Propeller pitch is the measurement of how much a propeller will move forward with one complete turn through the full 360 degrees. Pitch is also used for other pieces of equipment and tools that use rotation to move, such as screws. What this means in layman’s terms is that if the propeller is said to have a pitch of 50”, then it will move 50 inches for each full turn of the propeller. This movement occurs through the displacement of a material and is measured using solid materials. As such, it is important to remember that the pitch given for a propeller may differ slightly from what is expected as propellers displace liquid and not solids. Pitch is always given in the unit of displacement, so, inches, centimetres (cm) or metres (m) for propellers, while the pitch of things like screws is often given in millimetres (mm).
For any boat that has a propeller, it is important that both the pitch and diameter are properly calculated. Choosing a propeller with either of these calculations done incorrectly will result in an inadequately running motor. A propeller with a pitch that is too small is referred to as a light propeller. In this case, the propeller will not be able to use all of the energy produced by the engine. If a propeller has a pitch that is too large, it is too heavy for the engine. In this situation, the propeller needs more power than the engine can provide to turn.
In order to choose a propeller with the correct pitch, there are a number of different factors that must be examined. This goes beyond differentiating fishing boat propellers from sail boat propellers. Power, RPM, size of the boat and particular use of the boat are all important considerations in determining the best propeller pitch for a boat. Changing just one of these factors could have a drastic effect on the propeller that is chosen.
Changing Propeller Pitch
The pitch of a propeller can be changed if better performance is needed, but it is usually by no more than one or two inches. The amount that the pitch can be increased or decreased by usually depends on the material the propeller is made of. For example, aluminum and bronze propellers can often be changed by up to two inches, while stainless steel propellers are usually changed by one inch. In most cases, an increase in pitch of one inch will result in a loss of 200 RPM and a decrease of one inch will provide an increase of 200 RPM.