The issue of "prop-singing" is a harmonic, set up by the leading edge of the blade vibrating as it bites the water
. The cause is often because someone has sharpened the leading edge. It could be that the edge was always sharp, but the re-pitching has placed the sharp edge at a different angle to the water
. What is happening is not unlike a person playing a saw with a bow. A sharp edge can cause all sorts of issues, but believe it or not, Balance is one of them. Sharp leading edges can cause the blades to "bite" at different rates and cause issues that feel like unbalanced props, inducing vibration, exccessive noise
and cavitation. The singing is not cavitation, but cavitation may or maynot also be an additional issue. Remember, a prop is a rotary wing. It is the surface of the prop that gives the propulsion
, not the bite into the water.
The remedy is simple. Round the leading edge. It is what actually should be in the first place. It is easy to do with emery cloth. Just remove any edge and ensure you have a round'ish edge.
trailing edges are not usually the issue, but I am not saying never. However, it is best to also have a round edge on the trailing edge. Or the same issue in reverse can occur. However, due the shape of some blades, cavitation can be a major issue when the prop is swinging hard in reverse, creating a heck of a vibration noise