Choosing the Correct Boat Props
The best boat props size for your boat and engine combination is based on the recommended operating range at wide open throttle for your engine, which you can found in the owners manual. The owners manual will refer to it in a RPM.
Proper boat prop selection will determine the correct propeller style and size that will maximize performance for your boat, while allowing your engine to operate in the recommended RPM range. The correct boat prop will prevent the engine from over-revving and allow it to reach the minimum RPM at maximum horsepower.
Run the motor at wide open throttle under normal operating load to determine the maximum RPM you are able to obtain. Your boat will need a tachometer to complete this test. Adjust the motor trim for the optimum performance. If your test results in your boat being able to over-rev the engine, you need to increase the pitch of the boat prop. Increasing the pitch increment by 1″ will result in approximately 200 RPM drop. If your testing shows, however, that you are only able to obtain a RPM somewhat lower than the maximum rating given by your engine manufacturer, you would need to decrease pitch. Decreasing pitch would increase your RPM. For every 1″ of pitch size, the effect will be approximately 200 RPM. Switching from an uncupped to a cupped propeller will also reduce your RPM. The cupped propeller of the same pitch and diameter will typically reduce your RPM by approximately 200.
Once your wide open throttle RPM falls within the recommended range of the engine manufacturer, you have a boat prop that is suited correctly for your boat with respect to RPM. If you use your boat for fishing, cruising and skiing, one prop probably won’t do all three things equally well. It is best in circumstances like this to have two propellers. One to accommodate one set of circumstances and the other to perform best under the different load. It is imperative that the wide open throttle RPM fall within the range specified by your engine manufacturer.