to amidships (from 40% to 50%aft of the bow, versus 33% for a sloop).

cycloneA low-pressure area (lowest pressure near the center) in whichthe wind is spiraling inward in a counterclockwise pattern(Northern Hemisphere). This may be an extratropical cyclone,usually involving fronts and contrasting air masses, or a tropi-cal cyclone (hurricane). See anticyclone.

cylinderA round chamber bored to receive a piston.

cylinder blockThe largest single part of an engine. Basic or main mass ofmetal in which cylinders are bored or placed.

cylinder headA detachable portion of an engine fastened to the top of thecylinder block.

cylinder head gasketSeal between engine block and cylinder head.


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cylinder liner or sleeveA machined sleeve that is pressed into a block, and withinwhich a piston moves up and down. Provides a renewablewearing surface.

Ddaggerboard

A sliding board that can be lowered through the keel to re-duce leeway.

Danforth® anchorAn anchor with long, pointed, hinged flukes with great holdingpower. (Fortress®, West Marine®, Hooker®, and Suncor® areother examples of this type of anchor.)

dangerous semicircleIn a tropical cyclone (Northern Hemisphere), the dangeroussemicircle is the side to the right of the storm track as youface the direction the storm is moving. Wind in this semi-circleis the sum of the cyclonic wind plus the velocity along thestorm track. In the Southern Hemisphere, the left side is knownas the dangerous semicircle.

datum1. The reference plane from which depths of water are mea-sured and recorded on charts (as “charted depths” or “sound-ings”) and, in coastal waters, to which height of tide is addedalgebraically to determine depth of the water. 2. A mathemati-cal representation of the irregular, not-quite-spherical shapeof the earth; defines latitude and longitude for the area cov-ered.


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davitCrane-like device for hoisting small boat, anchor, or cargo.

Davy Jones’ lockerThe bottom of the sea.

daybeaconUnlighted fixed aid to navigation.

daylight saving timeA variation of standard time in order to make better use ofdaylight.

daymarkUnlighted fixed aid to navigation.

daysailorSailboat without amenities, such as a cabin, used for shortduration sailing or racing.

day shapeAn object of specified shape and size used to describe a ves-sel and its activity in daytime.

day tankA small tank used to supply fuel or water to be used immedi-ately.

dead reckoning (DR)Calculating a boat’s position based on its course, speed, andtime run from a previous position.

dead reckoning trackThe intended course line of a vessel as plotted on a chart.


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deadriseThe athwartships angle between a line horizontal to the top ofthe keel and a line from the top of the keel to the chine or theturn of the bilge above it. Usually expressed in inches for eachfoot of distance from the keel.

dead runThe point of sail on which a boat has the wind blowing straightover the stern.

decarbonizingThe process of removing carbon deposits from the inside ofan engine.

decibel (db)The standard unit for measuring relative power levels. Whenan electrical signal is “attenuated” or “amplified,” the loss orgain is measured in decibels. It is also used to indicate powerlevels above or below a standard reference level. The unit isbased on a logarithmic function so that every time the poweris doubled, three decibels are added.

deckThe portion of the boat that covers the hull.

deck beamHorizontal support running athwartships on which the deckrests.

deck drainOpenings in the deck or cockpit to drain water overboard.

deck houseA partially enclosed structure erected on the deck.


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deck logJournal kept on board to record all information pertaining toboat movement, position, and important events.

declination (Dec)Angular distance of a celestial body north or south of the ce-lestial equator; the arc of an hour circle between the celestialequator and a point on the celestial sphere.

decompression leversLevers that hold the exhaust valves open so that no compres-sion pressure is built up, making it easy to turn the engineover.

deep cycleTerm used for a battery designed to supply a relatively lowcurrent for an extended period as opposed to an automotivetype battery that supplies very high starting currents for shortperiods.

deepeningThe lowering of the central pressure within a cyclone. Deep-ening is often associated with increased wind velocity and in-creased size of the low. Opposite of filling.

demodulationThe process of recovering information from a radio signal.

densityThe mass per unit volume. At sea-level pressure, air is com-pressed and therefore has a higher density than it would havealoft under lower pressure. (At sea level, air density is ap-proximately 1.2 ounces per cubic foot.)


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departure (p)Term used in sailing for distance made good to the east orwest on any course, expressed in nautical miles. Do not con-fuse with difference of longitude (DLo).

depressed poleThe celestial pole below the horizon; it is of contrary name tothe latitude.

depth sounderAn electronic device that uses the time lapse between a trans-mitted ultrasonic pulse and its received echo to determine thedepth of water.

desalinatorA device used to remove salt from seawater to make it po-table.

detonation (pinging)A too-rapid burning or explosion of the mixture in the enginecylinders (gas engine); usually audible upon acceleration ofthe engine.

deviation (D, Dev)Disturbing effect of boat’s magnetic field upon its compass.Numerical difference in degrees, measured east (E) or west(W), between the magnetic value and the compass value of agiven direction.

dewWater condensed on objects the temperature of which hasfallen below the dew point of the surrounding air.


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dew pointThe temperature to which the air must be cooled at constantpressure and moisture content to reach saturation.

dew-point spreadThe difference between the air temperature and the dew point.Small dew-point spreads are an indication radiation fog mayform.

DGPSDifferential Global Positioning System. An extension of GPSwhere a correction signal is transmitted from a surveyed lo-cation to improve the positional accuracy of the GPS fix. Seedifferential GPS receiver.

diaphragmA flexible partition or wall separating two cavities used in cer-tain type pumps.

dielectricA non-conducting material with few free electrons. A mate-rial with insulating properties.

dieselingThe continued running of an internal combustion engine afterignition is turned off.

difference of latitude (l)The shorter arc of a meridian between the parallels of twoplaces, expressed in angular measure.


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difference of longitude (DLo)The shorter arc of a parallel between the meridians of twoplaces, expressed in angular measure. Numerical value is thesame at any latitude. Do not confuse with departure (p).

differential GPS receiverA receiver of signals from ground-based transmitters that can-cels out the random errors introduced into Global PositioningSystem satellite signals through “selective availability.” SeeDGPS.

diffractionThe optical phenomenon caused by light from a point sourcebending around an obstacle. The resulting shadow is outlinedwith multiple bands of light and dark (diffraction bands).

diffusion of lightThe scattering and crisscrossing of light rays, producing gen-eral illumination rather than direct radiation.

digital1. Values of measurements displayed as numbers. 2. Circuitsthat perform their operations using binary mathematics, as dis-tinct from analog circuits.

digital displayA number flashed on the display unit representing a physicalvariable.

dinghySmall open boat used as a lifeboat or tender.


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diodeAn electronic device that permits current to flow in one direc-tion.

dip (D)The angle of depression of the visible sea horizon from thehorizontal due to the elevation of the eye above sea level.

dip short (DS)A sight in which the body is brought down short of the naturalhorizon.

direct current (DC)A current of either constant or variable magnitude that flowsin one direction only.

discontinuityThe surface between two transparent substances, such as airand water or air and glass, at which refraction occurs.

dismastTo lose a mast on a sailing craft.

dispersionAn optical phenomenon in which white light is separated intospectral (color) bands on passing through a nonuniform me-dium. This occurs because different wavelengths are bent (re-fracted) at different angles, spreading out into a spectrum.

displacement1. The total swept volume of an engine’s cylinders, i.e., throughwhich the pistions actually move, expressed in cubic inches orliters. 2. The weight of water displaced by a floating vessel;hence, the weight of the vessel itself.


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displacement hullA hull that maintains its full displacement of water whether atrest or moving at various speeds (as opposed to a planing hull,which generally decreases its displacement with increasedspeed).

distance (D)The spatial separation of two points and the length of the linejoining them. Navigators use nautical miles. See nautical mile.

distress callSee Mayday.

distribution systemA system for the distribution or routing of power to variousbranch circuits. It may contain switches and overload devicesfor each circuit.

distributorSee ignition distributor.

distributor pointsSee breaker points.

distributor pumpA type of fuel injection pump using one central pumping ele-ment with a rotating distributor head that sends fuel to eachcylinder in turn.

ditty bagA small bag for stowage (e.g., sewing kit, or small tools).

diurnalDaily phenomena that occur in 24-hour cycles—such as maxi-mum and minimum temperatures—are said to be diurnal.


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diurnal pressure changeA regular daily change in the pattern of barometric pressure.This change, usually small and not due to moving highs or lows,is most prominent in lower latitudes and in settled weather.

divergenceExpansion in the volume of a mass of air over a region, result-ing in a net horizontal outward flow. This may be caused bythe sinking of the air aloft. Opposite of convergence.

dock1. Area of water, within which a vessel can be made fast,between two landing piers or wharves. 2. To guide a vesselalongside a float, pier, or wharf.

documentationThe registration of a vessel with the United States Govern-ment (U.S. Coast Guard) to establish ownership and national-ity.

dog watchA watch of half usual duration; used to stagger the watchperiods from day to day.

doldrumsThe belt of comparatively low pressure, calm or light winds,and high temperatures within a few degrees of the equator.

dolphinA small group of piles, in the water, tied together into a singlestructure, generally used for mooring or as part of an aid tonavigation.


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dolphin strikerA short spar fitted below a bowsprit, over which the bobstayis fitted.

Doppler radarRadar system that not only detects the presence of moisture(or other objects) but also measures their quantity and direc-tion, enabling measurement of rainfall and determination ofserious conditions such as tornadoes.

Doppler shiftChange in the frequency with which waves (e.g., sound, light,or radio) from a given source reach an observer, caused bythe change in the relative speeds of the source and observer.

dorade ventA ventilator that does not permit water or spray to pass belowwith the fresh air.

double sheet bendA sheet bend with a second round turn on the standing bight.

double side bandUsually refers to amplitude moderated (AM) radio transmis-sion signals containing both upper and lower side-band com-ponents.

douseTo lower quickly.

downburstA strong downdraft that induces an outburst of damaging windsat or near the surface of the earth.


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downdraft1. A downward current of air, comparatively strong and lim-ited in horizontal extent, commonly found within thunderstormsand other areas of atmospheric turbulence. 2. Type of carbu-retor in which the fuel-air mixture flows downward to the en-gine.

downhaulA line or tackle used to exert a downward pull on a sail orspar.

draft1. Depth of water needed to float a boat. 2. The fullness or“belly” of a sail.

drainage windsCold air accumulated inland, usually at high elevations, thatflows down through valleys under the influence of gravityrather than pressure gradient.

drift (DFT)1. Movement of a boat due to wind and current. 2. Velocity(speed) of current.

drizzlePrecipitation consisting of numerous tiny droplets, sometimescalled mist.

drogueDrag deployed off the stern of a boat to create resistance andslow the boat’s speed in heavy weather, but still allow steer-ageway. Often conical or a series of conical shapes. Com-pare sea anchor.


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DR positionA position determined by dead reckoning.See dead reckoning.

dry bulbName given to an ordinary thermometer, or the thermometerwithout wicking in a psychrometer. Also, the air temperaturetaken with a dry-bulb thermometer, as distinguished from thewet-bulb temperature.

DSCDigital selective calling. A semi-automated system for estab-lishing communication with selected radio stations.

ductingA phenomenon affecting the transmission of VHF signalswhen they become trapped between upper-air temperatureinversions. The signals may travel for great distances beforereturning to earth.

Dutchman®A mainsail furling system that automatically flakes the sail onthe boom, using vertical polymer lines that pierce the sail onalternate sides.

duty cycleThe length of time a piece of equipment may be turned on ascompared to the total time it is on and off.

dwell angle/period (cam angle)Number of degrees of rotation of the distributor shaft duringwhich contact points are closed.


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Eearing

A line that secures a cringle to the boom.

earth groundA point that is at the same electrical potential as the surround-ing earth.

easeTo let out a line gradually.

easterliesBelts of persistent easterly winds such as the trade winds andthe polar easterlies.

easterly waveA zone, or trough, of low pressure and convergence embed-ded in the trade-wind belt, drifting from east to west.

ebb currentA tidal current in which the flow of water is generally awayfrom the major land mass toward the open sea (often incor-rectly termed “ebb tide”).

eclipticThe great circle in which the sun appears to move on the ce-lestial sphere by reason of the annual revolution of the eartharound the sun. This great circle is inclined at about 23°27' tothe celestial equator, because the axis of rotation of the earthis not perpendicular to its orbit. This makes the sun seem tomove north and south during the year and gives the earth itsseasons. See equation of time.


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eddyA small whirl or circling movement of a fluid, such as air orwater, embedded within a larger current.

electrolysis1. Chemical effect caused by the flow of an electric currentthrough a conducting fluid. 2. The decomposition of metalcaused by stray electrical current.

electrolyteAny liquid solution that is capable of conducting an electriccurrent.

electrolytic actionA plating and eroding action that occurs when dissimilar met-als are immersed in an electrolyte such as salt water.

electromagnetic fieldA field of force produced around a conductor when a currentflows through it.

electromagnetic spectrumThe entire range of wavelengths of energy (including light)reaching the earth’s outer atmosphere from the sun. The at-mosphere, ozone, and ionized gas layers, as well as the earth’smagnetic field, shield us from much of this radiation. Of theenergy that reaches the earth, 90% is in the visible portion ofthe spectrum.

electromotive force (E or EMF)Voltage or electrical difference of potential generated by abattery or a generator. See potential difference.


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electronsThe negatively charged particles surrounding the nucleus ofan atom. These particles determine the chemical and electri-cal characteristics of the atom.

electrostatic fieldElectric force field set up in the vicinity of electrically chargedmatter.

elevated poleThe celestial pole above the horizon, agreeing in name withthe latitude.

elevatorA wedge, either attached to or built into the bottom of a ves-sel, for the purpose of keeping its stern up when underway.Called “trim tabs” when adjustable.

El NiñoA slackening of the (easterly) trade winds accompanied by aflow of warm sea water from the western Pacific back to theeast. These changes in water temperature are known to alterair temperature, rainfall, and storm formation patterns overmuch of the earth. See La Niña.

ellipseThe curve formed when a cone or circular cylinder is cut acrossat an angle. When the angle is other than perpendicular to theaxis of the body being cut, the curve is not a circle.

embarkTo go aboard.


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Emergency Position Indicating Radio BeaconSee EPIRB.

energyThe capacity for performing work.

ephemerisA collection of tables or data showing the position of the plan-ets or heavenly bodies for every day of a given period; also,an astronomical almanac containing such tables.

EPIRBEmergency Position Indicating Radio Beacon. A small trans-mitter operating on aircraft, marine, or satellite emergencychannels used in cases of distress.

equation of time (EqT)The amount of time by which the mean sun leads or lags be-hind the true sun at any instant.

equatorial air massAn air mass that originated and derived its temperature andmoisture characteristics from the region of the equator.

equilibriumThe state of air in vertical balance with other surrounding air—that is, having no tendency to move vertically with respect toits surroundings.

equinoctialSee celestial equator.

estimated position (EP)Most probable position of a boat based on a single line of po-sition (LOP) or based on incomplete or questionable data.


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evaporationThe physical process of transformation of a substance fromthe liquid to the gaseous state. Opposite of condensation.

even keelA boat is on an even keel when it is floating level.

exhaust manifoldThe assembly that conducts exhaust gases out of the combus-tion chambers of an engine.

extratropical cycloneA low that develops outside the tropics. Involves two or moreair masses, with the fronts that separate them.

eyeThe center of a tropical storm (a hurricane or a typhoon). Theeye is a roughly circular area of low pressure, with compara-tively light winds and fair weather.

eyeletA small grommet through which lacing is passed.

eye of the windDirectly upwind.

eye spliceA permanent loop in the end of a rope.


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FFahrenheit (F)

A temperature scale on which 32° represents the freezing pointof water and 212° the boiling point at standard pressure. For-mula for converting to Celsius (Centigrade):

C° = 5/9 (F° – 32)

fairing blockAn underwater block mounted on the hull of a vessel andshaped to provide a laminar flow of water around an obstruc-tion.

fairleadFitting to route controls, lines, or cables.

fairwayNavigable channel in a body of water.

fake down1. To coil a line so that it will run free. 2. The mainsail may befaked or faked down onto the boom.

fallThe line in a tackle.

fall offTo change course so as to bring the relative wind directionfarther aft.

family radio serviceAn unlicensed (non-marine) type of two-way radio.


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faradThe unit of capacitance. A unit of measurement of the amountof electrostatic charge that can be stored.

fastSecure: to make something fast is to secure it.

fasteningsScrews or nails used to hold parts of a boat together.

fathomNautical linear measure equal to 6 feet.

FCCFederal Communications Commission.

fenderProtective device between a boat and another object.

fender railA fixed molding on the hull to keep a vessel from scraping thewharf.

fetchThe unobstructed distance over the water on which the windmay act to build wind waves.

F-head engineAn engine designed with one valve in the cylinder block at theside of the piston and one valve in the cylinder head above thepiston.

fiberglassA glass fiber usually impregnated with a synthetic resin suchas polyester or epoxy; may be woven or random fibers.


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fidTapered tool used in splicing. See marlinspike.

field coilA coil of insulated wire surrounding the field pole of a genera-tor or starter.

figure-eight knotA stopper knot; used to keep the bitter end of a line fromrunning through a block.

fillingThe process of normalizing or increasing the central pressureof a low so that it becomes less intense or disappears. Fillingis the opposite of deepening.

filter (oil-water-fuel)A unit containing an element or screen to retain foreign par-ticles in the fluid.

fin rudderA rudder fastened directly to the after edge of a keel.

first point of AriesAn imaginary point at the intersection of the celestial equatorand the ecliptic, where the declination of the sun is zero as itpasses over the equator on the way northward at the begin-ning of spring (about 21 March). Also, the instant when thesun crosses the equator. Often called Aries. Same as vernalequinox.

fisherman’s bendA knot used to connect a rode to an anchor.


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fixRelatively accurate position determined without reference toany former position. Usually determined by nearness to aknown charted object or by crossed (intersecting) lines ofposition.

flakeTo fold a sail on the boom.

flame arrestorA screen-like metal fitting fastened over a carburetor air in-take to keep backfire flames from flashing out into an enginecompartment where they could ignite gasoline fumes.

flare1. Outward curve of the hull towards the deck. 2. A blaze offire or light used to attract attention. 3. The device that pro-duces the blaze.

flatboatHigh-speed, outboard-powered fishing boat used in shallowcoastal waters.

flaxA natural fiber used in packing, e.g., around a shaftlog or rud-derpost at the point of hull penetration.

float levelThe predetermined height of the fuel in a carburetor bowl,usually regulated by a needle valve or float.


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float planAn outline plan of a cruise left at one’s home port or withfriends to provide a description of the boat, list of persons onboard, list of safety equipment carried, and, most importantly,where the boat will be cruising and when it is expected toreturn to home port.

flood currentA tidal current in which the flow of water is generally to-wards the major land mass from open sea (often incorrectlytermed flood tide).

flukeFlattened end of an anchor arm that bites into the ground.

fly1. A masthead wind pennant. 2. Length of a flag or burgeeperpendicular to the luff. 3. Part of flag farthest from the hoist.

flybridge (flying bridge)A steering position located atop the cabin of a powerboat.

flyweightA small pivoted weight used in mechanical governors.

flywheelA heavy wheel (usually steel or iron) in which energy is ab-sorbed and stored by means of momentum.

foehn windName given to a warm, dry wind caused by a rush of cold airdown the mountainside; the air warms due to an increase inpressure produced by decreasing altitude. (Pronounced like“foon wind.”) See chinook, Santa Ana wind.


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fogMinute water droplets suspended in the atmosphere. Differsfrom a cloud only in that it is at the surface instead of aloft. Icefog (minute ice crystals) may occur in extremely cold regions.

footThe lower edge of a sail.

force1. A mechanical push or pull or an electromagnetic attractionor repulsion. 2. A unit of measure of wind velocity.

foreDenoting at, near, or toward the bow.

fore-and-aftParallel to the centerline or keel.

foredeckThe deck of a boat located near the bow.

foreguyA line led from the foredeck to a spinnaker pole to limit itsupward swing.

forepeakA stowage compartment in the very forward part of the boat.

forereach1. To carry way (continue to move by momentum) upwind asa boat heads into the wind with sails luffing. 2. To overtakeanother vessel while on the same course. 3. To make head-way while hove to.


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forerunnerAn observable phenomenon such as cirrus clouds or a long,low swell moving ahead of a severe storm. In the case of ahurricane, the swell is marked by an abnormally long period.

foresailThe sail set from a schooner’s foremast.

forestayA stay below and aft of the headstay on a sailboat.

forestaysailA sail, similar to a jib, set on the forestay.

foretriangleThe area formed by the headstay, mast, and deck.

forwardToward the bow.

foulJammed, entangled, not clear; being hindered or impeded.

foul groundBottom that is unsuitable as an anchorage because holdingqualities for an anchor are poor or obstructions exist.

foul-weather gearRain suit, sometimes including boots and/or hat.

founderTo swamp and sink to the bottom.

four-cycle engineSee four-stroke-cycle engine.


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four-stroke-cycle engineEngine in which explosion in a given cylinder occurs everyother revolution of the crankshaft. A cycle, also known asOtto cycle, is considered 1/2 revolution of the crankshaft.Strokes are intake, compression, power, and exhaust.

fractional rigA sailboat with the jibstay attached to the mast at a point otherthan the masthead.

frameAthwartship structural member (rib) that gives shape and stiff-ening to the hull and to which planking is attached.

freeboardVertical distance from the water surface to the lowest pointwhere unwanted water could come aboard.

free electronElectron that is loosely bound to the nucleus of an atom and isfree to circulate among the other atoms of a substance.

frequencyNumber of cycles per second.

frequency modulation (FM)Transmission of intelligence by varying a radio frequency aboutits center frequency or “carrier.”

frequency rangeA portion of the electromagnetic spectrum—for example, therange of frequencies for infrared, visible, or ultraviolet radia-tion, or the frequencies used for radio and radar.


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frontThe zone of transition between two air masses of differenttemperature characteristics.

frontal fogFog associated with a front, usually formed by rain fromwarmer air aloft falling through and saturating a cooler layerof air next to the earth.

frontal depressionAn area of slightly lower pressure along a front. This oftenindicates the formation of a new low-pressure center.

frontal surfaceThe three-dimensional surface separating two air masses ofcontrasting characteristics, extending from the surface posi-tion of the front up over the colder air mass.

frostA deposit of crystals of ice on cold objects by direct sublima-tion from the vapor.

fuel injection pumpA device for metering precise quantities of fuel at precisetimes and raising them up to injection pressures.

full and byClose-hauled with all sails full and drawing.

funnel cloudA tornado aloft, not touching the ground.

furlGather and lash sails to a spar.


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fuseA protective device containing a metal alloy of low meltingpoint designed to melt and break a circuit under conditions ofexcessive current flow.

fusionTransformation of a substance from the solid state to the liq-uid state by the induction of heat, as in the melting of ice.

fusion temperatureThe temperature at which a solid melts.

Ggaff

1. Spar that supports the upper side of a fore-and-aft four-sided sail. 2. Long-handled hook to bring fish aboard.

gaff rigAny sailboat with a four-sided mainsail.

galeA wind in the range of 39 to 54 miles per hour or 34 to 47knots. See Beaufort scale.

galleyArea where food is stored and cooked; nautical kitchen.

gallowsA frame to hold the boom when the mainsail is down or isbeing reefed.


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galvanic actionDecomposition of metals in an electrolyte due to their differ-ent electrical potentials. Although salt water is usually consid-ered as the electrolyte, rivers and lakes will have similar actiondue to pollutants in the water.

galvanic corrosionChemical destruction of metal due to galvanic action causinga current flow between two different metals connected to-gether and immersed in an electrolyte.

galvanic seriesA scale of potentials developed between different metalsbonded together and immersed in an electrolyte. For marinepurposes the electrolyte is assumed to be salt water.

galvanometerA meter or other device used for measuring very small cur-rents.

garboard strakeThe longitudinal plank next to the keel.

gasket1. A strip of cloth or line used to secure a furled sail to aboom (also, stop). 2. Anything used as a packing to form aseal between two metal surfaces.

gaugeWire size. American Wire Gauge (AWG) is a numerical siz-ing system based on the cross section of a conductor.


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gearGeneral name for all non-permanent nautical equipment, in-cluding crew’s clothing and personal effects.

gear ratioThe number of revolutions made by a driving gear comparedto the number made by a driven gear of different size.

gelcoatThe outside color coat used in fiberglass construction.

General gas lawThe relationship of the temperature, pressure, and volume ofa gas, including the atmosphere. One form of this law,P1V1 ÷ T1 = P2V2 ÷ T2, predicts how changes in atmosphericproperties occur.

generatorSimilar to an alternator with a wound coil rotor but providedwith a commutator to obtain direct current output.

Genoa blockThe movable block that enables adjustment of the sail’s luffcurvature to optimize sail performance.

Genoa jibA large overlapping headsail.

geographical position (GP)A point on the earth’s surface aligned with the center of theearth and a body’s position on the celestial sphere.

geoidal horizonThe intersection of the celestial sphere and a plane tangent tothe earth at the observer’s position.


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geometrical horizonIntersection of the celestial sphere and a cone tangent to thesurface of the earth, with the apex at the eye of the observer.

ghostingSailing in very light winds.

giga (G)Metric prefix meaning one billion (109).

gimbalsMounting supports permitting an object, such as the compass,to remain level when the boat heels.

give-way vesselA term, from the Navigation Rules, used to describe the ves-sel that must yield in meeting, crossing, or overtaking situa-tions. Compare stand-on vessel.

glazeA clear, solid coating of ice created by the impingement ofsupercooled liquid raindrops onto exposed surfaces. Glaze isusually associated with large water drops such as those foundin cumuliform precipitation.

Global Positioning System (GPS)A satellite-based world-wide navigation system using simul-taneous signals from three or more satellites to establish highlyaccurate positioning.

GLONASSGlobal Navigational Satellite System. The Russian Global Po-sitioning System, with better performance near the polar re-gions.


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glow plugA heating element installed in diesel engine combustion cham-bers to aid in cold starting.

GMDSSGlobal Maritime Distress and Safety System. Part of the In-ternational Maritime Organization (IMO) to define radio pro-cedures.

go aboutTo tack or come about.

gooseneckA mechanism used to attach a boom to a mast. To be effec-tive it must permit the unattached end of the boom to swing inany direction without restraint.

goosewing jibeA jibe in which either the top or bottom of the sail has failed tojibe with the rest of the sail.

governorA device to regulate or control engine speed, regardless ofthe load. It may be mechanical, hydraulic, or electrical.

gradientIn weather terminology, change in an atmospheric property,such as pressure with horizontal distance.

grapnel anchorAn anchor with four or five arms to snag projections on thesea bed; often called a grappling hook in smaller sizes.


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graticuleThe network of lines representing parallels and meridians ona map, chart, or plotting sheet.

gravity switchA switch, usually electrical, that is closed when the contact ispulled down by gravity.

great circleA circle on the surface of a sphere, produced by the intersec-tion of a plane that passes through the center of the sphere.

great circle chartA gnomonic-projection chart on which a great circle appearsas a straight line.

green flashA momentary flash of green color seen just as the sun’s upperlimb dips below the horizon. When seen, it is usually over oceanwaters in the tropics or subtropics, in fine, clear weather.

greenhouse effectMuch of the atmosphere is transparent to short-wavelengthheat energy from the sun. As solids and liquids on the earth’ssurface absorb this energy, they heat up and also give off heatenergy, but at longer wavelengths. Certain gases, such as car-bon dioxide, reflect or absorb this longer-wavelength energyand trap it, causing further warming. Glass in a greenhousealso does this; hence, the term.


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Greenwich apparent time (GAT)Local apparent time at the Greenwich meridian. The angle atthe celestial pole between the lower branch of the Greenwichcelestial meridian and the hour circle of the apparent or truesun.

Greenwich hour angle (GHA)The angular distance of the hour circle of a body westwardfrom the celestial meridian of Greenwich, measured from 0°to 360° along the celestial equator. GHA values for the sun,moon, planets, and stars are tabulated in the Nautical Alma-nac.

Greenwich mean time (GMT, UT)Zone time at the Greenwich meridian. Greenwich hour angleof the mean sun, expressed in time units, plus 12 hours. Usedin the Nautical Almanac for tabulation of GHA and declina-tion of celestial bodies.

Greenwich meridian (G)The meridian through Greenwich, England, serving as the primemeridian and the reference meridian for Greenwich time.

Greenwich sidereal time (GST)Local sidereal time at the Greenwich meridian, the upperbranch of the Greenwich celestial meridian, and the hour circleof the vernal equinox.

grommetA metal ring fitted into a hole in a sail or in canvas. Seecringle.


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ground fault circuit interrupter (GFCI)An AC circuit-breaker-like safety device that interrupts thepower conductor of an AC circuit when there is leakage cur-rent to earth over 5 milliamperes, which would indicate thatan appliance housing had shorted to the energized conductorand could shock anyone touching it.

groundingConnecting by a conductor to a point of zero voltage, such asthe earth.

ground swellSteepening and increasing severity of ocean swells as theycross a shallow area such as a reef or bar.

ground tackleAnchor with rode and related gear.

ground waveA radio wave traveling essentially parallel to the earth’s sur-face.

group velocityThe speed at which an entire group of waves moves.

gudgeonOne half of a rudder hing. The other half is the pintle.

gudgeon and pintleA metal eye and matching pin used to mount a rudder on thesternpost or the transom of a boat.


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Gulf StreamA well-defined, narrow, swift-moving, warm ocean currentflowing from close along the Florida coast northeasterly alongthe continental slope off Cape Hatteras, then east of the GrandBanks to become the North Atlantic Current.

gunwaleUpper edge or rail of a boat. (Pronounced “gun’el.”)

gustA sudden, brief increase in wind speed.

gust frontA cooled downdraft pushed ahead of a thunderstorm. The coldair forces the lighter, warmer air at the surface up rapidly andcools it to saturation, adding to the cloud formation and rain-fall. The effect is similar to that of a cold front.

guyThe control line connected to the tack (windward clew) of aspinnaker.

Hhack watch

A watch used for timing observations of celestial bodies.


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hailFrozen water droplets that are formed by strong updrafts inthunderstorms and fall toward earth when they become heavy.If they do not melt in the warmer air below, they reach theearth in the form of pea-sized balls. In strong storms, severaltrips may be made up to freezing level and down to gathermore water, and much larger hailstones (up to four inches indiameter) may form.

haloA broad ring around the sun or moon, caused by the refrac-tion of light passing through cirrus-type ice-crystal clouds.

halyardLine for hoisting sails or flags.

ham radioAmateur radio, licensed for non-commercial use only.

hand1. A member of the crew. 2. To lower a sail.

handrailA grab rail provided for the safety of personnel.

hankClip sewn to the edge of a sail and used to attach it to wirerigging, such as a forestay.

hard aleeThe command given when the helmsman pushes the tiller toleeward to tack.

hardenTo haul the sheets in and sail closer to the wind.


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hard overTo put the helm over as far as possible.

harmonicMultiple of a basic or fundamental frequency.

harnessWebbing with D-rings worn around the torso and attached to astrong point on a boat to prevent falling overboard. The web-bing may be incorporated into a flotation device.

hashElectrical interference that produces a scratching or fryingsound, usually caused by vibrating or rotating electrical ma-chinery.

hatchClosable opening in the deck.

haul1. The wind hauls when it draws more ahead. 2. To removethe boat from the water.

hawsepipeA hole in the bow through which an anchor rode is led.

hawserA heavy rope or cable used for various purposes, such astowing or mooring large vessels.

hazeFine salt, dust, or other particles in the air, too small to be seenindividually, reducing visibility slightly or moderately but sel-dom below 3/4 mile.


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head1. Marine toilet and its compartment. 2. Upper corner of atriangular sail or upper edge of a four-sided sail. 3. Upper endof the mast, masthead. 4. Foremost part of a boat, bow.

headerA wind that shifts more ahead of a boat sailing close-hauled,forcing it to change course to avoid pinching or luffing. Usu-ally a good time to tack.

header tankA small tank set above an engine on heat-exchanger cooledsystems. The header tank serves as an expansion chamber,coolant reservoir, and pressure regulator (via a pressure cap).

head gasketThe gasket between a cylinder head and a cylinder block.

headingDirection in which a boat is pointing at a given moment.

head offHead away from the wind, bear off.

headsailAny sail flown forward of the mast.

headstayA stay that runs from the top of the mast to the bow or bow-sprit and is the outermost stay on boats with more than oneheadsail; a jibstay.

head upTo sail closer to the wind.


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headwayBoat’s forward motion.

heatEnergy in the process of being transferred from warmer tocolder bodies by virtue of their temperature difference. Heatmay be converted to or from other forms of energy, or storedwithin a system as internal energy.

heat exchangerA device containing a number of small tubes through whichraw cooling water is passed, while engine cooling water iscirculated around the outside of the tubes.

heat of condensationThe number of calories per gram given off by a substancewhen it changes from vapor to liquid (for water, 540 caloriesper gram—numerically same as heat of vaporization).

heat of fusionThe number of calories per gram required to melt a given sub-stance (for water, 80 calories per gram).

heat of vaporizationThe number of calories per gram required to vaporize a sub-stance (for water at sea-level pressure, 540 calories per gram).

heave1. Rise and fall of a boat due to wave action. 2. To pull on aline to tighten it. 3. To throw an object (e.g., a line).

heave toTo bring a boat into a position where there is little or no head-way, usually with the bow into the wind or current.


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heaving lineA light line used for throwing.

heavy weatherStorms and rough seas.

heelIncline to one side due to force of wind or waves.

height of tideThe vertical distance between the surface of tidal water at agiven moment and the datum (reference plane) from whichdepths are measured and recorded on charts of the area. Val-ues may be + or – and are added algebraically to the charteddepth to give the depth of water.

helmThe tiller or wheel and related steering gear.

helmsmanThe one who steers the boat with the tiller or wheel.

henry (H)Unit of measurement for inductance.

hertz (Hz)The unit of frequency of an alternating current. One hertzequals one cycle per second.

highAn area of comparatively high pressure with winds spiralingoutward in a clockwise direction in the Northern Hemisphereand a counterclockwise direction in the Southern Hemisphere.An anticyclone.


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high aspectHaving much greater height than width.

hiking outMoving a crew’s weight outside the boat to the windward side.The technique is used on small boats to decrease the heelingangle.

hitch1. A knot used to secure a rope fast to another rope or otherobject. 2. A connection between a vehicle and a trailer. 3. Toconnect a trailer with a vehicle. knot used to secure a ropefast to another rope or other object.

hockleShort tight bend or kink in a rope or line that has been twistedtoo hard or drawn off coil too fast.

hoggedDescribes a boat whose bow and stern have sagged.

hoistTo raise a sail or flag. Also, the length of a sail’s luff.

holedAn accidental hole in the hull, tending to cause flooding.

hole-type nozzleAn injector nozzle with one or more very fine holes generallyused in direct (open) combustion chambers.

homingPiloting a vessel in the direction of the source of a signal orreflected signal by use of an electronic device (e.g., RDF orradar).


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hook echoCondition seen on Doppler radar as a result of strong, mois-ture-bearing winds blowing in opposite directions in close prox-imity to each other. The appearance on radar is similar to afishhook and is a very strong indication of tornado formation.

horizonSee visible horizon.

horizontal parallax (HP)The parallax of a body—usually the moon—when it is on thecelestial horizon. See parallax.

horse latitudesEither of two belts of high pressure centered roughly at 30°Nand 30°S latitude, characterized by fickle winds and lack ofrain. So named because becalmed sailing ships often jettisonedhorses to conserve drinking water.

horsepower (HP)The amount of power or the rate of doing work. One horse-power is equal to the energy required to lift 550 pounds 1 footin 1 second. One electrical horsepower equals 746 watts.

horseshoe buoyA horseshoe-shaped life preserver, generally to be thrown tosomeone in the water.

hot wireA wire not at ground potential.


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hour angle (HA)The angular distance from a celestial meridian to an hour circle,measured westward through 360°. Designated local or Green-wich, according to origin. Called sidereal hour angle if mea-sured from the hour circle of the vernal equinox, or first pointof Aries, to the hour circle of a body.

hour circleA great circle through the celestial poles and a point or bodyon the celestial sphere. Moves with the celestial sphere,whereas a celestial meridian remains fixed with respect to theearth.

hour glassThe twisted shape of a spinnaker that has not been hoistedproperly.

hour meterA meter that records engine running time.

hullBasic structure and shell of a boat.

humidityA measure of the water-vapor content of the atmosphere.

hunting (rolling, lopping)A rhythmical cycling up and down in speed of a governedengine.

hurricaneA tropical cyclone with wind speeds of 73 miles per hour orgreater or 64 knots or greater.


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hurricane surgeA sudden rise in the level of the sea along a coast line as aresult of the approach and passage of a hurricane. See stormsurge.

hydrofoil1. A device designed to deflect a water stream, thereby gen-erating a force from it, e.g., rudder, keel, centerboard. 2. Avessel designed to have its hull raised clear of the water, whenmoving at high speed, by hydrofoils extended below the hullon struts.

hydrolantUrgent radio notice of dangers to navigation, Atlantic Ocean.

hydrologic cycleThe cyclical process of evaporation, transportation, conden-sation, and precipitation that maintains a water balance in theatmosphere.

hydrometerA float type instrument used to measure the specific gravityof the battery electrolyte to determine the percent of charge.

hydropacUrgent radio notice of dangers to navigation, Pacific Ocean.

hygrographAn automatic hygrometer that measures atmospheric humid-ity and continuously records readings on a clockwork-drivenchart.


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hygrometerA device for measuring atmospheric humidity. The hair hy-grometer and sling psychrometer are most common.

hygroscopicThe property of attracting or absorbing moisture from the air.

hyperbolic systemA navigation system, such as loran, in which the hyperbola-shaped lines of position (LOPs) are determined by the differ-ence between the arrival times of pulses from two transmittersin different locations.

Iice

Water in its solid form.

ICWSee Intracoastal Waterway.

ignition distributorAn electrical device usually containing a circuit breaker (points)for the primary circuit and providing a means for conveyingthe secondary or high-tension current (from the rotor and dis-tributor cap) to the spark plug wires.

ignition systemMeans for igniting fuel in cylinders; includes spark plugs, wir-ing, ignition distributor, ignition coil, and source of electricalcurrent supply.


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impedance (Z)The total opposition that a circuit offers to the flow of an al-ternating current due to the combined effects of resistanceand reactance. Symbol Z; measured in ohms.

impellerA rotor or wheel with blades used to pump water or other fluids.

impressed current systemAn alternate method of providing cathodic protection by theselective application of controlled electrical potentials usingelectronic circuitry. This method is common to large metal-hulled vessels.

inboardToward the centerline of the boat; inside the boat.

inboard/outboard (I/O)Propulsion system consisting of an inboard engine connectedthrough the transom to an outboard drive unit. Also see sterndrive.

inches of mercury (Hg)A unit of barometric pressure measurement related to theheight of the column in a mercurial barometer, usually rangingbetween 27.00 and 31.00 inches reduced to sea level.

1 inch Hg = 25.40005 millimeters Hg1 inch Hg = 33.86395 millibars

index correction (IC)Correction applied to a sextant reading (hs) to compensatefor the sextant index error.


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index error (IE)Error in a sextant reading caused by sextant misalignment.

induced currentCurrent caused to flow in a conductor due to its presence in achanging magnetic field.

inductanceProperty of a circuit that tends to oppose any change in thecurrent flow in a conductor because of a magnetic field causedby that current. Symbol I; measured in henrys.

inductionThe influence of magnetic fields of different strength not elec-trically connected to one another.

induction coilEssentially a transformer that, through the action of induction, cre-ates a high-tension current by means of an increase in voltage.

induction motorAn AC motor usually without brushes or commutator, the ro-tor being energized by transformer action.

inductive reactanceThe amount of opposition offered to the flow of an alternatingcurrent due to the presence of inductance in a circuit. SymbolXL; measured in ohms.

in ironsHeaded directly into the wind with no headway, the boat can-not be trimmed to fill on either tack. Also, “in stays.”

injection pumpSee fuel injection pump.


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injection timingThe relationship of the beginning point of injection to the rota-tion of the crankshaft.

injectorA device for atomizing diesel fuel and spraying it directly intoa combustion chamber.

injector nozzleThat part of an injector containing the needle valve and itsseat.

injector nutThe nut that holds a fuel line to an injector.

INMARSATInternational Marine Satellite Organization, commercial com-munications system.

inlet valve (intake valve)A valve that permits a fluid or gas to enter a chamber andseals against exit.

inline pumpA series of jerk pumps in a common housing, operated by acommon camshaft.

insolationRadiant energy received from the sun at the earth’s surface.INcoming SOLar radiATION.


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instability1. The state of the atmosphere when the vertical temperaturedistribution is such that an air parcel, if set in vertical motion,tends to move away from the original position of the parcelwith increasing speed. 2. The inability of a boat to return natu-rally to its original waterline position after being disturbed.

intended trackIntended or expected direction of travel of a boat over thebottom, with consideration given to current effects.

insulatorA material with few free electrons; a nonconductor.

intake manifoldThe assembly on an engine that conducts air or fuel-mix mix-ture into an engine.

intercept (a)The difference in minutes of arc between computed altitude(Hc) and observed altitude (Ho), representing the differencein length of the radii of the computed and observed circles ofaltitude. Labeled T (toward) if observed altitude is greater, A(away) if computed altitude is greater. Also called altitude dif-ference and altitude intercept.

internal combustionBurning of fuel within an enclosed space.

International Date Line (IDL)The boundary between the –12 and +12 time zones, corre-sponding approximately with the 180th meridian.


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Intertropical Convergence Zone (ITCZ)A zone within the tropics where the northeasterly trade windsof the Northern Hemisphere meet the southeasterly tradewinds of the Southern Hemisphere. The zone lies within theequatorial air mass, near but usually not on the equator.

Intracoastal WaterwayA section along the Atlantic seaboard that allows a mostlysheltered, close-to-shore way for boats to travel between NewYork to Florida.

inversionA phenomenon in which an overlying layer of air is warmerthan the underlying layer.

inverterA mechanical or electronic device used to convert direct cur-rent to alternating current.

ionAn electrically charged particle.

ionizationThe process of imparting an electric charge to a particle.

ionosphereThe atmospheric shell characterized by high ion density. Itsbase is about 50 miles in height and it extends to outer space.

isallobarA line connecting points on a weather map with the samevalues of three-hour barometric pressure change.


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isobarA line connecting points on a weather map with the samevalues of barometric pressure.

isolation transformerA transformer having two or more separate windings that arenot electrically connected.

isothermA line connecting points on a weather map with the samevalues of temperature.

Jjack stay

A stay that runs forward from the masthead over a jumperstrut and back to the mast to stiffen the upper portion of afractional-rigged mast.

jack wire/lineA line run from the cockpit to the foredeck, and firmly at-tached on both ends, to permit a crew member’s safety har-ness lanyard hook to have uninterrupted fore-and-aft travel.

jerk pumpA type of fuel-injection pump that uses a separate pumpingelement for each cylinder.


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jet streamA meandering, tube-like system of high-velocity winds (50 to300 knots). The jet stream flows west to east and may be 100miles wide and one or more miles high. It usually exists be-tween 30,000 and 40,000 feet of altitude and is responsible forsteering air masses and weather fronts across the middle lati-tudes (20° to 60° N and S). Two jet streams exist in the North-ern Hemisphere—subtropical and polar.

jettyA small pier or landing place.

jibTriangular sail set on a stay forward of the mast.

jib downhaulA line from the head of a jib to a block near the tack and thenleading aft. Used to douse the jib without going forward.

jibeTo change from one tack to another by turning the stern throughthe wind.

jib-headed (sail)A triangular sail.

jibstayThe forwardmost stay supporting the mast, extending fromthe bow or bowsprit to the upper part of the mast.

jiffy reefingA method of reducing the area of a sail by means of downhaulsled through reef cringles in the sail and through fairleads onthe boom to winches or cleats.


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jiggerThe mizzen or the mizzenmast.

jon boatA type of simple flat-bottomed fishing boat.

journalThe part of a shaft or crank that rotates inside a bearing.

jumper stayA stay that runs forward from the masthead over a jumperstrut and back to the mast, to stiffen the upper portion of afractional-rigged mast.

jumper strutHorizontal spar on the forward side of the mast to which thejumper stay is attached to keep the upper part of the maststraight; used with fractional rigs.

junctionA connection common to two or more components in a cir-cuit. A contact between two dissimilar metals as in a transis-tor.

jury rigMakeshift repair or substitute.

Kkedge

1. Move a boat by pulling it up to its anchor. 2. Type of an-chor.


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keelMain centerline structural member (backbone) of a boat. Also,downward extension of hull to increase lateral resistance andstability.

keeper1. The hand that can be set to indicate an initial reading on ananeroid barometer dial. 2. Small metal, dish-shaped piece thatholds a valve spring assembly on a valve stem.

Kelvin temperatureA temperature scale using Celsius units based on a theoreti-cal absolute zero. K = C + 273.16° or 0°C = 273.16°K

ketchA two-masted sailboat with the after mast stepped forward ofthe rudder post.

kilo (k)Metric prefix meaning one thousand (103).

kilowattOne thousand watts.

kilowatt hourA measure of power consumed or generated at the rate of1,000 watts per hour.

knife switchAn open, pivoted-blade-type switch.

knockA general term used to describe various engine noises causedby loose internal parts, or from pre-ignition detonation of thefuel.


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knockdownA temporary condition of excessive heel resulting from a sud-den increase of wind force.

knot1. General term for securing a line to an object, another line,or itself. 2. Unit of speed of one nautical mile per hour.

known position (KP)Triangular symbol with a dot in the center used to identify aposition ashore, on a dock or pier, or boat tied to them. Usedonly in sight folders for USPS celestial navigation courses.

Llag or lead

The displacement in time or in electrical degrees (0°-360°)between two electrical signals of the same frequency.

lake breezeA local wind blowing from a lake to the land as a result ofmore rapid warming of the land; similar to sea breeze.

lake effectChanges in temperature, humidity, stability, and momentum ofan air mass as it blows across the Great Lakes, and the result-ing weather phenomena over downwind areas.

land breezeA local wind blowing from the land to the sea. It is caused byrapid cooling of land surface as compared to water surface.Generally begins near midnight and lasts till daylight.


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laneA band approximately 8 nautical miles wide or one-half thewave length of an Omega navigational signal operating at 10.2kHz.

La NiñaA strengthening of the (easterly) trade winds off SouthAmerica accompanied by the retention of warm sea water inthe western Pacific, causing the waters in the eastern Pacific,off Peru, to cool. These changes in water temperature areknown to alter temperature, rainfall, and storm formation pat-terns over much of the earth. See El Niño.

lanyardShort line used as a handle or to secure an object.

lapstrakeType of hull construction with overlapping planks. See clin-ker-built.

latent heatHeat absorbed or given off by a substance during a change ofstate (as from a solid to a liquid). Since no temperature changeoccurs, the heat is hidden or latent.

lateral planeAny surface or appendage that serves to inhibit lateral motionof a boat through the water.

latitudeAngular distance on the earth's surface, measured north andsouth from the earth’s equator.


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launch1. To place a boat in the water. 2. A boat used to carry peoplebetween shore and a moored vessel.

lay1. To lay a mark is to be able to reach it in a single tack,close-hauled. 2. The lay of a line is the direction in which itsstrands are twisted.

lay lineAn imaginary line that brings a closehauled boat directly tothe mark.

lay toTo lie without headway either to a sea anchor or to linesstreamed over the side, or merely drifting (lying ahull).

layer instabilityThe condition of a layer of air —usually moist in the lowerportion, but dry in the upper—that leaves the layer stable aslong as it is not lifted but results in instability if the layer un-dergoes a general uplift.

lazaretteStorage compartment in stern of boat.

lazy jacksA series of lines attached on both sides of the boom, allowingthe mainsail to flake down in a neat pile.

lee (lee side)Side away or opposite that from which the wind blows. Also,area sheltered from the wind.


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leeboardA pivoting board attached to the side of a boat to providelateral resistance.

leechThe after edge of a sail between the clew and the head.

lee helmA condition of balance that requires the tiller to be held toleeward to keep a boat on a straight course.

lee shoreOne onto which wind or current can force a boat; shore toleeward of boat.

leech lineA light line sewn through the tabling in the leech of a sail, usedto control leech flutter.

leewardDirection away from the wind; downwind. (Pronounced“loo’ard.”)

leeway1. Sideways movement of a boat through the water caused bywind. 2. The angular difference due to wind between the truecourse steered and the true course over ground.

leg1. The distance covered on one tack when sailing to wind-ward. 2. On a race course, the course between two succes-sive marks.

lever armThe distance through which a force acts.


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licenseA government authorization for operation of a radio, radar, orother type of signal-generating station.

lie ahullTo lie without headway, more or less broadside to the wind,with no sail set, drifting freely. A means of coping with heavyweather.

lie toSee heave to.

life jacketA buoyant vest; called a Personal Flotation Device (PFD) inthe United States.

lifelinePlastic-coated wire rigged to stanchions around the peripheryof the deck to help keep personnel aboard.

life preserverA buoyant coat, suit, vest, ring, horseshoe buoy, or cushion;called a Personal Flotation Device (PFD) in the United States.

lift1. A force generated on the leeward side of a sail by air pass-ing over the airfoil shape. It is the force that drives a sailboat.2. When sailing close-hauled, a wind that shifts aft to allow achange of course toward the wind direction.

lift pumpA low-pressure pump that feeds fuel from a tank to an injec-tion pump.


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lightningA visible electrical discharge, generally produced during a thun-derstorm.

light sailsSails made of lightweight fabric for use in light wind.

limber chainA chain run through the limber holes for the purpose of loos-ening clogged debris.

limber holesOpenings through frames to permit water to drain to the low-est part of the bilge.

line of position (LOP)1. A line, usually plotted on a chart, along which the boat lies,as determined from a single observation. 2. A visual or elec-tronic reference line from or about a known navigational aid.

line of sightLine of direct visual observation from one point to another.

linkageAny series of rod, yokes, levers, etc., used to transmit motionfrom one unit to another.

line stopper (rope clutch)A quick-release, clamp-type device that contains a cam forsecuring a line.

listInclination of a boat to one side due to weight distribution.


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LOALength overall.

loadA device that uses power from a source to perform a usefulfunction.

loading coilA coil placed in a circuit to lower its natural resonant fre-quency. Used to increase the electrical length of an antenna.

lobeAn off-center or eccentric enlargement on a shaft to convertrotary motion to reciprocating motion. Also called a cam.

local apparent noon (LAN)The instant at which the apparent (true) sun is over the upperbranch of the local meridian. Time of sun’s maximum altitudeat the observer’s location.

local apparent time (LAT)The angle at the celestial pole, between the lower branch ofthe local celestial meridian and the hour circle of the apparentor true sun.

local hour angle (LHA)The angular distance of the hour circle of a celestial bodymeasured westward from the celestial meridian of the observer,measured from 0° to 360° along the parallel of declination.

local mean time (LMT)The time based on the mean sun with reference to the localcelestial meridian (not the zone meridian).


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local safety groundA terminal or conductor in direct contact with or at the samepotential as local earth or water.

local sidereal time (LST)Local hour angle of the first point of Aries (vernal equinox)expressed in time units; the angle at the celestial pole, be-tween the upper branch of the local celestial meridian and thehour circle of the vernal equinox. LST is LHA of Aries ex-pressed in time units.

lockerA storage or stowage compartment.

log1. Instrument to measure distance or speed through the wa-ter. 2. A book in which all matters pertaining to a vessel’snavigation are recorded.

log bookSee deck log and log.

longitudeAngular distance on the earth's surface, measured east or westfrom the prime meridian (000°) extending through Greenwich,England.

longshore currentA current along a shoreline, caused by waves approachingthe bottom contours of the coast at an angle.

long spliceA method of permanently joining the ends of two ropes with-out increasing the diameter.


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loomFlexible insulating wrap for enclosing wiring.

loomingAn optical effect due to a marked increase in air density nearthe surface, causing objects or lights that are actually belowthe horizon to appear above it.

loose-footedA sail that is attached to the boom only at the tack and clew.

loranAn electronic navigation system for establishing lines of posi-tion by using the time difference between the reception ofsignals from different terrestrial locations.

lowAn area of comparatively low barometric pressure with windsspiraling inward in a counter-clockwise direction in the North-ern Hemisphere and a clockwise direction in the SouthernHemisphere. A cyclone.

lower branch (m)The half of the observer’s meridian, measured from pole topole, passing through the nadir of the observer, i.e., the pointvertically below on the opposite side of the celestial sphere.

lower limb (LL)The lower part of the circumference of the sun or moon.

lubber’s lineA compass reference oriented parallel to the fore-and-aft keelline of the vessel.


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luff1. To alter course towards the wind; to head so close to thewind that the sails flutter. 2. The forward edge of afore-and-aft sail.

luffing (to luff or luff up)Head into the winds causing sails to flutter.

luff wireA piece of wire rope sewn into the leading edge of a jib.

lunch hookA light anchor used for short stops in settled conditions.

LWLLoad waterline; the length of a vessel from bow to stern atthe waterline. See waterline length.

Mmackerel sky

A sky covered with cirrocumulus clouds with shapes that re-semble the scales of a mackerel.

maelstromAn extremely violent storm, usually a hurricane. Also, a whirl-pool.

Mafor codeWeather forecasts on the Great Lakes and St. Lawrence Sea-way are broadcast in Mafor code. A special chart is neededto decode the forecasts.

magnetic fluxMagnetic lines of force.


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magnetic fieldThe flow of a magnetic force, or magnetism between the op-posite poles of a magnet.

magnetic meridianAn irregular curve on the earth’s surface, passing through themagnetic poles and representing the direction of the magneticfield at that location.

magnetoAn electrical device that generates current when rotated byan outside source of power.

magnitudeRelative brightness of a celestial body; the smaller the num-ber indicating magnitude, the brighter the body.

main bearingA bearing within which a crankshaft rotates and which sup-ports the crankshaft within an engine block.

mainsailBoat’s principal sail, set aft of mainmast. (Pronounced“mains’el.”)

mainsheetA line that controls the mainsail.

mammatusClouds with hanging protuberances, like pouches, on their lowersurface. Typical of turbulent, tornado-forming conditions.

Marconi rigA rig having a three-sided or jib-headed mainsail.


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mares’ tailsLong, well-defined wisps of cirrus clouds resembling a horse’stail.

marinaA facility in which recreational boats may be berthed.

maritime air massAn air mass that originated over water.

markAny object required by sailing instructions to be passed by aboat on a specific side.

marlineTwo-stranded twine used for whipping and seizing.

marlinespikeA pointed steel tool used in splicing to open the strands of arope or cable. See fid.

marlinespike seamanshipGeneral knowledge of knots, bends, hitches, splices, and careof the lines (rope).

mast1. A vertical spar on a sailboat, on which sails may be set. 2.A vertical spar on a powerboat, from which may be flown asteadying sail or flags and burgees, and which can be usedwith a boom to lift gear.

master stationThe station of a loran chain from which the first group ofeight pulses plus a ninth signalling pulse are transmitted.


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master switchA switch that disconnects a battery or generator from the elec-trical system.

mastheadThe top of the mast.

masthead rigA sailboat with the jibstay attached to the top of the mast.

mast hoopsTraditional wood rings used to secure a gaff sail to the mast;seized with marline to grommets in the sail.

mast stepFitting into which the butt of the mast sits.

matFiberglass material formed of irregular chopped filaments ofglass.

maximum draftThe greatest amount of fullness in a sail.

MaydayThe term used to signify an urgent distress communicationunder international radiotelephone procedures; the highest pri-ority transmission, indicating there is immediate danger to avessel or to someone on board.

mean sunAn imaginary sun traveling westward around the celestialequator at a uniform rate equal to the average rate of the truesun.


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mean timeTime based on the rotation of the earth relative to the meansun.

med mooringTo secure a boat with an anchor at the bow and the stern at adock.

mega (M)Metric prefix for one million (106).

mercurial barometerAn instrument used to measure barometric pressure. Consistsof an evacuated tube, closed at one end, about 36 inches long.The open end is inverted into a bowl of mercury; atmosphericpressure forces the mercury up the tube. The distance themercury is raised is the barometric pressure in inches of mer-cury.

meridianA great circle of the earth passing through both poles in thedirection of true north or true south.

meridian angle (t)The angular distance, measured east or west (whichever dis-tance is shorter) through 180°, between the upper branch ofthe local celestial meridian and the hour circle of a celestialbody. Labeled E or W.

meridian transitThe passage of a celestial body across a celestial meridian,usually that of the observer.


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meridional difference (m)The difference in meridional parts between two given lati-tudes, expressed in minutes of arc.

meridional parts (M)The length of the arc of a meridian between the equator and agiven parallel of latitude on a Mercator chart, expressed inunits of one minute of longitude at the equator.

mesopauseThe interface or boundary layer between the mesosphere andthe thermosphere, approximately 55 miles high.

mesosphereThe atmospheric region from approximately 30 to 55 mileshigh. Air temperature normally rises with altitude in this re-gion.

messengerA light line used to haul a heavier working line.

meteorologyStudy of atmospheric phenomena; particularly, the study ofchanging conditions in the lower atmosphere.

meter1. An analog dial or digital type of instrument for measuringvoltage, current, pressure, etc. 2. A metric linear measureapproximately equal to 39.37 inches or 3.28 feet.

micro(µ) (Greek letter mu)Metric prefix meaning one one-millionth.


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microburstA strong downdraft that induces an outburst of damaging windson or near the surface of the earth. The outflow at the sur-face is normally less than 4 kilometers across, with peak windslasting only two to five minutes.

micro processorThe control and processing portion of a computer containinglarge numbers of transistors, integrated circuits, diodes, andother components.

mid-latitudeThe middle latitude; half the sum of the latitudes of two placeson the same side of the equator.

milli (m)Metric prefix meaning one one-thousandth.

millibar (mb)A unit of barometric pressure measurement; atmospheric pres-sure usually ranges between 950.0 and 1,050.0 millibars re-duced to sea level.

1 millibar = 0.0295299 inch of mercury1 inch of mercury =33.86389 mb

millimeter (mm)A linear unit of barometric pressure measurement (height ofthe mercury column) usually ranging between 700 and 800millimeters reduced to sea level.

1 millimeter = 0.039370 inch1 millimeter mercury = 1.33322387 millibars


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mirageAn optical effect due to refraction in the atmosphere, causingthe image of an object to be displaced from its true position.

mistName for drizzle in North America. In Great Britain and someother countries, light fog.

mizzenThe fore-and-aft sail set on the mizzenmast. Also called ajigger.

mizzenmastThe mast immediately aft of the mainmast, sometimes calleda jigger.

mizzen staysailA triangular sail set from the mizzen masthead to the deckforward of that mast.

modificationA change in the character of an air mass or a portion of themass. This usually occurs when the air mass stalls over a sur-face with different temperature and humidity characteristicsfrom those of the mass. When the underlying surface causeschange, the air mass is said to be modified.

modulationThe impression of intelligence on a signal or carrier wave.

moleculeThe unique combination of atoms that constitutes the basicchemical building block of a substance.


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momentA force acting through a distance, tending to cause rotation.

monel metalCorrosion resistant alloy of nickel, copper, iron, and manga-nese. Sometimes used for propellers and shafts.

monsoonA seasonal wind flowing between ocean and continent, causedmainly by the temperature disparity between them. When theseasons change, the disparity reverses, as does the wind di-rection.

mooringPermanent ground tackle to which a boat is attached ormoored. A place where vessels are kept at anchor.

motorA rotating machine that converts electrical or combustibleenergy into mechanical energy.

mountain windAir flowing down the side of a mountain owing to nighttimeradiational cooling of the air next to the slope.

mushroom anchorAn anchor shaped like an inverted mushroom, often used toanchor permanent structures in soft sea beds. It tends to sinkdeeply into the bottom if left for long periods.


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Nnadir (Na)

The point on the celestial sphere directly below the observer,or 180° from the zenith.

nano (n)Metric prefix meaning one one-billionth.

Napier diagramA graphic plot of compass deviation values.

naturally aspiratedRefers to an engine that draws in air solely by the action of itspistons, without the help of a supercharger or turbocharger.

Nautical Almanac (N.A.)A yearly publication tabulating the position of celestial bodiesand other astronomical data useful to marine navigators.

nautical mile (nm)A unit of distance equal to 1 minute of latitude and equal toapproximately 6,076.1 feet or 1.15 statute miles.

nautical twilight (NT)The period of incomplete darkness that ends in the eveningand begins in the morning when the center of the sun is 12°below the celestial horizon.

navigable semicircleIn a tropical cyclone in the Northern Hemisphere, the navi-gable semicircle is the side to the left of the storm track as youface the way the storm is moving. Wind in this semicircle isdiminished by the velocity of the storm track. In the SouthernHemisphere, the right side is known as the navigable semicircle.


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navigation lightsLights shown by a vessel that indicate course, position, andstatus, such as fishing, towing, or at anchor.

Navigation RulesThe Rules of the Road of the United States, governing navi-gation lights, rules for vessels meeting or passing, sound sig-nals, and distress signals.

navigational triangleThe spherical triangle solved in computing altitude and azi-muth and great circle problems.

needle bearingAn anti-friction bearing using a great number of rollers of smalldiameter in relation to their length.

needle valveA type of valve in which a pointed shaft (the needle) moves tocontrol the flow of fluid; used in injector nozzles and carbure-tors.

negativePolarity of a cathode. Electrode from which electrons flow tothe anode.

negative polePoint from which an electrical current flows as it passesthrough a circuit; designated by a minus sign (–).


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neutralA conductor that is grounded at the marina service and car-ries return current from the appliance served by the energizedconductor. Must be white or gray in color, or both ends can betaped white.

Newton’s laws of motion1) If the resultant force acting on a body is zero, it will con-tinue to do whatever it is doing: (a) If it is at rest, it will remainat rest; (b) If it is in motion, it will continue to move in a straightline with constant velocity. 2) If the resultant force on a bodyis not zero, it will have an acceleration proportional to and inthe direction of the resultant force. 3) For every action thereis an equal and opposite reaction. That is, if body A exerts aforce on body B, then body B exerts an equal and oppositeforce on body A.

nicopress fittingA sleeve that is crimped onto wire rope to form a connection.

nicopress toolA tool used to compress cable fittings.

nimbostratusClassified as a middle cloud but also occupies lower levels.Formed by the downward thickening of altostratus. Charac-terized by its grey color and no well-defined base. Normallyassociated with steady rain.

NOAANational Oceanic and Atmospheric Administration.


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noble metalA metal that is passive in the galvanic series and, therefore, isresistant to corrosion. Gold, platinum, etc.

noiseUnwanted disturbances, superimposed upon a useful signalthat tend to obscure its information content.

North Atlantic currentA continuation of the Gulf Stream as a broad, slow-movingdrift of comparatively warm water toward the British Islesand the Iberian Peninsula.

northerA wind in the southern United States, especially in Texas andthe Gulf of Mexico, occurring from November through April.This strong, cold, northerly wind frequently causes a rapid dropin temperature.

Northill® anchorA lightweight anchor similar to the Danforth® anchor, but withthe flukes opposite each other as on a kedge anchor.

NOSNational Ocean Service.

Notice to MarinersA weekly newsletter, published by the United States CoastGuard, advising of changes or discrepancies relating to navi-gation.

no wake zoneAn area where boats are prohibited from causing a wake.


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Ooblate spheroid

A sphere flattened or depressed at the poles—such as theearth.

observed altitude (Ho)Sextant altitude after all corrections have been applied.

occluded frontA zone in which an advancing cold air mass overtakes a re-ceding cold air mass and, in the process, forces the warm airpreviously separating them up above the surface. Two types:1) Cold-front occlusion, in which the advancing cold air massis colder than the receding cold air mass. 2) Warm-front oc-clusion, in which the advancing cold air mass is warmer thanthe receding cold air mass.

occlusionSame as occluded front.

octane numberA unit of measurement on a scale intended to indicate ten-dency of a fuel to detonate or knock in a gasoline engine.

offingA position on the sea at a safe distance from land.

offshoreA direction away from the shore, as offshore wind.

off the windSailing on a reach or run.


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ohm (ΩΩΩΩΩ) (Greek letter omega)The unit of electrical resistance.

ohmmeterInstrument used to measure resistance.

Ohm’s lawBasic law for circuit analysis: E = IR; I = E ÷ R; R = E ÷ I.

oil pumpingA term used to describe an engine that is using an excessiveamount of lubricating oil.

oil-skinsFoul weather gear.

omnidirectionalIn all directions.

OmnirangeNavigation system in general use for aircraft.

onshoreTowards the shore, from seaward, as onshore wind.

on the windClose-hauled.

open circuitA break or opening in an electrical circuit that stops passageof a current.


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optical hazeA restriction to visibility caused by refraction of light whilepassing through layers of air with differing densities. The re-sult is a haze-like distortion. Atmospheric particulates are notinvolved in optical haze.

orographic liftingMoving air mass forced upwards by the presence of moun-tains. This rising air cools and saturates, often causing rainfallupwind of the mountains. This now drier air warms as it de-scends the downwind side, often creating desert conditions insuch areas.

oscillatorCircuit used to produce an alternating signal of desired shapeand frequency.

oscilloscopeAn electrical measuring instrument that can display the ampli-tude of rapidly varying signals as a function of time on a cath-ode ray tube similar to a tv picture tube.

outboardToward the outside of a boat.

outboard motorAn engine with propeller attached, designed to be fastened tothe transom of a boat.

outhaulDevice or line used to tension the foot of a sail.


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out of phaseTwo or more wave forms that have the same frequency butdo not pass through corresponding values at the same time.

overboardOff the boat and in the water.

ozone layera rarefied layer or layers of triatomic oxygen, or ozone, in theupper stratosphere. This is an unstable form of oxygen cre-ated and sustained by ultraviolet radiation from the sun. Theozone layer shields the earth from a large part of this damag-ing radiation.

Ppainter

Line tied to the bow of a dinghy to tow or secure it.

palm1. See fluke. 2. Leather glove-like device used as a thimbleto push a needle through a line or a sail.

Pan-PanThe term used to signify an urgent communication under in-ternational radiotelephone procedures; the second highest pri-ority transmission, indicating that the safety of the vessel orsomeone on board is involved, but the need for assistance isnot immediate. (Pronounced “pahn-pahn”)

parabolic pathThe curve followed by a stone thrown into the air; found inmany other applications as well.


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parallactic angle (X)The angle of the navigational triangle at the celestial body—i.e., the angle between the hour circle and the vertical circle.

parallax (P)The difference in the apparent direction of a celestial bodywhen viewed from a point on the earth’s surface as comparedto viewing it from the center of the earth.

parallel circuitA circuit that has two or more branches or paths for currentflow.

passive deviceA device that does not radiate any electrical signals but usesexternal signals to perform its function.

passive metalA metal that does not react readily in an electrolyte.

pay offTo head away from the wind.

pay outSlacken or let out a line gradually.

peak1. The corner of a gaff-headed sail formed by the head andthe leech. 2. The aft end of a gaff to which the peak of a gaffsail is set.

peak envelope power (PEP)Power of an alternating current at the maximum amplitude ofits cycle.


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pelican hookA snap shackle often used for lifeline gates and to attach orremove an inner forestay.

pendantShort rope serving as an extension of a line, chain, or cablewith descriptive name based on use, e. g., mooring pendant.(Usually pronounced “pen’ant.”)

pennantA tapering flag. Compare burgee.

personal flotation device (PFD)A buoyant coat, suit, vest, ring, horseshoe buoy, or cushion.

personal watercraft (PWC)A Class-A boat using an inboard motor to drive a water-jetpump for motive power, designed to be operated by a personsitting, standing, or kneeling on the vessel.

phaseOne of the stages in the appearance of the moon during itscycle.

phase or phase angleNumber of electrical degrees by which one wave form leadsor lags another.

pico (p)Metric prefix meaning one-trillionth.

pierStructure extending into the water from shoreline to providedockage.


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pig-tail adapterA short electrical cord, used to connect to shore power.

pileA pole or post driven vertically into the bottom, usually to sup-port a pier or float or to moor a boat.

pilingA structure of piles often used to protect wharves and piers.

Pilot ChartA planning chart of a major ocean area published by the De-fense Mapping Agency. It includes as much information onweather, seas, and winds as can be put on a single sheet ofpaper.

pilotingA means of navigating using reference points that may beseen or determined from information on charts—such as depths,heights, or ranges.

pilot ladderA ladder with steps joined with ropes, lowered from a ship’srail.

pin1. A metal peg used to fasten rigging together, usually se-cured with a cotter or circular pin. 2. A floating flag on a poleused as a racing mark, particularly to mark the end of thestart-finish line.

pinchTo sail too close to the wind.


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pin endThe end of the sailboat race starting or finishing line, markedby a float with flag (pin) or other suitable mark, opposite therace committee (boat).

pintleSee gudgeon and pintle.

pintle nozzleAn injector nozzle with one central hole; generally used inengines with pre-combustion chambers.

pipImage of a reflected signal on a radar screen.

pistonA pumping device used to generate pressure in a cylinder.

piston collapseAn abnormal reduction in diameter of the piston skirt due toheat or stress.

piston crownThe top of a piston above the rings.

piston displacementThe volume of air moved or displaced by moving the pistonfrom one end of its stroke to the other.

piston hanksA hank with a spring-loaded plunger sewn to the luff of a sailand used to attach sail to a stay.

piston landsThose parts of a piston between the piston rings.


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piston pinThe part that connects the piston to the small end of an engineconnecting rod; also known as wrist pin.

piston ringSpring-tensioned ring set in a groove in the circumference ofa piston; this ring pushes out against the cylinder wall to makea gas-tight seal.

piston ring gapClearance between ends of piston rings.

piston ring grooveThe slot in the circumference of a piston into which a pistonring fits.

piston skirtPart of piston below the rings and bosses.

piston slapRocking of a loose-fitting piston in a cylinder, making a hol-low, bell-like sound.

pitchingOscillation around the transverse axis; the rising and falling ofthe bow in waves.

pitchpoleCapsize end-over-end.

pivot pointThe theoretical fulcrum of a vessel around which it revolvesin making a turn.


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plan position indicator (PPI)Radar display on a cathode ray tube as the terrain would ap-pear if viewed from above.

plane sailingMethod of computing courses, distances, and positions by treat-ing the earth as flat and applying plane trigonometry. Seesailings.

planing hullA hull designed to climb toward the surface of the water assufficient power is applied and to skim along the water at agreatly reduced-displacement attitude.

plankingBoards used for covering the bottom, sides, and deck of avessel.

plow anchorAn anchor shaped like a farmer’s plow.

plugA piece of wood fitted into a hole in a vessel, usually belowthe waterline.

point1. To sail effectively close to the wind. 2. Any of the 32 equaldivisions, at 11°15’ intervals, marked at the circumference ofa mariner’s compass card that indicate direction.


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polar air mass (truly subpolar)An air mass having its origin over the colder, more northernportions of North America, Eurasia, and the oceans. Differ-entiated from Arctic air mass, which forms over the ArcticOcean and ice-covered polar regions.

polar distance (p)The angular distance from the observer’s elevated pole to theparallel of declination of a body. See co-declination.

polar easterliesCircumferential bands of surface air flowing from the polarhighs toward the subpolar lows. These winds are deflectedwestward (making them easterly) by the Coriolis effect. Thepolar easterlies occur in both hemispheres between the polesand roughly 60° latitude.

polar frontThe southern limit of the polar air mass. This boundary, whichfluctuates widely with the jet stream, is the turbulent interfacebetween the warm prevailing westerlies and the polareasterlies. The polar front is a spawning ground of extratropi-cal cyclones.

polarity indicatorA device for determining the polarity of an electrical terminalsuch as the “hot” side of an AC power outlet.

polarity of voltageA method of describing a voltage as either positive or nega-tive with respect to some reference point such as the earth.


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poppet valveA valve structure consisting of a circular head with an elon-gated stem attached in the center; designed to open and closea circular hole or port.

port1. Left side of a boat when facing the bow. 2. Toward theboat’s left. 3. Opening in a boat’s side (e. g., portlight). 4.Harbor. 5. In engines, the openings in the cylinder head orblock for valves, exhaust and inlet pipes, or water connec-tions; in two-stroke cycle engines, openings for inlet and ex-haust purposes.

port of entryA port where a vessel may legally enter a country.

port tackAny heading where the wind is on the port side and the mainsailis carried on the starboard side of the boat.

potential difference (E)Difference in voltage (V) or electromotive force (EMF) be-tween two points.

potentiometerA resistor placed across a source of voltage and having a vari-able tap to allow intermediate values of voltage to be taken off.

power (P)The rate of doing work.

power-driven vesselAny vessel propelled by machinery, including sailing vesselsunder both sail and power.


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Power LawP = E I = I2 R = E2 ÷ RWhere P = power in watts

E = voltage in voltsI = current in amperesR = resistance in ohms

pramFlat-bottomed dinghy with blunt bow.

precipitationForms of water particles, solid or liquid, that fall from the at-mosphere and reach the ground: drizzle, rain, snow, sleet, orhail.

precursory cirrusForerunner clouds of a tropical cyclone. These clouds occurat a high level and may be visible 400 to 500 miles in advanceof the eye of the hurricane.

preignitionIgnition occurring earlier than intended.

pressureForce per unit area. In weather, the weight of the air, oftengiven in millibars, or inches of mercury. See millibar, inchesof mercury.

pressure gradientA change in atmospheric pressure per unit of horizontal distance.

pressure tendencyThe change in atmospheric pressure within a given time, usu-ally three hours.


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prevailing westerliesA zone of winds flowing from the high pressure of the horselatitudes to the low pressure of the subpolar region. Thesewinds are deflected by the Coriolis effect (to the right, in theNorthern Hemisphere) so that they become westerly.

preventerA line run forward from the boom to a deck fitting to preventaccidental jibes.

primary cellA single galvanic cell producing a flow of current by chemicalreaction. The reaction is not reversible. It cannot be recharged.

primary windingA wire that conducts low-tension current and will create, byinduction, a high-tension current in a secondary winding of anignition coil or magneto armature.

primary wiresWiring circuit used for conducting low tension or primary cur-rent to points where it is to be used.

prime vertical (PV)The vertical circle that passes through the observer’s zenithand the east and west points of the horizon.

prognostic chartA weather map projected into the future, showing pressuresystems, fronts, and other features as they are expected to beat a given time.

prony brakeA machine for testing power of an engine while running againsta friction brake.


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propellerA multi-bladed, rotating wheel that furnishes propulsion.

propeller shaftA rod that transmits power from engine and transmission topropeller.

protonA particle that is the nucleus of the hydrogen atom and ispresent in the nucleus of other atoms. It has a positive chargeequal and opposite to the charge of an electron.

pseudo-cirrusThin, wispy, cirrus-like clouds blown from the anvil top of cu-mulonimbus clouds by upper winds. The pseudo-cirrus cloudstrail off the anvil top in the direction of movement of the cell.

psychrometerAn instrument for measuring humidity. Consists of two identi-cal thermometers, one with wet muslin around its bulb to al-low cooling by evaporation, the other with a dry, uncoveredbulb. The instrument is ventilated by fan, or by manually whirl-ing it (sling psychrometer). The difference in the readings ofthe two thermometers, called the wet-bulb depression, is usedin calculating dew point and relative humidity.

puffA short gust of wind.

pulpitA guardrail around the bow or stern.


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pulseA momentary increase in the amplitude of a voltage or cur-rent.

purgingThe process of removing air from a fluid system, such as purg-ing air from the fuel system of a diesel engine.

push-to-talkA spring-loaded switch used to energize a radio transmitter;in the released position, the receiver is energized.

pushrodA connecting link in an operating mechanism. Example: rodinterposed between valve lifter and rocker arm on an over-head valve engine.

pyrometerA gauge that measures high temperatures; often used to mea-sure the temperature of engine exhaust gases.

Qquarter

1. After part of a boat’s side, e.g., port quarter. 2. Direction45° abaft the beam.


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Rracing

The technical status of a boat entered in a race from the timeof her preparatory signal until she has finished and clearedthe finish line and marks, or has retired, or the race committeehas signalled a general recall, postponement, or abandonmentof the race.

raconA radar set of known position, usually shore-based. When ac-tivated by a ship’s radar signal, it will transmit an identifiablesignal that appears on the ship’s receiver screen.

radarAn electronic navigation system using reflected radio pulsesignals to determine the relative bearing and distance of anobject from the vessel.

radar reflectorAn arrangement of three metallic surfaces exactly at 90°angles to each other. Acts as a passive reflector to radar sig-nals.

radial cutTypical shape of spinnakers (radial head or tri-radial) wheretapered, parallel panels are sewn together to form a convexshape toward the head or all three corners.

radiateTo emit energy.


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radiationThe transfer of energy by electromagnetic wave motion. Nomedium is necessary for the process. Energy from the sun istransmitted to the earth by this method.

radiation fogFog formed by the radiational cooling of the earth at night andthe resultant conductive cooling to dew point of the adjacentlayer of air.

radio beacon (Rbn)An electronic aid to navigation of known position emitting codedradio signals for use by shipboard radio direction finders.

radio direction finder (RDF)A radio receiver equipped with a compass rose and a loop orother type of directional antenna to determine the direction ofthe source of a received signal.

radiofaxTransforms weather broadcasts into readable weather charts.Also known as Weatherfax.

radiometerAn electronic instrument used on a satellite to detect radia-tion.

radiosondeA device consisting of a radio transmitter and sensors, carriedaloft by a balloon, to measure pressure, temperature, and hu-midity. Information from the sensors, converted to radio sig-nals, gives the receiving station a profile of the pressure,temperature, and humidity aloft.


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raftA floating platform.

rafting upTying a group of boats together for a social gathering, usingonly a few anchors.

railA boat’s side above the deck line.

rainbowAn arc in the sky featuring a spectrum of color caused by therefraction, reflection, and dispersion of the sun’s rays in rain-drops.

rain catcherCanvas used to catch rain to augment a vessel’s water supplies.

rain gaugeAn instrument for measuring the amount of rainfall.

rakeThe slope from the vertical, in the fore-and-aft direction, of amast, transom, or deckhouse.

range1. Two or more objects in line. 2. Maximum distance at whichan object may be seen. 3. Distance between any two points,such as a radar range to a navigational aid. 4. Difference inelevation (height) between any successive pair of high andlow tides.

rating ruleA mathematical formula to establish handicaps for racing boatsof diverse size and design.


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ratingsManufacturer’s specifications regarding electrical and othercharacteristics of equipment.

raydistAn electronic system for hydrographic surveying that is char-acterized by short distance and high accuracy.

RDFSee radio direction finder.

reach1. A point of sail between close-hauled and a run. 2. A dis-tance, or fetch.

reacherA light-weight jib used for reaching.

reactanceOpposition to the flow of an electric current due to inductanceor capacitance.

ready aboutThe helmsman’s warning that he intends to tack.

reciprocalIn navigation, a bearing or direction 180° from another bear-ing or direction.

rectifierAn electrical device for changing AC current to DC current.

recurveThe generally parabolic path of the center of a tropical cy-clone, as it veers through west to northwest and north to north-east, is known as recurvature.


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reed valveA flat, springy valve covering ports between carburetor andcrank chamber in a two-stroke cycle engine.

reef1. To reduce the area of the sail. 2. A part of a sail that istaken in or let out to regulate sail area. 3. An underwaterbarrier, such as rock or coral.

reef knotSame as a square knot.

reef pointsShort pieces of line in a sail, placed in a line parallel to theboom and used to secure unused material when the sail isreefed.

reeveTo pass a line through a block.

reflectionGenerally obtained by a highly polished mirror surface, butany surface of any medium may return a portion of incidentlight at an angle of reflection equal to the angle of incidence.The remainder of the light may be refracted or absorbed withinthe medium.

refraction (R)An optical phenomenon in which light is bent at an angle uponundergoing a change in speed, as when passing from one me-dium to another. In effect, light is slowed upon entering a densermedium and refracted or bent toward the normal. An exampleis the focusing of light rays by a camera lens.


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relative bearingDirection of an object relative to a boat’s heading, measuredclockwise from dead ahead.

relative humidityThe ratio of the amount of moisture in the air to the amountthat would be present at the same temperature and pressureif the air were saturated, expressed as a percentage.

relayA switching device activated by an electromagnetic coil orsolenoid.

reserve powerThe number of minutes a battery can deliver 25 amperes ef-fectively to a load, at 80°F with no charging current applied.

resistance (R)The opposition an appliance or wire offers to the flow of elec-tric current, measured in ohms.

resonanceA condition that exists when an electrical circuit or mechani-cal device oscillates at its natural frequency.

resultant forceThe vector sum of two or more forces.

retardTo cause a spark to occur at a later time in the cycle of engineoperation. Opposite of spark advance.

rheostatA continuously variable electrical resistor, used to regulatecurrent.


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rhumb lineA straight line on a Mercator chart intersecting all meridiansat the same angle.

ribLandsman’s term for frame.

ridge of high pressureElongated region of high pressure.

rig1. To prepare a boat for sailing. 2. Arrangement of spars andsails, e.g., as on a sloop.

riggingAll the lines and gear used to support the spars and controlthe sails.

right ascension (RA)An angular measurement used by astronomers to locate stars,measured eastward from the first point of Aries, from 0°through 360° (24 hours).

righting armHorizontal distance between the vertical line through the cen-ter of buoyancy and the vertical line through the center ofgravity of the vessel.

righting momentThe righting arm multiplied by the vessel’s displacement.


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rimeA white, granular deposit of ice created by the impingementof small, supercooled drizzle or fog droplets onto exposed sur-faces. Associated with small droplet size such as is found instratiform precipitation.

ring buoyA ring-shaped life preserver, generally to be thrown to some-one in the water.

roachThe part of a sail that extends beyond a straight line from thehead to the clew.

rocker armIn engines, a lever located on a fulcrum or shaft, one endbearing on the valve stem, the other end on a pushrod.

rodeAnchor line or chain.

rod riggingStanding rigging made of connected rods rather than wire toreduce stretch; often used in racing boats.

roll cloudPart of the cloud base along the leading edge of a cumulonim-bus cloud. Formed by the rolling action in the wind-shear re-gion between the cool downdrafts within the cloud and thewarm updrafts outside the cloud.

roller furlingReducing the area of a jib by rolling it around its luff wire.


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roller reefingReducing the area of a sail by rolling it around the boom.

rollingOscillation of a boat around a longitudinal axis.

ropeCordage made of natural or synthetic fibers; can be made ofsteel wire.

rotorThe rotating part of an electrical or mechanical device, suchas motor, generator, or distributor rotor.

round turnA complete turn of line around a cleat, bitt, or post.

RPMRevolutions per minute, usually of an engine.

rudderA flat board, blade, or plate hung on the aft end of a boat andused to steer the boat.

rudder postThe shaft to which a rudder blade is attached. The tiller orother steering apparatus is affixed to the other end.

run (or running)Point of sailing with the wind astern.

running backstayStays that counteract forces from an inner forestay; must betended as the boat tacks or jibes.


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running fix (R Fix)Position derived by the crossing of lines of position that havebeen observed 20 minutes or more apart and one of which hasbeen advanced or retired to a common time.

running riggingThe lines used to raise, set, and trim the sails.

Ssacrificial anode

A galvanically active base metal, such as zinc, placed in elec-trical contact with a more noble metal, such as steel, to pre-vent corrosion or loss of the nobler metal due to galvanic action.

SAESociety of Automotive Engineers; also a term used to describecertain nuts, bolts, and wrenches, or other parts meeting thestandards of the SAE.

safety groundA terminal or conductor at the potential of the immediate localearth or water. It is not used to carry current for the operationof equipment.

sailing offAn intentional change of course away from the wind.

sailingsMathematical solutions of navigational problems involvingcourse, distance, difference in latitude, difference in longitude,and departure.


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sailing vesselAny vessel under sail, provided the propelling machinery, ifany, is not being used.

sail slidesMetal or plastic fasteners that are attached to the edge of asail and ride on a track on the mast or boom.

sail slugsSmall metal or plastic cylinders that are sewn to the edge of asail and fit into a spar cove to attach the sail to the spar.

sail twistTendency for the top of a fore-and-aft sail to sag farther toleeward than the foot of the sail.

salinityA measure of the percentage of salts dissolved in water. Seawater has about 3.5% salinity in the open ocean.

samson postA bitt in the bow of a boat. It often projects downward to thekeel and is secured to it for greater strength.

Santa Ana windA foehn-type wind that drains air from the western high desertareas to the coastal regions of southern California.

saturationIn weather terms, air that contains all the water vapor it canhold is saturated. In the absence of condensation nuclei, aircan hold much more water vapor and is said to be supersatu-rated.


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saturation temperatureSee dew point.

scatteringA process in which light rays are diffused in their movementby tiny particles, including air molecules, much smaller thanthe wavelength of light.

scavengingThe process of replacing the spent gases of combustion withfresh air in a two-stroke cycle engine.

schoonerA vessel with two or more masts rigged fore-and-aft, with theforward mast being shorter or equal in height to the aft mast.

scopeRatio of the length of the payed out anchor line (rode) to theheight of the chock above the bottom of the body of water.

screwIn nautical usage, an alternate name for a propeller.

SCUBASelf-Contained, Underwater-Breathing Apparatus; a type ofdiving gear.

scud1. Thin, low, fast-moving clouds. 2. To run off before a strongwind.

scullTo propel a boat with a single oar at the stern or with therudder.


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scupperAn opening in the rail or bulwark to permit water to drainoverboard.

seaA system of wind waves that are still being blown by the windforce that created them. Usually irregular and choppy, unlikeswell.

sea anchorA parachute-like device deployed off the bow to hold a boathead-to-wind with minimum sternway; often used in heavyweather to keep the bow into the wind and seas. Comparedrogue.

sea breezeAn onshore breeze; one that blows from over the water to-ward the shore. Caused by the more rapid heating of the landin the daytime, with resulting rising air currents over the landand an inflow of air from the sea to replace it.

seacockA valve to close through-hull openings below the waterline.

seakindlyDescribes a boat with easy motion.

sea-level pressureLocal barometric pressure corrected by adding or subtractingan amount to adjust the local pressure reading to what it wouldbe at sea level.

seamsThe spaces between planks in a vessel’s deck or sides.


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sea return (or sea clutter)Return or echo of a radar signal from the water surface closeto the radar station. Waves cause interference and loss ofresolution near the center of the screen.

sea roomNavigable water sufficient for safe maneuvering.

sea smokeSee steam fog.

seaworthySaid of a boat that is in fit condition to put to sea.

secantA straight line intersecting a curve at two or more points.

secondary cellAn electrolytic cell in which the chemical process of chargingand discharging is reversible. A component of a storage bat-tery.

secondary lowA low that forms along either the cold front or the warm frontof a mature extra-tropical cyclone. The secondary low formsat some distance from the central low.

secondary stationsTwo to four stations in the loran chain that transmit in se-quence at precisely predetermined intervals after the masterstation’s transmission.


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secondary windingIn an ignition coil or magneto armature, a wire in which asecondary or high tension current is created by induction dueto the interruption of the current in the adjacent primary wind-ing.

secureTo cover openings and make movable objects fast.

SecurityThe term used to signify a message concerning the safety ofnavigation under international radiotelephone procedures; thethird priority transmission. (Pronounced “sea-cur-i-tay.”)

seicheA rapid change in the water level of a landlocked body ofwater, usually associated with a rapid increase in the velocityof wind or a change in its direction, and a rapid change inbarometric pressure. Water may oscillate following the initialrise or fall.

seize1. To bind by many wrappings of small line. 2. To secure thepin of a shackle using non-corrosive wire.

seizingSmall stuff for binding.

seizureThe process by which excessive friction brings an engine to ahalt (may be caused by lack of lubrication).


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selectivityAbility of a receiver to reject unwanted signals, designated byband width.

self-destructing metalsAlloys, such as brass, in which one or more of the constituentmetals is destroyed by galvanic action when immersed in anelectrolyte.

self-tendingA sail that will change position by itself during a tack or jibe.

semidiameterOne half the diameter of an observed celestial body, mea-sured in minutes of arc.

semi-dieselA semi-diesel engine operates on a comparatively high com-pression ratio and utilizes fuel similar to diesel engines; how-ever, it does use an electrical ignition system rather than dependsolely upon heat generated by compression to furnish ignition.

semi-displacement hullA boat hull that performs effectively at low speeds as a dis-placement hull and at high speeds as a planing hull.

sensible horizonThe circle formed on the celestial sphere by the intersectionof a plane through the eye of the observer, perpendicular tothe zenith-nadir line.


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sensitivityA receiver’s ability to receive weak signals. Measured by theamount of signal in microvolts required at the antenna termi-nals to provide a designated output from the receiver.

sentinelA weight suspended from the rode to help keep the pull on theanchor as horizontal as possible to prevent dragging.

separationDetachment of air in a smooth laminar flow from the leewardside of a sail. See also stall.

series circuitA circuit with only one path through which current can flow.

set1. True direction toward which a current flows. 2. True di-rection of movement of a boat due to wind and current.

sextantA precision hand-held instrument used to measure angles to ahigh degree of accuracy, e.g., altitude angles of celestial bod-ies, heights of objects, and horizontal angles between objects.

sextant altitude (hs)The angle indicated by a sextant before corrections are ap-plied.

shackleA U-shaped metal device with removable pin, used to connecta sail, chain, line, or fitting.


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shaftA cylindrical rod. A rotating shaft is used to transmit powerfrom the engine to the propeller.

shaft horsepower (SHP)The actual power output of an engine and power train mea-sured at the propeller shaft.

sheaveThe roller over which a line passes as it goes through a block.(Pronounced “shiv.”)

sheerThe fore-and-aft vertical curvature of the deck.

sheer strakeThe topmost plank on the sides of a boat, just under the gun-wale.

sheetLine used to adjust a sail relative to the wind, e.g., a jibsheet isused to adjust a jib.

sheet bendA knot used to join two ropes, usually of unequal diameter.

sheet stopperA clamp-type device that contains a cam for securing a sheetor line. It can be released quickly.

shimA specially cut piece of shim stock used as a spacer in spe-cific applications, generally engine alignment.


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shim stockVery thin, accurately machined pieces of metal.

ship’s papersA boat’s registration papers, and often radio licenses, cus-toms papers, and other documents needed for internationaltravel.

shoalA localized shallow spot, often built up of sand or other loosematerial.

shoaling watersShallow water over a sandbar or rocks.

short circuitA path with little or no resistance connected across a powersource.

short spliceA method of permanently joining the ends of two ropes.

shroudStanding rigging that supports a mast laterally.

shroud rollerA tube installed on the shrouds and designed to turn freely inorder to minimize chafing of the sheets and sails.

shunt1. May be used interchangeably with “parallel” when speak-ing of circuits. 2. A low resistance placed across meter ter-minals to limit current flow through the instrument.


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sidelobesSide echoes detected by a radar set, resulting in multiple tar-gets.

sidereal hour angle (SHA)The hour angle of a body, measured westward from the firstpoint of Aries.

sidereal timeTime based on the rotation of the earth relative to the firstpoint of Aries. May be designated as Greenwich or local, ac-cording to the reference meridian.

sightA sextant observation of altitude of a celestial body.

sight reductionProcess of deriving from a sight, along with data from theNautical Almanac, the information needed to establish a lineof position (LOP).

skegAn extension of the keel, or a keel-like projection at the aftend of the hull, for protection of propeller and rudder.

skin effectTendency of radio frequency currents to travel along the sur-face of a conductor rather than be distributed equally through-out its cross section.

skipperCaptain, or often the boat owner.

sky waveRadio wave refracted back to earth from the ionosphere.


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slack1. Not carrying any load, loose. 2. To ease or pay out a line.

slack waterMinimum velocity of a tidal current (sometimes abbreviated“slack”).

sleetGenerally transparent, globular, solid grains of ice formed fromthe freezing of raindrops (North American definition). In GreatBritain, a mixture of rain and snow.

sling psychrometerAn instrument consisting of a wet-bulb thermometer and adry-bulb thermometer mounted on a frame that is connectedto a handle through a swivel so that the instrument may bespun to fan the wet-bulb thermometer. Accurate wet-bulb anddry-bulb readings are then taken to calculate relative humidityand dew point.

slip1. The difference between the theoretical movement of a pro-peller through the water and the actual movement. 2. A berthfor a boat between two piers or floats.

slip ringsContinuous contact rings used for the transfer of current,through brushes, to the rotor coils of an alternator.

sloopSingle-masted sailboat with the mast stepped less than 40%of boat length aft of the bow and setting a mainsail and, nor-mally, one triangular headsail.


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slot effectThe effect on air flow between two sails, caused by the mu-tual interaction of the sails.

small stuffLight line, marline, waxed twine, etc., used for whipping, seiz-ing, and serving.

smogFog contaminated by industrial pollutants; a mixture of smokeand fog.

snap shackleA shackle with a retractable spring-loaded pin.

snatch blockA block with a hinged hook that can be opened to receive orrelease a line.

snowPrecipitation in the form of white or translucent ice crystalsoften clustered in large snowflakes, formed by sublimation.

snubTo stop a line running out by taking a turn around a bitt orcleat.

sodium silicateAny of various soluble silicate glass compounds, used as apreservative for eggs. Also called “water glass.”

SOLASSafety Of Life At Sea conventions, which set internationalstandards for life rafts, life preservers, flares, etc.


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soleA cockpit or interior cabin floor.

solenoidAn iron core, surrounded by a coil of wire, that moves due tomagnetic attraction when electric current is fed to the coil.Often used to actuate a mechanism by electric means, e.g.,starter solenoid.

solid injectionSystem used in full diesel and semi-diesel, where fuel in fluidstate is injected into a cylinder rather than a mixture of air andfuel drawn from a carburetor.

solid statePertaining to components and circuits using semiconductors.

solsticeThe point on the ecliptic at which the sun reaches maximumdeclination—that is, farthest north (at summer solstice, about21 June) or farthest south (at winter solstice, about 22 De-cember) of the celestial equator. Also, the instant at whichthe sun reaches one of the solstices.

soundingA measured or charted depth of water, or the measurementof that depth.

source regionThe region in which an air mass originates and acquires itsinitial characteristics.

spade rudderA rudder attached to the boat only by its stock.


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sparAny shaft or pole for the attachment of a sail, such as themast, boom, yard, or sprit.

sparkA momentary arc of electrical current across a gap.

spark advanceTo cause spark to occur at an earlier time in the cycle ofengine operation. Opposite of spark retard.

spark gapThe space between the electrodes of a spark plug across whichthe spark jumps.

spark plugAn electrical or electronic device inserted into the combustionchamber of an engine to deliver a precisely-timed spark todetonate a fuel-air mixture.

specific gravity1. The measure of the density of the electrolyte in a battery;that is, the strength of the acid and therefore the battery’sstate of charge. 2. The ratio of the weight of a given volumeof a substance to the weight of the same volume of water.

specific heatThe amount of heat in calories necessary to warm 1 gram ofa particular substance 1°C. Water has a specific heat value of1 cal /g / °C—higher than most land-mass substances.

speedRate of motion.


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speed gradientA marked difference in speed between two adjacent winds.

speed made goodThe resultant speed from a point of departure to a point ofarrival.

speed of advanceIntended or expected speed along the track.

speed of electromagnetic wavesThe speed of electromagnetic waves in space is the same asthe speed of light. However, the speed of propagation of elec-trical signals along wires is somewhat slower. For example, itis 3/5 of the speed of light along coaxial cable.

speed of lightApproximately 186,000 miles per second, or 300,000 kilome-ters per second.

speed of sound1,076 feet per second through air at standard pressure andtemperature; 4,800 feet per second through water.

speed over the groundActual speed being achieved relative to the ground.

spinnakerA large, lightweight nylon headsail set forward of the headstayand used when reaching or running. Spinnakers may be sym-metrical or non-symmetrical.


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spinnaker netA web of light line hung in the foretriangle to prevent a spin-naker wrap where the spinnaker tends to wrap itself aroundthe forestay.

spinnaker poleA pole of fixed length to hold the tack of a spinnaker to wind-ward.

spinnaker sockThe fabric tube pulled over a spinnaker to safely furl the sail.

spliceJoining lines by interweaving the strands.

spray railDevice built into or attached to the hull of a boat, forward andabove the waterline, to deflect spray outboard and downward.

spreaderA strut fitted to the side of the mast to hold one or more shroudsaway from the mast.

spring lineA line leading forward or aft from a vessel to a piling or fittingon a pier to prevent the boat from moving ahead or astern.Compare breast line.

spritsailA quadrilateral sail with its upper aft corner held by a shaftthat extends from the mast diagonally across the sail to thecorner.


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spurious emissionsAny radiation of radio energy outside the authorized channelor band.

squallA sudden, violent burst of wind, often accompanied by rain orsnow. Persists longer than a gust, usually two minutes or more.

squall lineAny non-frontal line or narrow band of active thunderstorms.A squall line that is associated with and precedes a cold front—and likely parallels it—is a pre-frontal squall line.

square knotA knot used to join two lines of similar size; also called a reefknot.

squelchA circuit that mutes the output of a receiver in the absence ofa received signal.

SSB radioSingle-side-band radio used in marine communications overlong distances on the high seas.

stability1. A state in which the vertical temperature distribution of theatmosphere is such that an air parcel resists displacement fromits original level. 2. The ability of a boat to resist heeling andoverturning.

stableDescribes the tendency of a parcel of air to resist verticalmotion and, if displaced, to return to its original level.


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staffAn upright pole to which a flag or light is fixed.

stainless steelGeneral term for the many alloys of steel, chromium, nickel,and other metals that are corrosion-resistant.

stallThe condition that arises when the airfoil (sail) or hydrofoil(rudder) is turned too far off and the flow of air or waterseparates from the foil; there is no longer an attached flow.

stanchionA vertical post or column used to support a deck, cabin top, orlifelines.

standThat phase (or portion) of the tidal cycle during which there isno vertical change in the height of tide.

standard timeThe legal established time for a given zone, often used withadjectives, such as Eastern Standard Time (EST).

standing partThe part of a rope that is made fast.

standing riggingShrouds and stays supporting the mast.

stand-on vesselA term, from the Navigation Rules, used to describe the ves-sel that continues its course in the same direction at the samespeed during a crossing or overtaking situation, unless a colli-sion appears imminent. Compare give-way vessel.


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starboardSide of a boat, or direction, to the right when facing towardthe bow.

starboard tackAny heading where the wind is on the starboard side and themainsail is carried on the port side of the boat.

starterAn electric motor attached to an engine to provide the meansto turn the engine over by gearing to start the engine.

static electricityStationary electricity, a charge in equilibrium.

station modelThe arrangement of symbols used by weather observers andforecasters when posting station observations on a weathermap.

stationary frontThe line of discontinuity between a cold air mass and a warmair mass when neither is moving.

statorThe stationary armature on an alternator within which the ro-tor spins.

stayStanding rigging that supports a mast fore and aft.

staysailA fore-and-aft triangular sail normally set upon a stay otherthan the headstay or jibstay.


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steamPopularly, the visible condensation that occurs when watervapor passes from a warm to a colder environment; an ex-ample is steam fog.

steam fogFog formed when cold air moves over a relatively warm wa-ter surface. Also called sea smoke, or arctic smoke.

steeragewayEnough speed to steer the boat.

St. Elmo’s fireA discharge of static electricity, usually from pointed objectssuch as masts and spars, into the moist, highly ionized air sur-rounding them. The visible energy released by recombiningelectrons with ionized air particles produces the glow.

stemThe upright structural member or post of the bow, attached tothe foremost part of a vessel’s keel.

stemheadThe top of the timber that forms the bow of the boat.

stemhead fittingA metal casting made to fit over the top of a stemhead.

stepThe act of installing a mast on a boat or removing (unstepping)a mast.

sternAfter end of a boat.


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stern driveAn inboard/outboard drive system with the engine inside theboat. See Inboard/Outboard.

stern lineA mooring line that runs from the aft end of a boat to a float,a piling, or a pier.

sternwayMovement stern first, as opposed to headway.

stiffStable, resistant to heeling. Opposite of tender.

stockThe crossbar of an anchor. Also, a rudder shaft.

stopStrap, line, or shock cord used to lash a rolled sail.

storm jibA very small, strongly built jib set in heavy weather.

storm surgeA great dome of water, often 50 miles wide, that comes sweep-ing across the coastline near the area where the eye of a hur-ricane will make landfall. This abnormal rise of the sea isprimarily due to the winds of the storm.

storm trysailA small, strongly-built sail used in place of a mainsail in stormconditions.

stowTo put something in its proper place.


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strakeA continuous longitudinal line of planking on the bottom orside of a boat.

strand1. To drive a vessel ashore or aground. 2. One of the lays ofa rope (the wound yarns or fibers that are woven with otherstrands to make a rope).

stranded wireHelically laid small conductors forming a flexible cable or con-ductor of larger cross section.

stratocumulusA form of low cloud composed of a continuous or broken layer,with rounded top elements or undulations giving the appear-ance of a simple wave system. Often caused by the flatteningand spreading of cumulus clouds.

stratopauseThe interface or boundary layer, approximately 28 miles abovethe earth’s surface, between the stratosphere and the mesos-phere.

stratosphereThe region from approximately 6 to 28 miles above the earth’ssurface. It is characterized by increasing temperature withaltitude. The lower third of the stratosphere is the upper limitfor weather clouds, cirrus and cumulonimbus.

stratusA form of low cloud appearing as a gray, sheetlike layer witha uniform base, characteristic of stable air.


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stray currentLeakage of a current through a path other than the one in-tended.

stray-current corrosionCorrosion caused by stray or unwanted current action.

streak lightningOrdinary lightning of a cloud-to-ground discharge that appearsto be concentrated in a single, relatively straight lightning chan-nel.

stringerA fore-and-aft structural member of a hull.

strobe lightA brilliant, flashing white light.

strokeDistance traveled by a piston from bottom dead center to topdead center.

strutA bracket that supports the propeller shaft; located outside ofthe hull.

studA rod with threads on both ends, such as a cylinder head stud.

stuffing boxFitting through which the propeller shaft or rudder post pen-etrates the hull; designed to prevent water from entering thebilge and to provide lubrication for the shaft or post.


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sublimationThe transition of a substance directly from the solid state tothe gaseous state, or vice-versa, without passing through theliquid state. See snow.

subordinateTerm applied to a secondary station of a loran chain.

subpolar lowsCircumferential low-pressure belts with varied weather char-acteristics, located at approximately 60°N and 60°S latitude.

subsidenceDescending motion of a layer of air in the atmosphere, usuallyover a substantial area.

superchargerA mechanically driven blower used to compress the inlet air.

supercooledA liquid, such as water, is supercooled if its temperature isbelow the temperature at which the liquid normally freezes.This is usually due to lack of crystallization nuclei.

supersaturationA condition in which the relative humidity exceeds 100%, usu-ally due to lack of sufficient condensation nuclei.

superstructureCabins, deckhouses, etc., above the deck.

surfThe surface wave activity between the outermost line of break-ers and the shore.


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surface dispersionSpread of current over earth or water surface resulting froma lightning strike.

swabA seagoing name for a mop. You “swab down,” not “mopup.”

swampTo fill with water, but not to founder.

sweatThe soldering of two surfaces together by the application ofheat, which causes molten solder to flow between the sur-faces by capillary attraction.

sweat upTo tighten a line as much as possible.

swellA wave system that has outrun or is no longer being actedupon by the wind that created it. A swell is characterized byregular, smooth-crested wave forms, usually of long wavelength.

switchA device used to open or close a circuit, stopping or allowingcurrent to flow.

synchronous speedTurning or vibrating at the same speed as another device. Asapplied to electric motors or generators, it means that the speedof the motor is “locked in” with the powerline frequency.


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synoptic chart or mapA map showing the state of the weather over a large area ata given moment.

systematic windA wind created by flow from one air mass into another asopposed to wind created by localized heating and cooling.

Ttabernacle

A hinge at the base of a mast; permits lowering the mast.

tachometerA device for measuring and indicating the rotational speed ofan engine.

tackThe lower forward corner of a mainsail or jib or the corner ofa spinnaker that is near the spinnaker pole. Also, with “star-board” or “port,” describes the side over which the windpasses. Also, to change course by turning the bow throughthe eye of the wind.

tackleAn arrangement of line and blocks used to provide increasedmechanical advantage.

taffrailThe bulwark or rail across the stern of a vessel.

tailTo haul on a sheet around a winch being cranked by anothercrew member.


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tallboyA tall, narrow sail set close to leeward of the main, usuallywhen a spinnaker is set.

tangA metal strap used to attach standing rigging to a mast.

tapAs applied to a coil or resistor, a connection somewhere be-tween the start and finish of the winding.

tappetA cylinder, rod, or lever that moves or is moved by intermit-tent contact, as with a cam in an engine or machine.

telltaleA wind direction indicator made of a bit of cloth or other lightmaterial.

temperature (T)A measure of hotness. In a gas, temperature is proportional tothe average random kinetic energy of the molecules, but not ameasure of heat or total energy. The thermometer measuressome property that varies with temperature. The thermom-eter can then be calibrated by assigning arbitrary values tofixed temperatures such as the freezing point and boiling pointof water at standard pressure.

tender1. A small boat accompanying a vessel, used to transport per-sons, gear, and supplies, a dinghy. 2. Lower than average re-sistance to heeling.


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terminalA connecting point on a device or component.

thermal expansionPhysical property of a material by which it expands as it isheated. Conversely, it contracts as it cools.

thermoplasticA plastic that softens when heated. Used as an insulator forelectrical wiring.

thermosphereThe region above 55 miles altitude, characterized by an in-creasing temperature with altitude, but containing an air den-sity so low that sound is not carried.

thermostat1. A heat controlled valve used in the cooling system of anengine to regulate the flow of water. 2. An element, such as abimetal strip, responding to ambient temperature changes, tomake or break a circuit.

thimbleA grooved metal loop, around which a rope or wire rope maybe spliced, thus making the spliced eye more resistant to chaf-ing.

third brushIn DC generators, an auxiliary brush placed on a commutatorto control current output of the generator.

throatUpper, forward corner of a quadrilateral fore-and-aft sail.


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throttleSee butterfly valve.

throwDistance from the center of a crankshaft main bearing to thecenter of the connecting rod journal.

thunderThe sound emitted by rapidly expanding air along the channelof a lightning discharge.

thunderstormA storm produced by a cumulonimbus cloud. The thunder-storm is accompanied by lightning and thunder, usually withstrong gusts, heavy rain, and sometimes hail.

thwartA transverse seat in a boat. See thwartships.

thwartshipsAt right angles to the fore-and-aft line. See athwartship.

tidal currentThe horizontal movement of water caused by tidal action. Seealso ebb current and flood current.

tidal rangeDifference in height of tide between any successive pair ofhigh and low tides.

tideThe vertical rise and fall of ocean water (most noticeable incoastal regions) resulting mainly from the gravitational attrac-tion of the moon and sun.


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tillerA lever attached to the upper end of a rudder stock, used bythe helmsman to turn the rudder.

timberA general term for the rectangular pieces of wood used inship building.

time1. The instant of the day reckoned by the position of a celes-tial reference point, such as the mean sun, relative to a refer-ence celestial meridian. 2. A measure of the elapsed timeinterval between two events.

time diagram (TD)A diagram in which the celestial equator, as viewed from thesouth celestial pole, appears as a circle, and the hour circlesappear as radial lines.

time difference (TD)The difference in time of arrival, measured in microseconds,of two loran signals, one from the master station and the otherfrom one of the secondary stations.

time to arcThe converting of time in hours (hh), minutes (mm), and sec-onds (ss) to arc in degrees.

time zoneA zone, usually 15° of longitude wide, in which the same timeis kept throughout.


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timing chainChain used to drive camshaft and accessory shafts of an en-gine.

timing gearsA group of gears that are driven from the engine crankshaftto cause the valves, ignition, and other engine-driven appara-tus to operate at the desired time during the cycle.

toe railA small raised section around the perimeter of the deck.

toggle1. A U-shaped fitting linking a turnbuckle with a chainplate;permits movement so the turnbuckle is not bent when the stayis pulled out of line. 2. A small wooden crosspiece at the endof a line, passed through an eye or loop in another line aswhen bending a flag onto a halyard.

top dead center (TDC)The position of the crankshaft when the piston is at the verytop of its stroke.

topping liftA line used to support a boom end and/or spinnaker pole.

topsideOn or above the weather deck (wholly exposed to the ele-ments).

topsidesThe sides of a vessel between the water line and the deck.


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tornadoA violently rotating column of air attended by a funnel-shapedor tubular cloud hanging beneath a cumulonimbus cloud. Acyclonic storm of devastating destruction in the Northern Hemi-sphere, the tornado varies from about 20 yards to a mile wideat the surface. Wind speeds are estimated from 100 to over300 miles per hour.

torqueAn effort devoted toward twisting or turning.

torque wrenchA special wrench that measures the turning force applied to anut, bolt, or shaft.

traceTo analyze or follow current flow or signals through a circuit;also, the fluorescent line or image on an oscilloscope screen.

track1. A metal piece on the after edge of a mast or top of a boomon which sail slides run. Also, a metal strip on deck for carswith blocks for adjusting sheet leads. 2. The actual path of avessel over the ground, such as may be determined by track-ing. See intended track.

trade windsCircumferential bands of surface winds flowing from thehorse-latitude highs toward the low-pressure doldrums at theequator. These winds are deflected westward (making themeasterly) by the Coriolis effect. Trade winds occur in bothhemispheres between 30° and 0° latitude.


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transceiverA radio set that combines transmitting and receiving modes inone package. It may have circuits or components common toboth.

transducerElement of a depth sounder that translates electromagneticpulses into ultrasonic pulses and vice versa. Usually containsa barium titanate crystal.

transfer portIn two-stroke cycle gasoline engines, an opening in the cylin-der wall permitting the fuel mixture to enter from the crank-case.

transformerAn alternating current device to magnetically couple one cir-cuit to another. It can provide higher or lower output voltagerelative to the input voltage.

transistorA semiconductor device having three or more elements. Ithas the capability of amplification, rectification, or switchingvoltage or currents.

transit (Tr)The passage of a celestial body across a celestial meridian.See meridian transit.

transmitterAn electronic unit capable of generating a signal for propaga-tion through a medium.


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transomThe portion of the hull at the stern that is at right angles to thecenterline of the boat.

trapezeA line from the mast that can be used by the crew of a smallboat to suspend themselves outboard of the boat, thereby in-creasing the stability of the boat.

travelerA sail-positioning system composed of a track on which slidesa car attached to blocks to permit positioning sheet leads un-der load.

traverseA series of directions and distances, such as those taken by asailboat tacking to reach a destination windward.

traverse sailingA method of determining course and distance made good by atraverse.

trim1. To adjust the set of the sail. 2. Refers to the attitude of aboat at rest in the water.

trimaranA boat with three hulls connected by a deck or crossbeams,resulting in a wide beam and good stability; the middle hull isusually larger than the two outer hulls.


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trip-freeA circuit breaker that will function to interrupt a circuit underoverload conditions even though the reset mechanism may beset against the trip.

trip line1. A buoyed line attached to the crown of an anchor for thepurpose of freeing it when fouled. 2. A line fastened to thesmall end of the cone of a sea anchor to collapse it, thus spill-ing the water from the cone.

trochoidCurve traced by a point on or inside the rim of a wheel rollingalong level ground.

tropical air massWarm air that has its source in the low latitudes, chiefly in theregions of the subtropical high-pressure systems.

tropical cycloneAn intense, violent storm spawned on the outer fringes of thedoldrums (6° to 10° latitude) and fed by the warm, moist air ofthe maritime tropics. Also called hurricane, willy-willy, typhoon,baguio, and/or Mexican or Indian Ocean cyclone.See hurricane.

tropical depressionA rotary circulation at the surface; highest constant wind speednot more than 38 miles per hour (33 knots).

tropical disturbanceA moving area of thunderstorms that maintains its identity for24 hours or more. This is a common phenomenon in the tropics.


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tropical stormA distinct rotary circulation; constant wind speed of 39 to 73miles per hour (34 to 63 knots).

tropopauseThe boundary between the lowest layer of the atmosphere,the troposphere, and the next layer, the stratosphere. Rangesin altitude from 5 to 12 miles above the surface.

troposphereThe lowest layer of the atmosphere, characterized by a gen-erally decreasing temperature with increased elevation. Allordinary, visible weather phenomena occur in this layer. Itsthickness varies from about 5 miles at the poles to about 12miles at the equator.

trough of low pressureA flattened or elongated zone of low pressure that has notdeveloped into a cyclone.

truckThe top of the mast.

true course (TC)The course as plotted on the chart, measured from true north.

true windActual direction and speed of horizontal air motion in relationto the earth’s surface. Also, strength and direction of the windfelt on a boat not moving.

trysailSee storm trysail.


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tsunamiA seismically generated sea wave. Not observable at sea, butcan be extremely destructive when it strikes a coastline.

tumblehomeThe inward curve of a boat’s sides from bilge to deck line,particularly near the stern.

turbochargerA blower driven by an engine’s exhaust gas; used to com-press the inlet air.

turbulenceSmall-scale irregularities in the horizontal or vertical flow ofair.

turbulent flowIrregular, detached flow along a surface such as a sail or rud-der, which reduces the lifting characteristic of the foil.

turnbuckleA tension-adjusting device for tightening wire rigging or cable,composed of threaded rods extending from within a threadedbarrel.

turning blockA block used to change the direction of a line, such as a sheetor halyard, to make hauling more convenient.

turn of the bilgeThe lower outer part of the hull where the sides meet thebottom.


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twilightThe period of incomplete darkness following sunset or pre-ceding sunrise. See civil twilight and nautical twilight.

twistThe change in the angle of a sail’s chord with respect to itsfoot, which increases from tack to head.

twist-lockA connector or plug that is mechanically locked into its socketby twisting after insertion.

two blockA tackle that has been pulled to bring the blocks together.Also, a halyard that has been hoisted all the way.

type acceptanceApproval by FCC of a manufacturer’s design of a transmitterthat, when properly installed, will meet FCC technical stan-dards of performance.

typhoonA severe tropical storm in the western Pacific or the ChinaSea; same as a hurricane except for location.

Uultrasonic

Signal or pulse whose frequencies are above the limits of au-dibility.


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under bare polesSailing (underway) with no sail set and being driven only bythe force of the wind on hull, spars, and rigging; usually aheavy-weather precaution.

under powerA boat being propelled by an engine, even though sail may beset.

underway1. Making progress through the water. 2. Afloat but not atanchor, aground, or made fast to the shore.

universal motorAn electric motor that will operate on either AC or DC power.

Universal Time (UT)See Coordinated Universal Time (UTC).

unshipTo remove an object from its regular place of use.

unstableApplied to an air mass or parcel of air, this term refers to thetendency of the air to remain in vertical motion once set inmotion.

updraftAn upward current of air, usually generated by convectionnear or within a developing cumuliform cloud, or by turbu-lence within a layer of air. Also, a type of carburetor whereinthe fuel-air mixture flows upward to the engine.


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upper branch (M)The half of the observer’s meridian, measured from pole topole, passing through the zenith of the observer.

upper limb (UL)The upper part of the circumference of the sun or moon.

upslope fogFog formed when warm, moist air is lifted by moving overhigher ground and is cooled to its saturation point.

urgent callSee Pan-Pan.

USYRAUnited States Yacht Racing Union.

Vvalley wind

Air flowing up the side of a mountain as the result of heatingof the mountain slope and the air next to it during the daytime.

valveDevice for allowing gases in and out of an engine cylinder atprecise moments.

valve clearanceThe gap between a valve stem and its rocker arm or tappetwhen the valve is fully closed.

valve faceThe part of a valve that mates with and rests upon a seatingsurface.


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valve grindingA process of mating the valve seat and valve face, performedwith the aid of an abrasive.

valve key or lockKey, keeper, washer, or other device that holds the valve springcup or washer in place on the valve stem.

valve lifterSee tappet.

valve overlapThe period of time in which an exhaust valve and inlet valveare both open in a cylinder at the same time.

valve seatMating surface upon which valve face rests.

valve springA spring attached to a valve to return it to its seat after thelifting force is released.

valve stemA portion of a valve that rests within a guide.

valve stem guideA bushing or hole in which the valve stem is placed that al-lows longitudinal motion only.

valve timingIndicates the position of the valves (open or closed) relativeto the travel of the piston, in crankshaft degrees.

vangA line or purchase used to assist in positioning a boom or gaff.


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vaporA substance in the gaseous state.

vaporizationThe change of state from a liquid to a gas (i.e., water to watervapor).

vapor pressureThat portion of the total atmospheric pressure (gas pressure)caused by water vapor. This value is correctly called “partialpressure” of water vapor. The vapor pressure given indew-point tables refers to the maximum, or saturation, pres-sure at a specific temperature.

variation (Var)The angle between the geographic meridian and the magneticmeridian at a given locality. Variation is easterly or westerly,as the direction of the magnetic meridian is toward the east orwest of geographic north.

vectorA quantity that has both magnitude and direction, commonlyrepresented by an arrow. The length of the arrow representsthe magnitude; the direction in which the arrow flies repre-sents the direction in which the quantity acts.

vector sumThe “addition” in which the tail of one vector is placed at thetip of a second vector, and a new vector is taken from the tailof the second vector to the tip of the first.


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veer1. Change in direction of the wind in a clockwise direction inthe Northern Hemisphere and a counterclockwise direction inthe Southern Hemisphere. 2. To change direction or course.See back.

veering windA wind whose direction changes progressively in a clockwisemanner (for example, N-NE-E-SE) in the Northern Hemi-sphere; opposite of a backing wind.

velocityA vector quantity that includes both rate of motion and direc-tion of motion—as contrasted with speed, which does not in-clude direction.

venturiTwo tapered, streamlined tubes joined at their small ends soas to reduce the internal diameter. Also, a constriction in theair passage of a carburetor, which causes an increase in ve-locity and a decrease in pressure.

vernal equinoxSee first point of Aries.

vernierA short auxiliary scale alongside the graduated scale of amarine sextant to allow precise reading of fractions of aminute.

vertical circleA great circle of the celestial sphere, running through theobserver’s zenith and nadir in a plane perpendicular to thehorizon. See prime vertical.


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vessel engaged in fishingAny vessel fishing with nets, lines, trawls, or other fishing ap-paratus that restrict maneuverability. But the term “vessel en-gaged in fishing” does not include a vessel fishing with trollinglines (or other fishing apparatus) if these lines do not restrictmaneuverability.

vessel not under commandA vessel that, through some exceptional circumstance, is un-able to maneuver as required by the Navigation Rules and is,therefore, unable to keep out of the way of another vessel.

vibration damperA device to reduce the torsional vibration that occurs alongthe length of the crankshaft of an engine.

viscosityThe measure of a fluid’s resistance to flow.

visibilityThe greatest distance at which one can see and recognizeprominent objects.

visible horizonJunction where earth and sky appear to meet.

voltA unit of electrical force that will cause a current of one am-pere to flow through a resistance of one ohm.

voltmeterAn instrument for measuring voltage in an electrical circuit.


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volumetric efficiencyThe efficiency with which an engine replaces the spent gaseswith fresh air (diesel) or gasoline and air (gasoline engine).

voluntarily equippedApplies to vessels having radio or other safety equipment in-stalled even though not required by government regulations.

vortexThe funnel-shaped profile formed by a spinning fluid.

Wwake

Water surface turbulence left by a moving boat.

warm frontThe discontinuity or boundary at the trailing edge of a retreat-ing cold air mass that is being displaced by an advancing massof relatively warm air.

warm sectorThat sector, usually the smaller portion of the area surround-ing a wave cyclone, in which the warmer air mass is in con-tact with the earth’s surface.

warningIn weather advisories, a bulletin indicating that a hazardouscondition, such as a tornado or flash flood, is in existence andmay threaten some portion of the warning area.


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warp1. To maneuver a boat by hauling on a line fastened to a fixedobject, such as a cleat, pile, bollard, or anchor. 2. The lineused in warping. 3. A line streamed behind a boat to slow itdown.

wash1. Turbulent flow of water left by a moving boat or as a resultof a boat’s turning propeller(s). 2. The rush or sweeping ofwaves on a bank, shore, or vessel.

watch (W)1. In weather advisories, a bulletin indicating that a hazardouscondition, such as a tornado or flash flood, may develop. 2. Aperiod of time during which a crew member is on duty.

watch time (WT)The hour of the day as indicated by a watch or clock; usuallyexpressed in 24-hour time, or a 12-hour cycle and labeled AMor PM.

water vaporWater in its gaseous state.

waterline length (LWL)The measurement along the waterline from the point wherethe bow touches the water to the point where the stern emergesfrom the water.

waterspoutA whirlwind or spiraling uplift of sea or lake surface waterinto the lower atmosphere, caused either by a tornado thathas moved out over the water or by convective air motion(either cyclonic or anticyclonic).


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wattThe unit of electrical power.

watt-hour meterInstrument used to indicate total electrical energy consumed;usually designated in kilowatt hours.

wave1. In water, a series of undulations that move along the sur-face. 2. Other periodic occurrences, such as the successionof compressions and rarefactions of the air in sound waves orthe cyclical radiation of energy in electromagnetic waves.

wave heightThe vertical distance from the trough of a wave to the crest.

wave length1. The distance between successive crests of a wave. 2. Dis-tance occupied by one full cycle of an alternating current orelectromagnetic wave.

wave periodThe time interval between passages of two successive crests.

wave systemA collection of waves all due to the same disturbance, havingsimilar heights, lengths, and periods.

wayMovement of a vessel through the water, such as headway,sternway, or leeway.

waypointA defined point, such as a buoy or latitude/longitude position,entered in a Loran-C or GPS device.


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wearChanging tacks by jibing instead of tacking.

weather1. The condition of the atmosphere at any given time and place.2. The windward side.

weather deckA deck wholly exposed to the elements.

weather helmA condition of imbalance that requires the tiller to be held toweather to keep the boat on a straight course.

well-foundHaving all necessary equipment on board, in good condition.

westerliesSee prevailing westerlies.

wetted surfaceThe portion of a vessel’s exterior that is in contact with thewater.

wharfMan-made structure parallel to the shoreline for loading, un-loading, or making fast.

whipTo bind the end of a rope with light line.

whisker poleA pole used to hold a jib to windward when runningwing-and-wing.


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winchA geared drum turned by a handle and used to pull lines suchas sheets and halyards.

windHorizontal motion of the air, reported in both speed and direc-tion.

windageResistance to the wind.

windlassA winch, powered by hand or motor, for hauling anchors.

wind’s eyeThe exact direction from which the wind is blowing.

wind shearThe rate of change of wind velocity (direction and speed) perunit distance. This term is conventionally expressed as verti-cal or horizontal wind shear.

wind shiftA sudden change in wind direction, usually occurring with thepassage of a front.

wind vaneA type of autopilot that uses the apparent wind to steer theboat.

windwardThe direction from which the wind is blowing. (Pronounced“wind’ard.”)


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wing-and-wingSailing on a run with the jib and mainsail set on opposite sides.

working endThe outboard or free end of a line, chain, or cable, used tomake knots, bends, and splices and to attach to an anchor,cleat, other vessel, etc. Compare bitter end.

working jibA normally-used jib that has an area about equal to the areaof the foretriangle.

woven rovingLoosely woven fiberglass cloth.

wrist pinThe journal for the bearing in the small end of an engine con-necting rod that also passes through piston walls.

X

Yyardarm

The end of a spar crossing a mast horizontally.

yawTo swing off course due to wave action or bad steering, usu-ally to one side and then the other.

yawing momentA force tending to rotate the hull about a vertical axis, usuallydue to displacement of the center of effort of the sails out-board of the center of resistance of the hull.


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yawlA two-masted vessel whose mizzen is stepped abaft the rud-der post.

Zzenith (Z)

The point on the celestial sphere directly overhead of the ob-server.

zenith distance (z)Angular distance along a vertical circle, from the zenith to abody. Same as co-altitude. The side of the navigational tri-angle between the assumed position (AP), dead reckoningposition (DR), and the geographical position (GP) of the body.

zodiacBand of the sky extending about 8° to either side of the eclip-tic. Each of the zodiac’s 12 areas is named for a constellationand represented by a sign. Important to astrologers but of nosignificance for navigation.

zone description (ZD)A number, including a positive or negative sign, added to orsubtracted from zone time, to obtain Greenwich mean time.

zone time (ZT)Local mean time kept throughout a designated zone, usuallybased on the nearest meridian with a longitude exactly divis-ible by 15°.


usps nautical glossary · 2006. 6. 7. · with a sextant. aspect ratio 1. square of the luff length...

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