To Flex or not to Flex...

druid

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Apr 22, 2009
837
Ontario 32 Pender Harbour
When I installed the Universal M-25XPB into my Catalina 36, Norm at WestByNorth (good guy!) HIGHLY recommended a flexible coupling for the shaft. The existing engine in my Ontario 32 (Yanmar 2QM20) has one, and I think it may be partly responsible for the quiet, smooth running of the 2-cyl. The Beta's COME with one.

But Yanmar recommends against them, saying they make it hard to get the alignment right.

What say you? Are flexible couplings A Good Thing?

druid
 
Jan 4, 2006
7,268
Hunter 310 West Vancouver, B.C.
I've never bothered with a flex coupling but have relied on checking the alignment every three years. The first alignment check after buying the boat new in 1998 was baaaaaaaaaaaad. Not surprising considering the boat had never seen water until it hit Vancouver.

After that first check, I've never seen much change except for a minor adjustment a few years ago. The radial alignment is a bit of a bear as the prop shaft coupling is about 0.050" bigger than the transmission coupling. That's aligned by touch only and not that critical. My real aim is to keep the angular alignment less than 0.001"/inch of coupling diameter (from the manual). So far, with a lot of cursing and swearing, I've managed to keep it around 0.002" with a 4" dia. coupling and all seems to run smoothly.
 
Jan 30, 2012
1,144
Nor'Sea 27 "Kiwanda" Portland/ Anacortes
Is the shaft supported anywhere between the cutless and the transmission output - thus rigidly mounted? If the motor is mounted on flexible mounts then a flexible coupling is nearly mandatory in that case

If the shaft is not supported anyplace other than at the 'ends' a flexible coupling is not helpful. For example, Volvo specifically says flexible couplings are required if the shaft is supported other than solely at both ends. While you say Yanmar seemingly does not favor flexible couplings I am pretty sure they would reconsider that advice where the shaft is rigidly mounted.

If you really like this stuff you will certainly like this article:
http://highseasyachtservice.com/wp-content/uploads/2011/12/Passagemaker-Article-on-Straight-Line.pdf

Also here are instructions concerning alignment . This is a much more accurate and reliable method than you may be familiar with

Charles
 

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Oct 9, 2008
1,742
Bristol 29.9 Dana Point
When I bought current boat it was my first 2 cylinder diesel. Ran it for a year with a definite but not excessive vibration. Thought it to just be the nature of a 2.
Then the cutlass bearing failed and set to a raucous complaining session. Took her immediately to the yard and, since she was out and we were in there anyway, replaced the stuffing box, cutlass, log hose, and rigid coupling - faced to the shaft. Also aligned all. Now it runs as smooth as a Benz. Check the "M-18" video in my profile, no vibration at all. You can feel the pulses if you put finger to thermostat housing, but the motor doesn't move.
I prefer the concept of proper alignment vs a buffer. But other installations may need different.
 
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