flexible drive couplings - worth it?

carrswood

carrswood

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Hi
I'd be interested to hear peoples experiences after retro fitting a flexible coupling to there shaft drive? Did it make a big difference?

I fitted a new Volvo D1-20 diesel last winter but still find the vibration in neutral is quite noticeable
 
carrswood said:
Hi
I'd be interested to hear peoples experiences after retro fitting a flexible coupling to there shaft drive? Did it make a big difference?

I fitted a new Volvo D1-20 diesel last winter but still find the vibration in neutral is quite noticeable
Click to expand...

A flexible coupling won't make much difference with the drive in neutral.
OTOH, it could improve matters with the engine in gear. I was very pleased with the improvement with my last boat when I fitted an Aquadrive to a Yanmar 2GM20.
With a 3 or more cyl. engine, I think that the benefit would be less.
 
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carrswood said:
Hi
I'd be interested to hear peoples experiences after retro fitting a flexible coupling to there shaft drive? Did it make a big difference?

I fitted a new Volvo D1-20 diesel last winter but still find the vibration in neutral is quite noticeable
Click to expand...
We fitted one and are very pleased with the results. I have worked very hard at the noise insulation, and the end result is a very very quiet and vibration free boat under power. If I could seal the bilges off, it would be even quieter, but the engine needs to get air from somewhere, and the mechanics of providing a breather and sealing the bilge just don't make it worthwhile.

Edit: I ought to make it clear that the coupling I fitted and referred to above was an Aquadrive system from Halyard Marine where the engine is allowed to float around and the actual prop shaft is driven by a thrust bearing mounted on a small bulkhead which was bolted and glassed into place on the end of the engine bearers.
 
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Flexible coupling

I fitted an R and D Marine coupling, to Volvo Penta 2003 shaft drive, to three blade folding Gori prop. noticed less noise/vibration, I think a worthwhile installation esp. at the price.
 
In neutral it could make it worse as it would let the engine move more

Speak to R&D for advice they do a relatively stiff unit

The rule of thumb us that any 2 of 3 out of mounts coupling stern gland is acceptable but one must be solid
 
I have a Quietlife coupling to fit this winter (from ASAP) and along with new R&D Supermounts and some Noisekiller soundproofing, can expect a very smooth and quiet running 4108.

I think ordinary polyurethane R&D couplings are not as good as more resilient ones like Quietlife or Bullflex. (I always regard them as just there to allow for a little misalignment rather than as noise and vibration isolator)

I
 
It depends on your shaft arrangement. If you have a rigid shaft (ie bearings front and back of the tube) and flexible mounts then a flexible coupling is essential to allow the engine to move independent of the shaft. If the shaft is not rigidly supported at the inboard end then flexible coupling not so essential. I have used both R&D and Bullflex and the latter is definitely better (and much more expensive), but if there was room I would consider a Centaflex type.
 
I had an Aquadrive fitted with new slim line engine mountings for my Perkins 4236. Made a huge difference to vibration and noise reduction.
 
If it's vibration in neutral then the type of coupling will make no difference.

I do just want to clarify for non engineers where Aquadrive sits in the options available.
I for one don't consider an Aquadrive as a flexible coupling, a flexible coupling is a coupling made of material that allows the shaft/engine to turn whilst out of alignment or it bends whilst still being capable of taking the thrust which pushed the boat forwards.

Aquadrive is a mechanical joint that allows movement at the engine side but it's key benefit is that it takes the shaft thrust to a bulkhead. On a conventional set up the boat is pushed by the prop and the attachement to the hull is through the engine mounts so the boat is pushed through the engine mounts, this means that as the engine is heavy it will bounce around as the boat moves and it will be twisted by the torque in the shaft resulting on loads of movement with the shaft well out of alignment for most of the time.

By fixing the inboard end of the shaft and taking thrust through the hull at this point you eliminate many of the noise generating issues. So the key part is that it is a thrust bearing for the shaft and a mechanical joint to the engine which allows the engine to move freely.

There are two main players in the thrust bearing option one is Aquadrive the other is Seatorque.
 
Avocet has a horrible, harsh single cylinder diesel which transmitted a huge amount of noise and vibration. It had hard rubber mounts on to the bearers and then a conventional coupling from the back of the gearbox to a conventional strentube with stuffing box. I bought some of those Vetus hydraulic couplings. They were MUCH softer, but the engine hopped about like a mad thing at idle - so much so that within a few hours, it had fretted the holes in the plywood bulkhead to shich the stern tube attached.

I then made up a flexible coupling using two rubber "Rotoflex" joints and added a thrust bearing arrangement which fed the thrust loads into the plywood bulkhead. That made a big difference. Having said that, I think keeping the hard engine mounts and just fitting the flexible coupling wouldn't have helped that much.
 
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