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Old 30-12-2010, 09:01   #1
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Location: Norfolk, UK and Turkey
Boat: beneteau 423
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Prop Shaft Flexible Coupling Question

My beneteau 423 has a volvo engine, with ZF Hurth gearbox driving the prop through a rigidly mounted shaft. All very conventional. I'm considering fitting an R&D flexible coupling (R and D MARINE : Flexible Shaft Couplings) in the gearbox/propshaft drive flange to eliminate minor shaft misalignment arising during motor sailing, also to reduce NVH. (ZF actually recommend a flexible coupling is used with their gearbox)
The flexible coupling will move the relative position of the prop rearwards by a few mm, I'm assuming this won't have any significant effect (other than having to re-set the rope cutter position)
However- the boat also has a volvo dripless gland seal. I'm wondering if fitting the flexible coupling will affect the gland seal- I'd appreciate if anybody can share their experience of using this combination?
thanks
Bryan
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Old 30-12-2010, 14:08   #2
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Did you already investigate whether the shaft mis-alignment is because the engine mounts need to be adjusted or renewed? Have you checked the integrity of your engine's bed and bolts through the bed? (Has the boat been repowered?) How is the wear on the cutlass bearing?

A heads-up about flexible couplings, here is a quote out of Dave Gerr's "Boat Mechanical Systems Handbook": "It's ... vital that the shaft coupling match the engine mounts. Soft mounts should have very flexible couplings, slightly flexible mounts should have slightly flexible couplings, and rigid mounts should have rigid couplings."

Also, for vibration reasons he recommends to not have more than one shaft diameter between the prop hub and bearing housing, however that's probably more a rule for power boats, and 1.5 to 2 diameters is probably ok for sailboats.
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Old 31-12-2010, 04:49   #3
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Thanks for the informative reply. I don't actually have a problem with alignment, however as I currently have the gearbox out of the boat to replace the engine rear oil seal and vibration damper between engine and gearbox, (the original has failed)I thought that adding a flexible coupling would be an improvement to the boat- In the med, we have to do a lot of motorsailing, which can result in temporary engine/prop misalignment due to hull flex caused by the stresses from sailing whilst the engine is running. The flexible coupling should overcome this. (I've bought the R&D vibration damper, and saw their flexy coupling while researching the vibration damperon the 'net). I will set up the engine/propshaft alignment when i rebuild it, the cutlass bearing is OK and the engine mounts and cradle are good.
I note the points you have raised from Dave Gerr's book, I reckon my engine mounts are pretty stiff as they currently have to reduce engine movement as much as possible and absorb 100% of prop thrust. Your point has made me think about the possibility of setting up some harmonic vibration within the propshaft assembly, which clearly wouldn't be good, so i think i may not use the flexy coupling until I've seen how it performs with the new vibration damper
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Old 06-01-2011, 15:45   #4
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This might be a case where a combination of flexible mounts and a flexible coupling would be the solution. Probably a good idea to check with the owner's group and see how other owners have solved this problem.
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