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Old 23-04-2012, 07:00   #1
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Flex engine/shaft couplings worth it?

About to buy my new shaft and prop, which my installer forgot to return and has now disappeared. I've got a nicely mounted Kubota on proper rubber mounts. I sort of assumed that the flex coupler would be nice, but since the engine is rubber mounted, wouldn't that keep any drivetrain vibes away too? Or are secondary vibes from the engine going to go through the shaft and give issues at/through the cutlass bearing? I'd just as soon use the $250 on something like, I dunno, food.
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Old 23-04-2012, 07:12   #2
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Re: Flex engine/shaft couplings worth it?

Nice to have, but shouldn't be necessary. Advantages:
  1. sacrificial weakest link to reduce engine damage in event your prop strikes immovable object (e.g. a submerged log)
  2. de-couples shaft and prop from your boat's electrical ground to reduce electrolytic attack
  3. can reduce vibration in event of SLIGHT misalignment -- but shouldn't be needed if shaft and engine are properly aligned.
It will not correct vibration from a bent shaft or imbalanced prop.
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Old 23-04-2012, 08:20   #3
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Re: Flex engine/shaft couplings worth it?

Quote:
Originally Posted by SailFastTri View Post
Nice to have, but shouldn't be necessary. Advantages:
  1. sacrificial weakest link to reduce engine damage in event your prop strikes immovable object (e.g. a submerged log)
  2. de-couples shaft and prop from your boat's electrical ground to reduce electrolytic attack
  3. can reduce vibration in event of SLIGHT misalignment -- but shouldn't be needed if shaft and engine are properly aligned.
It will not correct vibration from a bent shaft or imbalanced prop.
All of the above for smaller motors. But for larger power boats, they seem to brake down quickly.

BTW I have a slightly used one for sale that came off a Hurth ZF12, which was replaced with a Yanmar/Kanzaki.

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Old 23-04-2012, 08:51   #4
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Re: Flex engine/shaft couplings worth it?

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Originally Posted by SailFastTri View Post
Nice to have, but shouldn't be necessary. Advantages: de-couples shaft and prop from your boat's electrical ground to reduce electrolytic attack.
On this above point, I installed a coupler on my boat, but bridged the gap across the couplings with copper wire to maintain my electrical system ground, which on my boat is the prop/shaft.

Would it be better to install a through hull grounding plate instead? My keel is encapsulated so can't use a keel bolt. Should the plate be copper or could it be zinc instead?
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Old 23-04-2012, 09:36   #5
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Re: Flex engine/shaft couplings worth it?

The grounding plates I have seen were bronze.
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Old 23-04-2012, 13:17   #6
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Re: Flex engine/shaft couplings worth it?

Years ago I bought a J/32 with a flexible coupling and a folding prop. It vibrated to beat the band at 1500 rpm or higher. The surveyor noted that the owner had installed a flexible coupling but had not shortened the prop shaft accordingly. As a result the prop stuck out beyond the recommended 2 diameters behind the strut and cutless bearing.

I replaced the folding prop with a MaxProp and did away with the flexible coupling. The prop now runs smooth as silk. The real reason for the vibration was probably because the pin on the folding prop was bent. Maybe the owner tried to fix the vibration with the flexible coupling but probably made it worse.

The moral of the story is don't try to solve vibration problems with a flexible coupling. The only benefit as far as I can see to one is to have a weak link to prevent the prop shaft from breaking when you strike something. But that is a very rare occurance and I wouldn't have a flexible coupling on my boat.
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Old 23-04-2012, 13:32   #7
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Re: Flex engine/shaft couplings worth it?

I was always of the opinion that if you had soft rubber mounts on the engine, you needed some sort of flexible coupling (like a centraflex), as the engine movement could damage the stern tube/gland?

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Old 23-04-2012, 20:42   #8
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Re: Flex engine/shaft couplings worth it?

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Originally Posted by goboatingnow View Post
I was always of the opinion that if you had soft rubber mounts on the engine, you needed some sort of flexible coupling (like a centraflex), as the engine movement could damage the stern tube/gland?

dave
On my new yanmar I installed a Pythondrive. Now the thing can dance around to its heart content while the shaft stays inline.

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Old 23-04-2012, 21:41   #9
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Re: Flex engine/shaft couplings worth it?

Quote:
Originally Posted by goboatingnow View Post
I was always of the opinion that if you had soft rubber mounts on the engine, you needed some sort of flexible coupling (like a centraflex), as the engine movement could damage the stern tube/gland?

dave
+1

I'm with you on that one Dave

Solid mounted engine = solid mounted coupling

Flex mount engine = flex coupling
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Old 26-04-2012, 14:04   #10
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Re: Flex engine/shaft couplings worth it?

There is not a lot of room for opinion here as there are certain rules.
If you have flexible engine mounts and 2 shaft bearings you must fit a flexible shaft coupling.
If you have flexible engine mounts and 1 shaft bearing you must have a solid shaft coupling.
I have had to play catch up with many boats that have been set up incorrectly.
Do not be tempted to get lazy on the alignment if using a flexible coupling, it is just as critical as when using a solid coupling.
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Old 26-04-2012, 14:14   #11
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Re: Flex engine/shaft couplings worth it?

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Originally Posted by NoTies View Post
If you have flexible engine mounts and 1 shaft bearing you must have a solid shaft coupling.
Can you explain why this is?
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Old 26-04-2012, 14:25   #12
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Re: Flex engine/shaft couplings worth it?

Shafts requires 2 fixed points (shaft bearing and gearbox thrust bearing) for radial alignment otherwise shaft runout will cause vibration. Fitting a flexible coupling in this case removes one of the "fixed" points.
With 2 shaft bearings and flexible mounts and a solid coupling, the engine will put undue side loads on the gearbox thrust bearing and the shaft bearings that they were not designed to handle.
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Old 26-04-2012, 17:27   #13
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Re: Flex engine/shaft couplings worth it?

Quote:
Originally Posted by NoTies View Post
There is not a lot of room for opinion here as there are certain rules.
If you have flexible engine mounts and 2 shaft bearings you must fit a flexible shaft coupling.
If you have flexible engine mounts and 1 shaft bearing you must have a solid shaft coupling.
I have had to play catch up with many boats that have been set up incorrectly.
Do not be tempted to get lazy on the alignment if using a flexible coupling, it is just as critical as when using a solid coupling.
Agreed. I have an Aquadrive I am about to install in a steel sailboat, and Number One priority is in getting everything perfectly in line. The flexible coupler works with the thrust bearing to isolate the engine from the shaft, meaning you won't damage your transmission should you have to motor on a heel (you will, however, have to watch your oil circulation).

The soft mounts make for a quieter boat. A good friend with a similar steel boat can barely feel the engine now, it's so quiet by comparison with the solid linkage he used to have.

Perhaps the biggest plus for me, however, is the fact that I have a four-bladed feathering prop. It's a lot of mass. The Aquadrive means I can shift the engine without transferring that inertia of swinging bronze prop blades back up the drive train.

But mine is a fairly specific application. On my other boat, a f/g 33 footer, hard mounts and a solid coupling, regularly aligned to .003 or better, works perfectly well. They are, as has been said, "nice to have" for the average boater. But if you want to motor through biggish waves or have a folding/feathering prop, they become necessary. Many fishing and work boat have them as a matter of course.
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Old 26-04-2012, 20:02   #14
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Re: Flex engine/shaft couplings worth it?

Quote:
Originally Posted by NoTies View Post
If you have flexible engine mounts and 2 shaft bearings you must fit a flexible shaft coupling.
Very timely, I had ordered one today and was a bit iffy on it. This is my situation and your advice, and explanation, is appreciated.
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Old 22-02-2015, 22:22   #15
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Re: Flex engine/shaft couplings worth it?

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Originally Posted by NoTies View Post
Shafts requires 2 fixed points (shaft bearing and gearbox thrust bearing) for radial alignment otherwise shaft runout will cause vibration. Fitting a flexible coupling in this case removes one of the "fixed" points.
With 2 shaft bearings and flexible mounts and a solid coupling, the engine will put undue side loads on the gearbox thrust bearing and the shaft bearings that they were not designed to handle.
I remain confused. The problem with my very flexibly mounted Yannmar with one fixed point (cutless bearing) is that the gearbox bearing is anything but fixed. It is moving quite a bit. ZF recommends a flexable connection between the gearbox and propeller shaft.
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