Flexible couplings, what do you really gain?

BobnLesley

BobnLesley

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We're about to order a new propeller shaft and a chap in the boatyard's suggested that if we got it made an inch or so shorter and fitted an R&D flexible coupling, then the world would be wonderful; but what do you actually gain/achieve by fitting one of these things? I'm told they're more forgiving with reagard to engine alignment? Also that they'll reduce noise & vibration; from where, the shaft rather thean the engine/gearbox I'm assuming? Surely there can't be that much noise/vibration coming from the prop & shaft?
I'm also concerned that fitting a flexible coupling at the front would reduce support to the shaft, around mideay along the shafts length we have a stuffing box, mounted on the end of a flexible tube, if the front end of the shaft's felxing too, don't I risk disturbing that?
 
Just done the exact same thing, shortened the new shaft I was fitting by 35mm and fitted the R&D coupling, still aligned it close but the noise levels are lower and shaft does not move at all. Seems well worth doing it and from my engineering background sound practice. The only issue I now have is that the stern tube to coupling space would limit options for future shaft seal options as most need more than the 130mm I now have
 
Even better is to install a pair of couplings and a thrust bearing like the ones Halyard Marine sell. We fitted one and the noise transmission to the hull is almost totally eliminated. With some careful sound insulation it is possible to make a REALLY quiet engine installation.

I've no interest in Halyard Marine except as a customer.
 
R&D couplings have little effect on vibration but protect the stern gear in the event of propeller strike. Very precise alignment is required to ensure smooth running. Centaflex couplings are very expensive and like the Halyard couplings offer huge reductions in vibration.
 
R&D do seam to point out that their coupling is to stop small vibrations and shaft movement fore and aft not for miss alignment which from its design you can see. It does reduce any sharp thrust loads on the gear box
 
BobnLesley said:
We're about to order a new propeller shaft and a chap in the boatyard's suggested that if we got it made an inch or so shorter and fitted an R&D flexible coupling, then the world would be wonderful; but what do you actually gain/achieve by fitting one of these things? I'm told they're more forgiving with reagard to engine alignment? Also that they'll reduce noise & vibration; from where, the shaft rather thean the engine/gearbox I'm assuming? Surely there can't be that much noise/vibration coming from the prop & shaft?
I'm also concerned that fitting a flexible coupling at the front would reduce support to the shaft, around mideay along the shafts length we have a stuffing box, mounted on the end of a flexible tube, if the front end of the shaft's felxing too, don't I risk disturbing that?
Click to expand...

Why bother if you have not had one for many years and do not have a problem?

Www.solocoastalsailing.co.uk
 
Having searched through the archives on the use of R and D flexible couplings it is not always clear if those advocating them are abiding by the two-out-of three guideline (one of engine mounts, coupling, stern tube needing to be non-flexible).

I understand that they don't introduce much additional flexibility and so those with flexible engine mounts, a Volvo-type stern gland and cutlass bearing in a P bracket have sometimes used them. It would be interesting to hear of experiences - good, bad and indifferent - from those who have broken the guideline.
 
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I had a sort of resonating sound from my drive train. Fitting an R&D coupling removed it. I didn't need to shorten the shaft. just pushed it a bit further into the coupling and drilled a new dimple in the shaft where the retaining bolt is.
 
R&D appears to be the exception to the 2/3 rule
You still need to line them up as you would a solid coupling.
Might be worth carrying either a spare or a distance piece if cruising in distant locations?
 
I've just spoken to R and D Marine about their flexible couplings and the two-out-of-three rule. Their advice is that you can use their coupling with a Volvo-type shaft seal (and flexible engine mounts and a cutlass bearing / P bracket) as long as the distance between the flexible coupling and the cutlass bearing does not exceed 30 times the diameter of the shaft. So in my case, with a 1" diameter shaft, the distance must not be greater than 2 feet 6 inches. If it does exceed this then they recommend the additional use of a 'half-cutlass' in the stern tube.
 
scottie said:
R&D appears to be the exception to the 2/3 rule
You still need to line them up as you would a solid coupling.
Might be worth carrying either a spare or a distance piece if cruising in distant locations?
Click to expand...

What, pray, is the 2/3 or 'two out of three' rule?

I am looking at installing an R&D flexible coupling as part of the war I just declared on engine/ shaft vibration. Vibration is much lower when running at speed in neutral than at speed in gear.

Aware of the need to check engine mountings first. Boat is 39' Beneteau with Yanmar, 30mm (I think) shaft, rubber cutless bearing, Featherstream folding prop, Ambassador rope stripper.
 
My single cylinder engine dances around like mad when it is running. I think it would do untold damage without a flexible coupling. It broke one coupling and I was advised that I needed one bigger and more flexible on account of the fact that I was driving my boat with a "cement mixer engine". I'm ashamed to say I can't remember the detail, but it did involve an R&D coupling with an atypical spec and an atypical price.
 
BelleSerene said:
What, pray, is the 2/3 or 'two out of three' rule?
Click to expand...

1. Gearbox to shaft coupling
2. Shaft seal
3. Engine mount

Two out of three of these can be flexible but not all three otherwise you could get terminal wobbliness. (I'm not an engineer.) On my boat the shaft seal and engine mounts are flexible so ordinarily the gearbox to shaft coupling must be rigid. However according to R & D it is possible to use their flexible coupling as long as the distance between the shaft coupling and cutlass bearing does not exceed x30 the diameter of the shaft.
 
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