Two zinc or to over zinc, that is the question.
Frank, When finally getting our E31 trucked out to SoCal from Port Royal, South Carolina back in June of 1995 and into the local boat yard here, I too wondered about the whole mystery of how much to zinc the boat. There was a collar zinc on the 1" prop shaft and that was it. Another friend who was helping with the bottom paint, etc, suggested that I add two more, a sand dollar zinc on either side of the strut and that I did by drilling a 1/4" hole and mounting them. Fast forward a year or so and when hauling for some reason that's long slipped my memory, I was horrified to discover that two areas of the hull were devoid of bottom paint for a distance of 2 to 3 inches around. They were the strut as it entered the hull and a sintered bronze ground plate forward on the port side and bolted through the bottom of the hanging locker. What in the heck had happened? There was no sign of that when I applied the new bottom paint to the hull back in the summer of 1995. A quick call to my dive service cleared everything up, I was "too hot" with too many zinc's. What in the world did all that mean? I had no idea but bowed to those much wiser than I and took their advice which was to remove one of the sand dollar zinc's. That I did and our boat went back into the drink for another period of time, say...two years. Actually I really can't remember all that clearly how long but suffice it to say, it was longer than an additional year. With the boat again out of the water for reasons again I can't recall, my primary interest was in checking for that burning before anything else. Sure enough, there it was, the burning on both the strut and the ground plate again but to a much lesser degree, let's say only 1" around both. Ok, enough with this little science experiment, I took the other zinc off the strut and went back to what she'd had in the first place, just the one on the shaft. To this day, there is no burning in either place or anywhere else for that matter. What's the lesson her for you though? That's hard to say other than to adopt the philosophy that if it ain't broke, don't fix it. Well, that doesn't exactly apply here because there's noting broken on your boat but you get the idea. How about letting sleeping dogs lie? No that doesn't work either. I know, talk to local a dive service and get their opinion on the subject of being over zinced and the possibility of that burning I experienced from too many of them. Could it be that colder water is more kind to a zinc or are we talking strictly electrolysis in any temperature, etc? Had you considered removing one of the two as is the case now with our boat? Shoot, I don't know if any of what I wrote above has helped but good luck anyhow and please share the result of any answers or opinions you come up with in your research. Glyn Judson, E31 hull #55, Marina del Rey, CA
