How this calculator works
Slip compares actual advance to the distance implied by nominal pitch and propeller turns. It is an apparent geometric measure rather than propulsive efficiency. A stalled boat can show 100% apparent slip while the propeller still moves water. Use actual shaft reduction, accurate RPM and consistent speed measurements. Tidal current, hull wake and cupped blades complicate interpretation.
Formula
v_theoretical = pitch × engine RPM/(60 ratio); slip = 1 - v_actual/v_theoretical.
Worked example
At 3000 engine rpm with 2:1 reduction and 0.4 m pitch, theoretical speed is 10 m/s. An actual speed of 9 m/s gives 10% slip. At 2400 rpm, 2:1 reduction and 0.5 m pitch, 20% slip gives 8 m/s.
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Frequently asked questions
Is slip equal to efficiency loss?
No. Propulsive efficiency depends on thrust, power and water flow, not just the apparent pitch advance.
Which speed measurement should I use?
Speed through water fits this comparison best. GPS speed includes current.
What does an implausible slip suggest?
Check pitch marking, reduction ratio, tachometer calibration, speed units and measurement conditions.