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All calculatorsMethodology

Prop Slip Calculator

Calculate boat propeller slip from engine RPM, gear ratio, prop pitch and measured speed in mph or knots, with theoretical no-slip speed and prop shaft RPM.

Interactive calculator · Method PROP-SLIP-1.0 · Last reviewed: 2026-11-04

Calculate your scenario

Prop RPM = engine RPM ÷ gear ratio.

Your inputs

Try an example

Default worked example. Edit the inputs to calculate your scenario.

Default worked example

Prop slip 19.54%
Theoretical speed (no slip) 49.72 mph
Theoretical speed (mph / knots) 49.72 mph / 43.2 kn
Propeller shaft RPM 2,500 rpm
Speed lost to slip 9.72 mph

5,000 ÷ 2 × 21 in × 60 ÷ 63,360 = 49.72 mph; slip = (49.72 − 40) ÷ 49.72 = 19.54%.

How to use the calculator

Enter the engine RPM and the GPS speed read together at a steady throttle, the lower-unit gear ratio and the propeller pitch, and choose mph or knots.

What the result means

At 5,000 rpm through a 2:1 gearcase with a 21 in pitch prop, the no-slip speed is 49.72 mph, so a measured 40 mph means 19.54% slip.

Formula and units

Prop RPM = engine RPM ÷ gear ratio. Theoretical speed (mph) = prop RPM × pitch (in) × 60 ÷ 63,360 (inches per mile). Knots = mph × 1,609.344 ÷ 1,852. Slip % = (theoretical − measured) ÷ theoretical × 100.

Worked examples

5,000 rpm, 2:1, 21 in pitch at 40 mph: Engine RPM at the measured speed: 5000 rpm; Gear ratio (2.0 means 2:1): 2 : 1; Propeller pitch: 21 in; Measured boat speed (GPS): 40 mph or knots; Speed unit: mph.
Example result: Prop slip: 19.54%; Theoretical speed (no slip): 49.72 mph; Theoretical speed (mph / knots): 49.72 mph / 43.2 kn; Propeller shaft RPM: 2,500 rpm; Speed lost to slip: 9.72 mph.

5,500 rpm, 1.86:1, 19 in pitch at 35 knots: Engine RPM at the measured speed: 5500 rpm; Gear ratio (2.0 means 2:1): 1.86 : 1; Propeller pitch: 19 in; Measured boat speed (GPS): 35 mph or knots; Speed unit: kn.
Example result: Prop slip: 24.3%; Theoretical speed (no slip): 46.23 kn; Theoretical speed (mph / knots): 53.2 mph / 46.23 kn; Propeller shaft RPM: 2,957 rpm; Speed lost to slip: 11.23 kn.

Limitations and common mistakes

Uses the nominal pitch stamped on the prop and assumes the RPM and speed were read at the same moment; tachometers are often a few percent off, and current or wind change GPS speed. It reports a measurement only and does not recommend a prop, engine height, RPM range or safe speed.

Leaving out the gear ratio (using engine RPM as prop RPM) roughly doubles the theoretical speed and gives a slip that looks far too high.

Sources and assumptions

Method PROP-SLIP-1.0; last formula review 2026-11-04. Sources and their supported claims. Calculation methodology.