Propshaft vibration since lift-out

[riojasailer]

Our 2008 Oceanis 37, which we've had since new, had her annual lift out recently. Having recently retired I was able to clean and polish her myself for the first time but, since she was relaunched, there has been a worrying vibration from the propshaft when sailing with the gear in neutral (engine off) above 4 knots. In the past she has had a very slight vibration/hum when going well under sail but nothing like the new noise.

What have I done wrong?

When I cleaned the crustaceans etc. off the prop I think I got rid of it all but there were some areas of the shaft that I couldn't get to easily and now I'm wondering whether I did in fact get rid of it all. Could this be the cause?

For the meantime we shall sail with the gear in reverse but could there be anything else causing this vibration?

 

[colind3782]

Shaft anode? Cutless bearing?

 

[riojasailer]

Cutlass bearing replaced a year ago and no play when lifted out. Anode is on the propellor , not on the shaft.

 

[vyv_cox]

Very unlikely that a few barnacles on the shaft will cause any problems - they are too light and too close to the axis to have sufficient mass to affect a big weighty propeller. What kind of prop? Rope stripper? I assume nothing was done to alignment?

 

[riojasailer]

Thanks for helping, it's a three bladed fixed prop with a rope stripper fitted from new - and still surprisingly sharp! There doesn't appear to be any greater noise in the rear cabin, or greater vibration generally, at 4 knots under engine so I think the alignment is OK. However I couldn't be present when she was lifted out and in so I suppose a lifting strop could have been placed under the shaft but I think it very unlikely. Certainly the evidence on the hull of where the slings were when she was power washed in them implies they were correctly placed.

 

[Topcat47]

If you have a length of shaft exposed from the hull to a "P" bracket AND the boat was lifted with a Wise hoist, it is possible that the rear strop was around the shaft and has bent it. It wouldn't have to be bent very far for it to vibrate noticeably. If left it will wreck the cutless bearing. I bent the shaft on my long keeler by standing on it while turning around inside the engine compartment, underneath the cockpit sole. it was about 0,020" out of true and by golly what a difference it made.

It's also possible you have a broken engine mounting, this would also make itself felt by a change in vibration.

 

[Tranona]

Unlikely the shaft could be bent as it is in a substantial skeg with a short exposed stainless steel tube which holds the aft bearing (which is actually composite, not a cutless).

What sort of rope cutter is it? A plain disc or a proper Stripper? If the latter is the fixed cutter properly in the strker block and have the bearings been checked for wear?

 

[riojasailer]

The rope cutter is a plain disc. I would have thought that the difference between how she sailed before the lift out and afterwards is too great for it to be bearing wear - wouldn't that have made itself felt more gradually?

Could this just be a coincidence and we've picked up some rubbish on the prop during the early part of yesterday's sail?

Thanks for helping.

 

[Thistle]

You mention a vibration when sailing but is there any vibration when motoring?

 

[riojasailer]

None that we could detect - there was no noticeable increase in vibration as we increased speed up to five knots whereas when sailing it started at 4 knots and got more noticeable - my wife would say louder.

 

[Neeves]

You are sailing with the prop in neutral, fixed or folding. The vibration stops when you sail in gear.

Have I summarised correctly?

 

[riojasailer]

Fixed prop in neutral, vibration starts at around 4 knots. No discernible (extra) vibration under engine. Didn't try sailing in gear with engine off.

 

[Neeves]

I'd try sailing in gear with the engine off.

I suspect its the prop turning in neutral when the yacht is moving, fast enough - so over 4 knots.

 

[riojasailer]

That's what we've decided to do next. What worries us is why it should start now, just after a lift out, when it never did before. (and not knowing is allowing our imaginations to run riot with all sorts of dire consequences)
Not able to get out now for a week or so, just have to wait and see.
Thanks for your help

 

[Thistle]

None that we could detect - there was no noticeable increase in vibration as we increased speed up to five knots whereas when sailing it started at 4 knots and got more noticeable - my wife would say louder.

In that case I would suggest that it's an annoying noise rather than a problem. As others have suggested, try sailing with the gearbox in reverse.

 

[vyv_cox]

Is the difference simply due to the clean prop, which is now rotating at a higher speed than it was when encrusted with fouling?

 

[riojasailer]

Thank you Vyv! That has really lightened my day.:encouragement: