If you ride around on boats long enough, chances are you run across a set of propellers that will “sing.” Singing props make a sound similar to the one made when running a finger around the lip of a wine glass. A number of factors can create these strange harmonics, including propeller size, engine revolutions per minute and boat speed. Asymmetry of the prop, a difference in the prop’s shape between the front and back, creates vortices and eddies that create sounds. If changing your revolutions per minute and speed do not eliminate the noise, you have to add what propeller professionals call an “anti-singing edge.”
To add this edge when you are in the field, take the largest mill bastard file you can find and add a 40- to 45-degree angle on the forward side of the prop, extending from around the middle of the blade out to the end. You can’t balance the prop perfectly in the field, but if you count the amount of strokes you make as you take off the metal, you will keep the balance close, which, for some, is far better than hearing the singing.
— George Sawley, Fort Lauderdale, Florida