What You Need to Know About Propeller Pitch
Many recreational boaters enjoy getting the best performance from their boat engines. A key factor in maximizing torque is to select the right propeller. A little knowledge about what parts make up a propeller and how these interact with other engine parts, will help boat owners determine the best propeller performance characteristics for their boats.
Propeller pitch is a measurement that will assist in determining the right propeller to buy. This measure is the maximum distance a propeller will move a vessel forward through the water per rotation. Roughly translated, a prop with a pitch of twenty one inches will actually move a boat forward by only fourteen inches.
The Mathematics of Propeller Pitch
Ideally, to measure pitch you have to take the prop off the shaft and lay it flat on a table. If this is not possible, know that your measurement will be slightly inaccurate. First draw a line across the widest part of a blade, then measure the distance from the front of the hub to the points where the line meets each edge of the blade. Next, use a protractor, angle gauge, or carpentry square to measure the triangle formed by the two points at either end of the line drawn across the widest part of the propeller blade and the center of the hub. The narrow, pointy end should be at the center of the hub. Measure the angle between the two lines that radiate from the center of the hub.
Now take the first measurement and multiply it by 360. (That is the number of degrees in a full rotation, see where we are going?) Then take the result and divide it by the angle you found in the second measurement. The resulting number is the propeller pitch. Of course, the easiest way to find the propellers diameter and pitch is to read the markings marked on the hub. Since many boaters are seeking to upgrade their vessels, however, this cheat sheet will not be of much use.