Corroding conversation

There is a conference going on next to me, and I finally have to wander over and listen. The boat had just been pulled, and it's hanging in the slings. The yard supervisor, owner, dealership mechanic, and a couple other hangers-on are in the background. There is a lot gesticulation and finger pointing at both the hole that wan't there 8 weeks ago when the boat came in for a bottom job, and the missing paint. This is a twin engine boat with Volvo dual prop outdrives.

I listen patiently while the experts all pontificated. "It's an AC problem!" "No it's the metals in the paint.", "It's another boat in the marina that's leaking." I hunch down and look under the boat. The bottom paint is eroded away from the trim tabs on both sides, Hmmm, that's a new one for me.

The trim tab zincs are showing some wear, and there is some paint damage around the top of the tabs also.

Finally, I can't stand it any more, and I chime in, "Are the trim tabs attached to the bonding system?" The dealership mechanic say's "Yes, and I just checked the wires they are fine." How did the builder do the bonding?", I asked. "When the tabs were installed they used a very long bolt on each tab that goes through the transom. The bonding system is attached to that." I paused, but didn't say anything more.

I go back to the hole and look closely at it. It's different from most corrosion made holes I see. The top edges are sharp, and the hole is corroded inside for a fairly large distance. It appears that the metal is being dined on from the inside out, not the outside in.

But in the end, it's not my job to find out what actually happened, so all of this is just my mental mastication. I personally think the boat itself is leaking DC current in a big way, and it should be easy to find. A bad float switch, pump battery charger or the ilk is the culprit. The zincs on the outdrive are nearly gone after just two months. The reason I think this is the case is because this boat had multiple locations that appear to be damaged. Given the shallow water the boat docks in, I think the leakage was bleeding from several locations on the boat to ground. It is less likely that if the current originated from an adjacent boat that it would have taken multiple paths in. So goes my logic, such as it is.

The mechanic corners me for a few minutes, and asks some questions about how to test for the problem. I do my best to help, and then dragged my Calder's bible out of the truck. "You should have this around, and Nigel wrote a really good article about testing for corrosion in Proboat, go look for it." 

I was more than a little surprised that the trim tabs were bonded, because the manufacturer's specifically tell you not to. Although I applaud the builder's diligence in tying every piece of metal together in the boat, leave the trim tabs alone. That's why the zinc is mounted to it. The directions from Bennett Trim Tabs are clear on the subject. This is what ate the metal in the paint around the tabs, and cost the owner some additional bucks to have it touched up. 

Bonding systems can operate in mysterious ways at times, and I often wonder if a lot of boats are in effect bonded to death. More wire, more connections, more failure points. I'm going to do a controlled experiment with a boat that you will meet soon. The two large bronzed Perko engine strainers are electrically isolated from the boat, with the exception of the water that resides in the hoses. Both have been blasted back to shiny bronze. The bonding system is brand new. I'm going to bond one, and not the other one. I will take pictures every two weeks, and see if there is any difference in corrosion or patina. We will see.