How to make boat prop pitch calculations

Written by nathaniel miller
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How to make boat prop pitch calculations
Four blade boat prop with pitch dimensions labled on the barrel. (www.mackspw.com)

Boat prop pitch is the number of inches a propeller moves through the water in one revolution, so a 21-inch prop moves 21 inches with each turn of the prop itself. To effectively utilise your boat's motor, it is necessary to pick the correct prop pitch settings to maximise performance. The prop pitch settings affect your boat's speed and take-off ability, also known as its "hole shot." A boat equipped with the correct prop pitch will run at close to the maximum of its wide open throttle RPM range. An example is a 12-foot boat with a 100 HP motor and a recommended wide open RPM range of 4400 to 5000. A stock prop may allow a wide open RPM of 4500 on this boat, however a correctly pitched aftermarket prop could run at 5300 and provide a faster hole shot and increased overall speed. Determine the correct prop pitch for your boat through manual testing and pencil-pushing calculations.

Skill level:
Moderate

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Instructions

  1. 1

    Examine the owner's manual that came with your boat motor and determine the maximum wide open throttle RPM range. This is normally located in the back of the manual under "engine specifications." Look up the RPM range for your specific boat size reported either in feet or pounds. The RPM range is generally reported as if your boat was only carrying the driver and was not under load.

  2. 2

    Test your current prop configuration by taking the boat to an open stretch of water and running at least a quarter mile at wide open throttle conditions with you as the only passenger. Record the RPM that your boat truly maxes out at under wide open throttle conditions. You are looking for a prop pitch that will raise your RPM closer to its max, without over revving the engine.

  3. 3

    Subtract your current max RPM from the manufacturer's rated RPM. Now subtract 200 RPM from that number. The number that you have now arrived at is the number of RPM you need to raise to maximise your boat's wide open throttle RPM range. For every 2 inches of prop pitch, RPM will raise around 400. Thus going from a 21-inch prop to a 19-inch prop should raise RPM by 400 and improve the hole shot and acceleration of the boat.

  4. 4

    Find the prop pitch marked on your current prop by examining the blades, barrel, or front of the prop. Using the formula outlined above, calculate the new prop pitch required to upgrade your boat. Talk to manufacturers or boat mechanics and learn the prop manufacturers that are recommended for your application. Once you have chosen a manufacturer, shop around online or at your local boating goods store and purchase the prop. Install it and then take a test drive at wide open throttle to once again test the real world application of your prop.

Tips and warnings

  • Some boat yards will assist you in the determination of the proper propeller pitch for your particular boat application, and may even allow you to test different prop combinations before purchase.
  • When testing out your prop picks, remember that the "2 inch to 400 RPM" rule is only a generalisation and that different props by different manufacturers may sway from this. Be conservative and do not over rev your engine.

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