Prop Slip Calculator

Prop slip is the difference between a propeller's theoretical and actual travel.

How the Math Works

The Prop Slip Calculator uses the formula slip = (theoretical - actual) divided by theoretical, multiplied by 100 to express the result as a percentage. This calculation compares the speed a propeller is designed to achieve (theoretical speed) against the speed it actually produces in water (actual speed). The difference between these values represents the 'slip' — the distance the propeller moves through the water versus the distance the vessel actually travels. A higher percentage indicates more slip, which is normal due to factors like water resistance, cavitation, and hull dynamics.

Practical Applications

To use this calculator, first determine your propeller's theoretical speed based on its diameter, pitch, and RPM using standard marine engineering formulas. Then measure your vessel's actual speed through water using a GPS or pitometer. Input both values into the calculator: theoretical speed minus actual speed, divided by theoretical speed, times 100. This gives you the propeller slip percentage, which helps marine engineers optimize propeller selection, adjust RPM settings, and diagnose potential mechanical or hydrodynamic issues affecting vessel performance.

Day-to-Day Use

Propeller slip calculations matter in everyday boating and shipping operations because they directly impact fuel efficiency, travel time, and maintenance costs. A marina operator can use slip data to determine if their boat's propeller is properly sized or if engine tuning is needed, potentially saving hundreds of dollars in fuel annually. Recreational boaters gain insight into why their vessel might be slower than expected or burning excessive fuel, while commercial ship captains use slip measurements to optimize routes and delivery schedules based on actual versus predicted performance.

Worked example

79 mph theoretical vs 65 actual → about 17.7% slip.

FAQ