2. 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.
Propulsive Devices
7. Water Jet: 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:
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 (CPP): 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.
9. Water Jet propulsion
• Advantages
– Maneuvering ability
– Eliminates the need for reversing gear
– Quietness and reduction in external appendage drag
• Disadvantages
– Loss of volume inside the ship due to ducting and impeller,
or conversely the increase in size of the ship to restore the
displacement to its original value,
– Danger of fouling of the impeller by debris or weeds
– Difficulty of obtaining uniform flow into the impeller
when the water comes from the boundary layer around
bends in the ducts.
11. Pump Jet
• Impeller is external to the hull, taking the form of
a rotating impeller with fixed guide vanes either
ahead of it or astern, or both.
• Whole unit is enclosed in a duct or long shroud
ring.
• Duct diameter increases from the entrance up to
the impeller, so that the velocity falls and the
pressure increases: higher efficiency
• Incidence of cavitation and noise is delayed
13. Ducted or Nozzle propeller
• Accelerating flow type of nozzle
– Used extensively in cases where the ship screw is heavily
loaded or where the screw is limited in diameter.
– Increases efficiency
– Nozzle itself produces a positive thrust
• Decelerating flow type of nozzle
– Nozzle is used to increase the static pressure at the impeller.
– The duct will produce a negative thrust.
– May be used if retardation of propeller cavitation is desired.
– Reduction in noise level can be obtained
15. Vertical-axis propellers
• Kirsten-Boeing type:
– Blades so interlocked by gears that each blade is
constrained to make a half-revolution about its axis
for each revolution of the whole propeller.
– The propeller is assumed to be advancing from left
to right with uniform velocity VO and rotating with
uniform angular velocity in the
counterclockwise direction.
16. • With the blades set
as shown here, the
resultant velocity is
as shown and the
normal forces N
furnish a thrust T
for going ahead.
17. • Blade forces oppose
the motion of the ship
• Resultant thrust acts at
right angles to the motion
of the ship.
23. Water Jet Propulsion
Impeller or pump inside the hull draws water from outside,
imposes on it an acceleration, and discharges it astern as a jet at a
higher velocity.
24. Water Jet propulsion
• Advantages
– Maneuvering ability
– Eliminates the need for reversing gear
– Quietness and reduction in external appendage drag
• Disadvantages
– Loss of volume inside the ship due to ducting and impeller,
or conversely the increase in size of the ship to restore the
displacement to its original value,
– Danger of fouling of the impeller by debris or weeds
– Difficulty of obtaining uniform flow into the impeller
when the water comes from the boundary layer around
bends in the ducts.
26. Phenomenon
• Vapour pressure: At which water begins to vaporize
at particular temperature
• Cavitation: Vaporization of water forming cavities
filled with water vapour
27.
28. Phenomenon
• Propeller thrust is due to pressure difference between
face & back
• Pressure on the back falls lower with the increase in a
propeller RPM.
• If absolute pressure on back of blade becomes low
enough, water vaporizes and local cavities form. This
phenomenon is known as cavitation.
• Impurities in water may cause cavitation at higher
pressure (around 17 kPa) than pv (1.7 kPa for fresh
water)
• Manifested by ‘racing’ of propellers: increase in rpm
doesn’t increase speed of vessel
29. Condition for Cavitation
• Minimum value of pressure coefficient (CP)
should be equal to the local cavitation number (σ)
0
2
1
2
0 - total static pressure at propeller axis
- vapor pressure of water
v
R
v
p p
V
p
p
