Prop Slip Calculator
Propeller slip is the difference between the theoretical distance a prop should move through water per revolution (based on its pitch) and how far it actually moves. Some slip is normal and necessary for thrust. Typical slip ranges from 10% to 20%, but values outside this range can signal a prop mismatch, hull fouling, or engine issues.
The theoretical speed of a boat is calculated from RPM and propeller pitch. The formula is:
Theoretical Speed (mph) = (Prop RPM x Pitch in inches) / 1056
The constant 1056 converts inches per minute to miles per hour (12 in/ft x 5280 ft/mi / 60 min/hr).
Slip percentage is then:
Slip % = ((Theoretical Speed - Actual Speed) / Theoretical Speed) x 100
- 10-15% slip is typical for well-matched props on planing hulls
- 15-20% slip is common on heavier displacement boats
- Over 25% slip suggests the prop is too small, the hull is fouled, or the engine is underpowered
- Under 5% slip may indicate the prop is over-pitched, potentially overloading the engine
If you have a gear reduction lower unit, remember to divide the engine RPM by the gear ratio to get the actual prop shaft RPM before calculating. Many outboards and stern drives have gear ratios between 1.75:1 and 2.5:1.