lines plan

LINES PLAN Principal ship dimensions

LINES PLAN Stations

LINES PLAN Waterlines

LINES PLAN Buttocks

LINES PLAN Diagonal

LINES PLAN

Ship Steering

• Steering is a special case of maneuvering involved in keeping an underway ship on a desired constant heading.

• Elements of the steering system are the rudder and steering gear.

• Rudder have a streamlined section to give a good lift to drag ratio and are of double-plate construction.

Ship Steering

Rudder area • Rudder area would be taken as a percentage of LT; • For merchant ships, the area of the rudder is usually about 2

percent of the product LT for ships 120 m long and over;• For smaller ships it may increase to about 3 percent for a 30

m ship;• Ships requiring special maneuverability will have more

rudder area;• Seagoing tugs may have rudder areas of 4 percent; • Harbor tug boats may have rudder areas of 6 to 8 percent of

LT.

Ship Steering

Types of rudders Rudders can be categorized according to the degree of balance (How close the centre of pressure is to the rudder axis): • Unbalanced rudders have no area forward of the

rudderstock or pintles.• Semi-balanced rudders have area forward for part of the

rudder height (or span, as it is sometimes called).• Ballanced rudders have area forward for the full span.

Ship Steering

Types of rudders The choice of rudder type depends upon:• Shape of the stern, • Size of rudder required, • Capacity of the steering

gear available.• Ship or boat type and

size.

Ship Steering Types of rudders (High-lift rudders)a) Flapped aerofoilb) Schilling rudderc) Wedge at taild) Gurney flape) Jet flapf) Blown flap gapg) Rotating cylinder in isolationh) Rotating cylinder in

association with rudderi) Rotating cylinder in

association with flapj) End platesk) Robust simple rudderl) Use of double/triple ruddersm) Active rudder

Ship Steering

Types of rudders

Flap rudder

Flettner rudder

Balanced reaction rudder

Ship Steering

Rudder structure and supports

Ship Steering


Unbalanced rudder

Ship Steering


Semi-balanced rudder

Ship Steering


Structural layout: (a) Spade rudder; (b) Skeg rudder

Ship Steering

Rudder bearings

Ship Steering

Steering gear

Two types of hydraulically powered steering gear are in common use, • Ram types• Rotary vane type

Ship Steering

Steering gear

Two-ram steering gear

Ship Steering

Steering gear

Four-ram steering gear

Ship Steering

Steering gear

Rotary vane steering gear

Propeller

Propeller reference line• The propeller blade is defined about a line normal to the

shaft axis called either the ‘propeller reference line’ or the ‘directrix‘.

Propeller

Cylindrical blade section• The aerofoil sections which together comprise the blade of a

propeller are defined on the surface of cylinders whose axes concentric with the shaft axis; hence the term ‘cylindrical sections’ is frequently encountered in propeller technology.

• The section lies obliquely over the surface of the cylinder and thus the line connecting the leading and trailing edges of the section form a helix over the cylinder.

Propeller

Generator line • The point A where this helix intersects the plane defined by

the directrix and the x-axis forms one point, at the radius r of the section considered, on the ‘generator line’. The generator line is thus the locus of all such points between the tip and root of the blade. Occasionally the term ‘stacking line’ is encountered, this is most frequently used as a synonym for the generator line.

Propeller

Pitch• The term pitch in propeller technology refers to the helical

progress along a cylindrical surface.• If we consider a section of the propeller blade at a radius r

with a pitch angle and pitch P and imagine the blade to be working in an unyielding medium, then in one revolution of the propeller it will advance from A to A', a distance P. If we unroll the cylinder of radius r into a flat surface, the helix traced out by A will develop into the straight line AM.

• The angle will be constant for a given helix, i.e., at a given radius, but will increase in value from the tip of the blade inwards to the hub.

Propeller

Pitch• In practice the pitch is not always the same at all radii, it

being fairly common to have a reduced pitch towards the hub and, less usually, towards the tip.

• In such cases the pitch at 0.7R is often taken as a representative mean pitch, as this is approximately the point where the maximum lift is generated.

Propeller

Pitch

Propeller

Rake and skew• Following the ITTC code, the skew angle of a

particular section is the angle between the directrix and a line drawn through the shaft center line and the mid-chord point of a section at its non-dimensional radius (x) in the projected propeller outline; that is, looking normally, along the shaft centerline, into the y-z plane.

• Angles forward of the directrix that is in the direction of rotation, in the projected outline are considered to be negative.

• The propeller skew angle is defined as the greatest angle, measured at the shaft center line, in the projected plane, which can be drawn between lines passing from the shaft center line through the mid-chord position of any two sections.

Propeller

Rake and skew• Propeller skew also tends to be classified into two types:

balanced and biased skew designs. The balanced skew design is one where the locus of the mid-chord line generally intersects with the directrix at least twice in the inner regions of the blade. In the biased skew design the mid-chord locus crosses the directrix not more than one; normally only in the inner sections.

Propeller

Rake and skew

Propeller

Propeller outlines and area• In a marine propeller, the surface of the blade facing aft,

which experiences the increase in pressure when propelling the ship ahead, is called the face of the blade, the forward side being the back.

• There are four basic outlines which describe the propeller blade shape:• The projected outline• The developed outline• The expanded outline

Propeller

Propeller outlines and area• The projected outline is the view of the propeller blade that

is actually seen when the propeller is viewed along the shaft center line that is normal to y-z plane. Convention dictates that this is the view seen when looking forward. In this view the helical sections are defined in their appropriate pitch angles and the sections are seen to lie along circular arcs whose center is the shaft axis.

• The projected area of the propeller is the area seen when looking forward along the shaft axis.

Propeller

Propeller outlines and area• The developed outline is related to the projected outline in

so far as it is a helically based view, but the pitch of each section has been reduced to zero; that is the sections all lie in the thwart-ship plane.

• In general, the developed area is greater than the projected area and slightly less than the expanded area.

• The expanded outline is not really an outline in any true geometric sense at all. It could more correctly be termed a plotting of the chord lengths at their correct radial stations about the directrix.

• Blade area ratio is simply the blade area, either the projected, developed or expanded depending on the context, divided by the propeller disc area.

Propeller

Propeller outlines and area

Propeller

Blade thickness distribution

Typical representation of propeller maximum thickness distribution

Propeller

Propeller drawing methods• The most commonly used method for drawing a propeller is

that developed by Holst. • This method relies on being able to adequately represent

the helical arcs along which the propeller sections are defined by circular arcs, of some radius which is greater than the section radius, when the helical arcs have been swung about the directrix into the zero pitch or developed view.

• This drawing method is an approximation but does not lead to significant errors unless used for very wide bladed or highly skewed propellers; in these cases errors can be significant and the alternative and more rigorous method of Rosingh would then be used to represent the blade drawings.

Propeller

Propeller drawing methods• A series of arcs with center on the shaft axis at O are

constructed at each of the radial stations on the directrixwhere the blade is to be defined.

• A length 2⁄ is then struck off along the horizontal axis for each section and the lines AB are joined for each of the sections under consideration.

• A right angle ABC is then constructed, which in turn defines a point C on the extension of the directrix below the shaft center line.

• An arc AC is then drawn with the center C and radius . • The distances from the directrix to the leading edge and

the directrix to the trailing edge are measured around the circumference of the arc.

Propeller

Propeller drawing methods• Projections, normal to the directrix, through and meet

the arc of radius r, about the shaft center line, at and respectively.

• These latter two points form two points on the leading and trailing edges of the projected outline, whilst and lie on the developed outline.

• Consequently, it can be seen that distances measured around the arcs on the developed outline represent ‘true lengths’ that can be formed on the actual propeller.

Propeller

Propeller drawing methods

Propeller

Propeller drawing methodsThe design drawing for a propeller usually consists of four parts:a) A side elevation of the propeller, b) An expanded blade outline with details of the section

shapes. The section shapes are shown with their pitch faces all drawn parallel to the base line and at their correct radii from the axis.

c) The pitch distribution if it is not uniform,d) A transverse view.

Propeller

Propeller drawing methods

Propeller

Propeller drawing methodsRadius of curvature of the helix is given by

Propeller

Propeller drawing methods• Consider a section at radius r where the pitch is P. • If AB is set out equal to 2⁄ , and the line BCD drawn, the

angle ACB is the pitch angle and BCD is the pitch face line for that section.

• The transverse projection of the section EF will be , and if an arc of a circle of radius r is drawn with center A and

and are measured around this arc from C, and will be points on the transverse projected blade outline.

Propeller

Propeller drawing methods• In the same way, the longitudinal projection of the section on

the centerline plane will be . • If the offsets and are set off in view (a) as shown,

and then the points and are dropped down to the same levels as and in view (d), and will be points on the longitudinal projected outline.

• In this way the whole transverse and longitudinal projected outlines can be drawn.

Propeller

Propeller drawing methods• For most purposes the developed outline is obtained by

expanding the section along an arc of a circle which has a radius equal to the radius of curvature of the helix at the point C. If BG is drawn perpendicular to BC, then ⁄ , which is the radius of curvature of the helix at the

point C having a radius r and pitch P.• Hence if CE and CF are set off from C around the arc of a

circle struck from G with radius GC, the resultant points and will be points on the developed blade outline.

• This is nearly a correct construction for narrow and medium-width blades, but is not so accurate in the case of wide blades.

Propeller

Geometrical characteristics of propellersThe characteristics of propellers are customarily expressed in the form of nondimensional ratios, the most commonly used being:

•

•

• ,

• ,

,

Propeller

XX

• , ,

• ,

⁄

• ,

•

Propeller

Section geometry and definition • The mean line or camber line is the locus of the mid-points

between the upper and lower surfaces when measured perpendicular to the camber line.

• The extremities of the camber line are termed the leadingand trailing edges of the aerofoil and the straight line joining these two points is termed the chord line.

• The distance between the leading and trailing edges when measured along the chord line is termed the chord length (c) of the section.

• The camber of the section is the maximum distance between the mean camber line and the chord line, measured perpendicular to the chord line.

Propeller

Section geometry and definition • The aerofoil thickness is the distance between the upper

and lower surfaces of the section, also measured perpendicularly to the chord line.

• The leading edges are usually circular, having a leading edge radius defined about a point on the camber line.

lines plan

Documents