Q: I have a Bénéteau 35s5 that produces a lot of vibration and rumbling when motoring...

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Q: I have a Bénéteau 35s5 that produces a lot of vibration and rumbling when motoring, particularly over three knots when it is very noticeable. Is this cause for concern?
There is quite a lot of play where the shaft passes through the P-bracket and I am not convinced that the shaft is straight, as it isn’t turning evenly or smoothly where it passes through the rubber stern gland.
Also there is a solid coupling where the shaft is attached to the engine – a Volvo 2002. Can you tell me whether I need to replace the bearing in the P-bracket? If so, how difficult is it to do?
And finally, is it possible to introduce a flexible coupling between shaft and engine? Would it help?
Nick Hockling, Gosport

A: To answer your questions in order:
Rumbling could be caused by engine, shaft or stern-gear problems or by an interaction between the propeller tips and the hull. If it’s the propeller tips, you may need a change of prop diameter or, if the prop is the right size, you may have to live with the noise. If it’s the engine, shaft or stern-gear, then read on.
Play in the cutless bearing needs to be remedied. If the shaft log is not absolutely true to the shaft centreline, and it often isn’t, then any misalignment is taken care of by the flexible rubber tube that carries the stern-gland, so this is not necessarily an indication of a bent shaft. To check this, someone needs to turn the prop-shaft from inside the boat while another person watches the shaft at the P-bracket for any noticeable bend. Otherwise you need to set up a dial gauge to measure this.
In your case you do need to replace the worn cutless bearing in the P-bracket. The amount of difficulty in replacing it depends on how tight the cutless bearing is held in the P-bracket housing (see PBO 478, p97). It should considerably reduce any vibration if the wear is as much as you imply.
The use of a solid coupling is normal, but in this case, shaft alignment is critical. Any lack of engine/prop-shaft alignment will cause vibration, which will, in time, cause gearbox wear. You may or may not have room for a flexible coupling. You will need to check with a supplier such as A.S.A.P or Vetus to see exactly how much room is needed. You may also have to shorten the prop-shaft to reduce overhang.

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