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Old 03-12-2006, 16:04   #1
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Flexible Shaft Coupling

I recently heard that I need a flexible shaft coupling between my transmission flange and propshaft coupling because my new engine installation has engine mounts that have a rubber insert that helps with engine vibration and shock from shifting forward to reverse.
Is there a recommended coupling or should I just dial West Marine and order the really expensive one in their catalog?
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Old 03-12-2006, 16:30   #2
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The soft couplings are a good accessory but not required. The shaft still needs to be aligned using the hard couplers before installing the soft.

They do help with viberation but the shaft needs to be bonded to the system with a rub plate/brush, if you have a bonded system, to prevent electrolysis.

http://www.randdmarine.com/flexiblesc.asp

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Old 03-12-2006, 17:22   #3
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Caution

Caution - make certain that ALL of the bolts on the coupling can be tightened without pulling your boat.

Two of the six bolts that hold my flexible coupling to the transmission have become loose. I need to get to both sides of the bolts to tighten them; one side receives the nut, the other needs to be held in place with an allen wrench. The sleeve that locks to the shaft blocks access to the head of the bolt. My only option is to put the boat on the hard and pull the shaft partially out.

Lots of $$$$$$$$ to tighten two bolts!!!

Roger
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Old 03-12-2006, 17:29   #4
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I'm not a fan of flexible couplings.
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Old 03-12-2006, 18:41   #5
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Aloha Roger,
That sounds like a real problem. Can you back the shaft a little way out without hauling out?
JohnL
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Old 03-12-2006, 21:40   #6
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Never Monday, please elaborate.
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Old 03-12-2006, 23:05   #7
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My steering and prop guy's don't like 'em either and said I won't need them, but that's as far as I went. I just thought "you beudy, more money for beer"

Dave
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Old 04-12-2006, 12:53   #8
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The original Boatowner's Mechanical and Electrical Manual by Nigel Calder says "(It is essential to fit a flexible coupling if flexible feet are used)" but his third edition of the same title leaves that out.
I suppose there is a good reason why it was edited out. My boatbuilding guru says I need one. I originally was not going to install one. Now I'm thinking I'll do without and be very careful with engine alignment.
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Old 04-12-2006, 14:16   #9
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I installed a flexible coupling a couple of years ago and it had a metal clip that bonded two adjacent bolts together to tie the shaft to the engine electrically.

Richard
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Old 04-12-2006, 15:36   #10
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Quote:
Originally Posted by Kai Nui
Never Monday, please elaborate.
I feel they allow for a sloppy alignment. If you read R&D's instructions. It allows for a .010" tollerence when measured at the red bolt head. Normal standard is .001" per inch of coupler diameter.
When you have an engine on rubber mounts, arn't they all now? You will need to be more "in tune" with your boat and check/adjust the alignment. For the guys up north, this should be part of annual commisioning anyway. For us in more tropical climes, make it a once a year thing.
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Old 04-12-2006, 17:46   #11
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rleslie, Wondering why you need to pull the boat to move the shaft back a bit. I have done it in the water a number of times. Even if the prop has to come off because you need to get the shaft back further, I diver usually can do this cheaper than a haul out. We have use a "donut" for many years on our last two boats. It does keep down some vibration but it's main purpose is to shear off if the prop is fouled under high loads and not perhaps pull the engine off its mounts. That is the primary purpose and not to let you run out of alignment. The only down side we have seen is when doing an alignment you need to have a spacer machined to fit in its place so the alignment can be done properly. If someone wants to do the alignment with the prop saver in place you should decline. If this is done correctly then there is no sloppy alignment.
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Old 04-12-2006, 19:00   #12
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Mine is like the pic above, and it works just fine. It did slightly reduce vibration. and it has the hex bolts all the way thru with the cross plate on both sides. NO shearing on theses!

I did grind off about an 1/8" from the threaded end for clearence on the trany casting and I can pull the bolts without moving the shaft. The bolts should be installed with everything in place so one knows he can get them out while afloat.

rleslie,
You should switch over to hex bolts so you can use open end wrenches to tighten. Mine actually called for touque spec's even though it has nyloc nuts. If you provide a picture, MAYBE we can come up with a resolve.

And like I stated earlier, "The shaft still needs to be aligned using the hard couplers before installing the soft coupler". There should be enough room between the prop and support strut to pull the shaft forward for alignment. The coupler only takes up 1/2" to 5/8" depending on the size.

WE have a lot of drift wook here in the sound, and that's my main reason for the installation, to take up some of the shock if a suck up a chunck.
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Old 04-12-2006, 19:29   #13
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rleslie, looking at Del's post, I think I understand your problem. If clearance for the allen wrench is the issue, you can get ratcheting box ends in the size of allen you need. Then you can either buy an allen wrench socket, and remove the bit, or cut a proper length off an allen wrench of the right size. This will allow you to loosen the allens bolts with very little clearance. If still not enough room, you could loosen the motor mounts, and move the engine forward. Not easy, but cheaper than a haul out.
Never Monday, I understand the view that these are a substitute for proper alignm,ent procedures, but if the shaft is properly aligned and then the flex coupling used, is there any other reason they shouldn't be used? While I am sure it could happen, I have never seen a flex coupling fail.
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Old 04-12-2006, 20:08   #14
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Scott,
Like all things boat. It ends up being personal preference. I don't find the vibration excessive or anoying in a proper install. If you bond well around the coupler, go for it. I would allow a slightly larger than standard gap aft of the bearing. That way if it does need to come out, you'll have a slightly smaller than standard gap when you pull the coupler faces together and remove the flex coupling.
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Old 04-12-2006, 22:49   #15
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Works for me. I guess I was just wondering if there were any known isues in the industry with these things that I had not heard about. Personally, I am not planning on running one, but did consider it. I prefer solid metal connections where I can get them.
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