How to Choose Boat Props

No one propeller will be all things to all boats. Fishing, cruising, skiing or just going very fast on the water are all activities that require different propellers. The key to propeller selection is determining how to strike a balance between the desired recreational uses and then picking a propeller with characteristics that allow the boat to do most of those things reasonably well. These characteristics begin with the diameter and pitch of the propeller and continue into the material from which the propeller is made and the number of blades.

Things You'll Need

  • A tape measure
  • A calculator
  • Engine operator manual for the boat
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Instructions

    • 1

      Measure the prop currently installed on the boat. A prop is usually described by two numbers (14 x 36 or 12 x 22, for example). The first number indicates the diameter of the circle made by the spinning prop. In the first example "14" means that the prop is 14" in diameter. The second number, 36, means that for each full revolution of the prop, the boat moves forward 36" under ideal conditions.

    • 2

      Leave the docking or marina area and open the throttle to the "wide open" position in the operating range specified in the boat's engine operating manual. Because the idea is to select a prop that will maximize performance within the engine's operating range--the revolutions per minute, or RPM as measured by the boat's tachometer--the correct prop will let the engine operate at the minimum RPM, where the maximum power is produced and will prevent the engine from exceeding the recommended RPM range specified in the engine manual.

    • 3

      Do the math. Operating at the wide open throttle position, if the engine exceeds recommended RPM range specified in the engine manual, then a prop with greater pitch is needed. The rule of thumb is, "increase the pitch by 1 inch, and RPMs will decrease by 200." Conversely, if the engine fails to reach maximum RPM with the throttle in the wide open position, the pitch of the propeller should be decreased: reduce the pitch by 1 inch, and RPMs will increase by 200. This allows for a good balance between speed and pulling power.

    • 4

      Determine what blade configuration you want. Modern props come in 3, 4 and 5 blade configurations. The more blades, the faster the acceleration of the boat and the boat will stay on plane with fewer engine revolutions, which translates to a small fuel saving. However, 4 or 5 blade props exact a price for this enhanced performance. The gains in acceleration and planing mean that the top speed of the boat will be lower: a good 3-blade prop will be faster (by 2 or 3 miles per hour) than a 4 or 5 blade prop with the same pitch.

    • 5

      Evaluate prices. An aluminum prop costs about 1/3 of the price of a stainless steel prop. The advantage of a stainless prop is primarily increased performance over a wider range, because stainless props are available in larger pitch sizes and do not flex under power. Unlike the stainless steel prop, the aluminum prop is only available in pitch sizes up to 23 and, when placed under a load, the blades will flex and reduce the pitch by up to one full size.