Propeller care

Stu Davies has valuable tips on how to maintain your prop-and explains how this vital bit of kit works

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The next time your boat is on the hard for a scrub, antifoul or whatever, have a look around and you will see quite a lot of different propellers on your neighbours’ boats.

Made of aluminium, brass or bronze – even plastic in the case of small outboards – some have two blades, some have three and some, very occasionally, have four.

While most propellers have fixed blades some have blades that fold.

Propellers can be right-handed, where they turn clockwise as you go ahead, or left-handed where they turn anti-clockwise to go ahead. Look from the stern to determine your rotation; mine is left-handed.

These differences come about because different types of propellers are suited to different types of boats, engines and transmissions.

How it works

How a propeller works is quite complex. It transforms 'torque' from the engine into a linear motion. A boat's propeller is often referred to as a 'screw' but it is far from it.

It 'screws' its way through the water because the blades work similar to a wing on an aircraft and like a torque converter in a car's automatic transmission; once it is up to speed it almost actually has a positive 'grip' on the water.

A turning propeller moves water behind the blades, almost like a hand grabbing water and pushing it behind it. This produces part of the thrust.

Each blade has a distinctive curved shape which, as it turns, acts like a foil in the process 'pulling' the propeller and the boat forward.

It acts like a foil because the angle of the blade creates lift as it moves through the water, in a similar way that a wing creates lift through the air, with a positive pressure, on the underside and a negative pressure, on the top side.