Sounds like you have enough zincs as long as you change them at least once a year. I have a single shaft zinc and the zinc on my maxi prop and I change them twice a year. Zincs don't stop mussels and barnacles as you discovered. There are special antifouling paints for motor boat outdrives that might work on the shaft, but I don't think I know anyone who does that. You wouldn't want any of it on the shaft where you put your zinc, because you want metal to metal contact. I just scrape the shaft and prop off twice a year when I replace the zincs. It's just one of those things you have to do.
1983 Fraser 41
La Conner, WA
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Boating for over 25 years, some of them successfully.