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Old 08-12-2021, 06:20   #1
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Prop Shaft Flex Coupling

So prop shaft flex couplings:

- proc/cons
- experience stories to share

I am considering one, sometimes it seems in my mind as a good idea that may/may not do anything, other days in my mind it just seems a silly idea.
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Old 08-12-2021, 07:11   #2
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Re: Prop Shaft Flex Coupling

Ahoy sailorboy1:


I only had ONE failure in 18 years of sailing with a boat that had one.


I had a flexible coupling on my Farymann 24 hp diesel years ago.

After 10 "Canadian years" of sailing (only a 6 month season) I left Halifax, NS for Florida.
Just north of Beaufort, NC it gave out. Managed to sail the last 15 miles of the ICW to Beaufort, where I was able to order a new one--and a spare.


Lesson: ALWAYS carry a spare (duh, such an original idea, LOL)
PS: If the parts supplier says it must have been acquired by the builder don't believe them. On a page of the shop manual , there it was.
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Old 08-12-2021, 11:17   #3
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Re: Prop Shaft Flex Coupling

I would think no need for a spare since could just remove it.
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Old 08-12-2021, 13:25   #4
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Re: Prop Shaft Flex Coupling

Iíve got one of the pyi plastic flex plates. I feel itís a worthwhile addition to our boat at least. And as you say, if it breaks Iíll just yank it, pull the extra anode off of the shaft and carry on.
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Old 08-12-2021, 14:04   #5
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Re: Prop Shaft Flex Coupling

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Originally Posted by sailorboy1 View Post
I would think no need for a spare since could just remove it.
I couldnt, there's not enough play in the shaft to slide it forward that inch or so. Folks in that situation ought to carry a spare!
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Old 08-12-2021, 14:22   #6
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Re: Prop Shaft Flex Coupling

I suspect the one on our 28 YO boat is original, since everything else is. Previous boat did not have one. So all things considered, I don't see a material difference. I suppose if the shaft gets out of alignment it may help, or hide a problem, or both.
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Old 08-12-2021, 14:43   #7
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Re: Prop Shaft Flex Coupling

I have a Vetus type 6 flex coupling. It is on a Hurth/ZF Marine V drive transmission and is is the only flex coupling that I saw that would work with a V drive.

It is also (IMO) made much better than most. It is made of a cast iron piece for the shaft with four fingers. The cast iron piece clamps onto the shaft the way a collet holds a piece in a lathe. It is much easier to remove than a conventional coupler. The fingers have rubber pieces on them that in turn are captured by a two part aluminum outer ring that bolts to the transmission. The flex is that the rubber pieces will yield to accommodate any misalignment.

Their very limited instructions - little more than a drawing shows a pin through the shaft and coupler to prevent the shaft from turning in the coupler. I chose to have a keyway machined into it to match my shaft and a locking bolt into a dimple on the shaft to prevent the shaft from pulling out.

When I bought it, about fifteen years ago I also bought replacement rubber pieces as spares. I have not need them yet. I inspect it about once a year and have never seen an indication of wear.
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Old 08-12-2021, 22:14   #8
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Re: Prop Shaft Flex Coupling

I THINK there was at least one thread on this recently... try search.. I'll see if I can find it too...

there is this: https://www.cruisersforum.com/forums...nt-256109.html (I know that's not about flex couplings but may be useful)

https://www.cruisersforum.com/forums...er-255067.html
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Old 09-12-2021, 04:35   #9
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Re: Prop Shaft Flex Coupling

following. We have a PYI flex coupling installed, and we do carry a spare.
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Old 09-12-2021, 04:57   #10
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Angry Re: Prop Shaft Flex Coupling

I had the Vetus coupling for over 30 years. I had to replace the rubber doughnuts a couple of times and the roll pin sheared off when undocking in Singapore but no other problems until I had the engine replaced. The mechanic who installed the new engine left it so badly misaligned that the doughnuts completely failed with less than 20 hours on new ones.
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Old 09-12-2021, 06:41   #11
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Re: Prop Shaft Flex Coupling

Lots of floating fishing gear out there trailing all sorts of line. Much cheaper to replace a flex coupling than rebuild or replace a transmission if you manage to snag one. Just don't be tempted to use the coupling to "correct" a misaligned shaft.
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Old 09-12-2021, 07:51   #12
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Re: Prop Shaft Flex Coupling

I have a flexible coupling behind the output flange in my 3GM30F. Due to the Flex-o-fold prop having such close clearance to the skeg of my Pearson 36, I have no room to move the propeller shaft forward as the prop would hit the "P" strut.

Also, my Yanmar mechanic said he couldn't align the output and the prop shaft flange with the flexible coupling in place. So...I had an exact duplicate machined out of aluminum to aid the alignment. Does anyone have a better idea. I suppose if my flexible coupling fails, I can use the aluminum duplicate until I find a replacement.

What do you do about alignment with these couplings?
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Old 09-12-2021, 08:12   #13
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Re: Prop Shaft Flex Coupling

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Originally Posted by George DuBose View Post
I have a flexible coupling behind the output flange in my 3GM30F. Due to the Flex-o-fold prop having such close clearance to the skeg of my Pearson 36, I have no room to move the propeller shaft forward as the prop would hit the "P" strut.

Also, my Yanmar mechanic said he couldn't align the output and the prop shaft flange with the flexible coupling in place. So...I had an exact duplicate machined out of aluminum to aid the alignment. Does anyone have a better idea. I suppose if my flexible coupling fails, I can use the aluminum duplicate until I find a replacement.

What do you do about alignment with these couplings?
=====================================

I installed one of this

https://mackboring.com/products/powe...oup/aquadrive/

Repowered my Tartan 34 C with a Yanmar, the engine is mounted off the centerline and knew will never get it aligned properly.
This unit solved the issue, cruised with many motoring hours in 4 years.
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Old 09-12-2021, 09:14   #14
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Re: Prop Shaft Flex Coupling

I had a Farymann 10 hp diesel with a rubber coupling that sheared after picking up a 3" hawser in NY harbor. Good thing?
On other engines I have used a Shaft Saver. I suppose that would shear too and save the transmission. It is advertised to tolerate some shaft misalignment. But the reason I use them is to electrically isolate the shaft and propeller and prevent galvanic corrosion.
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Old 09-12-2021, 10:48   #15
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Re: Prop Shaft Flex Coupling

One of the advantages of this type of coupling is that you can isolate the engine electrically from the rest of the boat, your engine produces electric by way of the alternator.
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