vic prop dot com and made some assumptions:
Data Input
Waterline length in feet:31 feetBeam at the waterline in feet:9 feetHull draft in feet (excluding keel):2 feetVessel weight in pounds:16000 lbsEngine Horsepower:350 HPNumber of engines:1Total Engine Horsepower:
350 HP Engine R.P.M. (max):4200 RPMGear Ratio:1.9:1Shaft R.P.M. (max):
2211 RPM Number of shaft bearings (per shaft):2Desired speed in Knots:16 knots
Horsepower Calculations
This will calculate the maximum horsepower and torque available at the prop(s). Total available horsepower at the engine(s):350 HPTotal available torque ft/lbs at the engine(s): 438 ft/lbsHorsepower loss of 3% per gearbox:- 10.5 HPHorsepower loss of 1.5% per shaft bearing:- 10.5 HP Total horsepower available at the propeller(s):
329.0 HPTotal torque ft/lbs available at the propeller(s):
782 ft/lbsSpeed & Power Calculations
Basic displacement speed and horsepower requiredDisplacement hull speed (1.34 X sqrt of waterline length):7.46 KnotsMinimum horsepower required at propeller(s) for Hull speed:34.9 HP Calculations based on desired speed and available HPHP required at propeller(s) for
desired 16 knots speed:
313 HPEstimated maximum speed with
existing 350 horsepower:
This is the speed we will use for the propeller size.
16.23 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 Blade21.0X12.5 3 Blade20.0X12.4 4 Blade18.8X12.1 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.
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