This has been beat to death for 100 years. It is not rocket science guys.
It is just simple math that most people can do with a calculator.
the chart below is actual numbers from my boat.
It has the correct 4 blade prop and will run at 10.84 knots using 310 HP WOT.
Back it off to 9 knots and it requires 165 HP.
To cruise a hull speed 8.89 knots with
10% reserve power for variable loading and sea states requires 174.5 HP.
I have twin 155 HP engines @2200 RPM, but cruise at 8.9 knots the engines only turn 1750 RPM.
See the chart below
Data Input
Waterline length in feet:44 feetBeam at the waterline in feet:16 feetHull draft in feet (excluding keel):6 feetVessel weight in pounds:80000 lbsEngine Horsepower:155 HPNumber of engines:2Total Engine Horsepower:
310 HP Engine R.P.M. (max):2200 RPMGear Ratio:2:1Shaft R.P.M. (max):
1100 RPM Number of shaft bearings (per shaft):2Desired speed in Knots:9 knots
Horsepower Calculations
This will calculate the maximum horsepower and torque available at the prop(s). Total available horsepower at the engine(s):310 HPTotal available torque ft/lbs at the engine(s): 740 ft/lbsHorsepower loss of 3% per gearbox:- 9.3 HPHorsepower loss of 1.5% per shaft bearing:- 9.3 HP Total horsepower available at the propeller(s):
291.4 HPTotal torque ft/lbs available at the propeller(s):
1391 ft/lbsSpeed & Power Calculations
Basic displacement speed and horsepower requiredDisplacement hull speed (1.34 X sqrt of waterline length):8.89 KnotsMinimum horsepower required at propeller(s) for Hull speed:174.5 HP Calculations based on desired speed and available HPHP required at propeller(s) for
desired 9 knots speed:
165 HPEstimated maximum speed with
existing 310 horsepower:
This is the speed we will use for the propeller size.
10.84 Knots At this point it is important to note that all of the calculations above are based on full RPM and HP. Most engines are rated to run at a percentage of thier full RPM. This is what will determine your maximum cruising speed. The propeller sizing calculations below are based on 90% of full RPM. This gives the engine some reserve power to allow for variable loading in the vessel.
Propeller Size
Number of bladesDiameter (inches) Pitch (inches)2 Blade27.1X17.53 Blade25.8X17.34 Blade24.3X17.0 The propeller sizes shown above do not contain calculations for cavitation or blade loading.
If you find that the recommended propeller is too large to fit your vessel, you can try increasing the shaft speed. Failing this, you can reduce the diameter and increase the pitch at the expense of your propeller efficiency. The rule of thumb is 1 inch of diameter is equal to 1 1/2 to 2 inches of pitch.