Marine Prop Slip

Theoretical speed, slip percent, and planing vs displacement category from RPM, gear, pitch, and GPS speed.

Run the calculator

Example

You enter

Engine RPM 4500
Gear ratio 1.85
Prop pitch (in) 19
GPS speed (kt) 35

You get

Theoretical speed 38.03 kt
Slip 8.0 %

Details, formula, and sources

Theoretical knots = (RPM / gear_ratio) × pitch_in / 1215.2. Slip % = (theoretical − actual) / theoretical × 100. Planing 10–15% / displacement 25–30% category bands.

Classical marine-propeller theory; ABYC P-17 (Boat Propeller Selection) by name.

ABYC standards licensed; principles free in published marine-engineering texts and at boatus.org.

Vessel master governs. Verify against the stability letter and USCG-approved loading manual.

Field names used by the API: rpm, gear_ratio, pitch_in, gps_speed_kt, theoretical_kt, slip_percent

Constant 1215.2 inches per nautical mile / min per hr = (12 × 6076.12 ft/nm) / 60 = 1215.2 (the earlier 1056 was the statute-mile value, which gives mph, not knots)knots-from-RPM derivation