... to the maintenance of our Yanmar SD50 involves the oil/gear lube I use. I have abandoned Mac Boring's suggestion to use the blue GL-5 Mercury gear lube and instead use a synthetic GL-4 gear lube that allows the cone clutch to live a much longer life.
My first sail drive failed pretty spectacularly before it was even due for a clutch replacement. Now I get 800 hours before I even look at the clutch. The last one (replaced this spring) was not discernible from the new one at that time. I should have recognized that there was something wrong when I saw the Yanmar spec for either GL-4 or GL-5 oil. It is something the auto industry went through 25 years ago and I lived through that! Once the correct oil is used the Yanmar has been fine.

I would have specified a Volvo had I known of the issue when I purchased the boat. They went to a conventional clutch drive rather than a cone clutch years ago. It is a much better design. Today Yanmar has introduced the SD-60 sail drive that utilizes a conventional clutch pac too. Not sure what I would do today....

Bruce