How to Determine RPMs From Propeller Pitch

When you buy a boat motor, you'll notice that it comes with a stated rpm range, such as 4,000 to 5,000. However, when you actually begin using the motor, it likely won't turn at the maximum rpm figure quoted even if the throttle is fully open. To optimize the motor's performance, you'll want to run the motor at the top of its rpm range, but the only way to increase the motor's rpm output is by changing the pitch of the propeller.

Things You'll Need

  • Calculator
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Instructions

    • 1

      Determine the propeller's current pitch and maximum rpm at that pitch. For example, a propeller has a pitch of 20 inches and a maximum rpm rating of 4,800.

    • 2

      Add 200 rpms to the maximum rating determined in Step 1 for every one inch you lower the propeller's pitch. Using the example from Step 1, if you lowered the propeller pitch to 18 inches, you would add 400 rpm to 4,800 and get 5,200 rpm.

    • 3

      Subtract 200 rpms from the maximum rating determined in Step 1 for every one inch you raise the propeller's pitch. Using the example from Step 1, if you raised the propeller pitch to 23 inches, you would subtract 600 rpm from 4,800 and get 4,200 rpm.