Chap 7 Resistance and Powering of Ship. Recall Static Equilibrium! What are the forces in the x-axis of the ship? Resistance or Drag Thrust (Propulsion) We will first look at propulsion, then drag in this chapter. 41 knots!. Ship Drive Train and Power. 7.2 Ship Drive Train System. Engine.
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7.2 Ship Drive Train System
Horse Power in Drive Train
Brake Horse Power (BHP)
- Power output at the shaft coming out of the engine before
the reduction gears
Shaft Horse Power (SHP)
- Power output after the reduction gears (at shaft)
- SHP=BHP - losses in reduction gear
Delivered Horse Power (DHP)
- Power delivered to the propeller
- DHP=SHP – losses in shafting, shaft bearings and seals
Thrust Horse Power (THP)
- Power created by the screw/propeller (ie prop thrust)
- THP=DHP – Propeller losses
Typical EHP Curve of YP
What EHP is required for the 12 knot YP cruise?
Effective Horse Power (EHP)
Real ship : Turbulent flow exists from near the bow.
Model ship : Studs or sand strips are attached at the bow
to create the turbulent flow.
Coefficient of Viscous Resistance (cont)
Stern divergent wave
Bow divergent wave
Bow divergent wave
Transverse wave system : Waves perpendicular to wake
Wave-Making Resistance (cont)
Transverse wave System
Vs < Hull Speed
Vs Hull Speed
Hull Speed : speed at which the transverse wave length equals
the ship length.
(Wavemaking resistance drastically increases above hull speed)
Divergent Wave System: waves that angle out
Calculation of Wave-Making Resistance Coeff.
Reducing Wave Making Resistance
1) Increasing ship length to reduce the transverse wave
- Hull speed will increase.
- Therefore increment of wave-making resistance of longer
ship will be small until the ship reaches to the hull speed.
Reducing Wave Making Resistance (cont’d)
2) Attaching Bulbous Bow to reduce the bow divergent wave
- Bulbous bow generates the second bow waves .
- Then the waves interact with the bow wave resulting in
ideally no waves, practically smaller bow divergent waves.
- EX :
DDG 51 : 7 % reduction in fuel consumption at cruise speed
3% reduction at max speed.
design &retrofit cost : less than $30 million
life cycle fuel cost saving for all the ship : $250 mil.
Tankers & Containers : adopting the Bulbous bow
3) Stern flaps - helps with launching small boats, too!
Coefficient of total hull resistance: the C’s added
7.7 Basic Theory Behind Ship Modeling
Ship Length=100ft, Ship Speed=10kts, Model Length=10ft
Model speed to satisfy both geometric and dynamic similitude?
Chap 7.7 Basic Theory Behind Ship Modeling
Highly Skewed Propeller
7.9. Propeller Theory
Speed of Advance
~70 % for well-designed prop
- For a given T (Thrust),
(Diameter ) ; CT ; Prop Eff
The larger the diameter of propeller, the better the propeller efficiency
Navy Model Propeller 5236
Flow velocities at the tip are fastest so that pressure drop occurs at the tip first.
Large and stable region of cavitation covering the suction face of propeller.
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
A one horsepower cable-drawn ferry!