The propeller is easy to overlook. It's bolted on, it goes round, and as long as the boat moves, most people leave it alone. But the wrong prop can make a perfectly good outboard feel sluggish, burn more fuel, or rev too high and put extra strain on the engine.
Whether you're replacing a damaged prop or trying to get better performance, here's what the numbers mean and how to choose one that suits your boat.
Diameter and pitch
Every propeller is described by two main numbers, usually written like 10 x 13 or 10 1/4 x 12.
The first number is the diameter in inches. That's the width of the circle the blade tips make as the prop turns. Diameter is mostly set by the engine's power and gearbox, so for any given outboard, the choice is fairly narrow.
The second number is the pitch, also in inches. Pitch is the distance the propeller would move forward in one full turn if it were screwing through something solid, like a screw going into wood. A prop with a 13 inch pitch would, in theory, travel 13 inches per revolution.
In practice, pitch is the number you change to tune how the boat performs.
How pitch affects performance
Think of pitch like the gears on a bicycle.
A lower pitch is like a low gear. The engine revs more freely, the boat gets onto the plane faster and copes better with a heavy load. Top speed is usually lower.
A higher pitch is like a high gear. The boat can go faster at the top end, but it takes longer to get going, and with a heavy load the engine may struggle to reach its proper revs.
As a rough rule of thumb, going up one inch of pitch lowers the engine's revs at full throttle by around 150 to 200 rpm, and going down one inch raises them by about the same.
Getting the revs right
Every outboard has a recommended full throttle rpm range, listed in the owner's manual. The aim is to choose a prop that lets the engine reach somewhere in the upper half of that range, with your boat loaded the way you normally use it.
If the engine can't reach the bottom of the range at full throttle, the pitch is probably too high. The engine is labouring, which is hard on it and can cause overheating and extra wear.
If the engine hits the rev limiter, or goes beyond the top of the range, the pitch is probably too low. The engine is over-revving, and you're leaving speed and efficiency on the table.
A tachometer makes this easy to check. Try a run with a typical load on board, a sensible amount of fuel and the trim set as you'd normally use it.
Number of blades
Most small outboards come with a three-blade prop, and for general use it's a good balance of speed, efficiency and smoothness.
Four-blade props give more grip in the water. They can help a boat get onto the plane faster, hold the plane at lower speeds and run more smoothly in rough water. They're popular on heavier boats and for watersports. The trade-off is often a little top speed.
Two-blade props are mostly found on very small engines and some racing setups.
Aluminium or stainless steel?
Aluminium props are the standard choice on small and medium outboards. They're lighter and cheaper, and if you hit something, the prop tends to take the damage rather than the gearbox. That's no bad thing.
Stainless steel props are stronger and stiffer. The blades flex less at speed, which can give better performance, and they stand up better to sand and grit. They cost more, though, and because they're so tough, a hard strike passes more of the force into the lower unit.
For most leisure boaters on small to mid-sized engines, aluminium makes sense. Stainless is worth considering if you're after every bit of performance, or regularly run in shallow, sandy water.
Check the fit before you buy
This is where people catch themselves out. A propeller has to match your outboard's propeller shaft, not just its size. The number of splines on the shaft, the shaft diameter and the type of hub all need to be right, and they vary between engine makes and sizes.
The easiest way to get it right is to match the part number on your current prop, or the make, model and horsepower of your outboard. Many props have their size and part number stamped on the hub or the side of a blade.
When you fit the new prop, replace any worn thrust washer, nut or split pin, and grease the shaft with a suitable marine grease so it comes off easily next time.
When a prop needs replacing
Look at the blades every time the boat comes out of the water. Small nicks can be dressed with a file, but bent or cracked blades, large chips or missing pieces all mean the prop is out of balance. You might feel a vibration through the steering or notice the engine revving higher than usual for the same speed.
If the engine revs freely but the boat barely moves, the rubber hub inside the prop may have spun. That's the part designed to slip if you hit something, and it can be replaced on some props or means a new prop on others.
It's always worth carrying a spare prop, plus the nut, washer and split pin to fit it, if you go out of reach of help.
We stock replacement outboard propellers and a range of other outboard and marine parts. If you're not sure which prop fits your engine, get in touch with your outboard's details and we'll help you find the right one.
This article is a general guide only. Always check your outboard manufacturer's recommendations for propeller size and full throttle rpm range.