Boat Propeller Slip Calculator

Method and units

Theoretical speed (mph) = pitch (in) × engine RPM ÷ (gear ratio × 1,056). Apparent slip = (theoretical speed − measured speed) ÷ theoretical speed × 100. Negative results are retained and indicate the assumptions or measurements need review.

Worked example

A 19-inch pitch propeller at 5,000 RPM with 2.0:1 reduction gives 45.0 mph theoretical. At 36 mph measured, apparent slip is 20%.

Supported inputs and interpretation

Pitch 1–100 in; RPM 1–20,000; ratio 0.1–20; measured speed 0–200 mph. Results are rounded for display only; internal calculations retain full JavaScript precision. Entered measurements and assumptions determine the answer.

References and limitations

The equations here use the stated unit conversions and geometric or accounting relationships; no empirical vehicle limits or manufacturer presets are supplied.

Pitch marking, engine RPM, gear reduction and measured speed must be accurate and matched to the same condition. Trim, load, water, propeller deformation and GPS error matter. Apparent slip is not a performance target, safety assessment or modification recommendation.

Related calculators

• Engine Rpm Road Speed Calculator
• Gps Speedometer Calibration

This planner performs the stated arithmetic; it is not a vehicle inspection, engineering approval, manufacturer's recommendation or professional review.