propellers and propulsion.pptx

Propeller & Propulsion of Ships
The propulsion system of a ship is to provide the
thrust to the ship to overcome the resistance.

Introduction
• Propulsive Devices
Paddle-Wheels: While the draft varying with ship displacement,
the immersion of wheels also varies. The wheels may come out
of water when the ship is rolling, causing erratic course-keeping,
& they are likely to damage from rough seas.
Propellers: Its first use was in a steam-driven boat at N.Y. in
1804. Advantages over paddle-wheels are,
1) not substantially affected by normal changes in draft;
2) not easily damaged;
3) decreasing the width of the ship, &
4) good efficiency driven by lighter engine.
Since then, propellers have dominated in use of marine
propulsion.

History of propulsion
• In the year 1770 A.D. when the Scotsman,
James Watt, further developed the steam
engine so that it, a short time thereafter,
could be built into a ship.
• Un advantageously, at this period in time,
the propeller had not yet been invented.
And, what was more convenient to make
use of than the old-familiar paddle-wheel
of the flour and lumber mills.

Paddle Wheels Propulsion (Stern)

• The paddle-wheel propelled the ship, and
the steam engine drove the paddle-wheel
and – if you’d like – the coal-fired boilers
provided the steam for the steam engine.
• At this point, technically speaking, a
radical change took place in regard to ship
propulsion
History of propulsion

Paddle Wheels Propulsion (Midship)

Submarine "H.L. Hunley" on 17.02.1864
while attacking the "Housatonic
There were certainly
attempts at interim
solutions. This illustration is
not a comic drawing but
rather the then
contemporary depiction of a
submarine propelled by
muscle power, which sank
the "Housatonic“ in 1864.

History of the propeller
• In the year 1827, Joseph
Ressel had the ship propeller
patented. He was an Austrian
and had the exciting title of a
“Marineforstintendant .
• The iron propeller was a great
improvement as compared
with the wooden paddle-wheel
with its many fragile pieces.
And, even more importantly,
the propeller was able to be
adapted to all of the following
types of propulsion and output
standards of today

• Type of Ship Machinery
1. Steam Engine (no longer used in common)
Advantages: 1) good controllability at all loads, 2) to be
reversed easily, & 3) rpm (rotations per minute) matches
that of propellers
Disadvantages: 1.) very heavy 2.) occupy more space
3.) the output of power per cylinder is limited
4.) fuel consumption is high
2. Steam Turbine
Advantages:
1.) deliver a uniform turning torque, good performance for large
unit power output, 2.) thermal efficiency is high.
Disadvantages:
1.) is nonreversible; 2.) rpm is too high, need a gear box to
reduce its rotating speed

3. Internal combustion engines (Diesel engine)
Advantages: 1.) are built in all sizes, fitted in ships ranging from
small boats to large super tankers, (less 100 hp ~ >30,000 hp);
2.) High thermal efficiency.
Disadvantages: 1.) Heavy cf. gas turbines;
4. Gas Turbines (developed for aeronautical applications)
Advantages: 1.) Do not need boiler, very light; 2.) Offer continuous
smooth driving, & need very short “warm” time.
Disadvantages: 1.) expensive in cost and maintenance 2.) need a
gear unit to reduce rpm.
5. Nuclear reactors – turbine
Advantages 1.) do not need boiler, fuel weight is very small
2.) operate full load for very long time (submarine)
Disadvantages 1.) weight of reactor and protection shield are heavy;
2) Environment problem, potential pollution.

Marine propellers
A marine propeller is a fan like mechanism
that transmits power by converting the
engine’s torque into thrust. Usually
consisting of two, three, four or more blades,
the propeller spins around a central shaft to
create dynamics similar to a rotating screw
or hydrofoil. When the blades spin, a
pressure difference between the forward and
after surfaces is produced, accelerating the
water behind the blade to create force

Transverse thrust
• The thrust of the propeller blades has two
components (parts): a fore-and-aft one and a small
arthwatships one.
• The fore-and-aft component is the force that
moves the ship forward and the arthwatships one
is the force that drives the stern of the ship
through the water in a direction at right
angles to the ship's line of motion.
• This is also known as transverse thrust.
•

Transverse Thrust
• A propeller not only thrusts a boat forward,
it also causes the stern to turn to one side,
which side depends on whether the
propeller is right or left handed.
• This transverse thrust (also called the p-
effect, paddle wheel effect or propeller
bias or walk) results in the tendency of the
propeller to create a transverse force
causing the stern of the vessel to creep
either to port or starboard side depending
on the handing of the propeller.

• When going ahead from the dead in the
water, the bow of the ship will cant (turn) to
port as the headway becomes faster, the
swing of the bow decreases and may change
to starboard.
• When going astern from the dead in the
water, the bow of the stern cants strongly to
port and continues to do so when the ship
gains sternway until the rudder is used to
slow down a little by putting it to hard right.
Transverse Thrust

Transverse Thrust
• The effect is especially noticeable when the vessel is
on a stern board where there is greater vessel
resistance to move sternwards thus making it easier
for the propeller to push the vessel’s stern sideways.
• If the propeller is right handed (i.e. it revolves
clockwise in forward gear when viewed from astern),
the vessel will normally veer to port going ahead and
clockwise going astern and vice versa for a left
handed propeller.
• Thus it is impossible for a single propeller boat to go
dead straight without constant minor adjustments to
the rudder. When fully understood, good use of this
effect can benefit close manoeuvres

Propeller Geometry and Terminology
Boss
Back
Hubcap
Face
Number of Blades: 2, 3, 4, 5 ,6
Boss
Hubcap
Shaft

Propeller definition
• A propeller is a type of fan of radial, spiral shaped blades
attached to a central boss usually situated at the after end of
a vessel that transmits power by reason of their rotation in
water and the blade section angle of attack, converts the
torque provided by the engine and gearbox into thrust.
• A pressure difference is produced between the forward and
after surfaces of the hydrofoil sectioned blade and the water
is accelerated behind the blade.
• A right handed propeller is designed to rotate clockwise
(when viewed from astern) which, and vice versa for a left
handed unit.
• Larger diameter propellers with greater pitch and producing
more thrust are usually needed by narrowboats powered by
slow revolving traditional engines but at the cost of greater
Transverse Thrust (q.v.).

• Propeller Centre Line - Linear reference line passing
through hub center on the axis of propeller rotation.
• Propeller Centre Axis - Linear reference line that
locates the blade on the boss and is perpendicular to
the Propeller Centre Line.
• Propeller Rotation- Twin screw applications utilize
both left handed (port side) and right handed
(starboard side) rotating propellers and are said to be
outboard handed. Left handed propellers are primarily
used on twin engine boats to cancel the steering bias
that would be caused if both propellers spun in the
same direction.
Propeller terminology

Hub
• The hub of a propeller is the solid center disk that mates
with the propeller shaft and to which the blades are
attached.
• Ideally the hub should be as small in diameter as
possible to obtain maximum thrust, however there is a
tradeoff between size and strength.
• Too small a hub ultimately will not be strong enough.
• In America the boss is called the hub.
• This is the solid cylinder located at the center of the
propeller to which each propeller blade is attached.
• Boss shapes include cylindrical, conical, radius and
barreled.
• The center of the boss is bored to accommodate the
engine propeller shaft.

• Bore - The maximum diameter of the hole bored into the
boss to take the propeller shaft.
• Boss diameter – The blades at their lower ends or roots
are attached to a boss which in turn is attached to the
propeller shaft. The maximum diameter of this boss is
called the boss diameter .
• The boss diameter is usually made as small as possible
and should be no larger than the size sufficient to
accommodate the blades and satisfying the
requirement of strength. It is usually expressed as a
fraction of the propeller diameter.
• At one time propeller blades were manufactured
separately from the boss, but modern fixed pitch
propellers have the boss and blades cast together.
• However, in controllable pitch propellers it is of course
necessary for blades and boss to be manufactured
separately.
Boss terminology

• Boss Diameter - The diameter should be
measured and recorded at each end of the
boss.
• Boss Diameter Ratio- The mean diameter of
the boss divided by the propeller diameter.
Usually about 0.18 to 0.25.
• Boss Length - The distance between the
forward and after faces of the boss.
• BossTaper - The slope of the bore inside the
boss. Usually 1 in 12 in Imperial units and 1 in
10 in metric.
Boss terminology

Blades
• The blade is an ellipse shaped leaf that
extends outward from the propeller boss
or hub.
• Twisted fins or foils that protrude from the
propeller hub. The shape of the blades
and the speed at which they are driven
dictates the torque a given propeller can
deliver.
• The curvature of the mean thickness line
of a given blade section is called the
Camber

• The face surface of a blade is a portion of a holicoidal surface
• The helicoidal surface: Considering a line AB perpendicular to a
line AA’ and supposing that AB rotates with uniform velocity
about AA’ and at the same time moves along AA’ with uniform
velocity, the surface swept out by AB is a helicoidal surface.

Blade parts
• Blade Back: This is the suction side or forward side of
the blade.
• Blade Centre Axis: Linear reference line that indicates
propeller rake.
• Blade Centre Line: Reference line that intersects each
cylindrical section at the midpoint of the blade section
width is used to indicate propeller skew.
• Blade Face: This is the pressure side, pitch side or after
side of the blade.
• Blade Number: The number of blades attached to the
propeller boss, number one blade being that over the
key way. The blades are then numbered in accordance
with the direction of rotation of the propeller

• Blade Root: This is also called the fillet area
and is the area the where each blade attaches
to the boss, the region of transition from blade
surfaces and edges to the boss periphery.
• Blade Tip: This is the point of maximum radius
of the blade from the center of the boss and is
also the point of separation between the leading
and trailing edges.
• Blade Sections: These are the shape of a
cylindrical section through the blade and are
often referred to as Cylindrical Sections. The
boss and fillet area account for about the first
20-30% of the propeller’s diameter.
Blade parts

• Leading Edge: This is the edge of the propeller
blade adjacent to the forward end of the boss
and leads into the flow of water when the
propeller is providing forward thrust.
• Trailing Edge: This is the edge of the propeller
adjacent to the after end of the boss and is the
closest when viewing the propeller from aft. The
trailing edge retreats from the flow of water
when providing forward thrust.
• Blade camber: The curvature of the mean
thickness line of a given blade section is called
the Camber
Blade parts

• Meanline:- Half distance along a section
between the upper and lower surfaces of the
blade.
• Nose-Tail line:- Straight line connecting the
leading edge meanline point to the
trailing edge meanline point.
• Chord length:- Length of Nose-tail line.
• Camber height:- distance between nose-tail
line and meanline normal to the nose-tail line
(varies with chordwise position).
• Max. Camber:- Maximum camber height along
Blade parts

• Meanline Distribution: A standard
distribution of camber height as a function of
chordwise position starting at the section
leading edge. Quite often these are tabulated
forms such as a NACA A=0.8 Meanline, and
can be obtained from standard foil literature.
• Thickness: Section thickness along a line
normal to the meanline. Varies with
chordwise position.
• Max. Thickness: Maximum section
thickness
Blade parts

• Thickness distribution:
A standard distribution of thickness
as a function of chord length quite
often are tabulated forms such as
NACA 66 thickness form that can
be obtained from standard foil
literature.
• Midchord line:- ine produced
from the midchords (i.e. Midpoint
of section nose tail line)
of each section along a propeller
blade.
Blade parts

• Blade outline: it is decided by propeller series diagrams.
• “Expanded blade outline”
• Blade sections: they are radial sections through the blade.
The shape of these sections is then shaped when laid out flat.
•Blade thickness
•Blade width (Chord)
•Leading edge
•Trailing edge

Blade Sections
Blade Sections are of five different basic types: -
1. Circular back sections: Flat faced with symmetrically
rounded back.
2. Hydrofoil sections: Resembling traditional aeroplane wing
sections i.e., a rounded leading edge with the section’s
maximum thickness at about one third of the section’s length
abaft the leading edge.
3. Troost B sections: Sections that are hydrofoil from 0.20 to
0.40 r/R and circular black from 0.65 r/R to the tip and
transitional between. The most common modern commercial
sections.
4. Ogival sections: Elliptical in shape.
5. Supercavitating sections: A high speed application with a
sharp leading edge with the maximum thickness near the
trailing edge. Found on surface piercing propellers

PROFILE SHAPES FOR SCREW
PROPELLERS

Cylindrical Section
A cross-section of a
propeller blade cut by a
circular cylinder whose
center-line is the axis of
rotation. Aerofoil
sections which together
comprise the blade of a
propeller are defined on
the surface of cylinders
whose axes are
concentric with the
shaft axis

Multi-bladed propellers
• Multi-blade propellers offer advantages for high
horsepower, with providing additional bite and
stability at higher speed.
• They can also improve acceleration while
maintaining plane with fewer engine revolutions.
• Because there is more drag, multi-blade propellers
do, however, tend to reduce the vessel’s top end
speed.
• A single-blade propeller would be the most efficient -
if the vibration could be tolerated.
• So, to get an acceptable level of balance with much
less vibration, a two-bladed propeller, practically
speaking, is the most efficient. As blades are added,
efficiency decreases, but so does the vibration level

Blade parameters
• Blade Area Ratio - The British Admiralty
name for the developed or disc area ratio.
• Blade Thickness Fraction or Ratio: - The
maximum blade design thickness as
extended to the propeller center line and
divided by the propeller diameter. Blades
must have enough thickness to achieve
both the desired sectional shape and to
provide sufficient strength under loading.
Blades that are too thick produce a lower
propeller efficiency.

Diameter
• Diameter is determined primarily by the
RPM at which the propeller will be turning
and the amount of power that will be
delivered to the propeller through the
shafts and gears.
• The degree to which the propeller may
operate in a partially surfaced condition, as
well as the intended forward velocity, will
also play a role in determining the most
desirable diameter.
• Within a given propeller line, the diameter
usually increases for propellers used on
slower boats and decreases for faster
boats.
• If all other variables remain constant,
diameter will increase as power increases;
diameter will increase as propeller RPM
decreases (slower powerhead or engine
speed and/or more gear reduction); and
diameter should increase as propeller
surfacing increases.
Diameter is twice the distance
from the center of the boss
(hub) to the tip of the blade and
is the diameter of the circle
described by the blade tips as
the propeller rotates

Disc Area
• This is the area of the circle
described by the propeller
blade tips.
• Projected Area Ratio- The
projected area of propeller
blades divided by the disc
area and is the smallest area
ratio in common use.
• Developed Area Ratio- The
area of the blades rotated to
zero pitch divided by the disc
area and the most widely used
of the ratios
Expanded Area Ratio-
Similar to the disc area
ratio with the sections
unwrapped from the boss.
It is the largest of the area
ratios.

Developed Area AD
Expended Area AE

Pitch
• It indicates the distance the propeller would “drive forward” for
each full rotation.
• In reality since the propeller is attached to a shaft it will not
actually move forward, but instead propel the ship forward.
The distance the ship is propelled forward in one propeller
rotation is actually less than the pitch. The difference between
the nominal pitch and the actual distance traveled by the
vessel in one rotation is called slip.
• A lower pitch will have greater acceleration and “pushing
power” but a lower top speed, while a higher pitch prop will
provide less acceleration, but a greater potential for higher top
speeds.
• The correct propeller will allow your engine to reach the upper
portion of the WOT range specified by the manufacturer with
a normal-to-heavy load (without exceeding it)

Pitch
• Pitch is the distance that a propeller would move in one
revolution if it were moving through a soft solid, like a
screw in wood.
• Pitch is measured on the face of the blade. A number of
factors can cause the actual pitch of a propeller to vary
from the advertised pitch stamped on it. Minor distortion
may have occurred during the casting and cooling process.
• Adjustments or modifications may have been made by
propeller repair stations. And finally, undetected damage
may have altered the pitch.
• There are two common types of pitch: constant (also called
"true" or "flat") pitch and progressive pitch.
• Constant pitch means the pitch is the same at all points
from the leading edge to the trailing edge.
• Progressive pitch (also called blade "camber")starts low at
the leading edge and progressively increases to the trailing
edge.
• The pitch number assigned Is the average pitch over the
entire blade.

Pitch: it is the distance a propeller drives
forward for each complete revolution,
assuming it is moving trough a solid element,
just like a wood screw does.
For instance, if the propeller cover
100 millimeters per turn through a solid,
then its pitch is 100 millimeters.
There are three main propellers' families:
constant-pitch propellers folding propellers and
controllable-pitch propellers

Helix definition on a cylinder
of radius r

Pitch
Pitch Line: -This is a line that passes through the leading and
trailing edges of the blade and used as a reference for pitch angle.

Types of Pitch
1. Nose–tail pitch,
2. Face pitch,
3. Effective or ‘no-
lift’ pitch,
4. Hydrodynamic
pitch.

• The nose–tail pitch line is today the most
commonly used reference line by the
principal propeller manufacturers in order
to define blade sections, and it is normally
defined at a pitch angle θnt to the thwart-
ship direction.
• This line also has a hydrodynamic
significance too, since the section angles
of attack are defined relative to it in the
conventional aerodynamic sense
Nose-tail Pitch

Face pitch
• Face pitch is now relatively rarely used by the large propeller
manufacturers, but it will frequently be seen on older drawings
and is still used by many smaller manufacturers.
• Indeed many of the older model test series, for example the
Wageningen B Series, use this pitch reference as a standard to
present the open water characteristics.
• Face pitch has no hydrodynamic significance at all, but was a
device invented by the manufacturers to simplify the propeller
production process by obviating the need to ‘hollow out’ the
surface of the propeller mold to accommodate that part of the
section between the nose–tail and face pitch lines.
• The face pitch line is basically a tangent to section’s pressure
side surface, and therefore has a degree of arbitrariness about
its definition since many tangents can be drawn to the aerofoil
pressure surface.

Effective pitch
• The effective pitch line of the section corresponds
to the conventional aerodynamic no-lift line and is
the line that if the incident water flowed along, zero
lift would result from the aerofoil section.
• The effective pitch angle (θ0) is greater than the
nose–tail pitch angle by an amount corresponding
to the three dimensional zero lift angle of the
section.
• This is a fundamental pitch angle since it is the
basis about which the hydrodynamic forces
associated with the propeller section are
calculated.

Hydrodynamic pitch
• The hydrodynamic pitch angle (βi) is the angle
at which the incident flow encounters the blade
section and is a hydrodynamic inflow rather
than a geometric property of the propeller.
• Neither this angle nor the effective pitch angle
would, however, be expected to be found on
the propeller drawing in normal circumstances.

Relationship between pitches
• The three pitch angles, effective, nose–tail
and hydrodynamic pitch, are all related by
the equations.
• Effective pitch angle = nose–tail pitch
angle + 3D zero lift angle = hydrodynamic
pitch angle + angle of attack of section
+ 3D zero lift angle.

Mean Pitch
• The mean pitch of a propeller blade is calculated using
a moment mean principle.
• The reason for adopting a moment mean is a practical
expedient, which has been confirmed both
experimentally and by calculation.
• As a consequence it can be used, in the context of
effective pitch, to compare propellers, which may have
different radial pitch distributions, from the viewpoint of
power absorption.
• For continuous and fair distributions of pitch from the
root to the tip it will be frequently found that the
moment mean pitch corresponds in magnitude to the
local pitch in the region of 0.6 to 0.7R.

• For practical calculation purposes of equation,
because the radial pitch distribution is normally
represented by a well-behaved curve without
great changes in gradient.
• It is possible to use a lower-order numerical
integration procedure. Indeed the trapezoidal
rule provides a satisfactory procedure if the
span of the blade is split into ten intervals giving
11 ordinates.
• Then the mid-point of these intervals xj
(j = 1, 2, 3, . . . , 10) are defined as follows,
where x is the non-dimensional radius x = r/R
Mean Pitch

Pitch: P when the line AB makes one complete revolution
and arrives at A’B’. It traveled an axial distance AA’,
which represents the pitch of the surface. The propeller
blade is part of that surface and the pitch is also called the
pitch of the blade.
Pitch angle 1
tan or tan
2 2
Pitch ratio: tan
P P
r r
P PR
PR
D
 
 


  
 
 
 
 
P
o
A

2 r

The angle θ is termed
the pitch angle and
the distance p is the
pitch.

Pitch angle
• Geometric Pitch Angle - The angle
between the pitch reference line and a line
perpendicular to the propeller’s axis of
rotation.
• Angle of the pressure face along the pitch
line with respect to the plane of rotation
measured in degrees. Not to be confused
with pitch. Pitch angle decreases from the
blade root to the tip in order to maintain
constant pitch.

Angle of Attack
• If a wing with symmetrical airfoil is moved
through the fluid so that fluid moves
symmetrically above and below the wing,
there is equal pressure above and below
resulting in no "lift." The wing is said to be
operating at zero degree (0°) angle of attack.
• With an angle of attack there is a pressure
change or difference above and below the
wing which creates lift: negative (lower)
pressure on the top and positive (higher)
pressure below.

• Blades operating
with some angle of
attack create a
negative (lower or
pulling) pressure
on one side and a
positive (higher or
pushing) pressure
on the other side.
• The pressure
difference causes lift
at approximately right
angles to the blade
surface. Lift can be
divided into a thrust
component in the
direction of travel and
a torque component
in the opposite
direction of propeller
rotation.
Angle of Attack

Pitch distribution
• The variation of the pitch over the length of the
blade. The different types of pitch distribution are: -
1. Constant (fixed) pitch – the pitch is equal for
each radius.
2. Progressive pitch – the pitch increases along
the cylindrical line from the leading to the trailing
edge.
3. Regressive pitch – the pitch decreases along the
cylindrical line from the leading to the trailing
edge.
4. Variable pitch – the pitch is different at each
radius.

Pitch Ratio
• This is the most fundamental of the
various propeller ratios and is the mean
pitch divided by the propeller’s diameter.
• Pitch ratio
In case that the pitch, P, is not constant, then the pitch is
defined as P = Ptip (the pitch at the tip of a propeller).
• Blade area ratio = AD /A0
AD - Total (developed) blade area clear of that of the boss
P
PR
D

2
0 / 4
A D



Blade Skew
• The skew angle θs(x) of a particular
section, is the angle between the directrix
and a line drawn through the shaft center
line and the midchord point of a section at
its non-dimensional radius (x) in the
projected propeller outline; that is, looking
normally, along the shaft center line, 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 (θsp) 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.

• A blade center line transverse curvilinear
sweeping back from the direction of rotation. The
contour of the blade is not radially symmetrical
about blade center axis.
• A blade that is swept back versus a blade that is
radially symmetrical in contour is said to have
skew.
• Considerable skew (sweep back) is helpful in
allowing a propeller to more easily shed weeds.
Higher skew on a surfacing application reduce
the pounding vibration of a propeller blade re-
entering the water.
Blade Skew

Types of Skew
• Skew forms an asymmetrical shape that can be viewed by
looking at the prop blades directly from the fore or aft:-
– Aft or Positive Skew: The blade sweeps in the direction
opposite to the propeller rotation when moving the craft
forward.
– Forward or Negative skew: The blade sweep in the same
direction to the propeller rotation when moving the craft
forward.
• 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 contrast, in the biased skew design the mid-chord locus
intersects with the directrix not more than once; normally
only in the inner sections.

Blade Rake
• Rake is the angle of attachment of the blade to the
boss of the propeller and affects the flow of water
through the propeller.
• Axial distance from the midchord point at the hub
section and the section of interest.
• Rake has implications with respect to boat
performance.
• Higher rake can improve performance in higher engine
elevation and/or ventilating or cavitating situations.
• When a propeller blade is examined on a cut extending
directly through the center of the hub, the face side of
the cross section of the cut blade relative to a plane
that is perpendicular to the propeller axis would
represent blade rake

Blade Rake
• A propeller blade may rake either aft or forward from
the blade center axis.
• Aft or Positive Rake the blade tip rakes towards the
after end of the boss and such rake is common for
planing and combination hulls as it provides increased
top end speed while assisting in trimming the bow
upward for less wetted surface and frictional drag.
• Forward or Negative Rake the blade tip rakes
towards the forward end of the boss and such rake is
rare but may be found in commercial craft with
displacement hulls as it trades speed for steady power
that can aid in holding the bow of the boat down and
level.

• If the face of the blade is perpendicular to the
propeller hub, the propeller has zero degree
rake. As the blade slants back toward the aft
end of the propeller, blade rake increases.
• With standard propellers, the rake angle varies
from -5° to 20°. Basic propellers for outboard
engines and stern drives commonly have
around 15° of rake.
• Higher-raked (high-performance) propellers
often have progressive rake which may go as
high as 300 at the blade tip
Blade Rake

Blade Rake
• A higher rake angle generally improves the ability of
the propeller to operate in a cavitating or ventilating
situation, such as when the blades break the water's
surface.
• With such surfacing operation, higher blade rake
can better hold the water as it is being thrown off
into the air by centrifugal force, and in doing so,
creates more thrust than a similar but lower raked
propeller.
• On lighter, faster crafts, with a higher engine or drive
transom height, higher rake often will increase
performance by holding the bow of the boat higher,
resulting in higher boat speed due to less hull drag

Components of Rake
• Propeller rake is divided into two components:
generator line rake (iG) and skew induced rake
(is). The total rake of the section with respect to
directrix (iT) is given by
• That is, it is the distance, parallel to the
x-axis, from the directrix to the point where the
helix of the section at radius r cuts the x–z-
plane.

Skew induced rake
• Skew induced rake is the component, measured in the x-
direction, of the helical distance around the cylinder from
the mid-chord point of the section to the projection
of the directrix when viewed normally to the y–z-plane.
• It is possible then to define the locus of the mid-chord
points of the propeller blade in space as follows for a
rotating right-handed blade initially defined, φ = 0, about
the OZ-axis of the global reference frame:

CUPPING
• Cup is the small curved lip on the blade tip
and/or trailing edge. Used in proper amounts,
cup helps reduce ventilation and propeller
slippage, allowing for higher mounting heights
and greater bow lift.
• Too much cup, however, will cause excessive
steering torque and bow lift and limit the
engine’s ability to develop and maintain proper
RPM at a certain pitch.
• When the trailing edge of the blade is formed or
cast with an edge curl (away from the boat), it is
said to have a cup

• Originally, cupping was done to gain the same
benefits as just described for progressive pitch and
curved or higher rake. However, cupping benefits
are so desirable that nearly all modem
recreational, high-performance or racing propellers
contain some degree of cup.
• Cupping usually will reduce full-throttle engine
speed below the same pitch propeller with no cup.
• A propeller repair shop can increase or decrease
cup to alter engine RPM to meet specific operating
requirements on most propellers
CUPPING

• For a cup to be most effective, it should be
completely concave (on the face or pressure
side of the blade) and finish with a sharp trailing
edge. Any convex rounding of the trailing edge
of the cup, on the pressure side, detracts from
its effectiveness.
• Using a round-bladed propeller as an example,
if the cupped area intersects pitch lines, it will
increase blade pitch. Cupping in this area will
reduce RPM by adding pitch.
• It will also protect somewhat against propeller
"blowout"
CUPPING

• If the cup is placed so that it interests rake
lines, it then has the effect of increasing
rake.
• There is clearly some overlap where cup
effects both pitch and rake.
• Adjusting the cup on a cleaver-style
propeller is more difficult. Since the trailing
edge is very thick and runs straight out on
a rake line, any adjustment will have far
less effect on altering rake
CUPPING

• The added pitch created by the cup can be
reduced substantially by filing or grinding
away some of the cup. At the same time,
rake can be altered slightly.
• For less rake, decrease the cup in the
area close to the tip.
• For more rake, reduce the cup in the area
close to the hub.
• Obviously, any cup reduction will also
result in an RPM increase.
CUPPING

• Because under actual operating conditions the propeller’s
absolute forward movement (actual pitch) is less than
theoretical pitch and the difference is called the slip.
• Slip is the difference between actual and theoretical travel
resulting from a necessary propeller blade angle of attack.
• To create thrust there must be some angle of attack or slip.
The objective of propeller design is to achieve the right
amount of slip or angle of attack.
• Slip is the amount of “wasted” energy a particular prop
generates, meaning that the actual distance traveled in one
full propeller revolution is less than its pitch measurement.
It is normally expressed as a “percentage of inefficiency”. A
certain amount of slip is engineered into each line of
propellers to create different performance characteristics
PROPELLER SLIP

• If the propeller had no slip,
i.e. if the water which the
propeller “screws” itself
through did not yield (i.e. if
the water did not
accelerate aft),
• the propeller would move
forward at a speed of V = p
× n, where n is the
propeller’s rate of
revolution,
PROPELLER SLIP
However, as the water is a
fluid and does yield (i.e.
accelerate aft), the propeller’s
apparent speed forward
decreases with its slip and
becomes equal to the ship’s
speed V, and its apparent slip
can thus be expressed as p × n
– V.

• The apparent slip ratio
SA, which is
dimensionless, is
defined as
• The apparent slip ratio
SA, which is calculated
by the crew, provides
useful knowledge as it
gives an impression of
the loads applied to the
propeller under different
operating conditions.
• The apparent slip
ratio increases when
the vessel sails
against the wind or
waves, in shallow
waters, when the
hull is fouled, and
when the ship
accelerates.
APPERENT SLIP

• The real slip ratio will be greater than the
apparent slip ratio because the real speed
of advance VA of the propeller is, as
previously mentioned, less than the ship’s
speed V.
• The real slip ratio SR, which gives a truer
picture of the propeller’s function, is:
• At quay trials where the ship’s speed is V
= 0, both slip ratios are 1.0. Incidentally,
slip ratios are often given in percentages.
REAL SLIP

Blade Thickness
• A blade is thickest at the point where it meets
the hub (blade root). As the blade moves out
from the hub to the tip, it becomes thinner.
• The basic reason for this is that, as with any
cantilever beam, the load that any blade or
beam section must support is the load on the
blade or beam between that section and the tip
of the blade
• Thus, at the tip there is zero load requiring zero
thickness. However, to be practical, a given
minimum edge thickness is chosen for a given
propeller material and type of use

Blade Thickness
• The minimum blade
thickness is determined by
physical corrosion fatigue
testing through 109 cycles.
• Another method for
determining the thickness
is through finite element
techniques develop blade
stress distributions which
can be correlated more
readily with model and full-
scale measurement.

• This is largely of American use and is the mean
width of the propeller blade divided by the
propeller diameter.
• It is related to the developed (or disc) are ratio
dar by the above formula.
• The formula assumes that the mean boss
diameter is 0.20 x the propeller diameter.
Mead Width Ratio

6.3 Theory of Propeller Action
• Assumptions:
1) replacing the propeller with a stationary actuating disk across
which the pressure is made to rise;
2) neglecting the rotational effect of propeller
3) neglecting vortices shed from the blade tip, & frictional loss.
D
VA
VA(1+b)
VA(1+a)

Propeller theory
• Momentum theory
• Blade element theory
• Lifting line theory
• Lifting surface theory
• Vortex field theory

precondition and assumption
• the flow is inviscid and steady (ideal flow), therefore the
propeller does not experience energy losses due to
frictional drag
• also the rotor is thought of as an actuator disk with an
infinite number of blades, each with an infinite aspect
ratio
• the propeller can produce thrust without causing rotation
in the slipstream
• the flow is inviscid and steady (ideal flow), therefore the
propeller does not experience energy losses due to
frictional drag
• also the rotor is thought of as an actuator disk with an
infinite number of lades, each with an infinite aspect ratio
• the propeller can produce thrust without causing rotation
in the slipstream

Pressure field on the propeller

Momentum Conservation
Force = net momentum flux (horizontal)
 
   
 
0
2
0
1
1 = = 1 (mass conservation)
1
A A
A A f A a
A A
T Q V b V
Q a V A V A b V A
T QV b A V a b

 
  
 
 
  
  
Energy Equation
 
 
 
2
2
2
2
2
0 0
0
1
2 2
2 1
, , 2
2 2
1 1 or
2 2
A
A
A
A
V b
V P
g g
b b V
T
A P T T A b b V
A g
b b
a a





 

    
   

 
 
0
0
0
2
0
2
1 1
0
2 2
Efficiency of a propeller
(no friction & no rotationary velo. considered)
1
1 1
1 1
1 1 / 2
Defining the thrust loading coeff., , as
1
A
A A
I
A
I
T
A
T
A
V A
TV TV
TQ
Q P a V A a
A
a b
C
A V a b
T
C
V A



 
   
  
 
 

 
Ideal
 
2
0
I
4 1
1 1 2
Thus, &
2 1 1
With the increase in , the ideal efficiency decreases.
0 1 2 3 4
1.00 0.827 0.732 0.667 0.618
A
T
I
T
T
T
a a
V A
C
a
C
C
C


 
  
 
 

• Extension of momentum theory
Consider the rotation of the flow passing through the propeller
disc., the reduced ideal efficiency becomes,
2
2
2
1 '
& ' 0.
1
2
1 ' 1
2
where is the rotation velocity of flow after the propeller,
& is the rotation velocity of the propeller.
I
a
a
a
a




 



 

 

 
 
   

• Blade Element Theory
In the momentum conservation of a propeller, no detailed
information can be obtained with regard to the effects of the
blade section shape on propeller thrust and efficiency.
 
 
The total velo. at radius , , 2 .
Thrust: cos sin
Resistance: sin cos
Moment: & and
is a function depending on section shape (win
r A T T
T L D
F L D
F L D
r V V V V rN
d d d
d d d
q d r d d f
f

 
 


  
 
 
  
 
 
g
section theory). For a propeller, the relative advance velocity
of the fluid at the disc, is 1 & the rotation velocity is
1 ' .
A
V a
r a




aVA
α
α’
 
1 '
r a
 
'
a r
r
V r


A
V
a & a’ are determined by experiments

• A simple solution for unswept three-
dimensional wings can be obtained by
using Prandtl's lifting line model.
• For incompressible, inviscid flow, the wing
is modelled as a single bound vortex line
located at the 1/4 chord position and an
associated shed vortex sheet
Lifting Line Theory

• The span-wise lift distribution is assumed
to be elliptical with a small modification
due to wing planform geometry. The
assumed vortex line strength is thus a
Fourier series approximation.
•
Lifting Line Theory

The required strength of the distribution coefficients (An) for
a given geometry and set of free-stream conditions can be
calculated by applying a surface flow boundary condition.
The equation used is based on the usual condition of zero
flow normal to the surface. For 3-D wings the condition is
applied at several span-wise sections by matching flow and
surface angles. The local flow angle of incidence for a 2-D
section of the wing must be equal to the sum of the wing's
angle of attack, the section twist and the downwash
induced flow angle. This downwash component is caused
by the induced flow from the trailing vortex sheet
Lifting Line Theory

where α is the 3-D wing angle of attack, θt is the wing twist angle and wi is the
velocity induced by trailing vortex sheet.
Lifting Line Theory

Lifting Surface Theory
• In the lifting surface theory, the real
propeller geometry is dealt. The blade
mean surface is defined in terms of a
camber distribution. Thickness is added
symmetrically with respect to mean line at
each radius.
• If the aspect ratio of blades is high, and if
the rake and skew is zero (or at least
small), the lifting line theory is applicable.

Boundary Element Methods
• Boundary element methods, which are also known as “Panel
Methods”, are based on the approach developed by Hess and
Smith (1967). In these methods, the surfaces of propeller blades
and hub or foil surface are discretised by a number of small
quadrilateral panels having constant source and doublet
distributions. The trailing vortex sheet is also represented by similar
quadrilateral panels having constant doublet distributions. The
strengths of the source and doublet distributions are determined by
solving the boundary value problems at each of the control points,
which are located on each panel. They are inherently “Non-linear”
with either the thickness or the angle of attack, since they make no
assumption about the magnitude of these quantities. On the other
hand, the panel methods become very expensive, in terms of
computing time, in the case of three-dimensional geometries
(marine propellers) as the number of panels are increased
tremendously compared to the 2-D foils.

Similarity Law for Propellers
Although theoretical studies and CFD on propellers are very
important and provides valuable guideline for designing propeller,
a great deal of knowledge concerning the performance of propellers
has been obtained from propeller model tests. Hence, it is
necessary to examine the relation between model and full-scale
results as the case of resistance. In open water (not behind a ship),
 
, , , , , ,
- rotational speed, - diameter of propeller
- pressure in water, - dynamic viscosity
- speed of advancing, - Thrust
A
A
T f D V g n p
n D
p
V T
 



2 4 2
1
2
2
1
2
2 4
Using D.A, the non-dimensinal formula is given by,
, , ,
Froude #: , Euler #: , Reynolds #:
: , :
The
A A A
A
A A
A
A
T
V V V D
T p
f
n D nD V
gD
V V D
p
V
gD
V T
J K
nD n D
  
 

 
 
  
 
 
 
Advanced ratio Thrust coeff.
2 5
Advanced ratio is related to the slip ratio 1 .
Define as the to drive a propeller
.:
A
Q
V
nP
Q
Q
K
n D

 

 
 

Torque
The torque coeff

In open water, the propeller efficiency coeff.:
.
2 2 2
When all the dimensionless parameters are the same for the
two propellers, the two propellers
will b
A T A T
o
Q Q
TV K V K
J
nQ K nD K

  
  
geometrically similar
1
2
e .
Scale ratio:
For the same Froude #:
For the same advance ratio (most important)
indicating the model rotating faster.
s
m
As s
Am m
s As m
m Am s
D
D
V D
V D
n V D
n V D






 
  
dynamically similar

2 2
1 1
2 2
For the same Euler # :
If the cavitation performance is not an issue, this number is not
of importance & may be neglected in the dynamical similarity.
- wat
A A
m s
s os s w
os
p p
V V
p p H
p
 

   

   
   
  
 
 
2
2
er surface pressure, is the depth of a propeller.
In general, , .
Because 1, and .
has to be negative, thus the model test is carried out in
s
A m s
m s m s
A s
m s m om m w
om
H
V p
p p
V
p p p p H
p
 

 
  
    
a
vacuum (cavitation) tunnel.

1
For the same Re: ,
which is contradict to the similarity of Fr. Therefore, it is almost
impossible to satisfy the Fr & Re similarity laws simutanously.
Similar to the assumption made
As m s
Am s m
V D v
V D v 
 
in model resisrtance tests, we
assume viscous force is independent of other dynamic forces.
Hence, it may be computed separately. In reality, viscous force
is usually a small portion of the total force. The smilarity of
Re is neglected in propeller model tests.
Therefore, propeller model tests follows & (advance
ratio) similarity laws. If the cavitation is relevant, then the
Euler number sho
Fr J
uld be the same as well.

Propeller types
• Azimuth Propulsor:-
• Contra Rotating Propellers
• Cycloidal Propeller
• Ducted Propeller or Shrouded Propeller or
nozzle propeller

Jet type: Water is drawn by a pump & delivered sternwards as a
jet at a high velocity. The reaction providing the thrust. It’s use
has been restricted to special types of ships.
Other propulsion Devices:
1. Nozzles (Duct) Propellers: main purpose is to increase the
thrust at low ship speed (tug, large oil tanker)
2. Vertical-Axis Propellers: Advantage is to control the direction
of thrust. Therefore, the ship has good maneuverability.
3. Controllable-Pitch Propellers (CCP): The pitch of screw can
be changed so that it will satisfy all working conditions.
4. Tandem and Contra-rotating Propellers: It is used because
the diameter of a propeller is restricted due to limit of the draft
or other reasons (torpedo). The efficiency of the propeller
usually decreases.

Azimuth propeller
A propulsion unit
consisted of a
shroud attached to
the vessel’s rudder
stock with a pod
inside fitted with an
ordinary propeller
and capable of
delivering its thrust
through 360°.

Contra Rotating Propellers
• A pair of propellers fitted in tandem on the
same shaft and rotating in opposite
directions.
• The after propeller is usually somewhat
smaller in diameter than the forward
propeller.

Cycloidal Propeller
• A device fitted to vessels requiring a high
degree of maneuverability and consisting of a
number of fairly narrow vertical blades rotating
round their own centers which, in turn, rotate
around the vertical center of the device. Hence
they perform a cycloidal motion.
• Sometimes fitted with a horizontal hydrofoil
form shield below the blade tips.
• Often found under the propriety trade name of
Voith Schneider

• In outline, the advantages of the VOITH
CYCLOIDAL Rudder for warships:
– Low resistance rudder for high speed operation.
– Improved maneuverability in comparison to conventional
propulsion arrangement.
– As VCR is main propulsion for low speeds, CP-propellers
may be replaced by FP-propellers.
– Redundancy of propulsion and steering (take home
capability)
– Roll stabilisation even during stand-still of vessel is
possible.
– High shock resistance, low magnetic signature, low
radiated noise levels
– Ideal complement to advances propulsion systems
Cycloidal Propeller

Shrouded Propeller
• A hydrofoil sectioned steel ring fitted round
a specially designed propeller and found in
two forms:
– The fixed nozzle which is permanently welded
to the vessel’s hull.
– The nozzle rudder which is attached to the
vessel’s rudder stock and replaces the rudder.

Difference between fixed/constant
and variable pitch propellers

Fixed pitch propellers
• Propellers of the FP-type are cast in one block and
normally made of a copper alloy. The position of the
blades, and thereby the propeller pitch, is once and for
all fixed, with a given pitch that cannot be changed in
operation.
• This means that when operating in, for example,
heavy weather conditions, the propeller performance
curves, i.e. the combination of power and speed (r/
min) points, will change according to the physical
laws, and the actual propeller curve cannot be
changed by the crew.
• Most ships which do not need a particularly good
maneuverability are equipped with an FP-propeller

Constant pitch propellers: this type of
propellers blades are welded to the hub, and
their pitch, as suggested by the name, is fixed.
Their structure is surely the stronger, because
they are manufactured from a single casting,
usually through CAM (Computer Aided
Manufacture) assisted machinery and they
have no moving parts.
Folding propellers: they have folding blades;
under sail the hydrodynamic pressure keeps
them closed, thus considerably reducing drag.
Their astern maneuverability is poor.

Controllable/variable pitch propellers
• Propellers of the CP-type have a relatively
larger hub compared with the FP-propellers
because the hub has to have space for a
hydraulically activated mechanism for control of
the pitch (angle) of the blades.
• The CP-propeller is relatively expensive, maybe
up to 2-3 times as expensive as a
corresponding FP-propeller.
• Furthermore, because of the relatively larger
hub, the propeller efficiency is slightly lower.

Controllable pitch propellers: in this type of
propellers, the user can modify the pitch,
while underway, by mean of a hydraulic
mechanism or a direct mechanical linkage.
Feathering propellers, in particular, are a
special controllable pitch propeller type,
ensuring low drag, because of their
characteristic blade design.
Controllable pitch propellers are very
practical because by modifying the pitch they
allow for thrust optimization under different
load conditions. Most modern sailboats are
fitted with this type of propeller. Lets
discover together how to use it.

Controllable pitch propeller
• Most of the propellers that we come across are Fixed
Pitch propellers(FPP). But there are propellers known as
Controllable Pitch Propellers (CPP), that can move their
blades about their own axis. So what are the uses of
movable blades and how it is more beneficial than the
Fixed Pitch Propeller.
• The mechanism that controls the blades movement is
located in the boss of the propeller. This mechanism can
be operated from both, the engine room and bridge, with
the help of hydraulic cylinders. Incase the hydraulic
system fails, the blades can be locked in the ahead
position with the help of a locking device. Now let's have
a look as to how a ship can be propelled forward and
backwards just by movement of the blades.

The SCHOTTEL Controllable Pitch Propeller –the
reliable
propulsion system for all ships with up to 30,000 kW

SCHOTTEL Controllable Pitch
Propeller Systems (SCP) are available
in various designs, including:
• X-type, i.e. hydraulic cylinder
mounted in the propeller hub• Z-type,
i.e. hydraulic cylinder mounted in the
propeller shaftOil is distributed either
via an Oil Distribution (OD) box
mounted in front of the gearbox (G-
type), or via the W-type OD box, which
ismounted in the shafting.

THEREFORE DIFFERENT COMBINATIONS OF
HYDRAULIC CYLINDER ARRANGEMENT AND POSITION
OF OIL SUPPLY CAN BE IMPLEMENTED. THE MOST
COMMON IS THE X-TYPE HUB COMBINED WITH OIL
SUPPLY IN FRONT OF THE GEARBOX,THE SO-CALLED
“XG” CONFIGURATION.
OTHER SOLUTIONS ARE THE “ZG”VERSION, WITH
THE HYDRAULIC CYLINDER IN THE SHAFT AND THE
OD BOX IN FRONT OF THE GEARBOX, AND THE “XW”
VERSION, WITH THE CYLINDER IN THE HUB AND THE
OIL SUPPLY IN THE SHAFT.

THE “X” TYPE INCORPORATES A HYDRAULIC
CYLINDER WITH THE PISTON DIRECTLY CONNECTED
TO THE YOKE. HENCE THE DESIGN IS SIMPLE, WITH A
MINIMUM OF MOVING PARTS, AND ACHIEVES THE
HIGHEST RELIABILITY. TO OBTAIN OPTIMUM
STRENGTH THE HUB IS CAST IN ONE PIECE. THE
PROPELLER BLADES ARE MOUNTED ON LARGE-SIZED
BLADE CARRIERS TO MINIMIZE THE STRESSES IN THE
SYSTEM. THE YOKE MOVING INSIDE THE HUB IS
SUPPORTED BY SLIDING PIECES. CRANK PINS ON THE
YOKE OPERATE THE PROPELLER BLADE CARRIERS,
WHICH HAVE GROOVES GUIDING THE PINS. THE
PROPELLER BLADES ARE BOLTED TO THE CARRIERS.
THE HUB IS SEALED BY A WELL-PROVEN SYSTEM
CONSISTING OF A PRE-LOADED SEALING RING
BETWEEN THE HUB AND THE BLADE FOOT.

• The mechanism that controls the blades
movement is located in the boss of the
propeller.
• This mechanism can be operated from
both, the engine room and bridge, with the
help of hydraulic cylinders.
• Incase the hydraulic system fails, the
blades can be locked in the ahead position
with the help of a locking device.
Hub based pitch control mechanism

The hydraulic oil flows through an inner and
outer oil pipe, both mounted concentrically
inside the hollow-bored shaft. The movable
double oil pipe also functions as a feedback
system indicating the current pitch of the
propeller system. The Z-type hub with the
hydraulic cylinder within the propeller shaft
results in a considerably shorter propeller hub.
The shaft-integrated hydraulic cylinder moves
the yoke by means of a rod leading through the
hollow-bored shaftline. For all systems,
propeller blades and hubs are available made of
Cu-Ni-Al or even stainless steel

CONTROLLABLE PITCH PROPELLERS DESIGNED BY
SCHOTTEL OFFER THE FOLLOWING ADVANTAGES:•
BLOCKING VALVES FOR PITCH SETTING INSTALLED IN
THE CYLINDER SPACE OF THE HUB, EASILY ACCESSIBLE
WHEN DOCKED WITHOUT DISMANTLING OF THE HUB •
BLOCKING VALVES ALLOW OPERATION IN THE AHEAD
CONDITION WITH 100% ENGINE POWER WITHOUT
RESTRICTION • BLADES CAN BE DISMOUNTED IN A
NOZZLE WITHOUT PULLING THE SHAFT • THE BLADE
MOVING PIN IS PART OF THE CAST YOKE, WHICH
ACHIEVES A LARGER CONTROL STROKE NEAR THE END
POSITIONS OF THE BLADES, ALLOWING FINER PITCH
CONTROL. THIS ALSO RESULTS IN LOWER STRESSES IN
THE PIN. • OPTIMUM MATCHING OF MATERIAL BETWEEN
HUB AND BLADE CARRIERS • LARGER HUB IS CAST IN
ONE PIECE, GIVING A RIGID STRUCTURE

Cpp Blade action
The diagram shows the cross section of
blades. We will assume that the ship is
moving in the ahead direction and the arrows
shows the direction of the forces generated
that pushes the ship forward. When the blade
is at zero position, the propulsive forces
acting on both the sides are equal in
magnitude, but opposite in direction. Even
though the net propulsive force is zero, the
propeller absorbs a large amount of energy to
convert it to wake turbulence. If the ship is to
reverse, the blades are moved even further,
this will result in a propulsive thrust in the
forward direction, facilitating the ship to
reverse. The position of blades are adjusted
according to the load of the ship.

DESIGNING A CP PROPELLER BLADE IS A COMPLEX
PROCESS,REQUIRING AN EXTENSIVE RANGE OF EXPERT
KNOWLEDGE IN THE SPECIALIZED FIELDS OF FLUID
PHYSICS AND MECHANICAL ENGINEERING. IN ADDITION
TO HYDRODYNAMIC BLADE DESIGN, THE CALCULATION
OF HYDRODYNAMIC LOADS AND THEIR EFFECTS WHEN
THE BLADE PITCH IS CHANGED AND IN VARIOUS
OPERATING CONDITIONS ARE OF GREAT IMPORTANCE.
IN ORDER TO PROVIDE ADVANCED BLADE SHAPES AND
SATISFY EVER-HEIGHTENED REQUIREMENTS, USE IS
MADE OF STATE-OF-THEART CALCULATION METHODS,
REFINED CONTINUOUSLY BY MEANS OF RESEARCH
PROJECTS CARRIED OUT IN COOPERATION WITH
RESEARCH INSTITUTES.THE BLADE DESIGN IS INITIALLY
EXECUTED THROUGH THE USE OF CIRCULATION THEORY
VERIFICATION AND OPTIMIZATION TECHNIQUES

THE STRENGTH OF THE BLADE IS VERIFIED
THROUGH THE USE OF FEM (FINITE ELEMENT
METHOD), ACHIEVING THE OPTIMUM
COMBINATION OF MECHANICAL EXPEDIENCE
AND HYDRODYNAMIC EFFICIENCY.ALMOST
EVERY PROPELLER UNDERGOES EXTENSIVE
MODEL TESTS, WHERE IT MUST PROVE THAT
IT ACTUALLY POSSESSES THE REQUIRED
CHARACTERISTICS WITH REGARD TO
EFFICIENCY, CAVITATION AND PRESSURE
FLUCTUATIONS.

Here SCHOTTEL employs two tried-and-
tested methods developed at the HSVA in
Hamburg and the SVA in Potsdam, which are
currently the most powerful programs in
existence. Openwater diagrams, pressure
distribution, cavitation and pressure
fluctuation properties are calculated for all
relevant operating states in the vessel’s
wake.
In addition to close cooperation with
research institutes, SCHOTTEL also draws
on the invaluable years of experience of
leading experts in the field of propeller
design.

The strength of the blade is verified through the use
of FEM (Finite Element Method), achieving the optimum
combination of mechanical expedience and
hydrodynamic efficiency. Almost every propeller
undergoes extensive model tests, where it must
prove that it actually possesses the required
characteristics with regard to efficiency, cavitation
and pressure fluctuations.
In these tests the SCHOTTEL design regularly
competes head-to-head with technology from other
suppliers, and as the results show, SCHOTTEL
produces some of the best propeller designs on the
market.

The pump and motor unit forms an essential
part of the hydraulic system. This assembly
delivers the oil quantity needed for adjustment
of the propeller blades and produces the
pressure required for pitch control. Two
electrically driven pumps (1 active pump, 1
standby pump, each with 100% capacity) are
mounted on the cover of the hydraulic tank,
with the pumps running in the oil

The compact control block, incorporating all the indicators and the
individual instruments necessary for pitch control, is located on the
top of the tank. The piping between the pump and motor unit and
the oil supply unit is part of the shipyard’s scope of supply.
Lubrication oil is fed through the stern tube into the hub.
This system is not connected to the hydraulic system of the
controllable-pitch propeller unit. Optionally a two-pipe system can
be supplied, in which case the hydraulic oil is used to lubricate

The remote control system is designed to
provide automatic control of a SCHOTTEL
controllable pitch ropeller. The system is
based on a microprocessor-controlled
system architecture with 2-wire bus
communication between central unit, ECR
and bridge. An HMI (human-machine
interface) allows clear, user-friendly
control, set-up and maintenance of the
system.The system is type-tested to GL,LRS
and ABS (other classes on request) and
meets class requirements according to
AUT24 and UMS.Standard features

UMS.STANDARD FEATURES:
• CONTROL FROM ECR, BRIDGE AND WINGS
• COMBINATOR AND CONSTANT
SPEED MODE
• UP TO 3 ACCELERATION PROGRAMS
• LOAD CONTROL MANAGEMENT
• AUTOMATIC SLOW DOWN
• AUTOMATIC SHUT DOWN
• SELF MONITORING
• NON-FOLLOW-UP CONTROL FROM ECR AND BRIDGE
• PITCH MEASUREMENT SYSTEM
• M/E INTERFACE :THE SYSTEM IS POWERED WITH 24 V DC.
A SEPARATE SUPPLY SHOULD BE PROVIDED FOR THE BACK-
UP SYSTEM.OPTIONS:
• ENGINE TELEGRAPH SYSTEMS AND ELECTRIC SHAFT
SYSTEM IN THE WHEELHOUSE AREA
• CLUTCH CONTROL SYSTEM
• INTERFACE FOR DP SYSTEMS
• INTERFACE FOR MANOEUVRING

Additional Features
• A CPP can be connected to a Shaft generator. A shaft
generator can supply power on the ship till the time the
main engine is running. CPP can be used to maintain the
frequency of the generator as the Engine moves on a
constant rpm.
• In case during the navigation time additional energy is
needed, an auxiliary generator can be used to provide
additional power to the shaft generator. This is mainly
used during maneuvering. If this is done, main engine
should be disconnected from the reduction gear to
prevent it from getting damage.

6 Passenger/Container vessel ZI YU LAN,1 x SCP 1544 XG (15,000
kW)
Shipyard: Aker-MTW, Germany, Owner: Shanghai Shipping
Corporation,PR China

FOR THE MAJORITY OF ENGINE AND PROPELLER
MANUFACTURERS THE IDEAL PROPELLER WILL CAUSE A LOSS
OF 5 TO 10% IN ENGINE MAXIMUM REVOLUTION PER
MINUTE; IF, FOR INSTANCE, THE ENGINE RATED MAXIMUM
RPM ARE 3600, THE LOSS WILL APPROXIMATELY BE 200
RPM, IN CALM SEA, WITH NO WIND, WITH NO OVERLOAD ON
BOARD AND WITH A CLEAN HULL BOTTOM, WHILE IT WILL
BE ABOUT 360 RPM IN ROUGH SEA, STRONG WIND ETC...
IF THE TOTAL ACTUAL LOSS IS BIGGER, THEN THE
PROPELLER IS "OVERLOADED" AND SO IS THE ENGINE,
WHILE IF THE PROPELLER IS TURNING TOO FAST IT IS
"UNDER-LOADED" AND IS NOT USING ALL THE ENGINE
POWER. ON THE OTHER HAND SOMEONE BELIEVES THAT
ONE SHOULD KEEP THE PITCH AS LONG AS POSSIBLE IN
ORDER TO ACHIEVE THE CRUSE SPEED AT LOWER AS
POSSIBLE RPM.

For example, lets suppose that a 6 knots cruise speed is
reached at 2800 rpm. Increasing the pitch (and of course
keeping the diameter constant) the same speed could be
registered at 2000 rpm. In this case, advantages are: lower
engine speed, less shaft vibration, less noise thus longer
engine life. The question is: which is the right choice?
THE "HIGH PITCH AND LOW RPM" SOLUTION , ALTHOUGH
APPEARING INTERESTING, IS NOT THE CORRECT ONE. THE
ENGINE IS ACTUALLY RUNNING SLOWLY, BUT IT IS
OVERLOADED THUS LASTING SHORTER, MUCH SHORTER
THAN AN ENGINE RUNNING FASTER BUT WITH LESS "JOB" TO
DO

The first thing to do is to find in the owner's manual at
which rpm the engine reaches its maximum power (BHP).
Lets perfectly working injection system. This means, for
instance, that an engine which has lost say, for example,
that the maximum power is obtained at 3600 rpm.
Then we have to check which is the actual rpm reached
by the engine, accelerating in neutral. If a 3700/3750 rpm
are achieved, everything is fine, if not you have to adjust
your revolution counter to that value (in fact, and
normally, an engine should increase, in neutral, 3 to 4%
its maximum rated rpm, because, usually, the
manufacturer takes into account the loss due to the
reduction gear). All this is applicable to all well
maintained engines, and in particular to those with clean
fuel filters and compression will not achieve its top rated
rpm. Once the revolution counter has been verified, we
can start the trial which will allow us to know if and at
what rpm our engine is overloaded.

THE SEA STATE MUST BE CALM, AND NO SAIL SHOULD BE
UP. KEEPING A CONSTANT ROUTE, WE HAVE TO INCREASE
ENGINE SPEED WITH A 200 RPM STEP. WE WILL PLOT, FOR
EACH RPM RANGE, THE BOAT'S SPEED, OBSERVED AT THE
LOG (GPS COULD BE TOO INACCURATE FOR THIS PURPOSE).
SPEED SHOULD INCREASE CONSTANTLY FOR EACH RPM
RANGE. MEANTIME, WE SHOULD CHECK EXHAUST WATER
AND FUMES COLOR, WHICH MUST NOT CHANGE. IF SPEED
DOES NOT INCREASE CONSTANTLY OR DOES NOT INCREASE
AT ALL, THEN THE ENGINE IS OVERLOADED (BE SURE THAT
YOU HAVE NOT REACHED THE HULL SPEED); EXHAUST
FUMES QUANTITY AND WATER COLOR WILL PROOF THE
OVERLOADED ENGINE CONDITION

IN FACT, INCREASING ENGINE LOAD,QUANTITY,
DENSITY AND COLOR OF BOTH EXHAUST FUMES
AND WATER WILL BECOME DARKER AND DARKER,
TILL THEY RICH A BLACK COLOR, MEANING
PITCH IS TOO LONG. IN THIS SITUATION,
INCREASING RPM WILL NOT INCREASE SPEED,
SOME OF THE FUEL WILL NOT BE BURNED AND
FUEL CONSUMPTION WILL INCREASE WITHOUT
BENEFITS THE SAME TEST SHOULD BE CARRIED
OUT WITH ROUGH SEA AND WIND AND THE
RESULTS PLOTTED; THESE WILL INDICATE IF
YOUR PROPELLER'S PITCH IS CORRECT OR IF
IT SHOULD BE INCREASED OR DECREASED

THEN LETS CHECK AGAIN THE ENGINÈS
OWNER MANUAL, WHERE WE WILL FIND
THE MAXIMUM HORSEPOWER OUTPUT
AND THE HP/RPM RATIO. LETS, NOW, FIND
THE BEST HP/RPM RATIO. WE WILL ASSUME
OUR ENGINE WILL DELIVER THE
MAXIMUM HORSEPOWER OUTPUT AT 3600
RPM, AND THAT A 2 HP POWER INCREASE
IS ATTAINED FOR EVERY 500 RPM TILL 2800
RPM, THEN 1.5 HP TILL 3200 RPM AND THEN
1 HP TILL 3600 RPM. THE BEST HP/RPM
RATIO IS AT 2800 RPM

We know that cruise engine speed is 20% less
than its maximum speed (3600 rpm): the closest
we go to this value the better is our propeller pitch.
For instance, if our engine has its maximum
efficiency at 2800 rpm and its maximum full ahead
rpm are respectively 3500 in calm sea and 3300 in
rough sea, than our pitch is correct (3500 rpm
minus 20% equals to 2800 rpm). This is true if our
test result confirm that the engine has not been
overloaded in the 0 to 2800 rpm range, otherwise
the pitch has to be reduced

Design System
Propellers are designed with the most suitable method
satisfy the needs of each ship operation.
The methods include applying conventional planning
methods based on systematic model-testing of the series
of propellers, utilizing various databases, and analyzing
the propeller's efficiency and characteristics computed by
propeller theoretical calculation. In particular, the
Propeller Characteristic Analysis Method, employing Non-
linear Lifting Surface Theory supported by the Vortex
Lattice Method (VLM), estimates propeller characteristics
with pure logic based on the difference of blade profile
and blade section, and the variation of working condition
Thus, we immediately can obtain the effect of propeller
characteristic, and its performance according to different
environments.All the information thus gained is applied
to our designing work to pursue efficiency

Analytical System
Using various analytical software programs including
the Finite Element Method, Kamome Propellers
undergo strength analysis if need be, to establish the
efficiency, characteristics, and strength at the most
suitable states.We also use three-dimensional CAD to
examine the best form of section and in establishing
numerical data of the section and utilize the
collected information for development of product
with precise quality.

Manufacturing System
KAMOME'S CAM (BLADE
PROCESSING SYSTEM) IS
INTEGRATED WITH A CAD SYSTEM.
TWO INSTALLATIONS OF
SIMULTANEOUS FIVE-AXES NC
BLADE MILLING MACHINE THAT
PROCESS CPPAND FPP
RESPECTIVELY TO THE MOST
SUITABLE STATE CAN PROVIDE
ACCURATE PROCESSING. NOT
LIMITING THE PROCESSING, THE
PROPELLER'S OPTIMUM FORM
DECISION CAN BE VERY
FLEXIBLE.ALL MANUUFACTURING
DATA IS STORED IN OUR DATABASE,
AND BECOME AVAILABLE AT THE
TIME OF REPRODUCTION

Advantages of CPP
• A CPP can operate with minimum or negligible loss in
power. This helps to improve maneuverability of the
vessel.
• The direction from ahead to astern can be changed in a
matter of few minutes or even seconds depending on the
condition of the load of the ship.
• This not only helps to absorb all the power generated by
the engine but also helps to prevent wastage of fuel. In
some CPP the direction of the thrust can be changed
within 15-40 seconds.
• A CPP can also be connect to a shaft generator.
• It can be used for a wide range of rotational speed.

Disadvantages of CPP
• The main disadvantage of CPP is that it is
a highly complex system
• It is vulnerable due to numerous hydraulic
components and sealing rings. As the
sealing rings are outside the ship, damage
of a single sealing ring can result in oil
pollution.
• As the system is complex, repairs and
maintenance is difficult.

Propeller Model Test
A test on a model propeller is run either in a towing tank or a
running flow in a water tunnel (cavitation tunnel) without a model
hull in front of it, which is called “open water” tests.
1) VA – velo.of flow
2.) n - rotation of
motor
3.) po - pressure can
be controlled
Measure VA , Q, T,
and n.

KQ
KT
Testing results
0

A
V
J
nD

Slip ratio 1 , Pitch ratio , section types & # of blades.
A
V P
nP D
  
2 4 2 5
Trust coeff. , Toeque coeff. ,
.
2 2
T Q
A T
o
Q
T Q
K K
n D n D
TV K J
nQ K
 

 
 
 
Open - water efficient

Purpose of open-water tests
• It is usually to carry out open water tests on standard series of
propellers. Their features (such as # of blades, blade outline
shape, blade area ratio, blade section shape, blade thickness
fraction, boss diameter & pitch-diameter ratio) are systematically
varied. The result data are summarized in a set of particular
diagrams, which can be used for design purposes. We will study
how to use these diagrams later for designing a propeller.
•Studying the efficiency of a propeller and find a propeller with
better efficiency
•Studying the extent and development of cavitations over a
propeller.

So far in the study of the resistance of a ship & its propeller the
two have been considered separately. However, in reality the
propeller has to work behind the ship & in consequence one has an
interaction upon the other. How does the hull affects the water
in which the propeller is working? (later we will also study the
effects of a propeller on the hull)
A ship affects the water near its stern in 3 aspects:
1) pressure increase at the stern;
2) boundary layer (a propeller is in the boundary layer or way
of the ship);
3) Water particle velocity induced by ship generated waves.
Interaction between a hull & a propeller

Wake fraction: water particle velocity near the propeller is
not the same as the ship velocity.
 
( :ship velocity & flow velocity at its stern)
: , thus
1
: , thus (1 )
The relationship between Froude & Tay
s A s A
s A s
F A
A F
s A
T A s T
s
w V V V V
V V V
w V
V w
V V
w V V w
V
 

 


  
Froude
Tay
wake factor
wake factor
lor
lor wake factor:
or
1 1
When wake (most cases, a single screw)
When , wake (only for high speed ship)
F T
T F
F T
A s
A s
w w
w w
w w
V V
V V
 
 


positive
nagative

wT & wF, (wake factors) are determined by the measurements
made in a model test (near a hull’s stern) or in a real ship test.
Nominal wake: wake measured near the stern of a hull in the
absence of the propeller (using pilot tubes).
Effective wake: wake measured in the presence of propeller.
The measurements show that a propeller at a rotating speed n
behind a hull advancing at velocity, Vs, delivers thrust T. By
comparing it to the results of the same propeller in the open-water
tests, we will find that at the same revolutions n, the propeller will
develop the thrust T but at a different speed (usually lower),
known as effective speed of advance, VA. The difference between
Vs & VA is considered as the effective wake.
•Relation between nominal wake & effective wake.
Since propellers induce an inflow velocity which reduces the
positive wake to some extent, the effective wake factor usually is
0.03~0.04 lower than the corresponding nominal wake.

Wake factor of a
single screw ship
Averaged Wake Fraction

Wake factor of a
twin screw ship

• Definition of Power
Indicated horsepower (PI): is measured in the cylinders (Steam
reciprocating engines) by means of an instrument (an
“indicator”) which continuously records the gas or steam
pressure throughout the length of the piston travel.
pm - mean effective pressure (psi)
L – Length of piston stroke (ft)
n – number of working strokes per second
A – effective piston area (in2)
n – number of cylinders
/550
I m
P p L A n
   

Brake Horsepower (PB): is the power measured at the crankshaft
coupling by means of a mechanical hydraulic or electrical brake.
where Q – brake torque (lb-ft) & n – revolutions per second.
Shaft horsepower (PS): is the power transmitted through the shaft
to the propeller. It is usually measured aboard ship as close to the
propeller as possible by means of a torsion meter .
where dS – shaft diameter (in), G – shear modulus of elasticity of
shaft material (psi), θ – measured angle of twist (degree),
LS – length of shaft over which θ is measured & n – revolution per
second
2 /550
B
P nQ


 
4
13,033
S
S
S
d G n
P
bL



Delivered horsepower (PD): the power delivered to the propeller.
Thrust horsepower (PT):
T – Thrust delivered by propeller (lb)
VA – advance velocity of propeller (ft/s)
Effective horsepower (PE , or EHP):
RT – total resistance (lb)
Vs – advance velocity of ship (ft/s)
/550
T A
P T V
 
/550
E T s
P R V
 

• Propulsion Efficiency
Total propulsion efficiency
can also be replaced by or
A more meaningful measure of hydrodynamic performance
of a propeller is: a quasi-propulsive coefficient,
,
, where is the shaft
E
T S B I
S
D
E
D
D
D
S S
S
P
P P P
P
P
P
P
P



 


 transmission efficiency
and thus, .
- 98% for ships with main engine aft
- 97% for ships with main engine amidship
- smaller if a gear box is used.
T D S
S
  



• Relative Rotation Efficiency
The efficiency of a propeller in open water is called open-water
efficiency,
where VA is the advance speed, T the thrust, n the rotation speed
(# of rotations per unit time), & Q0 is the torque measured in the
open water test when the propeller is delivering thrust T at the
rotation speed n.
In the case the same propeller behind a hull, at the same advance
speed it delivers the same thrust T at the same revolution n but
needs torque Q. In general, Q is difference from Q0. Then, the
efficiency of the propeller behind the hull,
0
0
2
A
T V
nQ




2
A
B
T V
nQ





• The ratio of behind-hull efficiency to open-water efficiency is
called the relative rotative efficiency.
The difference between Q0 and Q is due to
1. wake is not uniform over the disc area while in open water, the
advance speed is uniform.
2. model and prototype propellers have different turbulent flow.
(Remember then Reynolds number are not the same)
1.0~1.1 for single-screw ship
0.95~1.0 for twin-screw ship
0
0
0
, thus
B
R B R
Q
Q

   

  
R


• The influence of the propeller on the hull
Thrust-deduction factor (fraction)
When a hull is towed, there is an area of high pressure over the
stern, which has a resultant forward component to reduce the total
resistance. With a self-propelled hull (in the presence of the
propeller), the pressure at the stern is decreased due to the
propeller action. Therefore, there is a resistance augment due to
the presence of the propeller. If T is the trust of the propeller & RT
is the towing resistance of a hull at a given speed Vs , then in order
that the propeller propel the hull at this speed, T must be greater
than RT because of the resistant augment. The normalized
difference between T and RT, is called the thrust-deduction
Fraction, and denoted by t.

 
1 , thus 1
- is the "naked" hull resistance
- the thrust after subtracting the resistance of the rudder & other
stern appendages.
measured in experiments depends, not
T T
T
T
T R R
t R t T
T T
R
T
t

    
only on the shape of the hull
& the characteristics of the propeller, but also the type of the rudder.

• Hull Efficiency
Hull Efficiency is defined as the ratio of the effective power for
a hull with appendages to the thrust power developed by
propellers.
1
1
where
- effective horsepower EHP
- "naked" hull resistance
- speed of the ship
- the work done by the propeller in delivering a thrust
- the speed of
T s
E
H
T A
E T s
T
s
T
A
R V
P t
P T V w
P R V
R
V
P T
V

 
  
 
 
the propeller w.r.t. the ambient water.

• Propulsive Efficiency (QPC)
“Quasi-propulsion” coefficient is defined as the ratio of the
effective horsepower to the delivery horsepower.
0
0
0
2 2
- delivered horsepower 2
- efficiency of a propeller in open water,
- relative rotative efficiency,
- hull efficienc
T s T s
E A
D B H R H
D A
E T s
D
D R H
R
H
R V R V
P TV
P nQ nQ TV
P R V
P nQ
     
 

   



        
 

  
y.

Propulsive coefficients
The propulsive coefficients of the ship
performance form the essential link between
the effective power required to drive the
vessel, obtained from the product of
resistance and ship speed, and the power
delivered from the engine to the propeller.

• The power absorbed by and delivered to
the propeller PD in order to drive the ship
at a given speed VS is
• where n and Q are the rotational speed
and torque at the propeller.
• the torque required to drive the propeller
Q can be expressed for a propeller
working behind the vessel as
• where KQb is the torque coefficient of the
propeller when working in the wake field
behind the vessel at a mean advance
coefficient J

• From above the delivered power can be
expressed as
• If the propeller were operating in open water
at the same mean advance coefficient J the
open water torque coefficient KQo would be
found to vary slightly from that measured
behind the ship model.
• As such the ratio KQo/KQb is known as the
relative rotative efficiency ηr

• Hence, delivered power PD can then be
expressed in terms of the relative rotative
efficiency as
• Now the effective power PE is defined as
• where the QPC is termed the quasi-
propulsive coefficient

• QPC can be expressed in terms effective
power as
• Now the resistance of the vessel R can be
expressed in terms of the propeller thrust
T as R = T(1 - t), where t is the thrust
deduction factor
• the ship speed Vs can be defined in terms
of the mean speed of advance Va as Va =
Vs(1 - wt), where wt is the mean Taylor
wake fraction.

• Since the open water thrust coefficient KTo
is expressed as
• with To being the open water propeller
thrust at the mean advance coefficient J.
• the QPC can be expressed from the above
as

• Open water efficiency of propeller is
• The quantity (1 - t)/(1 - wt) is termed the
hull efficiency ηh and hence the QPC is
defined as
• in terms of the effective and delivered
powers,

The division of the quasi-propulsive coefficient into three parts is
helpful in 1) understanding the propulsive problem & 2) in
making estimates of propulsive efficiency for design purposes.
( )
In the design, usually we let
(1 )
( ) ,
where is a correlation allowance, (or load factor). It depends
principally on the hull roughness of the newly
T
D
H R o H R o
D D H R o
D
R V
EHP
P DHP
EHP
P DHP
     

   


 

 
painted ship,
foaling, weather condition & the length and type of a ship.
Finally, the ,
where is the shaft efficiency.
s
s
DHP
SHP



main engine horsepower

Cavitation
Pressure (+)
Suction (-)
Back VR
face
As the pressure on the back of a propeller falls lower and lower
with the increase in a propeller’s n, the absolute pressure at the
back of the propeller will eventually become low enough for the
water to vaporize and local cavities form. This phenomenon is
known as cavitation. ( , vapor pressure of water)
v
P
A typical pressure distribution in a blade element is shown below,

• This term is primarily used in conjunction with propellers and rudders.
• Often confused with ventilation, cavitation is the phenomenon of
water vaporizing or boiling due to the extreme decrease in pressure
on the forward, or, suction side of the propeller blade.
• Partial cavitation is normal on most propellers but excessive
cavitation can result damage to the propeller’s blade surface.
• Cavitation can be caused by nicks in the leading edge of the blade,
bent blades, too much cup, sharp corners at the leading edge,
incorrect matching of propeller to the vessel and engine or propeller
imbalance.
• It is usually measured in terms of a non dimensional cavitation
number and can be reduced by an adjustment of blade area and/or
pitch distribution.
• When cavitating, the propeller will speed up but power is lost and/or
the rudder may lose steering action.
• Cavitation often occurs when turning and results from a loss of a
constant solid water flow.
• Power catamarans usually require deflectors when a single motor is
used, to direct a flow of water to the propeller.

Development
of cavitations
of a propeller
in a
cavitation
tunnel

• The initial cause of the low pressure may be nicks in the leading edge, too
much cup, sharp leading edge comers, improper polishing, or, sometimes,
poor blade design.
• Massive cavitation by itself is rare, and it usually is caused by a propeller
that is severely bent or has had its blade tips broken off resulting in a
propeller that is far too small in diameter for the engine.
• A sharp leading edge produces cavitation and resulting cavitation burn as
the bubbles condense further back on the blade face. Such cavitation burn
can usually be corrected by repairing or rounding off the leading edge
directly in front of the burn. Cavitation and cavitation burns can also form
on the side of your gearcase. This will almost always be the result of a
sharp edge directly ahead of the burn. Rounding off the sharp edge will
usually eliminate the problem.

• Cavitation on a propeller will
1. lower the thrust of the propeller, & thus decrease its
efficiency,
2. cause vibration of hull & the propeller and generate
uncomfortable noise, &
3. cause erosion of the propeller blade.
• Criteria for prevention of cavitation
Mean thrust loading coefficient
2
1
2
c
R p
T
V A



 
2
2 2
- density of water, - Thrust,
- project blade area, 1.067 0.229 ,
- the relative velocity at 0.7 of a propeller
2 0.7
p
p
D
R
R A
T
A P
A
A D
V R
V V R n


 
   

• Cavitation number
0
2
1
2
0 - presuure at some point of a blade
- vapor presuure of water
v
R
v
p p
V
p
p




The cavitation is most likely to occur at the tips of blades where
the relative velocity is the largest and the hydro-static pressure is
the lowest when blades rotate to the highest position. It can also
occur near the roots where blades join the boss of a propeller
because the attack angle is the largest.

Ventilation
• Sometimes the term cavitation is used when in reality
ventilation or air drawing is actually occurring. Ventilation is
air being drawn down from the water surface or the
introduction of exhaust gases into the propeller blades both
of which cause the propeller to race and lose thrust.
• Ventilation can be useful in the bottom end acceleration by
allowing the propeller to slip a regulated amount, allowing the
engine to revolve higher during initial acceleration. It is
usually achieved by ventilation holes at the root of each
blade or the use of an over hub design.
• Ventilation is for through hub exhaust propellers only.
• Causes of ventilation include excessively tight cornering,
over trimming of the engine and mounting an outboard motor
too high on the transom.

Propeller Design
Methods of Propeller Design
a. Design based upon charts (diagrams). These charts are obtained
form the results of open-water test on a series of model
propellers. (also upon software, such as NavCad).
b. Design using circulation theory and CFD (not studied here).
Methodical Series
A model propeller series is a set of propellers in which the principal
characteristics such as pitch ratio etc are changed in a systematic
manner. There are many series tested, and their results are
summarized and presented in the form of charts which can be used
in design. The most extensive model propeller series is Netherland
Ship Model Basin (NSMB) at Wageningen. This series test was run
from 1937 to 1964.

NSMB Series include
Series A: narrow blade tips, airfoil sections, high efficiency
only for light loaded propellers (not widely used)
Series B: wider tips, airfoil section from blade root to 0.7
radius, and circular back from 0.8 radius to tip.
Scope of series B is shown

Given below is the dimensions (outline, thickness) of
B.4 blade

The B series results are presented in the form of charts of
diagrams, known as diagram .
At upper right corner, the diagram gives 4.40 B. (indicating B
type, 4 blades & AE /A0 = 0.40, t0/D = 0.0045 (blade-thickness
fraction), d/D = 0.167 (diameter ratio of the boss to the
propeller), & the Pitch, P.
At low left corner, it gives the definitions of
P
B 

and
P
B 
 
0.5
2.5
, and (notice that )
- revolutions per min, - propeller diameter (ft)
- delivered at propeller
(1- ) - speed of advance (knots)
and are
D A
p
A A
D
A s
p
n P V
nD
B J
V V nD
n D
P
V V w
B


  

horsepower
dime !
nsional

diagram
Horizontal coordinate:
Vertical coordinate: ratio of the pitch to diameter P/D
Two sets of curves , and one optimal ( ) line
P
B 

0
&
 
P
B
0


• Propeller Design Based on Charts
-The information required for making a propeller design from
charts are:
1. Principal dimensions, & main coefficients of a ship used to
estimate wake, thrust factors, & relative rotative efficiency.
2. Speed of a ship
3. EHP (from model tests or estimated from other available data)
4. engine power (SHP) & rpm.
5. restrictions on the maximum diameter of propeller.
0.5
0.5
D
-Design Procedures
( )
1. Calculating , (assuming , for computing )
From the chart to find , pitch ratio that give the best efficiency.
(From , & pitch ratio )
D E
p D D
A
n P P
B P
V
D P




 
 

2. This will give a best propeller in open water. Since the
propeller works behind the hull, it is usually to reduce by
5%~8%, for single-screw ship, 4% for twin screw ship.
3.With the same value p
D
B
0
a smaller value ( ), use
the chart again to find efficiency and pitch ratio ( / ).
4. In the same way, we may use different chart & different
to see the effects (no. of blades, blade area r
A
nD
V
P D
n
 


0
0
0
atio) on .
5. After determining , we calculate (propulsive coeff).
1
where . Then we re-calculate ( ) .
1
D
E
D R H H D
D
P
t
P
w

 
    


  


6. If the newly computed, , is very close to the previous
assumed one, then we continue to examining the cavitation
of the propeller. If not, we use the newly computed to
repeat the above 1-5 ste
D
D
P
P
ps again.
7. Examining the condition of cavitation for the propellers.
If the condition is not satisfied, choose a propeller with larger
, or make other adjustments (such as reducing , & using
mul
E
A n
tiple screws).

Examples
Example a, Using the B4.40 chart to design a propeller suitable for
the following conditions. Also determine SHP. (knowing EHP, Vs to
determine , P, D)
Vs = 16 knots Taylor wake factor w = 0.3075
EHP = 5000 Hp thrust deduction t = 0.186
Allowance for appendage 6% Shaft loss = 3%
Allowance for weather 15% reduction in δ = 7%
n = 120 r/min relative rotative effi. 1.0
R
 
0

 
   
 
0.5 0.5
2.5
2.5
: EHP(1 ) EHP(1 0.06 0.15) 6050 hp
Assuming 0.65, (DHP) EHP / 9308 hp
Advance velocity 1 11.08 knots
120 9308
Taylor propeller coeff., 28.33
11.08
D D D
A s
D
p
A
P
V w V
n P
B
V

 
    
  
  

  
Solution

0
Checking B4.40 chart, 213, 213(1 0.07) 198,
1 0.814
0.597, 0.597 0.597 0.705.
1 0.6925
The previous is assumed to low. New iteration starts.
Let 0.71, EHP / 8521 hp,
opt
o D H R
D
D D D p
t
w
P B
 
    

 
   

        

   
0
27.1,
From B4.40 chart, 209, 209(1 0.07) 194.4,
0.814
0.602, 0.602 0.708 0.71
0.6925
This time the assumed is very close to the comupted one.
194.4 11.08
, 17.9 ft
120
opt
o D H R
D
A
A
V
nD
D
V n
 
    



   
       

   
 
, 0.85,
DHP 8521
0.85 17.9 15.2 ft, SHP 8784 hp
1 0.03
s
P
D
P


     


Example b. Give D (due to the restriction of draft) & using
B.4.40 chart to find the optimum n, P/D, and
A cargo Ship
L = 86 m Vs = 9 knots
B = 13 m EHP = 515 hp
T = 5.66 m w = 0.184
= 4500 m3 t = 0.125
= 1.0 = 0.97
D = 4m = 13.14 ft χ = 0.218 (load factor or allowance)

R
 s

D

   
 
1. 1 9 1 0.184 7.34 knots,
2. Assuming 0.69,
1 EHP
3. (DHP) 909 hp,
A s
D
D
D
V V w
P



     


 
Solution :

4. Try a range of rotation velocities, n
0.5
2.5
( )
D
p
A
N P
B
V

A
ND
V
 
1
1
D o R
t
w
  



No. Name Unit Value
1 n rpm 90 95 100 105 110
2 18.6 19.6 20.7 21.7 22.7
3 161 170 179 188 197
4 % 64.5 64.6 64.7 64.3 63.8
P/D 0.95 0.875 0.79 0.75 0.70
5 P = P/D*D m 3.8 3.5 3.16 3 2.8
6 0.691 0.692 0.693 0.69 0.688
0
From the chart 

Based on the results shown in the table, it is found that
the highest value is 0.693 when 100,
and it is also closest to the assumed .
Thus, 100 is the optimal rotation speed.
Pitch. = 3.16
D
D
n
n
P




m = 10.37',
(DHP) 909
SHP 937 hp.
0.97
D
s
P

  
A different problem: given the rotation velocity, n, to determine
the optimal diameter of the propeller.

propellers and propulsion.pptx

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