Flexible Shaft Coupling

[Amp1ng]

Last season was my first with a Westerly Fulmar powered by Volvo Penta 2002 and MS2 Gearbox combo. First trip out immediately highlighted vibration at various RPM settings so engine mount replacement and various alignement checks were conducted during the season none of which cured the problem. In the end I resigned myself to always motoring at around the 2700 rev range to give the least vibration and once ashore for winter would investigate the complete drive train. Since coming ashore I have run the engine with prop engaged, checked cutlass bearing, dismantled coupling shaft and shaft seal and examined the shaft for bends/damage. The only noticable issue was the end float on the shaft (at the cutlass bearing end) when applying up/down pressure by hand on the prop (approx 3mm of float). Everything appeared to run true, no signs of previous damage to the prop and it was not heavily fouled when she came out of the water.

The shaft seal is due for replacement and whilst looking on the net for the volvo replacement part I noticed that a "Flexible Shaft Coupling" was also advertised alongside the seal described as necessary when the propeller shaft is rigidly mounted and the engine is suspended on rubber mountings also stating that a conventional type of propeller shaft coupling is required alongside the flexible coupling. The current installation does not include one of these flexible couplings, is this the answer to my problem?

Thanks

 

[vyv_cox]

The only noticable issue was the end float on the shaft (at the cutlass bearing end) when applying up/down pressure by hand on the prop (approx 3mm of float).

Not quite sure what you mean here, do you mean radial or axial play? 3 mm of cutless bearing radial wear is a lot and will account for your problem. 3 mm of axial play suggests there to be a problem with the coupling.

 

[craic]

The current installation does not include one of these flexible couplings, is this the answer to my problem?...

Dunno if that is the answer to your problem, but it was the answer to my own bad vibration problem. I had a single cylinder Yanmar 1 GM with a rigid coupling, and that started vibrating so much it shook the boat badly. Loosened the hull/shaft bearing, started a bad leak there, almost sunk the boat. After a flexible coupling was installed all went smooth.

Good luck.

 

[scottie]

Generally speaking if you consider mounts,coupling and inner shaft gland any two of the three can be flexible but not all three. (R&D coupling may be stiff enough not to count though)

Volvo do a flex coupling but it's large expensive and prone to failure especially if you get something round the propeller
But their flex gland seal if you can fit it is considered one of the best.

It is also worth checking the rear engine mounts are cast metal


2000 series are known to vibrate

It is also worth checking if you have a lot of wear in a 2 blade folding propeller.

 

[kacecar]

Based on my own, similar, experience I doubt if the lack of a flexible coupling is the answer. As others have said 3mm vertical movement in a cutlass bearing is a lot - change it and re-align the engine to the shaft.

When faced with a similar problem I fitted an R&D coupling - it worked well but didn't remove as much of the vibration as hoped. Things only got better once I replaced the cutlass bearing (it wasn't as worn as yours) and re-aligned the engine from first principles - using a home-made spacer to centre the shaft in the tube through the hull. I was astonished at just how small the tolerances are at the engine / shaft join.

 

[Amp1ng]

OK, thanks everyone. So the worn cutlass bearing is most likely to be the cause. The 3mm play is radial, my first instincts were to assume the bearing badly worn and to be the sole cause of the vibration but others who also witnessed the float in the shaft didn't share the same view. I will do as suggested and replace the bearing, align the shaft and re assess

 

[Tranona]

Vibrations at specific RPM are difficult to trace. In your type of set up the longish shaft with only the P bracket for support will suffer vibration if the cutless is worn as it is the only support for the shaft apart from the engine/gearbox. Also worth checking the prop for damage and balance when you have it off. You may also find replacing the shaft seal with a Volvo seal (if it does not already have one) will help as it provides more support for the shaft at the inboard end but still enough flexibility to move with the engine.

 

[mitiempo]

Also cutlass bearings are water cooled - the engine shouldn't be run in gear when hauled out. The result will be accelerated cutlass wear and possible shaft damage.

 

[Neil_Y]

I'm guessing the shaft bearing is rubber? as a guide to running clearances (what is a worn bearing) you can check new clearances on our calculator here http://www.h4marine.com/Downloads/Maritex AQUARIUS General Calculator REV 20100519.xls This is for a Maritex water lubricated marine grade shaft bearing which does not give under water pressure or shaft pressure if there is any movement.

With a 25.40mm shaft in a 45.00mm carrier a new Maritex bearing will be between 25.56 and 26.66 ID(or min 0.16 MRD minimum radial displacement) If this clearance approaches 0.635mm we would advise changing the bearing, sooner if you get vibration. If you add to this the fact that a rubber bearing will allow more movement for the same clearances this is probably your likely cause.

 

[fisherman]

I had a similar problem. Checked and double checked everything, but still the shaft was running out. This was shown by running dead slow in gear and holding a long lever, boathook, broomhandle, against the shaft and against a point near to it, so amplifying the movement. Cheaper than a DTI, some engineers don't like to lend them for me to put them in the bilge. It showed it running true at the stern gland but out at the G/B coupling. In the end the G/B coupling female register was found to be 10 thou off centre, the second time I have found this on different boats. The first time PRM replaced the coupling FOC, next time they calmly told me ten thou was within parameters. I had it bushed and rectified. A local engineer had a shelf full of couplings, common problem he said.

 

[JerryHawkins]

Prop balance

You mentioned a lot of things you checked, but did you check the prop itself was 'true'? Is a blade bent or is it out of balance? Maybe worth getting this professionally checked. Certainly if you currently have a solid gearbox-to-shaft coupling it is well worth inserting an 'R&D' flexible 'disk' coupling. This made a vast improvement on my last boat (after, in my case, replacing a bent prop shaft!).

 

[Billjratt]

Another downside of prop vibrations, or so I've been led to believe, can be failure of the rear gearbox casing.
The gearbox is, after all, where the power is transferred to the hull. (Unless you've a real boat with proper thrust bearings!)
Probably due to metal fatigue over a period of vibration, and may fail well after the original cause has been rectified.

So don't 'live with' a shoogle!

 

[Caer Urfa]

One tip 'if' you do buy an R&D Coupling is to make sure you assemble it the correct way round.

I had to replaced my gearbox last summer and could not figure out what I had done wrong when I re assembled the shaft/coupling after stripping it down for a good clean as the new bolts did not tighten up equally and also found the gap at the back of the bolt heads were not equal.

After speaking again to R&D (excellant service and support) they confirmed there is a 'right and wrong way' to re-assemble thier coupling.

I also assume from your post you checked or replaced the engine isolation mountings, I also have seen other boats with simular problems and after spending months replacing various items it was the gearbox thruster/drive plate which was the problem.

Mike

 

[Spyro]

One tip 'if' you do buy an R&D Coupling is to make sure you assemble it the correct way round.

I had to replaced my gearbox last summer and could not figure out what I had done wrong when I re assembled the shaft/coupling after stripping it down for a good clean as the new bolts did not tighten up equally and also found the gap at the back of the bolt heads were not equal.

After speaking again to R&D (excellant service and support) they confirmed there is a 'right and wrong way' to re-assemble thier coupling.

I also assume from your post you checked or replaced the engine isolation mountings, I also have seen other boats with simular problems and after spending months replacing various items it was the gearbox thruster/drive plate which was the problem.


Mike

Is the electrical continuity wire the one supplied by R&D or have you made that yourself? I've just fitted a new coupling and made up something similar

 

[Caer Urfa]

Is the electrical continuity wire the one supplied by R&D or have you made that yourself? I've just fitted a new coupling and made up something similar

Hi Spyro
Yes made it up myself, first attempt no good due to wire not thick enough, so went for a thicker wire as shown and so far so good.

However been thinking since I put it in, do I really need it? as the bolts from one side to the other will surely do the same job??, but will leave it in for now.

Mike

 

[Spyro]

Hi Spyro
Yes made it up myself, first attempt no good due to wire not thick enough, so went for a thicker wire as shown and so far so good.

However been thinking since I put it in, do I really need it? as the bolts from one side to the other will surely do the same job??, but will leave it in for now.

Mike

No they dont, If you look carefully there is no electrical continuity from the gearbox flange to the shaft coupling. I suppose that's what makes it flexible. looks like you have the wire on either end of the same bolt. You need to bridge the flexible coupling by connecting two different bolts. One that is holding the coupling to the gearbox flange and another that is holding on the shaft coupling.

 

[Genoa]

R&D v Aquadrive

Please escuse the ignorance...
The R&D disc coupling has been talked about here. Does it do a similarjob to the aqua drive from halyard? I was looking at the latter but even with their small version, I don't have the room to install it, and I'd like to find a way of reducing the vibration between engine and prop.

 

[Tranona]

No. It just introduces some flexibility into the shaft. It is very simple and can be effective, particularly if you have a fixed shaft and flexible engine mounts. There are much better, more sophisticated couplings such as the Centaflex or Bullflex (but inevitably much more expensive). The Aquadrive isolates the engine completely from the shaft and takes the thrust of the prop. It needs to be very firmly attached to the boat and as you say takes up a lot of room.

You may well find that you can reduce vibration by attending to the normal causes - loose or collapsing engine mounts, out of balance shaft and/or prop and worn stern cutless bearing.

 

[Caer Urfa]

No they dont, If you look carefully there is no electrical continuity from the gearbox flange to the shaft coupling. I suppose that's what makes it flexible. looks like you have the wire on either end of the same bolt. You need to bridge the flexible coupling by connecting two different bolts. One that is holding the coupling to the gearbox flange and another that is holding on the shaft coupling.

Hi Spyro

Thanks for that I see what you mean, will change the wiring the weekend, infact I did have the wiring as you say 'before' the gearbox went, so again in addition to assembling the flexable wrong also re-wired the electrical continuity wrong.
I will put it down to another 'senior moment' !!

Mike