Marine Prop Slip
Theoretical speed, slip percent, and planing vs displacement category from RPM, gear, pitch, and GPS speed.
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 %
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
Formula or tableTheoretical knots = (RPM / gear_ratio) × pitch_in / 1215.2. Slip % = (theoretical − actual) / theoretical × 100. Planing 10–15% / displacement 25–30% category bands.
Source and editionClassical marine-propeller theory; ABYC P-17 (Boat Propeller Selection) by name.
Free accessABYC standards licensed; principles free in published marine-engineering texts and at boatus.org.
What governsVessel master governs. Verify against the stability letter and USCG-approved loading manual.
API field namesrpm, 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