Hi, all.

When I moved to South Florida from Atlanta last year, I added a prop shaft zinc. It corroded away and fell off the shaft after less than a month. My diver added a couple of new ones, but they also corroded away rapidly. The standard assumption is that the zinc is sacrificing for other boats in the marina through the AC ground wire, so I added a galvanic isolator. (See other thread on that subject.) However, the zincs STILL kept corroding away at an abnormally high rate and we replaced them again. When I asked him what they looked like, he said they were "shiny and corroded" which usually indicates stray current leak somewhere, i.e. not just the typical corrosion pattern from protecting other boats in the marina.

So, the diver brought in his half-cell corrosion meter/tester, which I had read about in Nigel Calder's famous book "Boatowner's Mechanical and Electrical Manual". The diver took a stab at the problem, but wasn't able to pinpoint it, so he let me borrow his book, which came with the tester (ProMarine). At first I couldn't understand it, but after a while I started to "get it" and see its potential....so I bought one through West Marine.

Using the tester, reading Calder and the tester manual, and tinkering (and I managed to do all this without shocking myself :-)), I discovered that the problem was in the AC ammeter and/or reverse polarity indicator, which had been connected to the AC (green) ground bus. (See attached picture. The white wires look a bit yellow.)

The manual says that it is ok to connect white to ground bus for high-resistance reverse polarity indicators, but NOT for low-resistance reverse polarity indicators, since for any significant AC circuit, the AC white (neutral) is only supposed to connect to AC ground back on shore...far back on shore.

Before unhooking the white wires from the ground bus, the increase from hull potential was about +200 millivolts on the test meter. After I unhooked the reverse polarity indicator and the ammeter whites from the AC ground bus, the corrosion meter read exactly hull potential (about -1.0 volt)....which means that the zinc is fully protecting the in-water metals, even over-protecting, i.e. I would be better off to remove one of the zincs.

Last weekend, I had the boat out at anchor at Elliott Key and dove to check the zincs. They are both completely intact and are a dull grey color, and it's been about a month since I did the rewiring....so I think I've found/fixed the stray current problem.

Now I have a new problem: I have no AC ammeter and worse, no reverse polarity indicator.

I'm wondering if I can simply hook the AC ammeter back up but just not ground it to the AC ground bus. I did it for a few seconds and it works, but I'm too chicken to try it long-term. Need a good marine electrician to tell me what to do next...

...but at least the case of the disappearing zincs is solved. :-)