Singing propeller

[TopsailB]

Last August Vivie was hauled, after three years in the water, for some routine maintenance including replacement of the cutless bearing and bottom painting. In the process the prop shaft and coupling were removed, as was the stainless steel stern tube, and prop. The stuffing box packing was removed and replaced with Duramax Ultra-X packing. When the prop was removed it was thoroughly cleaned, polished, and painted with antifouling prop paint. Everything was reassembled without incident and the boat was splashed. When I went for the first run afterwards a low squeal was heard from the rear of the engine. Only when the boat was in gear, moving forward and the engine was at 1400 rpm, not higher or lower, and not in reverse.

Initial thoughts were that the packing was too compressed although the packing gland was cool and slightly wet on the bottom. Adjusting the nut or loosening it completely made no difference in the sound. Tied up in the slip I ran the engine and there was no squeal at 1400 rpm or any engine speed. My next thought was that the engine alignment was off due to the removal and replacement of the stern tube. Removal of the shaft coupling from the engine showed that the alignment was near perfect. The only thing I could imagine as the cause was perhaps bad engine mounts. Or something worse like an engine bearing. Another trip out with someone else driving and several checks with the boat moving forward at three knots then going to neutral for the engine to rev down, and backing the boat down hard didn't show any problem with the engine mounts.

Back at the dock we discussed the matter. No clue as to what was going on. I figured maybe components in the drive line just needed to settle in and the sound would stop. So I've been using the boat and feeling unsettled every time I heard the squeal. Recently I happened to talk with a retired diesel mechanic and shared what I had checked and the video below with him. He said it's just a singing prop. Not harmful, just annoying. An easy fix by checking and changing the taper at the trailing edge of the prop. I likely induced the singing when I cleaned and polished the prop and applied antifouling paint. He said it may even go away when the paint wears a bit or the prop becomes slightly fouled. If not, a prop shop can change the trailing edge and get rid of the sound.

https://youtube.com/shorts/djMMHSHPq1M

Wow. Singing propellers is a physics thing involving fluid vortices--who knew? A search provides good information about it on boats/ships of all sizes.

[slode]

If it is limited to the very small rpm band that is a simple resonant frequency noise.  Just run slightly over or under that rpm.  Shouldn't take more than +-50.

[soilgod]

This happens.  The trailing edge of the prop needs to be filed off a bit at an angle.  Look at props, say Michigan Prop< and see the slanted trailing edge.  With time and accumulation of gunk on the prop in the water the noise will go probably go away.