I am on to my second set of sail drives, SD50T'S, the first two gave up after only 750hrs, the Yanmar
"expert" couldnt explain why, there was no single
problem with them but a whole range issues which rendered them no longer servicable or even repairable.
Symtoms leading up to the failure was extreme vibration between 1800 and 2400rpm ,much worse when Gori props where in overdrive.
Constant slipping of both clutches.
Continuous leaking of oil
from top seal. Just to name a few that i can remember.
I allways had my suspicions that the Overdrive feature on the props may have been putting to much pressure on the allready fragile build of these units, but Gori and Yanmar deny this of course.
These days i just avoid using the overdrive feature just incase that was the problem, which is a shame because it is a great idea ,just maybe more suited to a shaft drive.