PROPELLER THEORY

All propellers can be described in basic terms using a set of two numbers: diameter x pitch. Hence a prop with an 18in diameter and 12in pitch is described as 18 x 12.

The diameter of the propeller is the circle which the tip of a single blade describes in a complete rotation.

It is found by measuring from the centre of the propeller boss to the tip of one blade and then doubling the result.

Pitch is the forward distance that a propeller would theoretically travel in a single rotation if there were no slip present – imagine a screw being driven into a piece of wood.

The angle at which the propeller blades are set governs the distance travelled. A fine pitch (meaning little angle on the blades) gives a lesser distance than a coarse pitch (greater blade angle).