Selecting the right propeller for your boat and engine. The proper propeller size for your boat and engine is based on the wide open throttle operating range for your particular engine (see owner's manual). This will be expressed in terms of certain horsepower at a certain RPM.
The goal in propeller selection is to determine what propeller will maximize performance for your boat, while allowing your engine to operate in the recommended RPM range. The correct propeller will prevent the engine from over-revving, yet allow it to reach the minimum RPM where maximum horsepower is produced.
Test your propeller and determine what is the maximum RPM you are able to obtain. If during this test, you begin to exceed the maximum rated RPM of the engine, reduce throttle setting to a position where maximum RPM is not exceeded.
If your test results in your being able to over-rev the engine, you need to increase the pitch of the propeller. Increasing the pitch increment by 2" will result in approximately a 300-400 RPM drop. However, if your test shows that you are only able to obtain an RPM somewhat lower than the maximum rating given by your engine manufacturer, you would need to decrease pitch. Decreasing pitch would increase your 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 propeller that is suited correctly for your boat with respect to RPM.
At all times you should have a spare propeller with you in the boat. Boating without a spare propeller is similar to driving a car without a spare tire. Once that propeller is damaged, you may be out of luck without having a spare propeller along.