stern bearing question

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seabright

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We have a traditional long keeled yacht (super sovereign 35) with a white metal stern bearing. We have sailed over to New zealand over the last two years so she has done alot of miles recently. She performed really well and gave us a very smooth ride. We took her out of the water for 3 weeks for some TLC and all went well. Once back in the water though you could hear a whine from the stern at very low revs. As soon as you increase the revs above minimum then no problem. We didn't touch engine, tranmission, stern gland of stern bearing (honest guv) so confused as to why the whine now. The whine is not harsh but more of a singing noise.

Since then, I have replaced enfine mounts and realigned the engine as part of general maintenance. We have a flexible coupling (rubbery thing) and the stern gland is in dead wood (ie not flexible). Noise better but still there. Very pleasant singing at very very low revs.

Not too worried but keen to have thoughts if possible. May replace it with a modern cutlass bearing soon. Current bearing is only 2 years old but have a hydraulic gearbox so shaft has done alot of spinning in the last 2 years. Noise only started after time ashore though and now the engine moves less and pretty aligned. Could the bearing be dry? Does it need to be, well, wet?
 
Did you touch the prop.? As if the blades are too "sharp" as they cut into the water it makes them "sing".I had this prob. and a forumite suggested I took the sharp edge off,which I did and the prob. went.
 
Yep I did clean the prop quite aggresivly with wet and dry and then prop shield was applied.

Be nice if this was the problem - it is a singing sound.

Thanks for the reply. I will round it off a little. The pessimist in me still wonders about the bearing,

Thanks for the reply.
 
This diagnosis is probably correct; the best confirmation is that the noise arises over about 500-700 revs range in the middle of the revs range - so you can drop revs and raise revs to eliminate it. Plus, the noise is not associated with any vibration in any other part of the gear.

If there were probs with the stern bearings I suggest you would not be able to rev away from the noise.

Working the tips of the prop will help, but beware balance is not disturbed. It is also not unknown for tip work to merely move the range in which the singing arises!

PWG
PS nice part of the world you sailed to! Envy....
 
No, just a old fashioned stern gland. Thanks for the thought though.

Read more about the singing propeller theory and the propellor is now very very shiny (I was quite pround of it) and prop shielded for the first time.

Only happens at very very low revs though. Oh, and doesn't happen when engine going astern at any revs. Haven't tried it whilst the boat is tied alongside though. I've read that this is a good test for a singing prop?
 
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