We did the same switch from SD40 to SD50 on one of our engines for the same reason: SD40 was full of water. Actually, the SD40 had filled with water a first time, when one of the bolts holding the lower part of the saildrive
had not been tightened. It filled up a second time, after the mechanic
who "fixed" the first problem, overtightened and broke the bolt without telling us.
Over time, owning a Yanmar saildrive
forces you to become a mechanic
, or at least knowledgeable enough that you can direct a mechanic in every detail of the needed maintenance
We have now "lapped the cone" on our SD40 and SD50: once each, preventively, after 600 to 800 hours. They would probably have lasted a good few 100's of hours more, as we handle the throttle with great care, but we had a golden opportunity to have it done recently in Tunisia
, with a great mechanic who knew the procedure and charged very reasonnably. They also seem to do the procedure routinely in Marmaris, Turkey
, for a decent price
If you go for the SD50 (I am afraid, other than changing the engine
, you probably won't have much choice), make sure to have the documentation
with you, that describes the cone lapping procedure: it will come in handy at some time.