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Old 28-11-2019, 03:26   #1
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Yanmar 2 GM20 shaft coupling

Ladies, Gentlemen,

some help on a 2 GM20 shaft coupling topic would be very appreciated.

When we bought the boat, the original split shaft coupling (25 mm shaft) was almost touching the shaft seal (a Volvo shaft seal). There was perhaps 1 mm of gap when the engine was at a standstill. It had worked for 24 years, but - in reverse, the coupling could well have rubbed on the seal, probably has rubbed on the seal.

We had a different split coupling installed last year, it is a bit shorter. See photo. I do not know the maker and model of this coupling - perhaps an older Buck Algonquin?

As you can see, there is not a lot of space available. If possible, I seek a solution without moving the engine forward. Going backward with the shaft seal is not possible, the hull and shaft tube touch right at the back end of the shaft seal.

Now, there are issues with this new coupling as well:
0. -> Question: I would appreciate help on identifying the currently installed coupling / confirm maker and model.

1. It is possible, that the shaft was not inserted far enough when this coupling was fitted. I have tried to find fittting instructions on the Buck Algonquin homepage and elsewhere, but I was not successfull. I have almost come to believe, that fitting instructions for marine shaft couplings are secrets ...

-> Question: So, I am looking for proper fitting instructions for this particular coupling.

2. The current coupling has slipped a few degrees when I change from forward to reverse to hard. This has happened 3 times so far. It is clamped with 4 x M8 8.8 bolts onto the shaft, these bolts are torqued to approx. 30 Nm each. Again - no manufacturer spec. available, I retorqued them by hand underway last summer after the first slipping and when checking with a torque wrench back home, I found that they would just start to budge at 30 Nm. There is no hole in the shaft for the security screw, I have been told by more than one pro that couplings will work properly without the security pin and that the hole in the shaft will only damage the shaft seal when pulling the shaft.

-> Question: What way forward would you go? Can you recommend a similar short building coupling that works with sufficient safety margin?

Help appreciated.
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Old 28-11-2019, 03:44   #2
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Re: Yanmar 2 GM20 shaft coupling

For me the space available looks OK. Depends of course on the stiffness of the engine mounts.
Could you take a picture with the engine running high rpm in reverse? That would show clearly why you are concerned.

Another solution of course would be to install stiffer engine mounts, but resulting in more vibration in the hull.
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Old 28-11-2019, 03:48   #3
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Re: Yanmar 2 GM20 shaft coupling

I am not familiar with the coupling brand. Is there not a keyway in the hub and shaft? If not I would consider adding one. Ask that the keyway depth be on the plus side so the clamping action is still to the shaft and not on the top of the key. You could go with taking some material but if someone ever replaces the key it might get missed.
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Old 28-11-2019, 03:50   #4
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Re: Yanmar 2 GM20 shaft coupling

I have no idea what damage a dimple in the shaft would cause. Seems like there should be a key between the shaft and coupling to prevent slippage. The “security screw” prevents shaft from spinning off.

Would love to see a better view..... show more. Appears adequate space. What type boat?
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Old 28-11-2019, 10:53   #5
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Re: Yanmar 2 GM20 shaft coupling

When I took my shaft off a couple years ago, the key had sheared but I guess there was enough to hold it in. No one had a clue how it had happened. If anything is slipping, I'd pull the shaft and check that the key is in there and all is good. The proper "fit" as I recall was to slide it in until it doesn't go any further. You'll just know it when you feel it, I believe. I'd also question why you have a gap in your coupling. I can't look at my set up at the moment (it's a 3GM30) but I don't recall such a significant gap like that. The distance between shaft and coupling looks like mine, but I wouldn't think spacing really matters as long as things aren't moving or vibrating.
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Old 28-11-2019, 10:59   #6
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Re: Yanmar 2 GM20 shaft coupling

I had a similar problem with a 3HM Yanmar that was replaced with a 3GM30. I was able to slide the engine forward and drill new holes for the mounts. Then I had to get a longer shaft. The old shaft was worn in the cutlass bearing so this was not a waste.
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Old 28-11-2019, 11:09   #7
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Re: Yanmar 2 GM20 shaft coupling

I don't understand the problem...? There is space there. Are you saying that gap completely goes away when in reverse? If so your motor mounts are way too soft.
IF no key is in there you can dimple the shaft, but not sure if that will solve your problem.... but it may.
Would a standard packing setup be shorter and better? I guess that wouldn't matter because your problem is really shaft movement. It should not be moving.
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Old 28-11-2019, 12:37   #8
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Re: Yanmar 2 GM20 shaft coupling

Some additional explanantions:

The photo shows the coupling we had installed this year. Spacewise, it is ok (the original coupling was longer and in reverse it touched the shaft seal - this coupling doesn't touch the shaft seal in reverse - the reason why we had it fitted).

The problem with the coupling on the photo is, that - in case I shift to hard between forward and reverse - as sometimes happens when berthing in a tight harbour with wind going - it slips a little bit on the shaft. The design of the coupling does not foresee a key - it is supposed to hold by friction only.
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Old 28-11-2019, 15:15   #9
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Re: Yanmar 2 GM20 shaft coupling

You should never go from forward to reverse without pausing momentarily in neutral, you will eventually damage your gearbox if you don't do that. Yanmar specifically mention this in their handling notes. My Yanmar 2GM20 has a similar coupling to yours and I have never had a problem with slippage, even though I sometimes have to use a lot of power in reverse to stop my boat quickly, as my normal berth is between two piles with a cross tide and crosswind acting together to push me off before I can reach over the side and pick up my mooring strops on their pick up line. If I approach to slowly, I just get blown off so have to adopt this technique. As I say I've never had a problem with the shaft slipping and it doesn't have a keyway, locking pin or grub screw.
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Old 28-11-2019, 16:28   #10
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Re: Yanmar 2 GM20 shaft coupling

Martkimwat, I am completely with you on the "you should never". And I try my best to avoid it, but I am a failable human being.

So, the "should never" is theorie. Reality is, that I have a single lever control below knee height and a tiller that is asking for swing room when steering. When berthing, I have to stand in the cockpit to see what's happening. I can either bend down (and loose my visual on what the boat is doing) and operate the lever by hand or I keep standing and operate it with my leg. When things get hectic, whether I want it or not, whether I operate the lever by hand or by leg, it does happen from time to time (not always, as mostly I do concentrate sufficiently on allowing for the "spin down time"), that I switch a bit fast. So, the fact, that "Yanmar specifically mention this in their handling notes" is - chickening / wiggeling out by them on a product, that is apparently not designed to take expected misuse. Because I am sure, that I am not the only poor sod who switches a bit hard on a single lever control from time to time when things get hot. I am being blunt on them here. It is not the first time, that I come accross a marine engine manual that includes legal wiggeling. Not too long ago, I read in the manual of a 6 hp Yamaha outboard, that that outboard was not to be used for stopping a boat by putting the outboard in reverse. Again - theorie and legal wiggeling out. I was close to bringing that outboard back to the shop and asking my money back, as it apparently was unfit for my needs. I appreciate everyone who is able to stop a boat without engaging reverse when they have a tailwind, but that was Yamaha and that is a different story. Here, we are with Yanmar.

So, your clutch does not move on the shaft. That is excellent. What clutch do you use? Brand and model? Do you have the manufacturers fitting instructions? Does it also have 4 bolts M8 to hold it on the shaft, or does it have 4 bolts M10? Or does it have 6 bolts?
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Old 28-11-2019, 18:12   #11
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Re: Yanmar 2 GM20 shaft coupling

Hestie, as you don't have key or a tapered shaft, you are relying only on friction to hold the coupling to the shaft - as you already know

Your new coupling is shorter so you now have less surface area - so the only options I see is

a. remove everything and have a key and keyway machined and fitted - big job


b. increase the friction component by increased torque on the four cap head screws. You would most likely need to get something tougher than 8.8, say 10.9 or 12.9 or whatever is available in your area. They don't need to be S/S, just keep them well greased and check regularly.

c. Check to see if any lubricant was used when the coupling was fitted, often these are fitted with grease (or similar) when fitted up but really they should be fitted dry if you are relying on friction.

In essence, you need more friction, especially as you now have a reduced surface area.
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Old 28-11-2019, 23:59   #12
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Re: Yanmar 2 GM20 shaft coupling

I have an almost identical coupling on my Yanmar 2GMF. But my coupling has a key way on the shaft and coupling. I got it done when I had my Autostream propeller fitted.
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Old 01-12-2019, 14:42   #13
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Re: Yanmar 2 GM20 shaft coupling

I agree with you the manufacturers keep installation instructions a trade secret.

The split coupling I installed for a one inch propeller shaft was made by PYI. They were very helpful over the phone. They told me that the propeller shaft should go into the coupling at least 2 inches and it did not need a key way or set screws. The torque for the bolts with self locking nuts on the coupling required 45 foot pounds. Using a cris-cross pattern in 15 foot pound increments, I brought them up to 45 foot pounds. The coupling over lap on the propeller shaft measured 2 1/4 inches which met their minimums. Like you the installation has limited space.

Having been second in command of the engine room on a CG Cutter I'm cursed with maintaining that level of maintenance on my boat. So after one hour run time I re-torqued the nuts and picked up 3/4 turn on the two bolts farthest from the coupling flange. Another call to PYI revealed thay had never had a customer re check the torque and they were concerned and told them I would give them updates.

Four hours of run time later using both forward and reverse a lot, the lock nuts maintained 45 foot pounds of torque and again at 65 hours the same result.

The design of the split coupling saved me a lot of work and money because the standard coupling would have required me to drop the rudder to remove the propeller shaft for a proper fit of the coupling to the shaft at a machine shop.

Looking at the photos of your coupling it looks like screw drivers were used to open the split coupling rather than steel wedges, which raises a red flag, as to the rest of the installation in my world. (Yes I know, cursed with attention to detail.)

Hope this info helps.
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Old 01-12-2019, 14:59   #14
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Re: Yanmar 2 GM20 shaft coupling

You could try and increase shaft diameter by wrapping a piece of thin iron foil around it. Something the thickness of a can of coke, but not coke because those are aluminium
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Old 01-12-2019, 23:57   #15
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Re: Yanmar 2 GM20 shaft coupling

Perhaps 25mm shaft versus 1 inch coupling (or vice-versa) .Check both with a micrometer.
These clamp type couplings should work OK if properly matched to the shaft and torqued.
The shaft should be inserted at least as far as the aft face of the flange.

If there is enough space inside the flange coupling , then next time you draw the shaft have a 10mm thread tapped in the end of the shaft and a fit security washer ( 6mm x 10mm x 30mm ) on the end of the shaft.

Are the engine mounts 24 years old ? They may be too soft now
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