We usually specify what is called an anti-singing edge on our props. Essentially this is a chamfer on the forward face trailing edge of the blades as shown above. Occasionally this is not quite the right shape and at certain RPMs and loadings the prop will start a high pitched, and very annoying, song.

The props on Wind Horse were singing to us on the trip down from Gibraltar, probably the result of the extremely low load under which they were operating. The song would occur each time we accelerated down a wave face. The answer is in the photo above. We roughed up the anti-singing edge and tuned out the harmonic which was causing the music.