Finding the right prop is a very dark art, particularly for planing boats. But if you can get to grips with the basics, some light should begin to shine. John Cooke explains . . .

Nothing in the world attracts so many myths and old wives’ tales as the propeller. And yet it deserves so much better because this humble chunk of metal is the very thing that enables us to do what we love. Without it, an engine is nothing but an inanimate chunk of combustion - and that makes your boat no more than a floating lounge. But get yourself the perfect set-up and the first time you drive it, your boat will feel brand new. Everything will snap into focus and finally, you will realise what you have been missing . . .

The basics
Props are defined by two numbers, the diameter and the pitch. A 14x21 prop has a diameter of 14 inches and a pitch of 21 inches. In theory (without any slip) this means that the boat will travel 21 inches for each rotation of the prop. The make and model of the prop will determine its other characteristics such as rake (the amount the blade is angled back), cupping (the lip on the trailing edge of the blade), and the number of blades.

In theory, a propeller with one blade would be the fastest, most efficient prop because the drag caused by the amount of metal in the water would be at its minimum. In practice of course, it is not feasible to use one blade because the vibration would be too severe. As a result, props tend to use between two and six blades. The more blades you have, the more drag you get but the smoother the prop feels as it ‘grabs’ more water with shorter periods of time between loads. The fewer blades you have, the less drag you get and the quicker (in theory) it will be.

For more pick up your copy of the August 2010 issue of Boat Mart