Occasionally propellers will “sing”. This high-pitched whine is the result of blade harmonics and the blade interaction with surrounding structure. If you have an even number of blades there is more chance of singing occurring. Typically this does not happen through the full RPM range, but it can be extremely annoying.
Prop manufacturers deal with this by putting on a wavy anti-singing edge. In our case, the current set of propellers have a wavy shape to their trailing edges. Our previous props did not have this and were very noisy. So, we removed them and had a slight cup added which got rid of the singing and effectively added a bit of pitch.
When we put on our latest set of props they were quiet except for the starboard prop at 1300 to 1500 RPM. Since we do not operate in that range we ignored the issue. But we found that at heavier displacement the singing would move up to the 1700 to 1800 RPM range and we do occasionally run at this speed.
Not wanting to remove the starboard wheel and take it to a prop shop we asked John Hall of Premier Propellers if there was anything we could do on the boat.
“File the aft side of the trailing edge for four or five inches (100 to 125mm),” was John’s reply.
Although we have a dry suit aboard for cold water maintenance, we waited until we were in Desolation Sound, with its “warm water”, to do the job. Using a medium coarse flat file we took six light passes at each blade. The amount of metal removed was minuscule. You could see the bronze colored flakes in the water and there was not much material. However, we figured it was best to do this a little bit at a time to see what would happen.
We picked up the hook and went for a test run and were pleasantly surprised to find no more singing.
So, if you have a singing prop, try a little underwater filing, on the aft side of the trailing edge.