Anyone here knowledgeable on the merits of Vetus vs Polyflex couplings on little engines?

The 2GM in my Compass 28 has a Polyflex red flexible disk in the coupling. Even after careful alignment there's still lots of noise and vibration, which annoys the crap out of me.

I'm considering replacing the disk with a green one, which is rated for much lower hp and torque and should do a better job of isolation owing to its greater flexibility. However in looking at the specs, it is still rated for several times the output of my little 13hp donk, so I'm concerned the improvement might be only marginal. The existing disk is quite old and feels very stiff, and probably has lost heaps of the plasticiser or whatever it is that keeps it squishy.

The lowest rated Vetus Bullflex is rated much closer, but costs three times as much. The extra $$ would need to give me a vastly better result, but I have no experience with the Vetus. Anyone done such a replacement?

Cheers, Graeme

Check your engine mounts. No flexible coupling is going to work if the engine mounts are not firm and can absorb vibrations. If they are rubber (like my Yanmar 3GM30) then they may need replacing. I use a Polyflex flexible coupling and have had no problems with it.

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zilla said..
Check your engine mounts. No flexible coupling is going to work if the engine mounts are not firm and can absorb vibrations. If they are rubber (like my Yanmar 3GM30) then they may need replacing. I use a Polyflex flexible coupling and have had no problems with it.

I did these a few years ago. I know enough to have eliminated the obvious causes like that.

Vetus 7.7kw 2cyl engine on my 28 footer Adams served me well for the time I had that boat. In the 8 year period I never ever had any major problem with the motor. I like Vetus.
I always used Vetus parts - which were available from Maxwell - and I was happy as Larry with their service. The only aftermarket part I used was the Ryco oil filter.
Cost should not be a factor when one's pride and joy is concerned, I think.

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wongaga said..
Anyone here knowledgeable on the merits of Vetus vs Polyflex couplings on little engines?

The 2GM in my Compass 28 has a Polyflex red flexible disk in the coupling. Even after careful alignment there's still lots of noise and vibration, which annoys the crap out of me.

I'm considering replacing the disk with a green one, which is rated for much lower hp and torque and should do a better job of isolation owing to its greater flexibility. However in looking at the specs, it is still rated for several times the output of my little 13hp donk, so I'm concerned the improvement might be only marginal. The existing disk is quite old and feels very stiff, and probably has lost heaps of the plasticiser or whatever it is that keeps it squishy.

The lowest rated Vetus Bullflex is rated much closer, but costs three times as much. The extra $$ would need to give me a vastly better result, but I have no experience with the Vetus. Anyone done such a replacement?

Cheers, Graeme

Graeme,
When did you last replace your Polyflex coupling?
Did you know they should be replaced every 3 to 5 years!
I had to replace my prop shaft cutlass bearing in 2019. Well, when we removed the shaft it was so worn we had to replace it, it was only three years old. So the new shaft and cutlass bearing didn't fix the problem so new engine mounts were fitted.

The mechanic did a hand rotation after he thought he had it all aligned and the engine went up and down significantly.
So after he question me on when I had last replaced the Polyflex connection, answer was "probably it was the original"

It had work hardened!

We replaced the connector and Seaka's engine and prop have never run so smoothly and quietly!
Talking to the previous owner of Seaka I reckon some 11K [only 2K for me] was spent for a $350 polyflex connector that should have been replaced on a regular plan.
Phillip.

Good info at coxeng.co.uk/stern-gear/

Note the comment about the maximum number of flexible elements in the drive train.

Thanks Phillip;

The Polyflex disk came with the boat 11 years ago, so I guess you could say it's earned a rest. I'd much prefer to simply replace it vs fitting a Vetus, as it would be much cheaper and being like-for-like much less work. WIll follow it up.

I must admit though that I am intrigued by the idea of a plastic item "work-hardening". I thought when plastics age they harden due to loss of the plasticiser additives, whereas repeated mechanical stresses would be more likely to soften them.

Is there a materials scientist on board?

Cheers, Graeme

I am not a materials scientist but have posted before that Polyflex couplings don't work harden - before posting that I rang Polyflex direct to check and they agreed - they also added "unless under severe vibration situations for a long time".

This excellent article

powerboatmagazine.co.nz/amp/smooth-runnings/

includes comment on engine mounts;

If your engine mounts are more than five or six years old or have been contaminated with seawater, engine oil or coolant, change them. Even if the mounts look perfect, the tough elastomer (rubber-like) material in the engine mount work -hardens over time and ceases to isolate the boat from the engine's vibrations. Seawater rusts the metal parts of the mount and can cause delamination of the elastomer and metal parts. Rust also weakens the mounting bolts and in extreme cases can cause bolt failure. Engine oil and coolant deteriorates the elastomer and also hastens delamination. Poorly aligned engine mounts can also cause bolt failure. And, if you have noticed screws backing out on your boat, or if your oil pressure sensor has failed recently, then the root cause may be increased vibration due to bad engine mounts or a misaligned shaft.

I did a search for polyurethane work hardening or strain hardening (same thing if I understand it correctly) but found not much except this

polymerdatabase.com/polymer%20physics/Strain%20Hardening.html

So maybe if your coupling has done the same thing it is a good idea to change it to potentially save $11k damage consequence. Before you do anything I would call Polyflex direct they are an excellent company.

My phone discussion with the very helpful Poly flex bloke confirms your comments r13: these couplings do not work-harden unless they have been subject to extreme stresses and according to him they can last quite some time (he quoted 20-year good examples that were still ok).

So I think I might have another crack at the alignment before I send the big bucks. But I'd still like to know if the Bullflex would give a smoother result than the Polyflex, on a like-for-like basis.

Cheers, Graeme

The complete process to assess which coupling would have more vibration damping would be to set them up in suitable a workshop jig and give them sharp rubber mallet whacks laterally, axially and torsionally. Accelerometer record the motion response and from the typical amplitude decay time waveform the damping can be calculated. For torsional it needs a twisting input and rotational angle measurement system. Not a five minute job.
Next best would be to simply find out the durometer Shore A hardness of the coupling resilient elements. The Polyflex Red polyurethane element is 90A as here;
www.brierleyhose.com.au/uploads/6/1/8/4/61849789/engine_couplings.pdf
The Vetus Bullflex element is rubber. I cannot find a hardness for it on the web. See the 4 below links for good general background polyurethane vs rubber. Centaflex couplings with rubber elements are normally in the range 50-60 Shore A hardness - 70 and 75 available on request. The Vetus factory would be able to accurately advise if contacted. Lower hardness normally means higher vibration damping so if the Bullflex element is in this 50-60A hardness range than nominally it should have better vibration damping characteristics than a 90A hardness element. The percentage difference would be difficult to nominate. If you do go down the Bullflex route watch out for the change of torsional / lateral / axial vibration characteristics of the whole drive train which will result with a softer coupling - all the natural frequency modes (torsional, lateral, axial) will reduce and one or more could cause a resonance or partial resonance issue which wasn't there before in the rev range. Vetus would probably be able to do calculations given details of your whole drive train but at considerable cost.

www.manuf-rubber.com/news/whats-the-difference-between-polyurethane-and-rubber/
gallaghercorp.com/types-of-polyurethane/
hannarubbercompany.com/durometer/
www.couplingtips.com/elastomer-couplings/shore-hardness-relate-elastomer-inserts-couplings/

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r13 said..
I am not a materials scientist but have posted before that Polyflex couplings don't work harden - before posting that I rang Polyflex direct to check and they agreed - they also added "unless under severe vibration situations for a long time".

This excellent article

powerboatmagazine.co.nz/amp/smooth-runnings/

includes comment on engine mounts;

If your engine mounts are more than five or six years old or have been contaminated with seawater, engine oil or coolant, change them. Even if the mounts look perfect, the tough elastomer (rubber-like) material in the engine mount work -hardens over time and ceases to isolate the boat from the engine's vibrations. Seawater rusts the metal parts of the mount and can cause delamination of the elastomer and metal parts. Rust also weakens the mounting bolts and in extreme cases can cause bolt failure. Engine oil and coolant deteriorates the elastomer and also hastens delamination. Poorly aligned engine mounts can also cause bolt failure. And, if you have noticed screws backing out on your boat, or if your oil pressure sensor has failed recently, then the root cause may be increased vibration due to bad engine mounts or a misaligned shaft.

r13 - thanks for the info. I had not previously considered the damage a slow diesel or oil leak might have on a bearing. I still have my incontinent Bukh with the leaky seals and cactus bearings. This gives me another reason to replace it.

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Yara said..
Good info at coxeng.co.uk/stern-gear/

Note the comment about the maximum number of flexible elements in the drive train.

Graeme,
Take a look at the sketches of the two typical arrangements of stern gear in the link above. Is the C28 not the "flexible shaft" arrangement?

r13 said this:

"I am not a materials scientist but have posted before that Polyflex couplings don't work harden - before posting that I rang Polyflex direct to check and they agreed - they also added "unless under severe vibration situations for a long time".

Which is exactly what my polyflex suffered for years. It was hard and distorted. So saying they last 20 years depends on how true they are running and any problems that occur can shorten that life considerably.

If I get the same problems I had before I would be replacing my engine mounts AND my polyflex connector before atempting any engine/shaft re-alignments.

Ok thanks for closing this out. Must have been a very bad situation to have the coupling end up distorted.

Agree the 20 years life would depend on many factors, including the yearly hours run. The "general industrial norm" for replacing mechanical components (gearboxes, couplings, seals, plummer block bearings etc) is 12 years but this also is very broad and assumes relevant and effective preventive maintenance actions are properly completed within that time frame. Agree mounts and coupling replacement at the same time.

Wongaga would appreciate your advice of what course you take and the result.

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Yara said..

Yara said..
Good info at coxeng.co.uk/stern-gear/

Note the comment about the maximum number of flexible elements in the drive train.

Graeme,
Take a look at the sketches of the two typical arrangements of stern gear in the link above. Is the C28 not the "flexible shaft" arrangement?

So, the Compass "bible" by Geoff Raebel describes the typical C28 prop shaft with no bearing on the inner end of the prop tube, just the gland mounted on a hose. If this is your case, then your problem could well be too many flexible connections, and the flexible coupling is redundant.

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So, the Compass "bible" by Geoff Raebel describes the typical C28 prop shaft with no bearing on the inner end of the prop tube, just the gland mounted on a hose. If this is your case, then your problem could well be too many flexible connections, and the flexible coupling is redundant.

Not sure I'd agree with that. With flexible engine mounts and a rigid coupling, the radial vibration components would have to be absorbed by the inboard end of the cutlass.

Agree Wongaga. The propeller shaft must be supported by 2 bearings suitably spaced and (ignoring seals which are a given) going fwd in the drive train from the inboard bearing is the flexible coupling to the gearbox output shaft. The gearbox and close coupled diesel on suitable mounts.

The 2nd schematic in the below link - the flexible shaft schematic - is ambiguous imho. The text before it talks about P bracket arrangements but the schematic does not include a P bracket or its bearing. So in a P bracket shaft drive arrangement the outboard shaft bearing would be in the P bracket, and the next shaft bearing going forward would be at the stern tube exit. So 2 bearings supporting the shaft. The text in some of the case studies is also imho ambiguous but I might be misinterpreting the text.

coxeng.co.uk/stern-gear/

How long is the stern tube bearing in a Compass 28?

Having a prop shaft mounted in two bearings and a soft mounted motor is a shtt arrangement. The prop shaft is trying to hold the motor still.
Ideally your better off with having one bearing at the prop so the prop shaft is able to move with the motor on the soft mounts. There are many applications in the industrial World where shafts are supported by only one bearing at the business end.
In saying that. Problems arise with sail boats because of inadequacies( poor engineering) as engine is not usually a priority and quite often installation of a second bearings at the inboard end of of a stern tube is a treatment to a symptom but not a fix.
Plenty of inboard stern drive power boats out there with one cutlass bearing and they pump out the HP without problems.
If you start trying to make things flexible or soft ,,,and your lazy in the process you will run into trouble.

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r13 said..
Agree Wongaga. The propeller shaft must be supported by 2 bearings suitably spaced and (ignoring seals which are a given) going fwd in the drive train from the inboard bearing is the flexible coupling to the gearbox output shaft. The gearbox and close coupled diesel on suitable mounts.

The 2nd schematic in the below link - the flexible shaft schematic - is ambiguous imho. The text before it talks about P bracket arrangements but the schematic does not include a P bracket or its bearing. So in a P bracket shaft drive arrangement the outboard shaft bearing would be in the P bracket, and the next shaft bearing going forward would be at the stern tube exit. So 2 bearings supporting the shaft. The text in some of the case studies is also imho ambiguous but I might be misinterpreting the text.

coxeng.co.uk/stern-gear/

How long is the stern tube bearing in a Compass 28?

This is Geoff Raebel's site suggest you make contact

groups.google.com/g/compass-yacht-group/c/JTX_cIgVQ1g/m/iUBdyM0cAwAJ

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wongaga said..

So, the Compass "bible" by Geoff Raebel describes the typical C28 prop shaft with no bearing on the inner end of the prop tube, just the gland mounted on a hose. If this is your case, then your problem could well be too many flexible connections, and the flexible coupling is redundant.

Not sure I'd agree with that. With flexible engine mounts and a rigid coupling, the radial vibration components would have to be absorbed by the inboard end of the cutlass.

Firstly, can you confirm that the prop shaft has only one bearing, the cutlass bearing at the outer end?
A cutlass bearing is not a tight fit and relies on a water film for lubrication.

You only need a flexible coupling if the prop shaft is rigidly held by two bearings.

Let's use the classic engineering analysis technique of looking at things at the extreme, and you have a prop shaft with one bearing and a flexible coupling and flexible engine mounts. Now imagine both of those flexible elements are very soft. Nothing to restrain the system from vibrating in all directions, with multiple possible resonant frequencies. My guess is that that is your problem.

Thanks all, I appreciate your desire to help but my old boat is what it is and I'm not about to re-engineer it all from first principles. I know Geoff and have owned his book and been an active member of the Compass forum for over ten years.

Keep the discussion going as you please, but all I really wanted to know is how well Vetus couplings work compared to Polyflex.

Cheers, Graeme