Power and Estimate speed
EHP - Effective Horsepower. This is very close to the amount of power required to tow the vessel.
IHP – Indicated Horsepower. – to drive vessel at a given speed. Ratio of EHP/IHP is usually 50 % (1:2).
BHP –break horsepower, max horsepower generated by the engine at a given rpm.
SHP – Shaft horsepower, transmitted along the propeller shaft to the propeller at a given RPM. Max SHP is the MAX power delivered to propeller, almost always at max attainable RPM.
*power lost in the gear box 3%
*power lost in the shaft bearing 1.5%
Power and Energy losses
35% lost in heat to the atmosphere
25% lost in heat and vibration to the water
2% lost at the propeller shaft
Torque – twisting force.
The lower the RPM and the higher the HP, the greater the torque
T = (5252 X HP) / RPM
SL ratio
SL – Speed length
Kts – Speed in knots
SHP – Shaft horse power at propeller
LB – Displacement in pounds
WL – Waterline length in feet
SL ratio =
Displacement boat speed formula
SL ratio = = Kts/
Recommended RPM for Continuous Operation
Type of engine
|
% of max RPM
|
Light – duty petrol and diesel automotive conversions
|
70~ 80%
|
Light – duty or high out – put marine diesels
|
80 ~85%
|
Intermittent – duty marine diesels
|
88 ~ 92%
|
Continuous – duty marine diesesele
|
98 ~ 100%
|
The Torque Curve
The Max torque of most engines occurs below max RPM. For that reason, the most economical and efficient speed of operation of many engines is around 70 to 85% of top rated RPM. – choose cruising speed.
Measuring to find Displacement
Must be sure to include the weight of:
1. Full crew and passengers
2. All normal ship’s stores and gear
3. 2/3 of all fuel and water tank
4. 2/3 of all cargo
Displacement hulls
Sailboats, typically have displacement hull. Dome powerboats, some powerboats cruising trawlers.
*Design to ride in (not ride on) the water.
*Generally more fuel efficient than planing hulls.
* Can carry heavy stuff.
*It offers soft ride, gently push water aside.
* Susceptible to roll from the ‘beam sea’ / powerboats are more vulnerable than sailboats –they install keel(with the ballast) small riding sails. Or roll stabilizers
Displacement hull speed
The wave move at a 1.34 times the square root
1.34 X
Speed Length ratio is 1.5 to 2
• DL ratio =DispT/(0.01 x WL)3
DispT – Displacement in long tons of
2,240 pounds
WL – water line in feet
• Max. speed-length ratio vs DL ratio
SL Ratio = 8.26/(DL Ratio)0.311
Semi –Displacement Hulls
Lift up forward that achieve higher speed than pure displacement hulls. However, it is high fuel consumption.
*Speed to length ratio in the 2.0 t0 3.5
Planing Hulls
Lighter than similar size of displacement or semi displacement
Tend to bounce around in choppy water.
Require substantial engine power to achieve planing speed. – Once planing, efficiency jumps – less power need/ total wetted surface drop – less friction
Planing Boat
• SL Ratio is over 2.5 or 3
• Quarter beam buttock is 0 to 2 degree
• Crouch’s Planing Speed Formula
Kts = C / (LB/SHP)0.5
Kts – Speed in knots
C – Constant chosen for the type of vessel
SHP – Horsepower at propeller shaft
Reference
Dave Gerr (1989) Propeeler Handbook. International Marine
Bob Sweet (2007) Powerboat Handling Illustrated. International Marine
