Mysterious knocking from stern gear

colind3782

colind3782

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On the first decent run out of the season I have noticed a mysterious knocking from the stern gear. We haven't been out since she was lifted out with new antifoul, shaft anode, cutless bearing and the feathering prop reassembled.

We took her out of the marina and ran the engine up to max RPM, around 3800, with no problems or vibration. Then we motored a couple of miles across to Roses and took on some diesel. On the way back the knocking started when the RPM went over about 2500 and was accentuated with rudder input in either direction.

Until I get a chance to either, go over the side and see if anything is amiss or pay for another lift out, I wondered if any of the assembled throng have any ideas as to the culprit?
 
A deep knocking can be the prop shaft hitting the stern tube, particularly as you describe it worsening with turning. Will also happen with following seas at times. Maybe that your engine mountings have settled a little.
 
colind3782 said:
Sounds like an idea Vyv but shouldn't the P bracket and new cutless bearing stop that? I've had a good pull and prod at the shaft inside the boat and can't find anything amiss/any play.
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Try to assess the clearance between shaft and tube inside the boat. Can be tricky with the typical rubber hose of a floating stern gland but in the past I have taken mine off, stemming the flow of water into the boat with a rag. You will get a few pints of water into the bilge but hopefully your pump will cope.

It's usually the internal clearance that changes, as I said when the mountings drop a little. Changing the cutless bearing should simply restore the status quo, so won't solve it or cause it.
 
If there is no debris around the prop, and presumably you didn't have the problem before the change of anode, cutlass bearing, and prop reassembled, its pointing to a change occurring during the maintenance of those items. I'm guessing the yard did the work, did they remove the prop shaft whilst doing so. Vyv may well be on the money with his suggestion, but it sounds like a thrust issue to me if the knocking is accentuated when turning the rudder. I don't know what sort of arrangement you have at your PTO/gearbox end, but as a long-shot check that your shaft is not moving for and aft in the coupling allowing either the prop or anode to hit your P/bracket when coming under thrust. As I say, its just a long shot, but stranger things have happened.
 
Robih said:
Just happened to me this season too. Horrid metal on metal sound at a specific rev range, sounded like bearing was shot. Paid for a lift out and found the 30mm shaft anode loose. Retightened anode and all good now.
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If you don't want to go for a haul out, (and don't fancy a snorkelling dive) , you could find somewhere you can dry out alongside for a tide- and take a look at LW? Have a spare anode ready?

Graeme
 
No rope cutter and I'm in the med, no tide!

I did find a loose locknut on part of the steering linkage which locks a ball/rose joint. When it was loose and I rattled it the noise was similar to that described. Needless to say I've tightened it and remain hopeful.

When the wind drops a bit, I'll take her out to some clear water and go over the side for a look. It's warm enough and the water in the marina is too manky to contemplate.
 
If the cutlass bearing was of the Phenolic type, they have quite a bit of clearance to start with as there is a "swelling" allowance so that when the boat has been in the water for about six weeks, there should be less clearance.

That is what I was told by the manufacturer / supplier. My boat has been in 3 month and it still rumbles at low speed. :(
 
I was thinking rudder, on friends 1/4 tonner the prop wash from a two bladed prop shook the rudder from side to side at higher revs, with loose bottom bearing it used to knock but you cold feel it through the tiller.

Also can I clear up the difference between phenolic and phenolic shelled, we get many people confused by bearings called phenolic when they are plain rubber bearings in a phenolic shell, the problem is we supply phenolic bearings, which means the bearing surface is phenolic not rubber.

With regard to moisture swell for our phenolic bearings, on a 2" shaft, new clearance should be min 0.22 min which takes account of moisture swell of 0.07mm on the diameter.
 
To check anodes etc go on Amazon and buy a cheap USB endoscope camera. We got one for under £20 to check our prop, and just connected to a laptop computer. Was able to film the folding prob when closed and spinning - not something you could safely do diving or ashore.
Saves a fortune compared to a lift if no problem found. Search USB underwater camera and you should find a thread I posted with the details and example picture quality
 
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