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Old 08-06-2015, 04:53   #1
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Cracked Flex Coupling

Looking over the engine and drive shaft this weekend and discovered that the shaft flex coupling is cracked. Eng. is 3ym30 yanmar "1" inch shaft.
Looking on line there appears to be 2 different types of couplings, Globe drive savers and R&D flexable couplings. What I have is a flexable coupling but can't see any markings on it for brand name.
Would be great if anyone with this particular engine who has installed one of these couplings could indicate which specific coupling part # they used and where purchased, as there are several models with the same configuration but different bolt size and diameter, otherwise I have to remove and take measurements.
Which brings me to the next question, the shaft will have to pull back a couple of inches to remove the coupling, boat is currently in the water, I am thinking that a couple inch movement wont hurt the stuffing box seal but have not done this before.
Suggestions would be greatly appriciated!
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Old 08-06-2015, 05:19   #2
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Re: Cracked Flex Coupling

Flex couplings are usually an owner mod. Take your measurements and call the sellers for advice and quotes. I bought mine from PYI Inc. I found them to be very knowledgeable and helpful, with fast service. (I have no interest in them other that as a customer).
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Old 08-06-2015, 05:23   #3
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Re: Cracked Flex Coupling

One other thing to add: if your flex coupling failed it's likely you hit something -- you might have prop or shaft damage also. Bring up your RPM slowly. If you have any unusual vibration after replacing the coupling then keep it very slow until you can haul out and deal with the other damage.
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Old 08-06-2015, 05:46   #4
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Re: Cracked Flex Coupling

I believe this occured last year, reversed into rope which caused the eng to shut off, revs were up, had to dive down and cut the rope. Checked the running on start up at the time and also motored the boat for a good month before haul out last yearwith no indication of a vibration, still runs very smooth. Other than this the prop has never been struck, very difficult to achieve where the prop is situated on the Nic.
Wish I had seen this crack before splash down but better now than finding out 50 km along the coast.
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Old 08-06-2015, 06:25   #5
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Re: Cracked Flex Coupling

With your transmission model number you can look up your bolt pattern. Call a yanmar marine dealer with it. You need the radius to the bolt centers and the bolt diameters themselves, of course.

Sliding the shaft in/out a bit shouldn't hurt the stuffing box packing too much, if you are just going /out/ with it, just clean up the bit in the boat that will ultimately pass into the stuffing box and now you know you won't be forcing any schmutz into it. Anyways it's an awfully forgiving seal.

It looks like your prop is in an aperture between the keel and rudder? You probably don't have loads of room to work with, then, but you may not need it. You can only go so far until you bump into the rudder or, in the other direction, bump into the stern (cutless) bearing. The placement of whatever zincs you have will also influence your clearance.

If you have the room, you could simply remove the coupling, bolt the couplings together without it, and off you go. Not if theres not enough clearance between the prop (or any collar zincs) and the stern bearing, however. Clearly you've learned that they are an important engine-saving device, so reinstall a new one ASAP!

These placement constraints also dictate the thickness of the flexible coupling, so make sure the new one isn't so fat it pushes your prop into the rudder, or so much thinner that you hit your stern bearing.

And a change in prop placement can also impact whether your rudder makes contact with it when turned hard over, may want to tweak your rudder stops if so.
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Old 08-06-2015, 06:36   #6
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Re: Cracked Flex Coupling

The Globe and R&D couplings look very different and are different colors. If yours looks like a red or blue smooth donut, it is a Globe. If it is beige with scalloped edges, it is a R&D.

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Old 08-06-2015, 07:17   #7
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Re: Cracked Flex Coupling

Thanks Chris9504, Yes just spoke with yanmar dealer and as you said he went by the trans #, they are pricey little things!! I don't think it will be an issue moving the shaft back for clearence, I can kick the rudder over to allow for an extra bit of room. Yes I would have probably done the hard mount but there is only about an inch of shaft between the prop and boat so can't pull in anymore. The coupling never did let go totally, although its clearley cracked across two bolt connections, not sure if this one has the failsafe feature or luckily holding on!!

Interesting tidbit, I spoke with the local yard mechanic and he mentioned the numerous inboard diesel eng.shafts he has worked on that only have a piece of "wood" spacer cut to join between the transflange and shaft flange. Said some have been there 10 years plus. He indicated the flex coupling is not really needed, unles you have an alinment issue which should be resolved anyway, but it can save you much more worries and $$$ if prop abruptly stops so I will be replacing.
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