So knowing these basic facts how do you determine what size propeller you need? You may think that a propeller with a very high pitch will give the fastest speed, and to some degree this is true. However pitch is directly connected to engine rpm. The higher the pitch the less the engine will rev. So if your engine should rev to say 5,000 rpm but you can only achieve 3,000 rpm then the pitch is too high. If you lower the pitch so you can get 5,000 rpm you will actually achieve a faster top end speed than with a higher pitched propeller. The equation is simple, you must achieve maximum rpm. This equals maximum horsepower development and therefore optimum performance. Under certain circumstances you may want to fit a propeller that will enable you to gain more rev's. If you are using your boat for watersking or wakeboarding for example, a propeller with a lower pitch will give increased acceleration and bite at lower speeds, ideal for pulling up waterskiers etc. However you must be careful not to exceed the recommended maximum rpm of your engine. Generally when you use a boat for skiing you do not run flat out so this is not a problem. As a rule of thumb the higher the pitch, the faster the top end speed but the lower the rev's. The lower the pitch the faster the acceleration and the higher the rev's.

A propellers blade shape will affect its performance. Most outboard and sterndrive propellers are constant pitched across the entire high-pressure face. Some are progressively pitched. A progressive pitched propeller is less prone to cavitation and will give increased acceleration and general performance. This is because the leading edge will be a lower pitch so the water is picked up under less pressure. The water then flows along the blade surface increasing in pitch to the trailing edge where it is forced into a tighter thrust funnel and this gives higher

PROPELLER BASICS

Have you ever wondered why there are so many different sizes and blade shape of propeller? Does it make any difference to the average boat user which propeller is used? Well the answer to both questions is a definite yes. The propeller converts precious power into forward motion. If it is mismatched or damaged in any way its ability to do this will be severely affected and this will reduce your boat's performance. There is no point in having an engine in tiptop condition if you run it with the wrong propeller. So, with so many different types and sizes of propeller available how do you choose the correct one? Unfortunately there isn't a simple answer. The following is only intended to be a rough guide to propellers, as it is beyond the scope of this article to cover every possibility, but you should begin to appreciate the different types and their applications. However at the end of the day it is always wise to seek professional advice.

Surprisingly the material the propeller is made from also affects performance. Most outboard and sterndrive propellers are made from aluminium. Fitting a stainless steel propeller will increase performance. The reason for this is simple. Stainless steel is a much stronger material than aluminium which enables the propeller to be manufactured with a thinner blade section. If the leading edge is thinner less air bubbles will be formed on impact and thus cavitation is reduced. A thinner blade is much more efficient that a thicker one. For racing applications the blade section can be thinned to such an extent that you could literally use the propeller to have a shave. For most applications such a thin blade section is not desirable as it is more prone to failure under stress. For the enthusiastic boat owner, simply changing from an aluminium propeller to a stainless steel one will give improved performance. For significant improvements try a stainless steel progressive pitched propeller with cupping.

For outboard and sterndrive propellers, diameter is related to pitch, Generally the lower the pitch the larger the diameter and vice versa. The reason for this is generally boats that require low pitches are slower and heavier and they need propellers with a larger diameter to give improved thrust. Faster lighter boats need less thrust to achieve planning speeds and therefore less diameter but larger pitches to give optimum performance.