Boat Propeller Slip Calculator
Calculate propeller slip percentage from theoretical and actual boat speed.
A useful diagnostic for identifying prop pitch mismatch or hull fouling.
What Is Propeller Slip?
A propeller with a 12-inch pitch theoretically advances 12 inches through the water per revolution, the same way a wood screw advances 12 inches through solid wood. But water is not solid. The propeller blades push backward against the water, and some of that water slips around the blade rather than being grabbed cleanly. The result is that the actual forward movement per revolution is less than the theoretical pitch distance.
Slip percentage = ((theoretical speed - actual speed) / theoretical speed) x 100
Theoretical Speed
Theoretical speed (knots) = (RPM x pitch_inches x 60) / 72,913
The constant 72,913 converts inch-revolutions to nautical miles per hour.
At 3,000 RPM with a 12-inch pitch: (3,000 x 12 x 60) / 72,913 = 29.6 knots theoretical.
Normal Slip Values
Typical prop slip for planing hulls: 5 to 15%. Displacement hulls: 15 to 25%. Sailboat auxiliaries: 20 to 30%. High-performance racing drives: as low as 2 to 4%.
Slip above the expected range indicates prop pitch is too high for the load (engine cannot reach full RPM), hull fouling, prop damage, or wrong propeller diameter. Low slip below the normal range usually means prop pitch is too low, allowing the engine to over-rev.
Fouled Hull Effect
A hull heavily fouled with barnacles can add 10 to 20% resistance, showing up as higher-than-normal slip at the same RPM. Cleaning the hull often restores 1 to 2 knots of top speed on affected boats.