As for how I averted disaster: The boat recovery was possible in large part to my quick acting mate (my wife) and someone on shore who recognized the dilemma, but kept focus and surged to action to protect the bow of the boat and gain control and guide me to an open slip. Fortunately I was in reverse when the mount broke and moving at slow velocity forward. My hand was still on the handle of the motor and I never even let go of the motor (it was also attached via starter wires and fuel lines), but I couldn't keep the 90lb motor it above water at such awkward angles from just the handle. With the boat safely into an open slip, my wife helped me pull the engine up onto the boat. Does marrying well count as luck or skill? :^) Mostly luck I think.
1) How big is the engine -- too big. A tohatsu 8 with electric start. Somewhere between 90 and 100 lbs. Also 8hp is way to much for the boat, and the prop is too small and high pitched, which is part of why I think I had so much trouble with the throttle.
2) Travel. On longer trips I was trying to take the engine off, but with the electric start this is a real PITA because there is no quick disconnect so it requires disconnecting he battery, taking the engine cover off, unbolting the 2 wires and snaking them out, etc. Then its a bear to move. So, yes I was lazy and typically trailered with the engine on unless more than about 75miles. Also, I know the PO traveled with it on, and don't know before that. But, I suspect the bouncing down roads was a significant fatigue factor. The motor always felt solidly attached, so the actual motion on the mount, although powerful, would likely be small compared to the stretch in any rope. I think it was lot of small motions rather than large ones. In fact, I think a lot of the stress goes to the transom since when I bought the boat the transom had cracks that I professionally repaired (also reported on some other eclipses), so I was always nervous about traveling with the motor on, not so much because of concern for the mount, but that I might end up with more transom cracks (which so far I've been lucky with - the repair is holding).
3) The mount plastic looks very dry and brittle. I noticed this even when I bought the boat, but figured it more of a cosmetic item at the time. It looks like its seen a lot of sun and time.
4) The mount failed at bolt lines. A better design would reinforce with metal rods to strengthen along the horizontal axis.
I'm still looking into it. But...
The ComPac Parts list has something called "Motor Bracket, Eclipse" $258, but I suspect that is the entire bracket, not just the plate. All that is broken is the mount pad.
It looks like you can buy a similar replacement mount for about $50-100 searching google for "outboard motor mount plate," but I need to investigate more on specific sizing, etc. I might consider a hand build
I will likely take the motor to be serviced, if nothing else so that it can be sold, but am now strongly considering replacement with a smaller/lighter unit without the electric start like the SailPro, and then building a mount on the trailer so that I can move the motor off the transom during travel.
Will keep you posted.