2. Ship Drive Train and Power
Ship Drive Train System
EHP
Engine Reduction
Gear Strut Screw
Bearing Seals
THP
BHP SHP DHP
3. EHP
Engine
Strut
Reduction Screw
Gear Bearing Seals
THP
DHP
BHP
SHP
Brake Horsepower (BHP)
- Power output at the shaft coming out of the engine
before
the reduction gears
4. EHP
Engine
Strut
Reduction Screw
Bearing
Gear Seals
THP
BHP DHP
SHP
Shaft Horsepower (SHP)
- Power output at the shaft coming out of the reduction gears
5. EHP
Engine
Strut
Reduction Screw
Gear Bearing Seals
THP
BHP
DHP
SHP
Delivered Horsepower (DHP)
- Power delivered to the propeller
- DHP=SHP – losses in shafting, shaft bearings and seals
6. EHP
Engine
Strut
Reduction Screw
Gear Bearing Seals
THP
BHP DHP
SHP
Thrust Horsepower (THP)
- Power created by the screw/propeller
- THP=DHP – Propeller losses
- THP is the end result of all HP losses along the drive train
12. Basic Nomenclature:
• 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.
• Blades 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.
• Diameter The diameter (or radius) is a crucial geometric parameter in determining the
• amount of power that a propeller can absorb and deliver, and thus dictating the amount of
• thrust available for propulsion. With the exception of high speed (35 Knots+) vehicles
• the diameter is proportional to propeller efficiency (ie. Higher diameter equates to higher
• efficiency). In high speed vessels, however, larger diameter equates to high drag. For
• typical vessels a small increase in diameter translates into a dramatic increase in thrust
• and torque load on the engine shaft, thus the larger the diameter the slower the propeller
• will turn, limited by structural loading and engine rating.
13. Basic Nomenclature:
• Revolutions per Minute (RPMs) RPM is the number of full turns or
rotations of a propeller in one minute. RPM is often designated by
the variable N. High values of RPM are typically not efficient except
on high speed vessels. For vessels operating under 35Knots speed,
it is usual practice to reduce RPM, and increase diameter, to obtain
higher torque from a reasonably sized power plant. Achieving low
RPM from a typical engine usually requires a reduction gearbox.
• Pitch The pitch of a propeller is defined similarly to that of a wood or
machine screw. It indicates the distance the propeller would “drive
forward” for each full rotation. If a propeller moves forward 10inches
for every complete turn it has a 10inch nominal pitch. 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.
14. Screw Propeller
• Variable Pitch (the standard prop):
- The pitch varies at the radial distance from the hub.
- Improves the propeller efficiency.
- Blade may be designed to be adjusted to a different
pitch setting when propeller is stopped.
• Controllable Pitch :
- The position of the blades relative to the hub can be
changed while the propeller is rotating.
- This will improve the control and ship handling.
- Expensive and difficult to design and build
16. Suction Face
Leading Edge
Trailing Edge
Pressure Face
17. Propeller Walk
• Due to a difference in the pressure at
the top and bottom of the prop (due to
boundary layer), the lower part of the
prop works harder.
• This leads to a slight turning moment.
• Right hand props cause turns to port
when moving ahead.
20. Skewed Screw Propeller
Highly Skewed Propeller
Advantages
- Reduce interaction between
propeller and rudder wake.
- Reduce vibration and noise
Disadvantages
- Expensive
- Less efficient operating in
reverse
DDG51
21. Propeller Theory
Propeller Theory
• Speed of Advance Vwater= 0
P
Wake Region
VS VW
Q
Vwater= VS
• The ship drags the surrounding water so that the wake to
follow the ship with a wake speed (Vw) is generated in the
stern.
• The flow speed at the propeller is,
VA = VS − VW Speed of Advance
22. Propeller Cavitation
• Cavitation : Definition
- The formation and subsequent collapse of vapor bubbles
on propeller blades where pressure has fallen below the
vapor pressure of water.
- Bernoulli’s Equation can be used to predict pressure.
- Cavitation occurs on propellers (or rudders) that are
heavily loaded, or are experiencing a high thrust loading
coefficient.
25. Blade Tip Cavitation Navy Model Propeller 5236
Flow velocities at the
tip are fastest so that
pressure drop occurs at
the tip first.
Sheet Cavitation
Large and stable region
of cavitation covering the
suction face of propeller.
26. Propeller Cavitation
Consequences of Cavitation
1) Low propeller efficiency (Thrust reduction)
2) Propeller erosion (mechanical erosion as bubbles
collapse, up to 180 ton/in² pressure)
3) Vibration due to uneven loading
4) Cavitation noise due to impulsion by the bubble
collapse
27. Propeller Cavitation
• Preventing Cavitation
- Remove fouling, nicks and scratch.
- Increase or decrease the engine RPM smoothly to avoid
an abrupt change in thrust.
rapid change of rpm ⇒ high propeller thrust but small
change in VA ⇒ larger CT ⇒ cavitation &
low propeller efficiency
- Keep appropriate pitch setting for controllable pitch
propeller
- For submarines, diving to deeper depths will delay or
prevent cavitation as hydrostatic pressure increases.
28. Propeller Cavitation
• Ventilation
- If a propeller or rudder operates too close to the water
surface, surface air or exhaust gases are drawn into the
propeller blade due to the localized low pressure around
propeller. The prop “digs a hole” in the water.
- The load on the propeller is reduced by the mixing of
air or exhaust gases into the water causing effects
similar to those for cavitation.
-Ventilation often occurs in ships in a very light
condition(small draft), in rough seas, or during hard
turns.
29. Other forms of propulsion
A one horsepower cable-drawn ferry!
31. Ship rudder
• rudder is the most important ship control
surface
• a fin-like projection under the counter and
below the waterline – placed as far aft as
practical
• mounted onto a circular shaft referred to
as the stock – penetrates the hull through
bearings
34. Area and shape of rudders
• no fixed rule for determination of the size
• in practice, rudder area, expressed as a
fraction of the product of the length and
draft or centerline plane area, often
selected by comparison with a ship with
similar maneuverability requirements
• (Rudder Area)Cargo Ships = 0.017 * LOA * T
36. Area and shape of rudders -
types
• rudder consists of two parts: the blade (flat part) against which the
water pressure acts and the stock (shaft) which transmits motion of
the steering gear to the blade
• there are 3 types of rudders:
balanced: a portion of the blade area is disposed symmetrically through
the rudder height and fwd of stock
unbalanced: blade is entirely aft of stock
semi-balanced: area fwd of stock does not extend to the full height of
the blade aft of the stock – upper portion may be considered
unbalanced and the lower portion, balanced
37. Area and shape of rudders
Modern double-plate,
semi-balanced rudder
in a single screw ship
