Prop Shaft Knock

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smithy

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I removed a rigid stuffing box assembly from the stern tube and fitted a PSS seal. The pro shaft now starts to knock on the stern tube around 1800rpm The clearance around the shaft is only about 1/8" so I was careful to centre it when setting up the alignment.
I have tried renewing Beta engine mounts and cutless bearing (in carrier on end of tube). I am drying the boat out again and going to re centre shaft and realign engine, I can't do this in the water as I have to pull back the seal. If this doesn't work I plan to remove shaft and check straightness.
I was thinking of fitting a maritex bearing in the inboard end of the tube. Thoughts?
R & D coupling
Disk rope cutter tight
Prop tight
 
Had exactly the same issue and fitted a short piece of cutless bearing in the old bearing housing section of the stuffing box which previously had a white metal lining. Volvo type seal and Vetus Bullflex coupling . Worked well originally with a Yanmar 1GM then a Nanni 14. no reason why a composite bearing properly sized and a PSS should not work.
 
If you have a flexibly mounted engine, plus an R&D this will occur with that sort of clearance. It could just be the engine (shaft moving around on the mounts, it could be shaft 'whipping' (think of a skipping rope).

The previous stuffing box would have acted as a centre bearing to hold the shaft straight, so the only support for the shaft now is a cutlass bearing at the outboard end.

A bearing of some sort will be needed.
 
I have had just the same problem with a PSS seal, which provides no support. Careful alignment mostly eliminates the knocking, although in a following sea it will knock at around the same revs, 1900 in our case. It is far worse going astern, when presumably the lateral force changes. Inspection has shown no serious damage to either shaft or stern tube.
 
This was discussed in detail in another thread a few months ago, the issue is that there is too much flexibility in the system now that the support has been removed.
The recommendation (if not installing another support) is to remove the flexible coupling and replace this with a fixed bobbin or spacer. The R&D protected the system and allowed the engine to move freely when the shaft was fixed, now that the shaft is floating, this only allows excessive movement in the driveline.

It could also be that the clearance between shaft and stern tube is too tight irrespective, and in this case the movement of the engine on the mounts can allow the shaft to hit the tube. In this case some kind of shaft support should definitely be installed. It is noted that others have this happen and are OK with it, but I've seen shaft wear, tube wear and a cracked stern tube in my time so I wouldn't go to see with it like that.

Good luck with it ?
 
Moodysailor said:
This was discussed in detail in another thread a few months ago, the issue is that there is too much flexibility in the system now that the support has been removed.
The recommendation (if not installing another support) is to remove the flexible coupling and replace this with a fixed bobbin or spacer. The R&D protected the system and allowed the engine to move freely when the shaft was fixed, now that the shaft is floating, this only allows excessive movement in the driveline.

It could also be that the clearance between shaft and stern tube is too tight irrespective, and in this case the movement of the engine on the mounts can allow the shaft to hit the tube. In this case some kind of shaft support should definitely be installed. It is noted that others have this happen and are OK with it, but I've seen shaft wear, tube wear and a cracked stern tube in my time so I wouldn't go to see with it like that.

Good luck with it ?
Click to expand...
Not always as you say, although you are not wrong. My boat has an Aquadrive, so engine mount flexibility does not enter into it. I suspect that the 1900 rpm at which the problem occurs is the natural frequency of the shaft and prop. At this frequency it will tend to form a single node 'skipping rope' profile that allows it to contact the stern tube. The clearance is quite small on my boat.

I have spent considerable time trying to centralise the shaft in the tube but with only limited success in reducing the knocking.
 
vyv_cox said:
Not always as you say, although you are not wrong. My boat has an Aquadrive, so engine mount flexibility does not enter into it. I suspect that the 1900 rpm at which the problem occurs is the natural frequency of the shaft and prop. At this frequency it will tend to form a single node 'skipping rope' profile that allows it to contact the stern tube. The clearance is quite small on my boat.

I have spent considerable time trying to centralise the shaft in the tube but with only limited success in reducing the knocking.
Click to expand...

I'm a huge fan of an Aquadrive (or in fact, most methods of transferring the thrust to the hull directly vs via the engine mounts ). I eluded to the shaft whipping in my first reply, I don't often go into too much detail on that aspect as a) it seems some aren't too aware of it and b) it's usually quite hard to fix.

I may presume that in your case (and possibly for the OP), there is quite a long length of unsupported shaft? With a small diameter shaft, this will almost always lead to some whipping effect at a certain frequency (read: rpm). The fix is to either fit a larger diameter shaft (rarely feasible), or to install a centre bearing of some kind.

I remember a repower project we did years ago where, try as we might, we just couldn't get the shaft to avoid contacting the stern tube. As part of the repower we had replaced the stuffing box with a VP shaft seal. After many hours of trial & error and discussion, the fix was to fit the stuffing box back on. Sometimes the "fix" is not to fix anything.....
 
From the way the OP describes his problem it seems the stern tube might be quite short so the shaft could be relatively short from the PSS to the aft bearing. If the distance from the PSS to the coupling is also short then shaft whip is unlikely. It may well be that a Volvo type seal could provide enough support as an intermediate bearing to stop the knocking. The PSS bellows are much more flexible. However, that is a bit of my speculation and inserting a bearing into the forward end of the tube would be the best solution as I did when I converted my Stuart Turner stern tube. First iteration was to machine out the white metal in the stuffing box and attach a bellows type seal to the outside of the housing. This was not successful leading to shaft knocking because of the bouncy Yanmar 1GM. So had the housing bored out to take a 2" length of cutless and problem solved. Subsequently changed the seal to a Volvo which was shorter and allowed the fitting of a Bullflex coupling as the shaft was now effectively fixed. Vary successful with both the Yanmar and the Nanni that replaced it.
 
Cannot argue with the above ... shaft now in a less rigid setup.

Once lifted out - may I suggest a look see at any rope-cutter you may have fitted ?

I had to lift out one time for a mysterious knocking on shaft at certain revs ... mine turned out to the gator fixed part of rope cutter was loose and at certain revs hit the shaft cutter part.

Years ago - same boat when in Solent ... on way up to Newport - picked up a piece of net on the prop ... knock knock knock ... basically causing the prop to be out of balance ..

Just mentioning.
 
vyv_cox said:
Not always as you say, although you are not wrong. My boat has an Aquadrive, so engine mount flexibility does not enter into it. I suspect that the 1900 rpm at which the problem occurs is the natural frequency of the shaft and prop. At this frequency it will tend to form a single node 'skipping rope' profile that allows it to contact the stern tube. The clearance is quite small on my boat.

I have spent considerable time trying to centralise the shaft in the tube but with only limited success in reducing the knocking.
Click to expand...
Your comments re shaft amplitude and RPM make sense, in as much as the OP, by removing the fixed stuffing box, has lengthened the unsupported length of the shaft (and amplitude) for which it was not designed. Reintroducing a new bearing in its place as Tranona suggests, would solve the issue.
 
I'm interested in the idea of adding a new bearing of some sort into my set-up (engine on rubber mounts, R&D flex coupling, PSS seal, then a longish bit of
shaft before a cutless bearing in the P-bracket, and finally a folding prop). I've never had a stuffing box on this boat. There's little room for the PSS seal as it is, I can't get a Volvo type seal on the stern tube (or couldn't when I last looked at availability). There's no space for an Aquadrive either. Like the OP, I only have 3-4mm clearance around the shaft in the stern tube. What sort of bearing should / could I fit? For example, does it have to be complicated hi-tech or would a simple water lubricated shell do the job?
 
kacecar said:
I'm interested in the idea of adding a new bearing of some sort into my set-up (engine on rubber mounts, R&D flex coupling, PSS seal, then a longish bit of
shaft before a cutless bearing in the P-bracket, and finally a folding prop). I've never had a stuffing box on this boat. There's little room for the PSS seal as it is, I can't get a Volvo type seal on the stern tube (or couldn't when I last looked at availability). There's no space for an Aquadrive either. Like the OP, I only have 3-4mm clearance around the shaft in the stern tube. What sort of bearing should / could I fit? For example, does it have to be complicated hi-tech or would a simple water lubricated shell do the job?
Click to expand...
Depends on the stern tube. The suggestions above assume that the stern tube originally had a conventional stuffing box screwed on, in which case there would be a metal housing which could be bored out to take a bearing and then screwed back onto the stern tube. The external part of the housing would be machined to take the seal - in my case a Volvo seal.

I suspect though in your case the stern tube is bonded into the hull with an open end and the PSS clamped to the outside. If that is so it may not be possible to insert a bearing because of the limited space between the tube and the bearing. However it is worth exploring as I may be overly pessimistic.

Suggest you contact h4marine.com the owner posts here from time to time and is the expert on this subject.
 
Got the boat out. The shaft was centred when I launched but has dropped slightly. The back engine mounts seemed to have settled since I put them in a few months ago. The shaft is now centred and I have aligned within 4 thou (R & D limit is 10thou). There is no way of checking centering when back in the water and if I try adjusting the alignment again it will almost certainly knock the centering off. I will launch the boat tomorrow and see if the problem is cured.
If this hasn't worked I think I will look at epoxying a bearing into the stern tube. I've been onto the H4 site and tried the bearing sizes calculator. It appears the size is possible despite the thin wall, although I would speak to them before going further.
Anyone know how to accurately measure the ID of the stern tube when the shaft is in place, can't get verniers or internal calipers in.
 
Well boat back in the water and no improvement. My attention is now turning back to the cutless bearing although I did renewed this. There is some shaft wear, about 5thou. In total I have about 15thou movement of the shaft which will give a clunk if hit. Could this be the sorce of the problem? I have also noticed the knock does not seem to be apparent in the first few minutes of running although this seems illogical.
 
Would it not just be easier to refit the stuffing box than trying to engineer a solution to a problem caused by removing it?
 
Last edited by a moderator:
I would if I could remember where I put it, old age doesn't come alone! I'm resigned to taking the shaft out over the winter unfortunately got to lift the engine to do it.
 
Well got to route of the problem but not before lifting the engine and removing the shaft. I found a washer underneath the head of one of the bolts on the R & D coupling that shouldn't have been there. It meant the bolt coming into contact with the face of the coupling when the thrust went on, wear apparent on both surfaces. I could have sworn the noise was coming from far further back. Wish I could blame someone else, I had even checked the bolts for tightness several times and not spotted it.
 
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