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Propeller Slip Calculator

Work out how much of a propeller's theoretical advance the boat actually makes, from pitch, engine RPM, gear ratio and the speed you measured.

inches of nominal advance per turn, 4 to 40
tachometer reading at that speed, 400 to 9000 rpm
engine turns per propeller turn, e.g. 2.07 for 2.07:1
speed over ground from GPS, or speed through water
the unit of the measured speed
Enter the propeller and speed to see the slip.

Slip compares the distance the propeller would advance in one turn at its nominal pitch with the distance the boat really covers. The theoretical speed is the pitch in inches times the shaft revolutions per minute times 60, divided by the 72,913.4 inches in a nautical mile, where shaft RPM is engine RPM divided by the gear ratio. Slip is the theoretical speed minus the measured speed, as a percentage of the theoretical speed. Planing hulls commonly run 10 to 25% slip; displacement hulls at low speed run higher, and a negative figure means the boat is going faster than the propeller's nominal advance, which a following current, a tailwind or a mismatched tachometer or gear ratio can explain. Pitch is nominal, so a cupped or damaged propeller differs, and speed over ground includes any current while speed through water does not. Everything runs in your browser with no storage, uploads or network access.