Propeller Pitch, Slip & Boat Speed Calculator

How it works

Theoretical mph = (engine rpm ÷ gear ratio) × pitch (in) ÷ 1056; slip % = 1 − actual ÷ theoretical; pitch change ≈ rpm error ÷ 175 per inch

A propeller is a screw: with each revolution it should advance by its pitch, so shaft rpm times pitch gives a theoretical speed, and the shortfall against GPS speed is “slip” — partly the inevitable angle of attack the blades need to generate thrust, partly hull drag and load. Efficient planing hulls run 8–15% slip; pontoons and heavy displacement boats 25–45%. The same relationship drives prop selection: an engine that cannot reach its rated wide-open rpm is over-propped and lugging (bad for it), one that over-revs is under-propped, and each inch of pitch moves WOT rpm by around 150–200. Diameter, blade count, cup and rake matter too, which is why the calculation is a starting point for testing rather than a final answer.

Worked example

A 19″ prop at 5,600 rpm through a 1.85:1 gearbox: theoretical 54.5 mph; a GPS speed of 43 mph means 21% slip — high for a runabout. If the engine is rated to 5,800 rpm, dropping to a 17″ prop would let it rev out.

Frequently asked questions