class4 question & answers ship const

7/30/2019 Class4 Question & Answers Ship Const

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Question

Sketch a semi balanced rudder showing:

(i) Details of rudder support;

(ii) Details of the hinging mechanism about which the rudder turns.

State ONE advantage of the semi balanced rudder over other types.

Answer


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Locking pintle

Bearing pintle


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Rudder carrier

Advantage of the semi balanced rudder over other types.

In unbalanced rudder all rudder area has aft of pivot point

CE position results in high torque on stock and pintle


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In semi balanced rudder up to 20% of rudder area has forward of pivot point hence

torque on stock and pintle is reduced

Thus relatively smaller diameter stock and smaller capacity steering gear can be

used

Rudder stops - Three ways

Limit on telemotor - 35deg each way from midship

Arrangement such that actuators are put to stop by hunting gear control

In steering gear 37deg each way from midship

There are stop set to limit the angle to which the rudder can be moved by gear

Stops are provided on steering gear itself at 37deg in order that the rudder is not

forced against external stops in case of mal adjusted control gear

External stops 39deg each way from mid ship

They are provided at 39deg each side with the aim of preventing unlimited

movement of rudder in case of disconnection

Specially in view of protecting the propeller

Question

Work is being carried out in dry dock on a large sea water inlet chest and valves

Describe the inspection

As work has started

During and after work

Before starting work

Check condition of grating how much the grating is chocked with sea growth

Check condition of corrosion this will indicate whether MGAS is working properly


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Check galvanic protection is OK

Check condition of zinc block

As work commences and is progressing

At regular interval sea chest has to be examined whether surface is properly

cleaned of barnacles and sea growth

Corrosion entire sea chest is scrapped check for corrosion pitting etc

If ship is more than 5years old with consultation with surveyor [class] carry out

thickness gauging of sea chest

New zinc block are properly fitted and quality of zinc is good they should have

good contact with sea chest metal

If MGPS is provided fit new copper rods and zinc rods

Painting of sea chest should be properly painted two or three coat minimum

While painting paint should not be sprayed on zinc anodes and MGPS anodes

Air and steam valves to be checked for corrosion crack condition of sitting valve

seat etc

After work

Check no foreign material as paint brush jute is left inside

Vent line and air line should be blown through to make sure that they are clear

Air vent blowing valve and main sea chest valve are properly boxed up with new

joint packing etc

Sea chest valve can be checked for leakage

If everything is satisfactory put the grating from outside grating is cleaned

Fitted with stainless steel nut and nut to be welded and secured by split pin

Write short notes on

FREE SURFACE EFFECT


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When a tank on board a ship is not completely full of liquid, and the vessel heels,

the liquid moves across the tank in the same direction as the heel. The centre of

gravity of the ship moves away from the centreline, reducing the righting lever and

increasing the angle of heel.

The movement of the centre of gravity from G to Gx has been caused by the

transfer of a wedge of liquid across the tank. Thus if


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CAVITATION

The thrust of a propeller varies approximately as the square of the revolutions.

Thus as the speed of rotation is increased there is a considerable increase in thrust.

The distribution of pressure due to thrust over the blade section is approximately as

shown in Fig

The net pressure at any point on the back of the blade is the algebraic sum of the

atmospheric pressure, water pressure and negative pressure or suction caused by

the thrust. When this suction is high at any point, the nett pressure may fall belowthe vapour pressure of the water at water temperature, causing a cavity or bubble to

form on the blade. This cavity is filled with water vapour and with air which

disassociates from the sea water. As the blade turns, the bubble moves across the


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blade to a point where the nett pressure is higher, causing the cavity to collapse.

The forming and collapsing of these cavities is known as cavitation.

When the cavity collapses, the water pounds the blade material, and since the

breakdown occurs at the same position each time, causes severe erosion of the

blades and may produce holes in the blade material several inches deep. Cavitation

also causes reduction in thrust and efficiency, vibration and noise. It may be

reduced or avoided by reducing the revolutions and by increasing the blade area for

constant thrust, thus reducing the negative pressure. Since cavitation is affected by

pressure and temperature, it is more likely to occur in propellers operating near the

surface than in those deeply submerged, and will occur more readily in the tropics

than in cold regions.

Reserve buoyancy

A floating vessel must displace its own weight of water.

Therefore, it is the submerged portion of a floating vessel which provides the

buoyancy.

The volume of the enclosed spaces above the waterline are not providing buoyancy

but are being held in reserve.

If extra weights are loaded to increase the displacement, these spaces above the

waterline are there to provide the extra buoyancy required.

Thus, reserve buoyancy may be defined as the volume of the enclosed spaces

above the waterline. It may be expressed as a volume or as a percentage of the total

volume of the vessel.

Question

a) Sketch the fwd section of a ship to show the position of the following

component parts of the ships anchorage system; hawse pipe, cable stopper,

windlass and cable lifter, spurling pipe and chain locker.


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b) Describe a chain stopper and state it's purpose.

c) Show by means of a sketch how an anchor cable is attached to the ship.

d) Describe how a chain locker is drained of water, sand and mud.

ANS - a) SKETCH


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b)The chain stopper is used to hold the anchor chain in place once the anchor hasbeen lowered and it takes the strain on the anchor when the anchor is raised and in

place.

c)The final link of the anchor chain is secured to a half shackle which is welded to

a secure point of the ships structure in the chain locker, bulkhead or deck and is

known as the " bitter end

d)A chain locker is drained of water, sand and mud by means of a bilge eductor

which is normally driven by water from the fire main or general service main.

The reason for using a bilge eductor is that sand and mud mixed with water would

damage a normal pump.


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Question

Explain with the aid of sketch

Raise floor

Water tight bulkhead

Tumble home

Shelter deck

Free board

Tumble-Home - is the fall in of the sides amid ships.

It flattens out the stability curve and allows the ship to roll more easily.

Rise of Floor - is height from the base line to the bottom of the bilge radius.

It helps to drain the double bottom tanks.

Shelter deck - upper deck having no overhead protection from the weather, but

sheltering the deck below

Freeboard is the difference between the depth at side and the draught, that is it is

the height of the deck above the waterline.

The freeboard is usually greater at the bow and stern than at amidships. This helps

create a drier ship in waves. Freeboard is important in determining stability at large

angles.

Freeboard is the distance from the water load line up to the main deck.


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The main deck is the highest deck that is water sealed.

Water falling on upper decks may run down companion ways, but it cannot go any

farther down into the ship than the main deck. When the ship is in bad weather and

ploughing her way through the waves, she will at times displace a greater weight of

water than her own-weight, which will tend to push her up again. This extra weight

of water tending to push the ship up when in the above condition, is represented by

the freeboard.

This extra volume of water which the ship can displace in bad.weather and is in

reserve for that purpose, is measured by the water plane area multiplied by the

freeboard and is called the reserve buoyancy. Every ship must have reserve

buoyancy, as in case of heavy leakages into holds below the main deck, or bilge

pumps becoming defective or in case of collision, the hold would gradually fill up

with water. The reserve buoyancy, therefore, keeps the ship from immediately

sinking and allows time for repairs to be effected and thus save the ship and

incidentally the lives in the ship.

Question

Purpose of load line

Historical note

In 1876, Samuel Plimsoll introduced a law into Parliament that meant that ships

were assigned certain freeboard markings above which, in particular conditions,

they were not allowed to load beyond.

Prior to this law a great many ships were lost at sea mainly due to over loading.

In 1930 and in 1966, international conferences modified and expanded these

statutory regulations dealing with the safety of ships. These regulations have been

further improved over the years by conference meetings every 3 or 4 years up to

the present day.

One such organisation was the Safety of Life at Sea organisation (SOLAS).


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In recent years, the IMO has become another important maritime regulatory body.

The purpose of a 'load line' is to ensure that a ship has sufficient freeboard and thus

sufficient reserve buoyancy.

The freeboard on commercial vessels is measured between the lowest point of the

uppermost continuous deck at side and the waterline and this must not be less than

the freeboard marked on the Load Line Certificate issued to that ship.

All commercial ships, other than in exceptional circumstances, have a load line

symbol painted amidships on each side of the ship.

This symbol must also be permanently marked, so that if the paint wears off it

remains visible.

The load line makes it easy for anyone to determine if a ship has been overloaded.

The exact location of the Load Line is calculated and/or verified by a Classification

Society and that society issues the relevant certificates.

This symbol, also called an international load line or Plimsoll line, indicates the

maximum safe draft, and therefore the minimum freeboard for the vessel in various

operating conditions.

Question

State with reasons, the purpose of each of the following with regard to ship hull

forms.

a) bulbous bow

b) flare

c) bilge keels

d) sheer

e) camber

ANS


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a)The bulbous bow allows the ship to cut through the water better due to it's shape,

this give s the ship more speed and improves fuel economy, also it gives the ship

more buoyancy. It does this by reducing the bow wave.

b)Flare is the shape of the ships bow coming up from the water line, it helps keep

the bow above the water in heavy weather, especially on smaller ships when

pitching. It also allows the anchor to be lowered without hitting the ships side.

c)Bilge keels have the prime function of damping the rolling motion of the ship,

they also give added longitudinal strength at the bilge.

d)Sheer is the curvature of the ships deck from forward to aft, it gives increased

strength and keeps water off the deck.

e)Camber is the curvature of the ships deck from port to starboard, it keeps water

off the deck, it also gives added strength to the ships hull.


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Question

W.R.T. hull protection against corrosion describe how each of the following

operate.

SACRIFICIAL ANODES

IMPRESSED CURRENT

Means of preventing marine growth and fouling.

Answer.

Cathodic protection is a system of inhibiting hull corrosion by eliminating the

anodic area: on the steel surface. There are two systems in use, sacrificial anodes

and impressed current

SACRIFICIAL ANODES

Sacrificial anodes consist of streamlined blocks of zinc, magnesium or aluminium

alloy attached at strategic position around the hull, especially necessary in the

vicinity of propellers. The presence of the sacrificial anode causes the previously

anodic hull to become a cathode thus being protected. The anodes will of course

need to be regularly inspected and periodically replaced. This is the usual

protection system for smaller vessels, particularly around the after end where a

concentrated cell occurs.


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The anode material is cast around a steel strip to allow welding of the anode to the

hull. Alternatively, the anode is attached to studs welded to the hull, allowing in-

water renewal as well as renewal in way of non-gas free spaces.

IMPRESSED CURRENT

The impressed current system is generally used for larger vessels or luxury craft

since it is more expensive to install.

Electrical current from a power unit to a number of hull anodes which, unlike the

sacrificial anodes, are insulated from the hull, and are classed as non-consumable


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(actually having a life of about 15 years). The electrons from the d.c. power source

travel into the hull at the same rate that the anions are being formed at the cathode,

thus preventing a loss of electrons from the hull. Therefore the iron ions are not

formed and the hull does not anodically degrade. A reference electrode is used to

measure cell potential difference gives a signal to a controller that regulates the d.c.

power source which supplies electrons to the hull at the required rate. The circuit is

completed by the current passed through the anodes which are consumed at a very

slow rate (approximate life of 10 years). With lead-silver anode: (2% Ag) the

negatively charged hydroxyl ions are attracted to the anode causing the surface to

form a skin of lead-peroxide, after which the reaction reduces. The following

diagram shows the principle of the impressed current system.

The control equipment operates from reference electrodes port and starboard,

whose signal to the amplifier of the control equipment can operate separately, or in

parallel. In operation, the current varies with the ships hull condition, speed, water

salinity and temperature, automatically adjusting for the different potentials

occurring.


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Means of preventing marine growth and fouling

In the initial stages, fouling is in the form of a slime composed of bacteria and

single cell plants. Spores and larvae then become trapped in the slime and develop

into marine plants and animals. Some marine plants require sunlight and may only

be found on the sides of the vessel, e.g. green grasses, whilst others such as red and

brown grasses may be found on the lower sides and bottom. Most marine animals

do not require sunlight and may be found mainly on the bottom of the

vessel.

The extent of fouling depends upon time in port, location and time of the year.

Generally, fouling does not occur at speeds greater than 4 knots. Fouling can

increase hull resistance which results in a speed loss for a given power or an

increase in power requirement (and hence fuel consumption) for the same speed.

ANTI-FOULING COAT1NGS

Two anti fouling paints are in use; conventional and self polishing.

Conventional anti-fouling

These rely upon the physical dispersion of toxins incorporated in the paint binder.

As they leach out, a coarse insoluble matrix is left behind. This matrix hinders

leaching from the inner coatings, thus reducing the effective life of the system as

well as increasing the hull resistance. Some systems can be re-activated by

scrubbing the spent anti-fouling from the surface.

Self polishing anti-fouling coatings (SPC)

In this system the toxin is chemically incorporated into the binder. The coating is

water soluble at its surface, thus allowing the toxin to be released as the binder

dissolves. The water flow over the hull surface erodes the paint and 'rough' peaks

are smoothed to a surface that has less resistance than in the new condition, hence

the name SPC. The life of the paint coating is proportional to the applied thickness

but there is an environmental problem with the toxins used.


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Question

Define the following with respect to propeller

Boss is the part of the propeller post or stern post to which the stern tube is

secured in single screw ships.

Blade Tip Maximum reach of the blade from the center of the hub.


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Radius Distance from the axis of rotation to the blade tip.

Rake The fore or aft slant of a blade with respect to a line perpendicular to the

propeller axis of rotation

Skew The transverse sweeping of a blade such that viewing the blade from fore or

aft shows an asymmetrical shape.

Pitch The distance (usually expressed in inches) a propeller would move in one

revolution in a solid medium with no slippage

Question.

With regard to ship construction details for watertight bulkheads

a) State the purpose of this type of bulkhead.

b) State how the bulkhead's are tested for water tightness.

If it is necessary to penetrate the bulkhead, precautions must be taken to ensure

that the watertight integrity and strength of the bulkhead are maintained. With this

in mind, sketch how the following pass through bulkheads:

(i) Main transmission shaft;

(ii) Electric cable.

Answer.

a) The purpose of a transverse watertight bulkhead is to divide the ship into

watertight compartments, thus restrict the volume of water which may enter the

ship due to damage to the shell plating. They also serve to separate different types

of cargo. In the event of a fire, they greatly prevent the spread. They also increase

the strength of the ship transversely.


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Male and female halves of a gland around the cable are screwed together.

Neoprene seal and felt washer are compressed to make a good seal.(ii)

Over size hole in the bulkhead has a stuffing box fitted, fastened by studs to the

bulkhead. Tightening of the bolts compresses hemp packing onto the shaft.

May also use a mechanical seal as shown in the right hand sketch.


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Question

Describe suitable methods for testing each of the following for watertight integrity;

a) a hollow rudder

b) a bulkhead

c) a double bottom tank

d) a watertight door

e) shell plating

ANS

a)A hollow rudder would be tested in drydock by filling the rudder with water and

checking it for leaks, a small pressure may be applied to aid this, ie a head of

pressure from a stand pipe of 2metres or 0.2 BAR air pressure.

b)Bulkheads would be tested by means of a simple hose test. On construction,

bulkheads are hydrostatically tested.

c)Double bottom tanks are tested by filling them completely with water, including

the sounding pipe, by doing this, a small head of pressure is applied to the tank,

adjacent tanks, cofferdams around the tank being tested would be checked.

d)A watertight door is checked for watertight integrity by means of a simple hose

test, but on construction are hydrostatically tested.

e)Shell plating can be tested for watertight integrity by a vacuum test, where in

drydock, a sealed box is placed over welds and evacuated of air, loss of vacuum

showing leaks, welds on ships plating may also be x-rayed to check the integrity of

the weld.


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Question

a)describe how anodes prevent corrosion;

b)

i)state three suitable materials that can be used for anodes;

ii)state any restriction on the use of these materials as anodes in oil tankers.

ANS

a)Anodes prevent corrosion by becoming a sacrificial metal. Anodes are less noble

metals in the periodic table than the cathode, which is steel in this case. The

saltwater sets up a galvanic couple, galvanic current attacking the sacrificial

anodes, which conveys a protective coating around the steel work in the tank (the

cathode). The anodes eventually waste away and have to be replaced.

b)

i) These three metals are suitable fore anodes as they are lower in the galvanic

series than iron;

Zinc, Magnesium, Aluminum

ii) Restriction on these materials as use as an anode in oil tankers is that

Magnesium is banned as there is a risk of sparks, fires and explosion with

magnesium .

Question

With regard to the roll motion of the ship in seaway;

a)explain why roll motion is considered more serious than pitch motion

b)discuss the basic principles of roll damping utilised in the following anti-roll

systems;


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i)bilge keels

ii) activated fins

iii)passive roll damping tanks

ANS

a)Roll motion is considered more serious than pitch motion because with roll

motion, the ship can get into synchronization with the wave and while the wave

gets bigger, so will the roll, also slack tanks can be a problem with roll motion, or

can be a shift in cargo.

b)

i)Bilge keels offer a damping effect against roll motion by their surface area acting

against the roll.

ii)Activated fins work by offering a damping effect against the roll motion by

creating an opposite force to the roll, plus using the ships forward velocity to

achieve this.

Ie, if the ship were to roll to port, the port fin would be sent a signal by a gyroscope

to have it's leading edge above the axis of tilt, while the stbd fin is sent a signal to

have it's fin below the axis of tilt.

See sketch


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iii)Passive rolling tanks dampen the roll motion of the ship by using the effect of

water movement in a tank.

The tank contains perforated baffles to slow the motion in the tank from side to

side, thus using the force of the water in the tank to act against the motion of roll.

Question

Define the purpose of cofferdams.

State where cofferdams are most likely to be found on :

(i) Dry cargo ships;

(ii) Oil tankers.

(c)

(i) State what information is available about danger of entering void spaces.

(ii) Identify, with reasons, the precaution to be observed before and during entry

to cofferdams.

ANS


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a) The purpose of cofferdams are to prevent leakage of one tank or double bottom

to another ie, there would be a cofferdam between fuel tanks and fresh water tanks.

They also allow for inspection of tanks, cofferdams should also be sounded

regularly to check for leaks.

b)

i) On dry cargo vessels, cofferdams would be found between fuel tanks and cargo,

ballast tanks and cargo, also found around the main engine LO drain tank.

ii) On oil tankers, cofferdams would be found between the oil cargo tanks and the

accommodation or engine room, in some cases this can be the pump room, also

there will be a cofferdam around the main engine LO drain tank.

c) Precautions taken before entry into an enclosed space are as follows; An entry

permit must be obtained from a responsible officer, you may be the issuing officer.

The space is to be well ventilated and oxygen levels tested by an 02 meter. Safety

gear, such as self contained breathing apparatus sets must be at hand, rescue lines,

communications. Also while anyone is in the enclosed space, someone must be on

standby at the entrance who must be in communication with the people in the

space and also must be able to raise the alarm if they are to get into difficulties.

Official guidance on entry into enclosed spaces can be found in "M" notices and

also in the "Code of Safe working practices".


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Question DEC 2006

Describe the arrangement of tank top and double bottom in the machinery space

making particular reference to the structure and scantlings below the main engine.

Show the method adopted in the arrangement of D.B. tanks to avoid contamination

of fresh water, fuel oil and lube oil stored in D.B. tanks.

Answer

Double bottom in the machinery space

In the machinery space other factors must be taken into account. Forces of

pulsating nature are transmitted through the structure due to the general out of

balance forces of the machinery parts. The machinery seats must be extremely well

supported to prevent any movement of the machinery. Additional girders are fitted

in the double bottom and the thickness of the tank top increased under the engine

in an attempt to reduce the possibility of movement which could cause severe

vibration in the ship. For similar reasons the shaft and propeller must be well

supported.


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Great care must be taken in the machinery space to ensure that the main and

auxiliary machinery are efficiently supported. Weak supports may cause damage to

the machinery, while large unsupported panels of plating may lead to vibration of

the structure. The main engine bedplate is bolted through a tank top plate which is

about 40 mm thick and is continuous to the thrust block seating. A girder is fitted

on each side of the bedplate in such a way that the holding down bolts pass through

the top angle of the girder. In welded ships a horizontal flat is sometimes fitted to

the top of the girder in way of the holding-down bolts.


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MAIN - ENGINE HOLDING DOWN BOLTS

In motor ships where a drain tank is required under the machinery, a cofferdam is

fitted giving access to the holding down bolts and isolating the drain from the

remainder of the double bottom tanks.

Additional longitudinal girders are fitted in way of heavy auxiliary machinery such

as generators.

Access to these D\B tanks and cofferdam is provided by watertight manhole door.

Docking plugs are fitted in all D\B yanks for complete draining in drydock for

purpose of inspectionAir pipes and sounding pipes are also fitted to D\B tank and these tanks are tested

by maximum service pressure head

[The construction of the double bottom under the machinery space regardless of

framing system has solid plate floors at every frame space under the main engine

Besides the continuous center girder of greater thickness, additional side girders

arc-fitted outboard of the main engine seating. The double bottom height is usually

increased to allow for the alignment of the shafting system, as well as to provide

fuel oil, diesel oil lubricating oil and fr. Water tank of suitable capacities. Where

the double bottom u, increased in depth above the normal, continuity of strength is


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maintained by the gradual sloping of the tank top and intercostals girders between

the machinery space and the adjacent compartments.

Heavy plate (40mm) support with, bar stiffener is provided on the tank top for the

seating of main engine bedplate and thrust block, which are bolted direct to heavy

plates incorporated in the tank top, the bolts penetrating into cofferdam.

The lubricating oil drain tank, fuel oil tank, diesel oil tank, fresh water tanks in the:

double bottom space are separated by cofferdams to avoid contamination. Access

to these double bottom tanks and cofferdam is provided by water-tight manhole

doors-Docking plugs are fitted in all double bottom tanks for complete draining in

dry dock for inspection purpose. Air pipes and sounding pipes are also fitted to

these double bottom tanks and the tanks are tested by the maximum service

pressure head.]


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Question

Explain why weighted cocks fitted on tank sounding pipes,

Answer

Cocks and valves are designed to control or interrupt flow. This is done in cocks

by rotating the plug and in valves by lowering, raising or rotating a disc in relation

to a seating surface or by controlling the movement of a ball

Cocks


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Figure Example of a sleeve-packed cock

A cock may be straight-through, right-angled or open-bottomed as required by its

situation in a pipe system. Its plug may be tapered or parallel with tightness

achieved by lapping in or by resilient packing material (Figure 4.a) often in the

form of a ready made sleeve.

In machinery spaces, the short sounding pipes for fuel or lubricating oil tanks must

be fitted with cocks having parallel as opposed to tapered plugs.

This, together with the requirement for weighted handles which will automatically

close the cock when released, is for safety.

Tapered plugs, when tightened to hold the cock open for sounding and then

forgotten, have contributed to fires when tanks have overflowed.

Remote operated gear for bilge valve

In the case of flooding of engine room it is not possible to operate the valve from

local position it can be operated remotely

Emergency bilge valve is operated remotely

Ventilation pipe

While deballasting air should entre from outside to prevent vacuum formation in

tank and during ballasting air must escape to prevent tank getting pressurised

other wise tank will collapse

During rolling and pitching tank level continuously chanes since mass of water

movement which leads to development of air pr or vacuum


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At this point air should entre to release vacuum or should go out to avoid over

pressuriastion

Gauze are made of some good conducting material such as copper M.S. or S.S.

They act as flame arrestor because of expanded surface of gauze flame front will

get cooled down and there by extinguish the flame thus preventing flame or spark

from outside from entering the tank and preventing fire and explosion

If the mesh size is too big then efficiency of flame trap will reduce and it will not

serve the purpose for which it is fitted

If too small it will get chocked due too rust cargo dust hence tank will get

pressurised and it will affect the functioning of vent

Hence tanks may over pressuriese or go into vacuum

Question

State the purpose of each of the following and describe where they are located in

the ship structure;a) Duct keel; b) Stern frame aft heel; c) Forward and after sheer; d) Round of bilge

ANS

a) The duct keel is situated in the double bottom of some vessels, it runs from the

forward engine room bulkhead to the collision bulkhead, they are used to carry

pipe work, making accessible when cargo is loaded.

See Sketch.


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b) Stern frame aft heel is where the heel block is placed when the vessel is in dry

dock; this part of the vessel has added strength to support the after end of the ship.

See Sketch.

c) Forward and after sheer is the rise of the deck at the forward end of the ship and

at the after end, this shape provides added strength, plus keeps water off the deck.

See Sketch.

d) Round of bilge is found where the side of the ship meets the bottom, it is

rounded to give extra strength, plus it also reduces racking. (Side motion of the

ship)

See Sketch.

Question


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Define five of the following, stating how they affect the ship structure and what

component parts of the ship structure help to resist the effect of;

Answer

Racking.

When a ship rolls there is a tendency for the ship to distort transversely in a similar

way to that in which a picture frame may collapse. This is known as racking and is

reduced or prevented by the beam knee and tank side bracket connections, together

with the transverse bulkheads, the latter having the greatest effect.

Water pressure effect.

A considerable force is exerted on the bottom and side shell by the water

surrounding the ship. The double bottom floors and side frames are designed to

withstand these forces, while the shell plating must be thick enough to prevent

buckling between the floors and frames. Since water pressure increases with the

depth of immersion, the load on the bottom shell exceeds that on the side shell. It

follows, therefore, that the bottom shell must be thicker than the side shell. When

the ship passes through waves, these forces are of a pulsating nature and may vary

considerably in high waves, while in bad weather conditions the shell plating

above the waterline will receive severe hammering.

Panting

As the waves pass along the ship they cause fluctuations in water pressure which

tend to create an in-and-out movement of the shell plating. The effect of this is

found to be greatest at the ends of the ship, particularly at the fore end, where the

shell is relatively flat. Such movements are termed panting and, if unrestricted,

could eventually lead to fatigue of the material and must therefore be prevented.


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The structure at the ends of the ship is stiffened to prevent any undue movement of

the shell.

Pounding

When a ship meets heavy weather and commences heaving and pitching, the rise

of the fore end of the ship occasionally synchronizes with the trough of a wave.

The fore end then emerges from the water and re-enters with a tremendous

slamming effect, known as pounding. While this does not occur with great

regularity, it may nevertheless cause damage to the bottom of the ship forward.

The shell plating must be stiffened to prevent buckling. Pounding also occurs aft in

way of the cruiser stern but the effects are not nearly as great.

Local weights.

The deck must be designed to support the weight of accommodation, winches and

cargo, while exposed decks may have to withstand a tremendous weight of water

shipped in heavy weather. The deck plating is connected to beams which transmit

the loads to longitudinal girders and to the side frames. In way of heavy local loads

such as winches, additional stiffening is arranged. The shell plating and frames

form pillars which support the weights from the decks. The tank top is required to

carry the weight of the hold cargo or the up thrust exerted by the liquid in the

tanks, the latter usually proving to be the most severe load.

Vibration from engine and propellers

If the frequency of the main or auxiliary machinery at any given speed coincides

with the natural frequency of the hull structure, then vibration may occur

In the machinery space other factors must be taken into account. Forces of

pulsating nature are transmitted through the structure due to the general out of


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balance forces of the machinery parts. The machinery seats must be extremely well

supported to prevent any movement of the machinery. Additional girders are fitted

in the double bottom and the thickness of the tank top increased under the engine

in an attempt to reduce the possibility of movement which could cause severe

vibration in the ship. For similar reasons the shaft and propeller must be well

supported.

Question

Explain with the aid of sketch

Raise floor

Water tight bulkhead

Tumble home

Shelter deck

Free board

Tumble-Home - is the fall in of the sides amid ships.

It flattens out the stability curve and allows the ship to roll more easily.

Rise of Floor - is height from the base line to the bottom of the bilge radius.

It helps to drain the double bottom tanks.


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Shelter deck - upper deck having no overhead protection from the weather, but

sheltering the deck below

Freeboard is the difference between the depth at side and the draught, that is it is

the height of the deck above the waterline.

The freeboard is usually greater at the bow and stern than at amidships. This helps

create a drier ship in waves. Freeboard is important in determining stability at large

angles.

Freeboard is the distance from the water load line up to the main deck.

The main deck is the highest deck that is water sealed.

Water falling on upper decks may run down companion ways, but it cannot go any

farther down into the ship than the main deck. When the ship is in bad weather and

ploughing her way through the waves, she will at times displace a greater weight of

water than her own-weight, which will tend to push her up again. This extra weight

of water tending to push the ship up when in the above condition, is represented by

the freeboard.

This extra volume of water which the ship can displace in bad.weather and is in

reserve for that purpose, is measured by the water plane area multiplied by the

freeboard and is called the reserve buoyancy. Every ship must have reserve

buoyancy, as in case of heavy leakages into holds below the main deck, or bilge

pumps becoming defective or in case of collision, the hold would gradually fill up

with water. The reserve buoyancy, therefore, keeps the ship from immediately

sinking and allows time for repairs to be effected and thus save the ship and

incidentally the lives in the ship.


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Question

Show with the aid of sketch as to how a vessel which is stable will return to the

upright after being heeled by an external force.

Consider a ship floating upright in still water as shown in Figure (a).

The centres of gravity and buoyancy are at G and B, respectively.

Figure (c)shows the righting couple.

GZ is the righting lever.

Now let the ship be inclined by an external force to a small angle as shown in

Figure (b).

Since there has been no change in the distribution of weights the centre of gravity

will remain at G and the weight of the ship (W) can be considered to act vertically

downwards through this point.

When heeled, the wedge of buoyancy WOW1is brought out of the water and an

equal wedge LOL1becomes immersed. In this way a wedge of buoyancy having its

centre of gravity at g is transferred to a position with its centre of gravity at g1.

The centre of buoyancy, being the centre of gravity of the underwater volume,

must shift from B to the new position B1,such that BB1is parallel to gg1, and BB1

= v x gg1 / V

Where v is the volume of the transferred wedge, and V is the ships volume of

displacement.


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The verticals through the centres of buoyancy at two consecutive angles of heel

intersect at a point called the metacentre.

For angles of heel up to about 15the vertical through the centre of buoyancy may

be considered to cut the centre line at a fixed point called the initial metacentre (M

in Figure (b)).

The height of the initial metacentre above the keel (KM) depends upon a shipsunderwater form.

The vertical distance between G and M is referred to as the metacentric height.

If G is below M the ship is said to have positive metacentric height, and if G is

above M the metacentric height is said to be negative.


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gravity of the immersed and emerged wedges, to the new centre of gravity of the

underwater volume.

The force of buoyancy is considered to act vertically upwards through the centre of

buoyancy, whilst the weight of the ship is considered to act vertically downwards

through the centre of gravity.

These two equal and opposite forces produce a moment or couple which may tend

to right or capsize the ship.

The moment is referred to as the moment of statical stability and may be defined as

the moment to return the ship to the initial position when inclined by an external

force. A ship which has been inclined by an external force is shown in Figure 1

The centre of buoyancy has moved from B to B1parallel to gg1, and the force of

buoyancy (W) acts vertically upwards through B1.

The weight of the ship (W) acts vertically downwards through the centre of gravity

(G).

The perpendicular distance between the lines of action of the forces (GZ) is called

the righting lever.

Taking moments about the centre of gravity, the moment of statical stability is

equal to the product of the righting lever and the displacement, or Moment of

statical stability = W GZ]


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Correcting unstable and neutral equilibrium

When a ship in unstable or neutral equilibrium is to be made stable, the effective

centre of gravity of the ship should be lowered.

To do this one or more of the following methods may be employed:

1. Weights already in the ship may be lowered.

2. Weights may be loaded below the centre of gravity of the ship.

3. Weights may be discharged from positions above the centre of gravity.

4. Free surfaces within the ship may be removed.

Question

Write short notes on:

Cellular double bottom tank

Transverse watertight bulkheads

Keel, Bilge.

Cellular double bottom tank.


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The inclusion of a cellular double bottom in the vessel of the present invention

gives to the vessel a large "KM" which is the vertical distance above the keel at

which the transverse metacenter occurs, and investigations have established that

the "GM," the vertical distance from the center of gravity to the metacenter, which

will obtain, after free surface correction is made for the liquid contained in all the

cargo-oil tanks in the vessel which, in a loaded condition will be about 6.00 feet,

which is more than adequate to ensure complete safety under all conditions likely

to be encountered in service.

Question

Transverse watertight bulkheads

Transverse watertight bulkheads are some of the principal transverse strength

members of a ship.

They are very strong in construction and responsible for maintaining the transverse

form of the hull.

The forward most transverse watertight bulkhead is called the Collision Bulkhead.

As its name implies, this bulkhead is the strongest bulkhead and is designed to

protect the vessel in case of a collision.

It is usually fitted between 0.05 and 0.075 of the length of the vessel from the

forward end.

The space forward of this bulkhead is the Fore peak tank.

Above this bulkhead is the first hold or tank.

The After Peak Bulkhead situated aft and serves to enclose the stern tube in a

watertight compartment.

Other bulkheads will also be fitted on the forward and after sides of the machinery

spaces


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Question

Keel, Bilge.

A fin fitted on the bottom of a ship at the turn of the bilge to reduce rolling.

It commonly consists of a plate running fore and aft and attached to the shell

plating by angle bars.

It materially helps in steadying a ship and does not add much to the resistance to

propulsion.

Flare. The spreading out from the central vertical plane of the forebody of a ship

with increasing rapidity as the section rises from the waterline to the rail. Flam. A

term often used lo express the meaning as flare, but more properly used to denote

the maximum curl or roll given to the flare at the upper part, just below the weather

deck.

class4 question & answers ship const

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