Chapter VI. Propulsion of Ships

The propulsion system of a ship is to provide the thrust to the ship to overcome the resistance.

6.1 Introduction• Propulsive Devices (reading p205-209) 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.

Paddle Wheels Propulsion (Stern)

Paddle Wheels Propulsion (Midship)

Propeller (5-blade)

Propeller (5-blade) & Rudder

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.

Jet Propulsion

Nozzle Propellers

Vertical-Axis Propellers

Vertical-Axis Propellers

Controllable Pitch Propellers (CPP)

Contra-rotating Propellers

• 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 space3.) the output of power per cylinder is limited4.) fuel consumption is high

2. Steam TurbineAdvantages: 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 – turbineAdvantages 1.) do not need boiler, fuel weight is very small2.) operate full load for very long time (submarine)Disadvantages 1.) weight of reactor and protection shield are heavy;

2) Environment problem, potential pollution.

• 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 secondA – effective piston area (in2)n – number of cylinders

/ 550I mP 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 / 550BP nQ

4

13,033S

SS

d G nP

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)

/ 550T AP T V

/ 550E T sP 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

ET S B I

S

D

ED

D

DS S

S

PP 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

6.2 Propeller Geometry and Terminology

Boss

Back

Hubcap

Face

Number of Blades: 2, 3, 4, 5 ,6BossHubcapShaft

• 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.

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 1tan or tan2 2

Pitch ratio: tan

P P

r r

P PRPR

D

P

o

A

2 r

p180

Developed Area AD

Expended Area AE

Boss: (aka, Hub)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.

• 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

P181 figure 10.5

• Rake (a blade is perpendicular or titled w.r.t the boss )

• Skew (the skewness of a blade w.r.t. the center line)

• 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

PPR

D

20 / 4A D

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.

DVA

VA(1+b)VA(1+a)

• Momentum ConservationForce = net momentum flux (horizontal)

0

20

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

222

22

0 00

1

2 2

2 1, , 2

2 2

1 1 or 2 2

AA

AA

V bV P

g g

b b VTA P T T A b b V

A g

b ba a

0

0

0

20

21 102 2

Efficiency of a propeller

(no friction & no rotationary velo. considered)

1

1 1

1 1

1 1 / 2Defining the thrust loading coeff., , as

1

AA AI

A

I

T

AT

A

V ATV TVTQQ P a V A a

A

a bC

A V a bTC

V A

Ideal

20

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

TI

T

T

T

a aV A

Ca

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

21 ' 1

2where is the rotation velocity of flow after the propeller,

& is the rotation velocity of the propeller.

I

aa

a

a

• Blade Element TheoryIn 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 ' .

AV a

r a

aVA

αα’

1 'r a 'a r

rV r

AV

a & a’ are determined by experiments

6.4 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 4 212

212

2 4

Using D.A, the non-dimensinal formula is given by,

, , ,

Froude #: , Euler #: , Reynolds #:

: , :

The

A A A

A

A A

A

AT

V V V DT pf

n D nD VgD

V V Dp

VgD

V TJ 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

QK

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 To

Q Q

TV K V KJ

nQ K nD K

geometrically similar

12

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 21 12 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 Am 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 sm s m s

A s

m s m om m w

om

H

V pp p

V

p p p p H

p

a

vacuum (cavitation) tunnel.

1For 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.

6.5 Propeller Model TestA 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 flow2.) n - rotation of motor3.) po - pressure can be controlled

Measure VA , Q, T, and n.

Development of cavitations of a propeller

in a cavitation

tunnel

KQ

KT

Testing results

0

AVJ

nD

Slip ratio 1 , Pitch ratio , section types & # of blades.AV P

nP D

2 4 2 5Trust coeff. , Toeque coeff. ,

.2 2

T Q

A To

Q

T QK 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.