: That's correct, the two coupling faces did not separate. The entire coupling came off the back of the transmission via the output spline, after the locking nut backed off.
I agree that the transmission-side flange will need to be replaced, and I hope it's like you say, that the nut just got loose over time and allowed the teeth on the flange to be eaten away by the spline on the transmission output. The spline still appears to be in pristine shape, but your point about the specs on the spline is a good avenue to double check, thanks.
I can't envision how I'd drill and pin the flange to the spline, given how worn the existing flange's teeth are. Once I get a new flange and nut (whenever that will be, being in Mexico), the reassembly should hopefully be straightforward.
: The coupling you're describing with the set screws sounds like the prop shaft side of the 2-part coupling. Fortunately that part is still attached to the prop shaft and in fine shape with the set screws still in.
You're probably right about the vibration being a major contributor, but in the four years I've owned this boat I've never known it to *not* have some amount of vibration. It's a diesel after all, right? Do people out there actually have smooth diesels?
Here are pictures of the problem at hand:
-- The output coupling flange that fell out of the transmission. The side facing the camera goes into the transmission. Notice the tiny teeth way back there. From what I can gather, these are supposed to go all the way up what is now the smooth inner shaft of the flange. As it is now, the teeth barely engage the spline on the transmission when cranked all the way down with the . . .
-- Locking nut. This nut requires a special wrench to cinch down, and requires about 68lbs of torque. Supposedly these nuts are easy to find at industrial supply shops. Once spun down, the end of the nut needs to be "calked" into the keyway on the spline, as shown in the next photo.
Here is the output flange reassembled the first time. I was able to make an initial wrench out of a notched PVC pipe, followed by a sturdier one made out of pipe. At the time I didn't know about keying in the end of the locking nut, which is why you can see that gap, but I don't think I can truly reassemble it without replacing the flange due to the worn teeth. The nut worked itself back off after about 20 minutes of running in forward. There might have been some dragging on the packing gland nut or somewhere else on the shaft that provided enough resistance to help the nut come off. Dear God I hope not.
Also pictured is the other half of the coupling, which hugs the prop shaft and bolts onto the transmission side flange.