Prop Slip Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

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When you need a quick prop slip check, the best result is one you can explain, not just copy. Mechanical systems combine geometry, speed, force, losses, and material limits, so a quick equation is best treated as a transparent first model. The calculator below uses a narrow equation set and shows the output in a form that is easy to sanity-check.

What this calculator does

The Prop Slip Calculator is a focused solver for propeller slip using Engine/crankcase rpm, Gear ratio, Propeller pitch. Its job is to make the active equation and the quantities feeding it easy to inspect rather than model every possible real-world effect.

How to use it

Begin with Engine/crankcase rpm, Gear ratio, Propeller pitch, Actual boat speed. Do not strip the units from those values when copying them from a datasheet or measurement. Once calculated, vary the most influential input slightly and confirm that the response agrees with the equation before using the number elsewhere.

How the calculation works

Theoretical boat speed is formed from propeller RPM and pitch. Slip is then (theoretical speed − actual speed)/theoretical speed × 100%. A positive value means the boat is traveling slower than the ideal no-slip advance.

Example

Using the default example on the page (Engine/crankcase rpm = 5000 rpm; Gear ratio = 2; Propeller pitch = 20 in; Actual boat speed = 40 mph), the calculator returns propeller slip of 15.52 %. Try increasing one input while holding the others fixed; the response should match the dependence shown in the formula above.

How to interpret the result

A positive slip percentage means actual speed is below the ideal pitch speed. Very high values can point to setup, load, ventilation, cavitation, or input error, while small positive values are normal in many operating conditions; there is no universal ‘best’ slip for every hull and propeller.

Limitations and notes

Slip is an inferred comparison between ideal pitch advance and measured boat speed, not a direct efficiency measurement. Tachometer error, gear ratio, actual pitch, cup, hull condition, current, and GPS speed can all move the result.