Prop Pitch Calculator

Use this tool to calculate the prop pitch of a boat from its speed, gear ratio, engine speed, and propeller slip.

Theoretical speed54.171 mph87.18 km/h, 47.07 knots

Propeller speed3,010.8 rpm5,600 engine rpm ÷ 1.86

Advance per revolution19 incheswhich is what pitch means — the distance a perfect, slip-free propeller would screw forward in one turn

Slip15.08%normal for most planing craft

Actual speed46 mph

Speed lost to slip8.171 mph

Pitch that would give this speed with no slip16.134 inches

Pitch to diameter ratio1.3333a middling ratio, typical of general use

Tip speed57.06 m/shigh tip speeds provoke cavitation on a marine propeller and noise on an aircraft one

-2 inches of pitch48.469 mph theoreticalroughly 400 rpm of engine speed traded, in the usual rule of thumb

+2 inches of pitch59.873 mph theoreticalroughly 400 rpm of engine speed traded, in the usual rule of thumb

The formula

theoretical speed = pitch × rpm ÷ gear ratio; slip = 1 − actual ÷ theoretical

Slip is not waste

A propeller's pitch is the distance one revolution would advance it through a solid medium, like a screw in wood. Water is not solid, so the boat always advances less than that, and the shortfall is slip.

Slip is often misread as lost efficiency. It is not: a propeller generates thrust precisely by accelerating water backwards, and that requires the blades to move faster than the water. Zero slip would mean zero thrust. Typical figures run from under 10% on a light fast hull to over 30% on a heavy displacement one, and what matters is whether the figure is normal for that hull rather than whether it is small.

The pitch-to-diameter ratio sets the character much as a gear ratio does. Lower pitch is a shorter gear: better acceleration and load-carrying, lower top speed, higher revs. The usual rule of thumb is that two inches of pitch trade against roughly 200 engine rpm at wide-open throttle, which is how a prop is matched to bring an engine into its rated speed range.