Choose a propeller by pitch, diameter and WOT rpm

The right propeller lets the engine reach the correct rpm

Choose a propeller that lets the engine reach the manufacturer's wide-open-throttle range with the boat's normal load. Too much pitch can cause low RPM and heavy load; too little pitch can cause over-revving. According to Mercury, both can damage the engine and drivetrain. So start with the engine manual and a measurement on the water, not a guess about top speed.

What does 14 × 19 mean?

Mercury uses the marking 14 × 19 as an example: the diameter is 14 inches and the pitch 19 inches. The diameter is the circle traced by the blade tips. The pitch is the theoretical distance the propeller would screw itself forward in one revolution with no slip. That is not the same as how far the boat actually travels.

Higher pitch normally lowers wide-open-throttle RPM if everything else is equal. Lower pitch raises it. Diameter, number of blades, blade shape and material also affect the result; two propellers with the same pitch won't necessarily perform the same.

Do the maths – but use the result for comparison

In the propeller calculator you can compare pitch, gear ratio, engine rpm and speed. The gear ratio should be the drive's actual ratio, not an assumed standard value. Get it from the drive model or its labelling.

Slip describes the difference between theoretical and measured propulsion. A calculated value is a troubleshooting aid, not proof of a bad propeller. Inaccurate speed readings, a different gear ratio and currents can give misleading results. Compare the same boat before and after a change.

How to get the most out of a sea trial

1. Make sure the engine is serviced, the bottom is clean and the propeller is undamaged.
2. Load the boat as it is normally used, including people and fuel.
3. Choose open water and safe conditions. Wear a life jacket and use the kill cord.
4. Note the propeller markings, WOT rpm, GPS speed, trim and load. Stop if the engine runs outside its permitted range.
5. Compare acceleration and cruising speed, not just top speed.

Test the next propeller in conditions as similar as possible. A propeller for a heavy holiday load may be a better choice than the one that gives the highest speed with an empty boat.

Don't change the propeller to mask another fault

If engine RPM suddenly changes with the same load and propeller, troubleshoot first. The engine may have lost power, the hull may be fouled or the propeller may be drawing air. If the engine races and the propeller loses grip, read the difference between ventilation and cavitation. Also check the propeller hub and shaft after a grounding. You'll find parts and compatibility in the spare parts catalogue.

Check the fit before buying: hub system, splines, direction of rotation and clearance must match the drive. Keep the old propeller's markings and your sea trial notes so the next comparison is based on facts.

Frequently asked questions