Prop Pitch Vs. Speed

A boat's propeller transmits the power of a boat's engine to the water in the same way tires transmit the power of a car's engine to the road. In theory, the larger the circumference of the wheel, the farther one complete turn of the tire moves the car. The greater the pitch -- the angle of the blades -- of the prop, the farther one complete turn of the propeller moves the boat, in theory.

  1. Pitch

    • A propeller's pitch tells you how far forward the prop throws the boat with each full revolution, off any one blade on the prop. You can find the pitch stamped into the body of the prop, either on the inside or the outside of the hub that fits over the propeller shaft. The pitch is indicated by the second number in a group of numbers that looks like a simple math formula; for example, "12 X 18." In this example, the pitch is 18, which means that the propeller will -- in theory -- move the boat 18 inches for each full rotation.

    Pitch and Speed

    • Though pitch represents the distance the propeller moves the boat in ideal circumstances, those circumstances rarely occur in real life. This means you might make a reasonable guess as to the boat's speed, based on the pitch of the prop and the motor's rpm, but it's only a guess. The prop's interaction with the water may cause cavitation or ventilation, phenomena that, along with propeller slip, decrease the prop's efficiency.

    Propeller Slip

    • Although the greatest part of the engine's energy pushes the boat forward, a small part of that energy is used to turn the propeller. The difference between your boat's speed in an ideal word and its speed in the real world is propeller slip.

      If your outboard is humming along at 4,500 rpm and its final gear is 2.1-to-1, the propeller shaft -- and the prop -- will rotate 2,143 times each minute, or 35.6 times per second. If the prop has a 12-inch pitch, it should move you 429.6 inches, or 35.8 feet, per second -- about 21.28 nautical miles per hour, or "knots." If you only manage 20.15 nautical miles per hour, you lose 1.13 knots to propeller slip.

    Cavitation and Ventilation

    • A prop raised so far from the water it traps air from the surface as it turns is ventilating. The prop is turning in a sea of bubbles -- air trapped in water. Bubbles make a poor surface against which to push, degrading the prop's performance and efficiency, and the boat's speed. Cavitation is another phenomenon: as the prop turns, it leaves a vacuum behind its leading edge. The change in pressure causes the water to boil momentarily. Not only do the bubbles that form slow the boat, they burst like small bombs, hammering at the propeller. Continued cavitation can damage the prop extensively.