I have run into the same problem here in NJ. With all the rain we had in the spring the boat hung from the mooring under-used . The sun came out and I looked back to see that black exhaust cloud! I just figured it was time to don the wet suit and go for a swim. 5 minutes with the paint scraper and the problem was solved. A side note. My friend and I both being frugal, decided to test out a couple of the "urban legend" Anti foul remedies. He painted his prop with a Sharpie, I used Lanocoat. Somewhere on this site you can find someone extolling their virtues. Fugeddabout it! We will be doing manual prop cleaning every 5-6 weeks for the rest of the season. (not a big deal, really).