Flexible couplings, what do you really gain? (2024)

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

The R&D coupling could be seen as a win win as you gain the advantages of its flexibility without the drawbacks.

I have an R&D flexible coupling on Seminole, if you take it out and try to bend it you won't. OK it's more flexible than steel, but it's all a matter of degrees. Alignment is still critical.

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

+1 for the Centeflex. Notably reduction in noise and vibration. Less stress on the stern gland mounting.

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?

Yes from a professional perspective we supplied many successful installation using R&D discs.

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.

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.