Cutless bearing and shaft alignment question

[skyflyer]

I have about +/- 2mm prop shaft movement, vertically, in the cutless bearing, but almost none from side to side. (This is a 25mm shaft, with bearing in a P bracket).

This came to my notice (suspected) when I suddenly started getting a knocking noise, but only when astern, at 1800rpm or more. I cannot rationalise why it would only occur when going astern.

anyway, the boat is for sale and under offer so being an honest bloke I declared this to the surveyor today. He says it is most likely a shaft alignment issue, but still no explanation as to why only in reverse.

I had the shaft aligned 6 years ago following engine mount replacement and have had no trouble at all until suddenly mid season this year. One week the boat was running fine ahead and astern and the next we get knocking.

the boat is now on the hard and bearing replacement scheduled for Friday, but whilst it’s out the water, does anyone have any suggestions for any other possible causes that could be investigated?

many thanks

 

[bignick]

Have you checked the engine mounts?

 

[Boater Sam]

Was the alignment checked a short while after mounting replacement to account for natural settling of the mounts?
Rubber mounts do slowly collapse over time, regular checks are required.

 

[skyflyer]

Was the alignment checked a short while after mounting replacement to account for natural settling of the mounts?
Rubber mounts do slowly collapse over time, regular checks are required.

No, I wasn’t aware they needed checking (alignment) regularly! but it’s been a very sudden deterioration

 

[steveeasy]

At a guess With the movement you describe sounds like alignment is out a bit. As for the noise in reverse. Different forces on the shaft and exposing the wear of the cutlass bearing.
I had noise in reverse. Replaced cutlass bearing and still got it. Now it’s gone. Alignment out a bit now it’s settled itself with a bit of wear.

Steveeasy