Selecting The Correct Propeller
by Keith Krych
How important is choosing the right prop for your boat? Well the wrong prop can turn what should be the perfect outboard for your boat into a lemon. But how do you make sure you choose the right one? Although it seems complicated, it really isnít.
The correct propeller is one that provides the desired performance while the engine is operating within the manufacturer's recommended rpm range. The first step is to look in the operator's manual to find the recommended wide-open throttle (WOT) rpm range is. Then compare that to what you are getting with your existing prop at WOT.
If the engine is exceeding the recommended WOT range, it's necessary to go to a prop with more pitch, or if the blades on the existing prop are uncupped , switch to one with cupped blades. A 2-inch increase in pitch will decrease rpm by roughly 300 to 400. Changing to a cupped prop, while maintaining pitch and diameter, will typically reduce rpm by about 200.
Conversely, if the engine doesn't reach the recommended WOT rpm range, pitch should be decreased, or switch to an uncupped prop.
When the WOT rpm fall within the recommended range, the prop is correctly matched. Even so, some aspect of performance ó hole shot, top end ó may not yet be achieved. Experiment with different styles and materials, but remember that you'll have to match size to get within the proper WOT rpm.
What about stainless steel. Stainless is a much harder material than aluminum. So the blades can be made thinner and with more cup. With a stainless prop you can get a better hole shot and top end speed.
Remember, too, that one prop may not meet all your performance demands. You can either live with the one that offers the best compromise, or carry a second prop that meets the different requirements.