How to Size a Prop

Using the correct size of prop on your boat will get the best performance out of your engine.

When it comes to sizing the prop for your boat, bigger is not better. Propellers are referenced by two numbers in sequence. The first number refers to the diameter of the propeller. The second number is the pitch of the individual blades. The goal in selecting the right prop is to maximize the boat's performance while operating in the recommended RPM (Revolutions Per Minute) range.

Instructions

1. Measure the distance of the prop from the center of the hub to the outside edge of one of the blades. Multiply this number by two to get the diameter of the propeller.

2. Take your boat out on the water and run it under normal conditions. Write down the RPMs on the boat's tachometer during this test, also record the speed on the boat's speedometer for the RPM number you wrote down. The pitch of the propeller is the distance it moves the boat per one revolution.

3. Divide the RPM number determined in Step 2 by the gear ratio indicated in the owner's manual for your boat. Multiply this number by 1.2 then divide that number by the speed of the boat during the test. Round the number up to the next whole number and you have the pitch for the correct propeller for your boat.

References

• Propline: Selecting the Correct Propeller Size