the forces involved are way beyond the internal "friction" in the unit itself. For what it is worth I think you may be correct in your hypothesis that the wear takes place at engagement. That certainly seems logical but I think that the friction of the drive is just one component of the issue, I believe that you need to look at "system" friction and propeller mass too!. When you include the friction of most shafts, bearing and seals I think you'll find that in fact the saildrive has far less friction than your "average" sailboat drivetrain. Certainly in my Yanmar SD-50, the friction is pretty minimal. Maybe I'll measure the friction on my first saildrive (now living under the stairs at my shop, waiting for a new home) with a dynamometer I use to adjust pinion preload on transaxels I rebuild in my shop. I'm curious now but I am also pretty lazy and it is summer so I should be sailing....

I do wonder just what happens to the clutch when the propeller un folds and actually bites into the water. I suspect that this is actually where the wear happens. I bet a feathering/folding prop has a part in the extreme wear. I do know that I love the saildrive though. I'll bet that you see more of them in the future too...

One more question I'll ask here, We have all wondered about the big hole in the bottom of the boat that the saildrive resides in. I've looked at the construction and decided that the risks are minimal, as long as the boat sees an annual haul out for inspection I am not worried. Maybe ignorance is bliss? Has anyone here ever heard of a boat sinking from a failed saildrive gland? I have never heard of one but you hear of boats sinking or almost sinking from damaged stuffing boxes fairly regularly. Just curious to know what I am missing....

Bruce