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

[dur]

Very many apologies for this very long winded first post but I am desperate to find a solution so I though I would try and include the whole story so far.


I have spent thousands on a new engine installation and with hindsight I really should have rebuilt the Volvo. So apart from feeling like a complete fool I am so disappointed and fed up with the whole business,


My earlier post on this is here: http://www.ybw.com/forums/showthread.php?394215-Knocking-prop-shaft




Background:


Old Volvo MD11C had to be replaced. The prop is offset to starboard and so I wanted to retain the left hand prop as the boat already pulls strongly to port and I imagine it would be worse still with a right hand prop due to paddle wheel effect. Also the prop is feathering so expensive to replace. So the prop and tube was retained and pretty much everything else replaced.


The boat - 1929 shoal draft Maurice Griffiths "Wind Song" , 24' waterline 2'6" with the plate up and 8'6" beam. 5 tonnes.




The new setup is:


Beta 25 and TMC60 gearbox with oil cooler (so rated to run in reverse direction to give ahead propulsion). 2.45:1 going ahead and 2.83:1 going astern
Left hand (anti-clockwise) Darglow JF 15 x 12 feathering prop on new 1" stainless steel shaft, new cutless bearing, traditional fixed stuffing box with greaser with new packing. Installed by experienced Beta agent.
The idle speed is (now) about 1000 rpm when warmed up
Gearbox to shaft coupling intially R&D now Centaflex type M 127




On first trial the engine made a horrible knocking noise at tickover speeds when in gear (That position you get to when you click into gear either ahead or astern and you would use for slow manoevering or even gentle motor sailing). This occured both going ahead and astern though slightly worse going ahead.
If the throttle lever is pushed over the next spring position the engine runs at 1500 rpm and the knocking smooths out.


Beta's first advice was to increase the tickover to 910 rpm which I did but as soon as it is in gear the revs drop to about 700 rpm anyway and it knocks. If I hold the lever against the spring to say 1100 rpm it mostly smooths out- just a little rough.


There is no noise in neutral and if the shaft is disconnected there is no knocking noise in either forward or reverse


Alignment checked using the R&D feeler gauge technique. It was in limits but I got it a little closer still.
Didn't make any difference.


I rang Darglow props who immediately said that they would suspect torsionally vibration (caused by the new engine having virtually no flywheel). The low speed torsional vibration causes the pressure on the feathering baldes to fluctuate sufficiently they lift and then go back onto the stops causing the knocking. They said the same can happen with a fixed prop but the knock is transfered to the gearbox instead.


R&D coupling replaced by Centaflex Type M 127 coupling.
Immediately smooth and quiet in in reverse but still as noisy going ahead.




Other things looked at/considered:


Engine mounting studs - apparently earlier Beta studs could stick through the mounts and hit the bearers. I used dividers to compare the length of stud above the top of the mount and they are all the same. I concluded this is probably a red herring.


Another comment was that if the gearbox has a cone type selector then it could be more susceptible to low speed torsional vibration running in reverse (i.e boat going ahead) as the helical gear is trying to drive the cone out rather than in. I don't know how the TMC60 works etc. so not sure about this.


Beta tried to say the prop is too big but as they sized a 20 hp with the existing prop and I subsequently went for a 25 hp on the installers advice this doesn't make sense.
Checked again with Darglow and and they advised that they would have suggested a 15 x 11½” prop so the 15 x 12 is close enough as to make no difference.


Beta suggested that maybe the engine is quite high on its mounting studs and that it might be worth putting a packer under mounting feet. I think on the front of the engine any packer would have to be about 15 - 20 mm. I imagine this would slightly reduce the amount of movement of engine on its relatively soft mounts. I watched the engine and noticed that in reverse the engine (now) hardly moves and is very steady and relatively smooth. In ahead the engine moves about a lot more and at least some of this movement is transferred to the prop shaft.
So packing the feet might help but it doesn't feel like this is the actual problem - otherwise why did the Centaflex get rid of the problem in reverse. Makes me wonder about using stiffer engine mounts


Another thought - could the knocking be the prop shaft hitting the gearbox - but I don’t think that it could be because it is the same propshaft installed with the R and D coupling so it is a little bit too short for the Centa insert - maybe 20 or 25 mm short of the front of the insert. It certainly doesn’t stick through the front of the insert. Also why did it make the same knock with the R&D coupling.


The gearing is lower in reverse but only by about 15% so 800 rpm ahead ought to be the same as 700 pm in reverse. But it still knocks at 900 rpm going ahead.


Other thoughts - would a PRM150 have been better as it doesn't matter which way it runs. I did ask about one several times when Beta were specing the engine but they discounted it in favour of the TMC60.


I have spoken / emailed Beta many times and they have been pretty helpful but I don't think they know what to do next and have started asking the same questions we have been through several times before. I spoke to Centa and they spoke about a softer drive plate between engine and gearbox but Beta are baulking at that and I believe it is a tricky install with not much space. Darglow were great too. In fact everyone has been very helpful and sympathetic but I am getting nowhere. If it were a new car engine I would have taken it back - but its not of course.


If somebody can come up with a solution (that I can afford) I would be very very grateful and there would be a bottle of Scotch in it somewhere I am sure!

 

[pampas]

One thing that springs to mind is that the shaft clearance is to small and bends quite naturally at a vibration/ rev frequency caused by the soft engine mounts. I would think that the two aft mounts need to be the hardest, Only a suggestion.

 

[macnorton]

Is it possible that the flywheel on the old engine had more mass acting as a damper, it sounds like it could be back-lash in the power train at low rev's?

 

[charles_reed]

I think you need, before anything else, to locate from where the knocking is coming - use either a stethoscope or even a screwdriver.
There are about 40 different possible causes.
In my case it only happens motor-sailing @ >15 degrees. Finally traced to the oversize alternator, recently fitted, knocking against the engine-box.
With such a small diameter shaft and a fairly heavy prop, it could be torsional vibration - but that depends very much on the maximum unsupported length of shaft.

 

[JumbleDuck]

Is there any possibility of swapping out the Featherstream prop, perhaps for whatever you had before, so see if that makes any difference?

 

[pvb]

Is there any possibility of swapping out the Featherstream prop, perhaps for whatever you had before, so see if that makes any difference?

Yes, I think I'd try to eliminate that first as a potential cause.

 

[dur]

Is it possible that the flywheel on the old engine had more mass acting as a damper, it sounds like it could be back-lash in the power train at low rev's?

I think that is what I mean by torsional vibration - essentially the shaft speed is varying slightly or hesitating slightly with each engine revolution.

 

[dur]

I think you need, before anything else, to locate from where the knocking is coming - use either a stethoscope or even a screwdriver.
There are about 40 different possible causes.
In my case it only happens motor-sailing @ >15 degrees. Finally traced to the oversize alternator, recently fitted, knocking against the engine-box.
With such a small diameter shaft and a fairly heavy prop, it could be torsional vibration - but that depends very much on the maximum unsupported length of shaft.

I tried with both long screwdriver and the burgee pole. Seems to be well aft. Not the gearbox or stuffing box. I'll post a couple of pictures later of the setup.

 

[Hypocacculus]

Is there any possibility of swapping out the Featherstream prop, perhaps for whatever you had before, so see if that makes any difference?

I think he's using his original prop. However, I had the same thought, maybe a different prop might help.

I found this somewhere:
"TECHNODRIVE TMC60 marine gearbox unit is equipped with conical clutches mechanically operated, actuated by a servo-system; gears are specially constructed for quiet operation and bearings are amply proportioned. Ratios 2,00 and 2,45 are suitable for left-hand or righthand propeller rotation in “ahead”, making TMC 60 P particularly well suited to twin engine installations. For heavy duty especially when the reverse is used as the main direction of travel, we suggest a cooling system, available from us under request.

The gearbox used also has a major influence on the propeller used so please consult the prop manufacturer before selecting the appropriate model;

Disregarding propulsion system torsional compatability could cause damage to components in the drive train resulting in loss of mobility. At minimum, system incompatability could result in gear clatter at low speeds.The responsibility for ensuring that the torsional compatibility of the propulsion system is satisfactory rests with the assembler of the drive and the driven equipment."

So, it sounds like the components of your drive train don't like eachother.

FWIW, we suspect that much of our own problems with vibration are caused by a hydrodynamically poor set up, with our two bladed prop shielded by a skeg. This results in uneven forces and up and down wear on our shaft bearing. If your offset prop is ok in one direction and not the other, it might have something to do with the way the water flow is bouncing off your hull as you go forwards that is upsetting your new drive.

 

[dur]

Is there any possibility of swapping out the Featherstream prop, perhaps for whatever you had before, so see if that makes any difference?

I have the old fixed prop so I might try that. Also had a suggestion to borrow and try a right handed prop of similar size to see whether it makes that much difference to tendancy to veer to port.
So those are on the to do list unless another option appears first.

 

[CreakyDecks]

Engine mounting studs - apparently earlier Beta studs could stick through the mounts and hit the bearers. I used dividers to compare the length of stud above the top of the mount and they are all the same. I concluded this is probably a red herring.

If they are all the same, is it not still possible that they are all wrong?
You could probably quickly eliminate that as a source of noise using a stethoscope or a big screwdriver.

 

[JumbleDuck]

I have the old fixed prop so I might try that. Also had a suggestion to borrow and try a right handed prop of similar size to see whether it makes that much difference to tendancy to veer to port.
So those are on the to do list unless another option appears first.

I think it would be worth doing both of them if you can. Are you in a position to dry out and swap props or would it mean lifts?

My best guess is that there is a self-excited vibrations going on here. That's not quite the same as a torsional vibration, though a torsional vibrations may be involved. Basically, you put the engine into gear, the propeller unfeathers, this slows down the engine, the propeller refeathers, the engine speeds up, the propeller unfeathers, the engine slows down ... rinse, lather, repeat. If this is happening at a resonant frequency in torsion of the propeller - shaft - gearbox - engine system it could turn a small shudder into a full-blown banging around. Changing prop is a good step to finding out of this is the case.

If it is then perhaps a torsional damper on the drive shaft would help. My Citroen DS has them: split rubber dumb-bells which fit around the shafts and then have heavy steel rings clamped around the wide parts. The idea is that they resonate at the frequency causing the problem but, since the rubber is a damped spring, the energy gets dissipated as heat rather than banging around as vibrations. Many car engines have similar things - typically a drivebelt pulley with a steel core and rim and rubber bonded in between.

But I digress. A bit of propeller swappery will help localise the problem.

 

[Buck Turgidson]

Before changing the prop compare the knocking with the boat stationary and moving but at the same engine RPM. Also what is the frequency of the knocking? If it is RPMxblade number then it's feedback from the hull/prop interaction.

 

[dur]

I think it would be worth doing both of them if you can. Are you in a position to dry out and swap props or would it mean lifts?

My best guess is that there is a self-excited vibrations going on here. That's not quite the same as a torsional vibration, though a torsional vibrations may be involved. Basically, you put the engine into gear, the propeller unfeathers, this slows down the engine, the propeller refeathers, the engine speeds up, the propeller unfeathers, the engine slows down ... rinse, lather, repeat. If this is happening at a resonant frequency in torsion of the propeller - shaft - gearbox - engine system it could turn a small shudder into a full-blown banging around. Changing prop is a good step to finding out of this is the case.

If it is then perhaps a torsional damper on the drive shaft would help. My Citroen DS has them: split rubber dumb-bells which fit around the shafts and then have heavy steel rings clamped around the wide parts. The idea is that they resonate at the frequency causing the problem but, since the rubber is a damped spring, the energy gets dissipated as heat rather than banging around as vibrations. Many car engines have similar things - typically a drivebelt pulley with a steel core and rim and rubber bonded in between.

But I digress. A bit of propeller swappery will help localise the problem.

I see what you mean about the self-excited vibrations - it makes sense. I was wondering about why it doesn't do it reverse now when it did before - but I have a feeling that the pitch in reverse is steeper than going ahead (on the basis that you don't need high speed but do need stopping power). So maybe the different pitch would result has a different vibration frequency - or something.

Also what Hippocalculus suggests about disturbance to the water flow is a thought but I wouldn't have thought the supports for the stern tube would have that much effect.

I expect the old flywheel weighed about 40 or 50 kg which would smooth out all sorts issues.

I can dry out easily enough so should be able to change the prop in half an hour - if it goes right!
I don't think I can do it for 3 weeks so I will see how it goes.

 

[mtb]

when you said if disconnected from the shaft then no problems, you answered most questions for your self
is the deadwood area ok
can you check the cutlass bearing has been fitted true
if you can have the boat out with the shaft disconnected maybe turning the shaft from the prop end with a dial gauge on the inner end will show is there's a problem with the shaft, even new shafts can be bent !
I cant see why you'd have problems with the prop unless it is to close to the dead wood
I'd think harder mounts will help, have you checked the engine beds are good
perhaps a shock absorber attached from the engine block may stop some movement


I fitted a yanmar 2qm15 the engine jumps about at low revs I fitted new high quality mounts meant for a much bigger engine alignment is good its just the torque of the engine. Even with a good set up I still want to fit a shock absorber to smooth out the jumping.

 

[JumbleDuck]

I see what you mean about the self-excited vibrations - it makes sense. I was wondering about why it doesn't do it reverse now when it did before - but I have a feeling that the pitch in reverse is steeper than going ahead (on the basis that you don't need high speed but do need stopping power). So maybe the different pitch would result has a different vibration frequency - or something.

You also have different gear ratios forward and reverse, and that changes the apparent moment of inertia (equivalent of mass for torsional effects) as seen from the drive shaft. My guess is that you are just unlucky to have the shudder of the prop gripping hitting a resonant frequency of the systems, so any change, like a different gear ratio, would make a difference. That's why your previous heavy flywheel would have worked, too.

All speculation, but changing the prop has to be worthwhile if nothing obvious shows up in the next three weeks.

 

[dur]

A few pictures:

The original R&D coupling now replaced. Its about 10" from the back of the coupling to...

...the front of the stuffing gland:

The view outside:

The engine mounts:

 

[dur]

Before changing the prop compare the knocking with the boat stationary and moving but at the same engine RPM. Also what is the frequency of the knocking? If it is RPMxblade number then it's feedback from the hull/prop interaction.

It could be three knocks per turn (three blades) but frankly it sounds slightly more irregular but if it is a vibration with its own frequency then it could set up its own timing - I guess.

 

[dur]

You also have different gear ratios forward and reverse, and that changes the apparent moment of inertia (equivalent of mass for torsional effects) as seen from the drive shaft. My guess is that you are just unlucky to have the shudder of the prop gripping hitting a resonant frequency of the systems, so any change, like a different gear ratio, would make a difference. That's why your previous heavy flywheel would have worked, too.

All speculation, but changing the prop has to be worthwhile if nothing obvious shows up in the next three weeks.

Thanks - yes I get all that. What surprised me a bit was just how smooth it is in reverse now whereas everything jumps around more when going ahead.

 

[mtb]

those mounts look very very high !