Is it a worn cutless bearing ?

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Jonathanbell903

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Hope someone far more knowledgable than myself can diagnose the issue - at low revs there is a knocking sound coming from low down in the stern, the engine in neutral runs as it should, forward and reverse are slow to engage at times and the knocking sound disappears the higher the revs. To me it sounds like a worn cutless bearing ? Am I correct or going down the wrong path ?
 
WEll, it could be but the condition of the entire drive train is in question until you find the culprit. Ther could also be some movement in the flexible coupling, assuming you have one, and the engine feet do soften with age and can also delaminate allowing vibration to get through. To be sure, you need to dry out and actually test for side play in the cutless bearing. It can be felt as sideways movment of the shaft when you push it.

Rob.
 
Thanks for the reply Rob - at least I have a place to start and eliminate certain items. When in neutral the engine doesn't vibrate ? Could the engine feet be eliminated because of that ? Also the boat is a 30 foot Cobra and it seems the engine us at sufficient angle that there is no flexi coupling ? Would that be right ?
 
Now having heard or felt it, nor knowing whether this has only started recently, I would say that it could be just the 'critical speed' of your installation. There is often a narrow range of revs during which there is vibration; once past that range the vibration disappears. If this is the case it is simply a matter of avoiding those revs. In practical terms, assuming that there is a good flow of water through the grooves of the bearing - and that the groves are clear of any fouling - the bearings are quite tolerant of slack. I have had cutless bearings that had more than 0,75mm clearance (on a 25mm shaft) and still gave no problems.

Not giving advice but just my personal opinion.
 
Should of said earlier but this vibration has only just become apparent in the last two days - on route from Cowes to Lymington and we heard a knock knock knock develop in a short space of time, it seems the knock is relative to the prop revolutions , and as far as I can see there us nothing fouling the prop .....
 
Jonathanbell903 said:
Should of said earlier but this vibration has only just become apparent in the last two days - on route from Cowes to Lymington and we heard a knock knock knock develop in a short space of time, it seems the knock is relative to the prop revolutions , and as far as I can see there us nothing fouling the prop .....
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In that case I would rule out the bearing.

Do you have an anode on the exposed shaft (assuming that it is exposed and on a P-bracket)? Sometimes these work loose and drift down until they hit the bracket.

It could also be a worn key in the prop or the prop itself is loose on the shaft taper.

A whole list of 'could be' explanations but I'm afraid that you'll only know when you can physically examine the prop. Sorry I can't give you a proper answer.
 
A knocking sound as you describe it doesn't sound like a worn cutless bearing. If your engine alignment is out it could be the shaft hitting the stern tube. Try motor sailing on different tacks and see if it gets worse or better.
 
Jonathanbell903 said:
Hi Puffthemagicdragon,

No p bracket just a rope cutter and no anode
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In that case it could well be the rope cutter or some part of it that became loose and is fouling. Only guessing at this stage. Please do remember to tell us what it was when you find out. Good luck and I hope that it is not something serious.
 
Jonathanbell903 said:
Hi Puffthemagicdragon,

No p bracket just a rope cutter and no anode
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Difficult to tell exactly from the angle of the photo, but the rope cutter seems to be installed with the fixed cutter too far in the block. That would cause bearing wear as the shaft moves forward when in gear and the whole cutter becomes a thrust bearing and wears the delrin bearings. The noise you are hearing is probably the cutters rubbing against each other. The cutter should be moved back toward the prop so tongue is in the middle of the block. The details are on the installation instructions on www.ropestripper.com
 
Had a possibly similar loud 'clacking' noise at certain revs thought to be the rope cutter.Despite the fact that it sounded distictly mechanical it proved to be the longish grease feed flexible tube to the gland that was simply slapping the hull at certain narrow rev bands when the engine rocked on its mounts.
 
I will be able to post an update tomorrow as the boat is coming out for an inspection and hopefully it will be obvious as to what the problem is. The only issue I have now is I have convinced myself it is the rope cutter based on all the replies I've had !!
 
Robin said:
I still cannot open pics on the forum but which cutter is it? it is quite likely the cutter is the cause, had that in the past with the excellent ambassador one. Time for a swim perhaps to check things?
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something happend to the way pics open a few weeks ago I have found that I have to right click on the pic/link and open in new tab
 
The boat was lifted today and.................it was the rope cutter !!!!!

Only problem is it looks like it may have damaged the fitting housing the shaft, not sure how much because I wasn't there when she was lifted and I only have a poor photo from the marina. But the offending rope cutter has been removed and I am now awaiting a quote on the fitting to be repaired / replaced. However there was no water ingress whilst motoring back to the marina so I really need to check the damage out for myself before forking out for expensive repairs that may not ultimately be necessary..........thoughts anyone ?
 
Jonathanbell903 said:
..........thoughts anyone ?
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Yes. Forget the rope cutter and put a loosely fitting piece of plastic pipe (rigid, not hose) over the shaft in the space between the prop and the end of the stern tube. It will not cut the rope but it will prevent it from jamming itself onto the shaft. Nine times out of ten it can be simply unwound by pulling on the free(d) end.
 
Puff (if I can be so bold to call you that) what a great idea !!

I'm looking at the boat on Friday as the marina said they didn't want to put it back in incase there was an issue. But I motored for about 4 miles with the dam thing spinning and had no water ingress just the blasted noise - now the cutter has been removed I don't see that there can be any remaining problems ? Clearly I need to look first but at the moment I have asked an engineering company to quote me to repair but I think they will quote on replacing the fitting attached to the hull that the shaft passes through........ Not sure if the expense is necessary ?
 
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