We just bought a boat and the survey
wanted them changed as the previous owner had the boat for 12yrs and had never changed them (not sure how old they were).
tried to talk us out of it. We now have nice new seals and after looking at the old ones, we saved them as emergency
backups. If we do have a failure and can't get new ones, we can put those on temporarily. I think they were $300 each which is pretty offensive for a rubber donut.
I think part of the issue is rubber fails typically thru UV degradation but down in the engine
room under the outdrive, there is no UV. My understanding is the double seal effectively stops growth between the seals because most of the time there is almost no water flow. With no oxygenated water flowing thru, barnacles
can't survive. Other than a bit of anaerobic slime, I don't think much could grow in there.
This is our first saildrive
boat. We'll have to report back after we've gotten some more use out of her.