Flexible Couplings - SailNet Community

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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 09-18-2008
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Flexible Couplings

I'm thinking of installing a Flexible Couplings (R&D or Drivesaver) and have following questions:
1. will it help improve on prop/shaft vibration ?
2. do I need to shorten the prop shaft or just drop the Flexible Couplings in between the engine & shaft flange ?
3. is it a regret route ?
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Old 09-18-2008
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On land even with the abilty to do a laser perfect alignment there is allmost nothing that gets a rigid coupling like a boat


So it should reduce the stress on the transmission ,On the other hand most boats go a long time without one and have no issues
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Old 09-18-2008
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Had to put either a spacer or drive coupling on the shaft when I installed a Maxprop VP. Went with the coupling. Reaslly havn't noticed any change in vibration but like the additional security for the drive chain.
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Old 09-18-2008
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Hello,

My boat came with a drive saver. I can't comment on vibration, but the shaft was not shortened, and I certainly have no regrets.

The grounds straps that connect the coupling have broken. I replaced one (and need to do another). But that's really no big deal.

Barry
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Old 09-18-2008
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You will have to shorten the shaft if you push it back to install the coupling and the prop hits the rudder.
If your engine is not correctly aligned now then the coupling will not help much. The tolerance that a flex coupling will take is .010" or so but it will still be noisy. Best to align as perfect as possible then install flex coupling.

The straps are not ground straps but failsafe straps - so that if the coupling disintegrates you will still have propulsion.

The main purpose is to prevent shock loads from transferring to the gearbox...not to correct misalignment
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Old 09-18-2008
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Proper alignment is the best route.

I'm not suggesting that the value of these products is dubious or suspect, simply that a vibration indicates there is something out of step. The products you mention are not designed to eliminate vibration per se and as such may also come under undue stress by whatever mis-alignment is causing the vibration in the first instance.

Generally speaking, engines/gearboxes are mounted on easily adjusted up/down and left/right mounts and unless you have a failed mounting (which should be replaced first) the alignment from original can't be far away. Given that these drive lines are rigidly connected, it takes very little mis-alignment to cause a vibration. Here's what I would do:
  • Undo the propsaft coupling off the gearbox and move it back to inspect the inner surfaces. The bolt holes may be burred which could be the problem.
  • Move the shaft back into position to leave a small gap
  • Choose four diametrically opposed points, alongside the bolt-holes is a good start.
  • Insert a feeler gauge (or any other thin blade) at any one of the points and pull the coupling together against the blade
  • Now check the clearance at the other points by inserting the blade at each point in turn, trying not to move the coupling. This will show up any misalignment
  • Adjust the engine mounts until the feeler guage goes in with the same level of resistance on all four measuring points.
  • Reconnect the coupling
If the vibration still exists, start looking elsewhere for the vibration. Could be a damaged prop or a bent shaft in which cases nothing that you add into the drive line will help. Running the shaft in gear and holding a pointer against the shaft will soon identify a bent shaft.

The prop will need a visual inspection but be aware that damage to a prop is not always glaringly evident. It could be a bent rather than "injured" blade.

This process is easier than it sounds, give it a go, it'll save you problems down the line if you get it right and IMHO is better than add-in solutions.

Andre
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Old 09-19-2008
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Andre,
Thanks for the instructions. BTW I've taken a long hard look at my eng-shaft coupling and the packing gland. Its looks like I've 2 issues if I were to install the flexible coupling. 1.the shaft flange has 2 locking bolts that will hit the molded engine pan if it is shift sternward by 1-1.5" (which should be the thickness of the flexible coupling. 2.if that problem is solved, then the gap between end of shaft flange and packing gland nut is so small that I doubt I can repack the gland if I've a need to. Ofcourse I can shorten the gland assembly by cutting tthe "hose" length but I don't think I want to go for that kind of job. Sooo, Andre, I'll do as you've instructed above. Many thanks.
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