The thrust of a propeller is calculated as its pitch. In theory, the pitch is how many inches forward the boat should move for each full turn of the propeller. In practice, the thrust transferred to your boat may vary because of the size of your boat, the kind of hull you have--whether it sits in the water or rises up to plane on top of the water--and a factor called "propeller slip", which is the difference between how far the propeller should make the boat move and how far the boat actually moves.
1. Set the bottom (straight) edge of a protractor at the widest part of one blade of the propeller, so that the straight edge forms a line between the front of the widest part of the blade and the back of the widest part of the blade.
2. Note the angle between the edge of the propeller hub and the base of the protractor. This is called the pitch angle. Use your calculator to determine the tangent of the pitch angle. For example, if the pitch angle is 23 degrees, then its tangent is 0.4245.
3. Say our propeller has a 12-inch diameter. Multiply 0.4245 by 2; multiply the result by 3.1416 (Pi) and then by the radius (half the diameter) of the propeller. The pitch of a propeller is always stated in even numbers, so the result, 16.0033,, means the pitch of the propeller is 16, and for each full rotation of the propeller, the boat is--theoretically--thrust forward 16 inches.
Items you will need
- Clear plastic protractor
- Look on the side or end of the propeller hub for a pair of numbers, separated by a slash, like "12/16" or "10/14". The first number is the diameter of the propeller; the second is the pitch.
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