Propeller Slip Calculator
Enter engine RPM, propeller pitch, gear ratio and your GPS-measured boat speed to calculate propeller slip percentage, effective pitch and how much speed is lost to slip.
RPM
in
Speed unit
Normal — typical planing hull
- 1
Prop shaft RPM = engine RPM ÷ gear ratio
4,500 ÷ 1.85 = 2,432.4 - 2
Theoretical speed = prop RPM × pitch ÷ 1056
2,432.4 × 21 ÷ 1056 = 48.37 - 3
Slip = (theoretical − actual) ÷ theoretical × 100
(48.37 − 39) ÷ 48.37 × 100 = 19.4The percentage of theoretical advance lost to water slipping past the blades.
How does this calculator work?
Propeller slip % = (Theoretical speed − GPS speed) ÷ Theoretical speed × 100, where theoretical speed (mph) = Engine RPM ÷ Gear Ratio × Pitch (in) ÷ 1056. A typical planing hull shows 10–20% slip. Higher slip means the prop pitch is too aggressive or the hull/engine is under-performing.
Formula
How this is calculated
Propeller slip is the difference between where a propeller theoretically should travel per revolution (its pitch) and where it actually goes through the water. Because water yields under blade load rather than acting as a solid medium, the propeller always slips backwards relative to its theoretical advance. The slip percentage quantifies this loss: slip% = (theoretical speed − actual speed) ÷ theoretical speed × 100.
Theoretical speed is calculated from (engine RPM ÷ gear ratio × pitch_in) ÷ 1056, the same formula as the prop-pitch calculator. Actual speed should be a steady-state GPS reading at the same RPM — calm water, no current, with the boat at its normal load. Effective pitch is the actual advance in inches per revolution: actual_mph × 1056 ÷ prop RPM.
Typical slip values are 10–20% for well-matched planing hulls, 5–12% for high-performance vessels and 25–35% for pontoons, displacement hulls or heavily loaded boats. Very high slip may indicate the prop is over-pitched (not enough blade area), the bottom is fouled, or the engine is under-performing. Use the result alongside the prop-pitch calculator to select the optimal pitch for your engine's rated RPM band.
Frequently asked questions
A typical planing runabout or outboard boat sees 10–20% slip at cruise or WOT. High-performance racing hulls can achieve under 10%. Pontoons, fishing boats and heavily loaded vessels commonly run 25–35%. Consistently above 35% suggests the prop is mis-matched or the hull/engine has a problem.
Slip represents engine power that does not translate to forward motion — it is dissipated as turbulence and heat in the water. Higher slip means more fuel burned per mile. Reducing slip (by matching pitch to the engine's rated RPM range and keeping the hull clean) directly improves fuel economy.
Propeller slip is the main reason. Other contributing factors include current (for or against), wind resistance, hull fouling, engine not reaching rated RPM at WOT, and inaccurate gear ratio input. Always measure speed on calm water with no current and take an average of runs in opposite directions to cancel current effects.
Also known as
TG we-Calculate Editorial Team. (2026). Propeller Slip Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/prop-slip-calculator
TG we-Calculate Editorial Team. "Propeller Slip Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/prop-slip-calculator.
TG we-Calculate Editorial Team, "Propeller Slip Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/prop-slip-calculator
@misc{wecalculate_prop_slip_calculator, title = {Propeller Slip Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/prop-slip-calculator}}, year = {2026}, note = {TG we-Calculate} }
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