How does a propeller work?

Your prop is as essential as the engine that drives it; neither moves without the other.

Like the tyres on your car, the propeller on your boat converts precious power into forward motion - but do you know how to look for wear? How it affects performance? How to maintain it? Or indeed how it works?

The truth is your prop is as essential a part of your boat as the engine that drives it - if your prop is is mismatched or damaged in any way, it will severely reduce your boat's performance. There is no point in having a tiptop engine if you run it with the wrong propeller.

A propeller pushes the boat along by ‘grabbing' water and ‘pulling' itself through the water by virtue of its spinning blades. When the prop rotates in the water, it forces water behind it and carries itself (and everything attached to it - i.e. your boat) forward.

It works the same way as a threaded screw pulls itself into a piece of wood - this is where the term 'screw' comes from. The speed a prop does this depends upon two factors - its diameter and pitch.

You'll notice all props are reffered to with two sizes, e.g. 13" x 19". The first dimension, in our example 13", refers to diameter and the second 19" is the pitch. Diameter is simply the distance across a circle swept by the extreme tips of the propeller blades.

Pitch is the theoretical distance travelled for one revolution of the propeller, in this case 19". However as water is not a solid substance, this figure is less, due to slip. Blades have a leading edge which cuts the water first, and trailing edge from which the water flows away.

Many propellers have a slight upturn on the trailing edge, this is known as cupping. Cupping has the effect of reducing cavitation. Cavitation feels the same as the clutch slipping on your car, and is commonly confused with bush failure as the effects can be very similar - i.e. the engine's revs suddenly rises but you lose speed.

Cavitation is actually caused when the surface of the blades become covered in tiny vacuum bubbles. A smaller degree of cavitation will cause vibration, but if left in this condition for long periods it will cause 'cavitation burns'.

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