Title: Chapter VI. Propulsion of Ships The propulsion system of a ship is to provide the thrust to the ship to overcome the resistance.
1Chapter VI. Propulsion of ShipsThe propulsion
system of a ship is to provide the thrust to the
ship to overcome the resistance.
2- 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.
3Paddle Wheels Propulsion (Stern)
4Paddle Wheels Propulsion (Midship)
5 Propeller (5-blade)
6 Propeller (5-blade) Rudder
7- Jet type Water is drawn by a pump delivered
sternwards as a - jet at a high velocity. The reaction providing
the thrust. Its use - has been restricted to special types of ships.
- Other propulsion Devices
- Nozzles (Duct) Propellers main purpose is to
increase the thrust at low ship speed (tug, large
oil tanker) - Vertical-Axis Propellers Advantage is to control
the direction of thrust. Therefore, the ship has
good maneuverability. - Controllable-Pitch Propellers (CCP) The pitch of
screw can be changed so that it will satisfy all
working conditions. - 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.
8 Jet Propulsion
9 Nozzle Propellers
10 Vertical-Axis Propellers
11 Vertical-Axis Propellers
12Controllable Pitch Propellers (CPP)
13 Contra-rotating Propellers
14- Type of Ship Machinery
- 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
153. 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 gt30,000 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.
16- 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
17Brake 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), ? measured angle of twist
(degree), LS length of shaft over which ? is
measured n revolution per second
18Delivered 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)
19- Propulsion Efficiency
- Total propulsion efficiency
206.2 Propeller Geometry and Terminology
Number of Blades 2, 3, 4, 5 ,6 Boss Hubcap Shaft
21- 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.
22Pitch P when the line AB makes one complete
revolution and arrives at AB. 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
23p180
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25Expended Area AE
Developed Area AD
26Boss (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.
27- 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
28- 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
29- 6.3 Theory of Propeller Action
- Assumptions
- 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.
VA(1a)
VA(1b)
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31- Momentum Conservation
- Force net momentum flux (horizontal)
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33- Extension of momentum theory
- Consider the rotation of the flow passing through
the propeller disc., the reduced ideal efficiency
becomes,
34- 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.
35a a are determined by experiments
366.4 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),
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416.5 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.
42Development of cavitations of a propeller in a
cavitation tunnel
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46- 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.