For the engineers - torsional vibration? - long winded! - Help!!

[JumbleDuck]

I then used the ensign staff as a listening stick. No undue noise from gearbox, stuffing gland and maybe just a bit from the shaft log. Then I put it on the nut which secures the one of the inboard ends of A bracket that holds the end of the shaft (which are just bolted through the middle of a plank). The knock was really loud and distinct. No doubt in my mind at all that that is where it is coming from. Doesn't do it going astern. I need to check that the prop is not moving forward and hitting the cutless bearing but it would have to be moving about 10 mm and there is no sign of it doing that from inside.

So what next. I could fiddle around and try another prop but suspect that will just come back to the same place again - needing some sort of torsional damping.

If you change the prop you'll change the torsional vibration characteristics of the system and with luck avoid the resonance. If I were you I'd try that just to be sure, though obviously you'll want the Featherstream back as soon as possible.

As I see it, there are three ways you can introduce some torsional damping into the system.

1. You could perhaps fit a damping drive plate in between engine and gearbox.R&D make 'em - http://www.randdmarine.com/downloads/RandD_Damper.pdf
2. You could replace the coupling - again - with a damped one. Polyflex do damped coupling plates (http://polyflex.com.au/Flexible-Drive-Couplings.php), but I'd start by asking R&D and Centaflex if they can suggest anything
3. You could add an external torsion damper. I don't know any makers, but I'm sure they'll exist.

Worth talking to Beta, Darglow and the engineers about this. Unfortunately you probably have a systemic problem: no individual part is faulty but the combination is behaving oddly, so nobody is directly responsible. However, it's in all their interests to find a solution and keep you happy!

 

[dur]

If you change the prop you'll change the torsional vibration characteristics of the system and with luck avoid the resonance. If I were you I'd try that just to be sure, though obviously you'll want the Featherstream back as soon as possible.

As I see it, there are three ways you can introduce some torsional damping into the system.

1. You could perhaps fit a damping drive plate in between engine and gearbox.R&D make 'em - http://www.randdmarine.com/downloads/RandD_Damper.pdf
2. You could replace the coupling - again - with a damped one. Polyflex do damped coupling plates (http://polyflex.com.au/Flexible-Drive-Couplings.php), but I'd start by asking R&D and Centaflex if they can suggest anything
3. You could add an external torsion damper. I don't know any makers, but I'm sure they'll exist.

Worth talking to Beta, Darglow and the engineers about this. Unfortunately you probably have a systemic problem: no individual part is faulty but the combination is behaving oddly, so nobody is directly responsible. However, it's in all their interests to find a solution and keep you happy!

'Morning

Yes I guess I should try the old prop - mostly for interest - and I will put some spacers under the engine mounts to reduce movement though I have to figure a way of checking the alignment with the Centaflex coupling - not as easy as the R&D coupling.

As a first step I will ring Beta again and see if they will talk to Centa.

 

[Hypocacculus]

It's difficult to tell from the photos, but what is the clearance between the prop blades and the hull? It looks quite close in the picture but I suppose it could be foreshortening. I'm sure you know but should be (I think) about 10-15% of the prop diameter.

 

[fisherman]

I had a 4cyl Ford sabre, it jiggled about because the rear flexi foot was on the flywheel housing, which, because of the weight of the gearbox was the centre of gravity of the whole thing, it just rotated around the aft foot. The solution, had I not sold it, would have been to put the engine on a raft with feet well forward and aft, and maybe three a side. If you have space to raise the flexi mounts you could put the engine on a length of channel or angle iron, with the flexi mounts re arranged further forward and aft. If you want to hold something still it's easier with a long lever.

On the other hand, remember the old Mini had a stabiliser bar from the cyl head to the bulkhead: a bar sideways from the top of the engine to a flexi mounting might cure it, though unorthodox.

 

[dur]

It's difficult to tell from the photos, but what is the clearance between the prop blades and the hull? It looks quite close in the picture but I suppose it could be foreshortening. I'm sure you know but should be (I think) about 10-15% of the prop diameter.

Yes I did look but didn't have a tape measure. I think it is about 2" so not quite 15%. I suppose with the lighter flywheel a slight pressure difference might show up when it didn't before. Another thing to check more closely.

 

[dur]

I had a 4cyl Ford sabre, it jiggled about because the rear flexi foot was on the flywheel housing, which, because of the weight of the gearbox was the centre of gravity of the whole thing, it just rotated around the aft foot. The solution, had I not sold it, would have been to put the engine on a raft with feet well forward and aft, and maybe three a side. If you have space to raise the flexi mounts you could put the engine on a length of channel or angle iron, with the flexi mounts re arranged further forward and aft. If you want to hold something still it's easier with a long lever.

On the other hand, remember the old Mini had a stabiliser bar from the cyl head to the bulkhead: a bar sideways from the top of the engine to a flexi mounting might cure it, though unorthodox.

Funnily enough I was thinking about those stabiliser bars when I ran it last night. I sat on it instead which reduced the movement but didn't stop the knocking.

Beta / Centa are investigating whether there is a softer damper plate which will fit in the bellhousing. Apparently there is not much room.

 

[jakeroyd]

Just a knee jerk idea.
Is it possible to fit 2 jubilee clips on the shaft with the 'heavy' threaded drivers at various angles relative to each other.
Land Rover propshafts used to be balanced on the car this way.

Or as othets have suggested both harder and softer mounts will change the resonants ftequencies of the whole system.

 

[dur]

Just a knee jerk idea.
Is it possible to fit 2 jubilee clips on the shaft with the 'heavy' threaded drivers at various angles relative to each other.
Land Rover propshafts used to be balanced on the car this way.

Or as others have suggested both harder and softer mounts will change the resonant frequencies of the whole system.

I think at the low speeds we are talking about - 285 rpm for the prop shaft with the engine at 700 rpm such weight on the shaft is not very significant. Alsl compared with the relatively significant weight of the prop itself which is also a wider diameter. I did ask Beta if fit is possible to add a flywheel to the engine and they say they already fit an inertia ring (sounds like a flywheel to me) and that it is not possible to fit a heavier one. Even if Centa come up with a softer damper plate and ignoring its cost and the cost of fitting which I guess could be an engine out job, there will be no guarantee it would work. The one Centa would prefer to use doesn't fit. It sounds like they are running out of ideas.

It would be interesting to know if any of the other makes of gearbox to shaft couplings offer a greater level of damping than the Centaflex M 127 which we have

 

[PaulJS]

Just my suspicious mind, but have you confirmed that both of the aft A-bracket securing bolts are firmly attached to solid wood?
I'm suspicious that a not unusual combination of new components is producing a very severe knock which appears to be emanating from the new stern tube, and the fact that it's worse in one direction COULD imply that there is some slack somewhere in the drive train that only reacts severely to ahead torque reaction.
I only suggest this as I can tell you are clutching at straws...
Good luck,
Paul

edit... also is the bracket itself in good condition?
2nd edit... sorry, just reread your posts and noticed that you said the worst knocking was from the A bracket bolts. I'm even more suspicious now.

 

[Tranona]

Couple of observations. Not sure it is worth changing the coupling if the knock is coming from the A bracket area. A Bullflex might have been a better choice of coupling as it allows for misalignment. However your shaft seems to be rigid in bearings either end so the purpose of the flexible coupling is to isolate the engine movement from the shaft. I have a similar arrangement on my Eventide - photos on Vyv Coxs site.

Looking at the photo of the external bits, you have a lot of overhang of shaft aft of the bearing and the cutless seems to be unsupported for half of its length. Is that a modification to replace a greased aft bearing in the stern tube? Have you had the shaft out and the bearing checked for wear? The shaft will move forward 3-4mm when you go in forward and a similar amount back in reverse so the shaft runs in the bearings at a slightly different point. Hearing the knock through the A bracket bolts suggests it is coming from that area. It could just be the prop blades chattering against the stops at that critical shaft rpm and not connected with the engine at all.

If it were my boat, I would change the prop first back to a fixed one to eliminate that before fiddling with couplings, damper plates etc. Also check for wear in the shaft and bearing by sliding the shaft back while you have the prop off.

 

[dur]

Just my suspicious mind, but have you confirmed that both of the aft A-bracket securing bolts are firmly attached to solid wood?
I'm suspicious that a not unusual combination of new components is producing a very severe knock which appears to be emanating from the new stern tube, and the fact that it's worse in one direction COULD imply that there is some slack somewhere in the drive train that only reacts severely to ahead torque reaction.
I only suggest this as I can tell you are clutching at straws...
Good luck,
Paul

edit... also is the bracket itself in good condition?
2nd edit... sorry, just reread your posts and noticed that you said the worst knocking was from the A bracket bolts. I'm even more suspicious now.

Thanks. That's another thing that needs checking when I've got her on the beach. I suspect that if the bolt were loose I would have water coming in and, thinking about it this hasn't happened gradually, it was there from the moment we put the new engine in gear the first time it was in the water. But still worth checking every part of the underwater gear particularly as it is more susceptible to damage than a deadwood mounted shaft.

(Not relevant to this but I think I should add another floor or at least a strap to beef up the A bracket fixing points. I could imagine getting a chain around the prop could pull a plank out, which could spoil one's day)

 

[dur]

Couple of observations. Not sure it is worth changing the coupling if the knock is coming from the A bracket area. A Bullflex might have been a better choice of coupling as it allows for misalignment. However your shaft seems to be rigid in bearings either end so the purpose of the flexible coupling is to isolate the engine movement from the shaft. I have a similar arrangement on my Eventide - photos on Vyv Coxs site.

Looking at the photo of the external bits, you have a lot of overhang of shaft aft of the bearing and the cutless seems to be unsupported for half of its length. Is that a modification to replace a greased aft bearing in the stern tube? Have you had the shaft out and the bearing checked for wear? The shaft will move forward 3-4mm when you go in forward and a similar amount back in reverse so the shaft runs in the bearings at a slightly different point. Hearing the knock through the A bracket bolts suggests it is coming from that area. It could just be the prop blades chattering against the stops at that critical shaft rpm and not connected with the engine at all.

If it were my boat, I would change the prop first back to a fixed one to eliminate that before fiddling with couplings, damper plates etc. Also check for wear in the shaft and bearing by sliding the shaft back while you have the prop off.

I did wonder whether the Bullflex was more flexible than the Centaflex. Vetus did'nt reply to my email which didn't help.
The cutless bearing extends to the very aft end of the tube. It is the same setup that was on the boat when I bought it in 2007 and I suspect many years before that and seems to work well enough. A new cutless bearing was fitted with the new stainless shaft with the engine change. I think the noise is just that - the blades clattering on their stops as the pressure comes on and off them. And the suspicion is that, as it didn't do it with the Volvo, it is the flywheel weight or lack of it which is the culprit.

I have just found the old fixed prop and it is the same size (15 x 12) so it should be a reasonable test.

 

[LittleSister]

(Not relevant to this but I think I should add another floor or at least a strap to beef up the A bracket fixing points. I could imagine getting a chain around the prop could pull a plank out, which could spoil one's day)

+1

Also even without picking up something round the prop, the normal vibration and other movement will be working the plank to which it is attached, I would have thought.

Are you also sure that the plank is not moving when you can hear your current knocking? Maybe the A bracket for the prop is rigid but what it's attacheed to isn't.

 

[dur]

+1

Also even without picking up something round the prop, the normal vibration and other movement will be working the plank to which it is attached, I would have thought.

Are you also sure that the plank is not moving when you can hear your current knocking? Maybe the A bracket for the prop is rigid but what it's attacheed to isn't.

I don't think so. We are hardly making much water at the moment - only a dribble. When I caught a line around the prop a few years back there was an obvious weep until I hardened the caulking next to the bracket. If the planking starts moving when the engine is at tickover there are rather more serious problems than a clattery prop!

 

[JumbleDuck]

Couple of observations. Not sure it is worth changing the coupling if the knock is coming from the A bracket area.

It sounds to me as if the knock is a symptom of unwanted resonance, rather than the problem itself. I think you are absolutely right on things to check, but just because the knock is apparent at the P-bracket doesn't mean the cause is there.

As a similar example, a car engine might knock the starter motor against something during a resonant vibrations: that doesn't mean that the problem is in the starter motor, or even in the vibration - tie bars are ugly kludges - but more likely to be a duff mounting.

 

[PaulJS]

Just my suspicious mind, but have you confirmed that both of the aft A-bracket securing bolts are firmly attached to solid wood?
I'm suspicious that a not unusual combination of new components is producing a very severe knock which appears to be emanating from the new stern tube, and the fact that it's worse in one direction COULD imply that there is some slack somewhere in the drive train that only reacts severely to ahead torque reaction.
I only suggest this as I can tell you are clutching at straws...
Good luck,
Paul

edit... also is the bracket itself in good condition?
2nd edit... sorry, just reread your posts and noticed that you said the worst knocking was from the A bracket bolts. I'm even more suspicious now.

After a reasonable night's kip on the flight home it occurred to me that the A bracket bearing/bush itself might have some play in it. Maybe worth checking?

 

[dur]

It sounds to me as if the knock is a symptom of unwanted resonance, rather than the problem itself. I think you are absolutely right on things to check, but just because the knock is apparent at the P-bracket doesn't mean the cause is there.

As a similar example, a car engine might knock the starter motor against something during a resonant vibrations: that doesn't mean that the problem is in the starter motor, or even in the vibration - tie bars are ugly kludges - but more likely to be a duff mounting.

No I agree. I am all but certain that the knock is the blades unfeathering and refeathering - either due to a torsional hesitation as each each piston reaches compression -as described to me by Darglow a little while back- or the self excitation thing - as you described in a much earlier post. One other possibility is the change in pressure caused by the proximity of the prop to the hull but which ever it is the blades hitting the stops and, as I see it, the real answer to all of these is more flywheel weight. Beta says this is not possible and what they are looking at with Centa is more torsional damping which is obviously a fudge. Either way it will cost and has no guarantee of success but what do you do?

 

[JumbleDuck]

Beta says this is not possible and what they are looking at with Centa is more torsional damping which is obviously a fudge.

Torsional damping isn't a fudge! It's a good, sound solution and used by many if not most car makers.

 

[fisherman]

Get a couple of long bits of 4x2 and use them as levers to wedge against the engine, see if restricting its movement helps. (But it does look like the problem is at the prop)

 

[dur]

Torsional damping isn't a fudge! It's a good, sound solution and used by many if not most car makers.

Ok - I'm not going to argue - you've been too helpful!