Propeller Pitch, Slip & Boat Speed Calculator

Propeller pitch is the distance a prop would advance in one revolution through a solid medium. Water yields, so the real advance is always less — the shortfall is slip. A well-matched planing hull runs 5% to 15% slip; higher figures suggest an over-pitched prop, excess load, ventilation or blade damage. As a tuning guide, one inch of pitch is worth roughly 200 RPM at wide-open throttle, so reducing pitch raises RPM back into the engine's recommended band. Worked example: at 4,200 engine RPM with a 1.9:1 gearbox the propeller turns at 2,211 RPM. A 19-inch pitch gives a theoretical speed of 2,211 × 19 ÷ 1,215.2 = 34.6 knots. If the GPS shows 28 knots the slip is (34.6 − 28) ÷ 34.6 = about 19%. Typical planing boats run roughly 10 to 25% slip, so this figure would be reasonable, while a very high slip may suggest the wrong pitch or a fouled propeller.