Singing stationary prop

[anthonyyearsley]

Last winter I replaced the cutless bearing on my 1989 Fulmar as wear had reached the limit. At the same time I fitted the Volvo cushion drive plate to the gearbox indrive on the 2002 although there was negligible wear on the splines at 1500 hours. Now when sailing at over 5.5 knots with the prop locked in astern, there is a clear tone, about middle C, from the prop shaft. Although useful to the skipper below in telling him when the crew on deck are not maintaining cruising speed, I would rather have silence. The prop is silent when motoring or freewheeling. Presumably the play in the worn cutless bearing prevented the resonance being maintained. Is the cure the same as for a spinning prop? The prop makers has never come across this before.

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[richardandtracy]

It sounds like an unwanted resonance.

There are several ways you could attack it.
1) Increase the frequency out of the audible range by stiffening it up (extra clamps on the shaft maybe, or add another bearing).
2) Decrease the frequency below the audible range by reducing the sytem stiffness or increasing its mass (like with lead sleeves clamped on the shaft in the shaft tube).
3) Add some damping material to absorb the energy. I think the old cutlass bearing allowed some play and the impact on the bearing as the shaft resonated absorbed the energy that created the sound. You could try to replicate this at the other end of the shaft. Is it possible to get to any part of the shaft and use a rubber lined clamp? The hysteresis of the rubber in the clamp could take out enough energy to damp the sound.
4) Go to a noise isolator specialist and get a tuned damper created for your shaft. It would consist of two steel rings (the inner one clamping to your shaft) separated by rubber. The aim would be to get the outer steel ring to vibrate like crazy while the shaft held almost still. This is not an easy option & would require some fair design work as it's unlikely that there would be an off the shelf system. If you are relatively handy you may be able to have a bash at this yourself using two steel rings and some silicone sealant - it would be rather hit & miss though.

Regards

Richard.


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[NigeCh]

There are 2 solutions

1) haul the boat, remove the prop and blunt/round the trailing edges...
1a) Replace prop, Pop the boat back in the water and test
1b) If it works then fine ..... Otherwise ......
1c) Re-haul boat and repeat ad infinitum till the singing stops

2) Fit either a folding or a feathering prop.

Both are expensive ways to cure a problem that should not exist.

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[iangrant]

Do nothing, Leave it alone it will naturally foul up a bit and the hum will stop

Ian

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[LORDNELSON]

Re: There are 2 solutions

Why not have the prop in ahead?

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[anthonyyearsley]

Re: There are 2 solutions

Many thanks for all the suggestions. I believe that I should be able to prevent the resonance starting, so will have a careful look at and possibly reshape the blade edges when the boat comes out in October. The gearbox is kept in astern to stop it - in ahead it would freewheel with risk of rope fouling.

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