A propeller is a gear ratio you can change. The correct one puts the engine at the top of its wide-open-throttle range with a normal load, which is where it makes rated power and where its lubrication and cooling were designed to operate.
The wide-open-throttle test
Every outboard has a published WOT operating range — commonly something like 5000–6000 RPM. With a typical load, trimmed for best performance, the engine should reach the upper half of that range.
- Below the range — the engine is over-propped. It is labouring, running hot, consuming more fuel and accumulating mechanical stress. Reduce pitch. - Above the range — the engine is under-propped. It will over-rev, potentially entering rev limiting, and will not carry load well. Increase pitch.
As a working rule, each inch of pitch is worth roughly 150–200 RPM at wide-open throttle, in the opposite direction to the pitch change. This is an approximation that varies with hull and load, but it is close enough to choose a test propeller.
Diameter and pitch
Diameter is the circle the blade tips describe. It is largely determined by the gearcase and the application: heavier, slower hulls need more diameter to convert torque into thrust. Pitch is the theoretical distance the propeller advances in one revolution. Higher pitch gives more speed per revolution and less acceleration; lower pitch gives stronger hole-shot and load-carrying.
Blade count
- Three blades — the efficiency and top-speed default for most recreational applications. - Four blades — better hole-shot, better grip in turns and at low planing speeds, better stern lift. Usually costs a little top-end. Often the right answer for heavy or load-carrying hulls. - Five blades — specialised, generally for high-performance and heavily-loaded offshore work.
Material
Aluminium is economical and easily repaired, and is the right choice for general recreational use and where impact damage is likely. Stainless steel is stiffer, holds its blade geometry under load, and typically delivers better performance and durability — at higher cost, and with more risk of transmitting impact damage into the gearcase rather than sacrificing the blade.
Rotation
On a single-engine installation, standard rotation is almost always correct. On twins, one engine is counter-rotating so that steering torque and hull-lifting effects cancel. A counter-rotating engine requires a counter-rotating propeller — this is not interchangeable, and it is worth confirming which engine on a twin rig you are ordering for.
What to send when requesting a recommendation
Hull make and model, length, typical load, engine model and horsepower, gear ratio, current propeller (diameter, pitch, blades, material) and the WOT RPM you currently achieve. With those, a propeller recommendation is straightforward; without the current RPM figure, it is guesswork.
Last reviewed September 2026. This guide is general technical reference. Always work from the manufacturer documentation for your specific engine.