Data Input 

Waterline length in feet:  feet 
Beam at the waterline in feet:  feet 
Hull draft in feet (excluding keel):  feet 
Vessel weight in pounds:  lbs 
Engine Horsepower:  HP 
Number of engines:  
Total Engine Horsepower:  0 HP 
Engine R.P.M. (max):  RPM 
Gear Ratio:  :1 
Shaft R.P.M. (max):  NaN RPM 
Number of shaft bearings (per shaft):  
Desired speed in Knots:  knots 
Horsepower Calculations 

This will calculate the maximum horsepower and torque available at the prop(s).  
Total available horsepower at the engine(s):  0 HP 
Total available torque ft/lbs at the engine(s):  NaN ft/lbs 
Horsepower loss of 3% per gearbox:   0.0 HP 
Horsepower loss of 1.5% per shaft bearing:   0.0 HP 
Total horsepower available at the propeller(s):  0.0 HP 
Total torque ft/lbs available at the propeller(s):  NaN ft/lbs 
Speed & Power Calculations 

Basic displacement speed and horsepower required  
Displacement hull speed (1.34 X sqrt of waterline length):  0.00 Knots 
Minimum horsepower required at propeller(s) for Hull speed:  NaN HP 
Calculations based on desired speed and available HP  
HP required at propeller(s) for desired knots speed:  NaN HP 
Estimated maximum speed with existing 0 horsepower: This is the speed we will use for the propeller size.  NaN 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 blades  Diameter (inches)  Pitch (inches)  
2 Blade  NaN  X  NaN 
3 Blade  NaN  X  NaN 
4 Blade  NaN  X  NaN 
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.  
Intructions This calculator is free to use
as often as you wish.
The calculations and results are based on imperical data and formulas. The results are only as acurate as the data you enter. The calculated propeller sizes are based on standard propeller designs; 2 blade = 30% blade area ratio 3 blade = 50% blade area ratio 4 blade = 69% blade area ratio 
Calculate a propeller size based
on the data you enter
No calculations for cavitation 