# Boat Propeller Calculator

You can use this boat propeller calculator to determine one of five variables: boat speed, propeller slip, propeller pitch, engine gear ratio, or engine revs.

To use the calculator, input four of the five variables. The calculator will automatically compute the 5th variable.

Boat Prop Calculator

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## Reference

Propeller slip provides an indication of the efficiency with which a boat is traveling through the water. However, it is not to be confused with propeller efficiency.

If there was an absence of slip as the propeller circulates through the water, the boat will, theoretically, move forward at a distance that matches the propeller pitch.

A propeller pitch can be understood by considering the action of a screw. The pitch indicates the distance that each rotation contributes to the extent to which the boat moves forward on each rotation. For instance, if the propeller moves forward 15 inches every time it completes a full turn, the nominal propeller pitch is 15 inches.

The propeller revolution rate is determined by dividing the engine rpm by the gear ratio. The propeller rpm multiplied by the propeller pitch indicates the distance forward a boat will move forward every minute.

However, as a result of propeller slip, there is a difference between actual and theoretical speed. Specifically, the actual speed is typically 10-20% slower than the propeller than the theoretical speed when the boat is traveling at its top speed.

The slip can be much higher at lower speeds, often above 50%. Typically, the slip reduces as the speed increases, generally diminishing to under 10%.

'Gear ratio' defines the number of drive shaft revolutions per propeller revolution. As such, if an engine has a gear ratio of 2:1, the engine drive shaft will turn twice for every propeller revolution. It is common for 200-HP engines to have a ratio of 1.86:1. Engines that have a smaller horsepower will have a higher ratio in the region of 2.33:1.

## Calculator Equations

The following equations are used within this calculator:

V = [RPM × PP × (1 − (PS/100))] / [GR × C]

RPM = [V × GR × C] / [PP × (1 − (PS/100))]

GR = [RPM × PP × (1 − (PS/100))] / [V × C]

PP = [V × GR × C] / [RPM × (1 − (PS/100))]

PS = [1 − (V × GR × C) / (RPM × PP)] × 100

Where:

V is the speed at which the boat is traveling,

RPM is the crankcase speed (rpm),

GR is the number of revolutions the crankshaft needs to produce one revolution of the prop shaft,

PP is the blade pitch of propeller (inches),

PS is the index of propeller performance (as a percentage),

C is the constant to convert inches-per-minute of revolution to boat speed V;

V (in mph), C = 1056;

V (in knots), C = 1215.2;

V (in kph), C = 656.