Boat propeller & speed calculator

How the propeller calculation works

A propeller is a screw. In one turn it would theoretically move the boat forward by its pitch — a 19in prop, 19 inches. Multiply that by how many turns the prop makes per minute (engine RPM divided by the gearcase ratio) and you have a theoretical speed. Water is not solid, so the real figure is lower: the difference is slip.

What slip figure to use

Slip is usually 8–12% on an efficient planing hull with a well-matched prop, 12–20% on heavier boats, pontoons and boats running a lot of bow lift, and higher again for displacement hulls. If you know your actual GPS top speed, adjust the slip value until the calculated speed matches — that gives you your boat's real number, which you can then use to test a different pitch.

Choosing pitch by RPM, not by speed

The right propeller is the one that lets the engine reach the top of its recommended wide-open-throttle RPM range with a normal load. Every inch of pitch you add costs roughly 150–200 RPM; every inch you drop gains about the same. If your motor over-revs, go up in pitch. If it will not reach its rated RPM, go down — a lugging engine is a damaged engine.

Where the gear ratio comes from

The gearcase ratio is printed in your owner's manual, commonly around 1.85:1 to 2.33:1 on modern outboards. Enter it as a decimal — 2.0 for a 2.00:1 gearcase.