Old as Dirt!
Some friends of ours have been cruising the southern Caribbean on a sister ship to ours for the last several years, leaving the boat on the hard in Trini for hurricane season and returning every November for another 6 months. On their last return, our friends announced they were trying a "new" way of dealing with growth on their running gear/folding prop. A coating of Egg Whites. I kind of laughed that off but, evidently, it's something the locals use to good advantage. N'any case, they gave their gear 7 coats with egg whites before re-launching last December. Since then, they have reported virtually no growth on their shaft and prop. Evidently, PropSpeed has a low cost competitor, eh?