Boat Propeller Slip Calculator

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

The Gear Ratio Is Not Optional

This is where most slip calculations go wrong, including plenty of published ones. The propeller does not turn at engine speed. Every outboard and sterndrive has a reduction gearcase between the two, commonly 1.75:1, 1.85:1 or 2.00:1, and it is usually cast or stamped into the gearcase housing. Feed engine RPM straight into the pitch formula and the theoretical speed comes out roughly double what the prop can actually deliver, which then reports a healthy boat as slipping 50% or more.

Propeller shaft RPM = Engine RPM / Gear ratio

Theoretical Speed

Theoretical speed (knots) = (prop shaft RPM x pitch_inches x 60) / 72,913

The constant 72,913 is the number of inches in a nautical mile, so it converts inch-revolutions per hour into knots.

Worked Example

A 22-foot center console holding wide open throttle: tachometer reads 5,800 RPM, the gearcase is 1.85:1, the prop is stamped 16 inches of pitch, and the GPS shows 36 knots.

Prop shaft RPM = 5,800 / 1.85 = 3,135 RPM Theoretical speed = (3,135 x 16 x 60) / 72,913 = 41.3 knots Slip = (41.3 - 36) / 41.3 = 12.8%, which is right where a planing hull should sit.

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.

Take the reading at wide open throttle on flat water, with the boat loaded the way you normally use it. A slip figure taken at cruise RPM, or with the bow still coming up, describes a boat that is not on plane yet and tells you nothing about the propeller.

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.

If the slip figure says your prop is wrong, the Boat Propeller Size Calculator works out which pitch to move to and what it costs you in top speed.