About

Propeller Slip is the difference between the theoretical distance a propeller should move through the water in one revolution (its pitch) and the actual distance it travels. While zero slip is physically impossible due to the fluid nature of water, understanding this metric is crucial for boat performance.

High slip numbers (>25%) usually indicate an incorrect propeller pitch, engine mounting issues, or a damaged prop, leading to wasted fuel and reduced top speed. A healthy planing hull typically sees slip figures between 10% and 15%. This calculator allows captains and mechanics to diagnose performance issues by comparing RPM, Gear Ratio, and Actual Speed against the physics of the propeller.

Formulas

First, we calculate the Theoretical Speed based on engine data:

V_theo = RPM × PitchRatio × C

Where constant C converts units (e.g., 1056 for mph with pitch in inches). Then, Slip Percentage is derived:

%Slip = V_theo − V_actualV_theo × 100

Reference Data

Vessel Type	Typical Slip Range (%)	Goal Efficiency
Racing Hydroplanes	3% - 8%	Maximize Speed
Light Planing Hulls (Bass Boats)	10% - 15%	Balanced
Cruisers / Runabouts	15% - 25%	Fuel Economy
Heavy Workboats / Barges	30% - 45%	Thrust (Torque)
Sailboat Auxiliaries	30% - 50%	Thrust

Frequently Asked Questions

The gear ratio is the number of engine revolutions required to turn the propeller shaft once (e.g., 2.0:1 means 2 engine turns = 1 prop turn). This is found in your engine's owner manual or stamped on the serial plate. Common outboards range from 1.75:1 to 2.33:1.

Negative slip is theoretically impossible in standard physics. If you see this result, it usually means your data is incorrect: either the tachometer is reading low, the speedometer is reading high (common with pitot tubes vs GPS), or the propeller pitch has been modified (cupped) to be effectively higher than the stamped number.

Yes. Heavy cupping on a propeller blade effectively increases the pitch. A 19-inch pitch prop with heavy cupping might act like a 20 or 21-inch prop, which can skew standard slip calculations.

Reducing slip often involves switching to a stainless steel prop (less flex), selecting a propeller with larger blade area, or adjusting the engine height (jack plate) to run in cleaner water.