Re: Transmission output flange fell off Yanmar 2QM15 - Theories re: why?
As an update, we received a replacement flange and locking nut from a friend who came to visit us from back home. We also decided to replace the engine mounts, as one of the adjustment bolts spun in its rubber housing when I tried to adjust the alignment.
As we all suspected, the new flange had much longer teeth and gripped the transmission output spline much better. There was no wiggle between the spline and the new coupling, so Gary's point about the spline being made of harder metal than the coupling (and remaining undamaged) appears to be true. The old flange must have just been reamed out by the spline due to engine misalignment.
To complete the repair, first I installed the new coupling and spun on the locking nut. On the advice of a local mechanic I used a flat punch and a hammer to get the last little bit of torque on the locking nut, then used a cold chisel to calk it in and lock it in place.
Next was to replace the engine mounts. I borrowed a car jack and lifted one corner of the engine at a time, sliding out the old mount and sliding in the new. This was a relatively painless process that took an afternoon.
Finally came realignment of the engine. There are other guides out there on how to do this properly, but suffice to say that when I first checked the alignment on the old engine mounts, the transmission output coupling was probably 1/4 to 1/2 inch below the prop shaft coupling. This would explain the vibration issues, the worn cutless bearing, and the eventual failure of the coupling. The vibration and noise should have been a clue a long time ago, but it took separating the two coupling parts to actually see how bad it was.
After completing repair, we took the old girl out in the bay for a sail. Immediately we could feel and hear the difference. Our feet felt barely a buzz through the cockpit floor. The prop shaft swished smoothly straight through the packing gland where before it was a vibrating blur. With the new mounts, the motion of the engine itself had changed from a battering shake to a calm rumble. As we motored along, the cat took to perching with his belly on the companionway stairs like he was snuggled up to a purring mother.
If you haven't checked your alignment recently, I highly recommend it.
PS - On that last trip, I did notice more vibration transmitted through the tiller than we remembered from before. I feel pretty confident about my alignment job, so could there be some other reason why the rudder has more vibration?