"Bag of zincs" ???? How does one carry a bag of zincs and maintain neutral buoyancy?We use a diver down here from May-Oct once a month. He cleans running gear, through hulls etc and carries a bag of zincs so if he is down there and they need replacing they get done. We get a report on condition of the bottom when he is done.
I have been using almost exactly the same technique as John describes in his first paragraph (Interlux InterProtect, then a hard bottom paint like Petit Trinadad) for the past several years, with the same excellent results that John describes.....I have used the same method for the past ten years and never have more than one or two barnacles on my prop or shaft. I clean the stainless steel shaft and bronze prop down to clean metal with a wire brush on an electric drill. Wipe with solvent and apply two coats of a non-metallic primer. I have used Interprotect successfully but other non-metallic primers work as well – even Rustoleum primer. I then apply two coats of any available copper based bottom paint and splash the boat. Don't prime or paint under or over the zincs. As I said, little to no fouling after six+ months. I use the boat almost every weekend and motor perhaps for 30-60 minutes total each time we take the boat out. By the end of the season, the paint has mostly worn off the tips and parts of the prop blades but is intact elsewhere.
I have tried this method without the primer and always got bad fouling of the prop and shaft. Interlux outdrive spray works well if used with their primer but is more expensive. I have heard others theorize that the electrolysis current set up between the metal shaft/prop and plain copper paint somehow keeps the copper inactive and thus allows barnacle fouling. A non-metallic barrier coat of primer stops this interaction and allows the copper to inhibit growth.
I sail in the salt waters of Barnegat Bay NJ and keep the boat in Forked River - a somewhat brackish river feeding the bay.