“Prop slip” may be the most widely used and most widely misunderstood term in boating. It sounds like something bad, but it’s a good thing when occurring in the proper percentage.
In simplest terms, prop slip is the difference between actual and theoretical forward travel resulting from a propeller’s angle of attack. Too much prop slip leads to poor performance and lousy fuel economy. Cavitation, or prop blow-out, will greatly increase the slip percent number. A prop diameter that is too small for the horsepower and load can also increase slip beyond desirable levels.
Too little slip can create other issues, not the least of which is destroying a drive or gear case components.
Confused yet? Try this: a 10-inch propeller actually advances only 8.5-inches in one 360-degree revolution. 8.5” is 85% of 10”, leaving a prop slip of 15%.
You can use some trigonometry, a rotational speed equation and a theoretical speed equation to calculate prop slip (and risk hurting your brain) OR you can punch in your pitch, gear ratio, engine rpm and actual speed to the BBLADES Prop Slip Calculator, and it will do the work for you.
Have other questions about Prop Slip? You can always give us a call and we’ll be happy to walk you through the process and suggest a propeller to better suit your boating style.