The boat is a zinc'er

The boat is eating itself alive. It's almost like it has a case of aluminum flesh eating bacteria. Purchased about a year ago, the owner had this 1999 cuddy cabin single outboard sent in for a new bottom job, and it was then was moved to the owners slip. A few months later, the owner's diver noted that the engine zincs needed to be replaced, and they were. A few months later, again the engine zincs need replacement, and they were again. About six months into ownership, a motor tilt hydraulic cylinder failed, and the boat went into a local marina for repair, and it was discovered that the motor mount was severely corroded, including chewing a hole through the lift cylinder housing, causing the failure. The owner was understandingly concerned, and he should have been. 

There is a flurry of activity from the marina. A new motor mount assembly was found, and installed. A failed amidships bilge pump is replaced. Hull potential was measured, bonding system is checked, and all is averred to be okay. New engine zincs are installed, the owner grimaces when he gets the bill, and the boat goes home. The diver changes the engine zincs again after only eight weeks in the water, and the owner calls the marina inquiring why zincs are still being eaten.

The marina calls me, and asks if I would check his slip for any stray current issues from dock wiring, or other boats in the neighborhood. Out I go, and there is nothing at the dock that is causing a problem. The nearest boat is 200' away, no notable current either AC or DC is in the water, and there is no AC leakage in the shore power system. So the slip isn't the issue, and I suggest that something was overlooked, and the boat comes back in, and it's put on a work rack.

There is lots of galvanic corrosion going on here, and it has now eaten a hole through the hollow aluminum cantilevered motor bracket, and other holes are starting. Now the question is why? I clamber up into the boat, and look for all of the usual suspects. The bonding system is in really good shape. The wire is flexible with little to no corrosion. I can't find any DC leaks to the grounding bonding system. The charger is checked, and no DC current is riding on the ground. I'm stumped. I stare at Nigel Calder's bible to see if I have missed something, and I haven't, at least I think I haven't. 

So what to do? I take may camera and I start to document all of the corrosion. All of the damage is below the waterline. The new motor mount has some damage after only ninety days in the water. The motor mount's cantilevered platform has about a dozen suspicious corrosion locations, one real hole, and several that will be holes shortly if this keeps up. The motor skeg is being chewed on. About the only thing that is not damaged are the trim tabs, and what the heck is this thing?

Above the port side trim tab, is a big brown lump covered in worm tubes. I poke at it with a screw driver, and underneath I find two nuts and bolts holding a long expired, and large zinc.

Zincs should be replaced when about half of it has been chewed away, or sooner, and what's left of this zinc, has been kaput for a very long time. I ask the owner if the diver had ever replaced this zinc, and he doesn't think so, he only bought engine zincs. I check the marina records for the boat, and all I find on the parts lists for the boat are engine zincs. So somehow, nobody noticed this large zinc, and over the past year, the boat was surviving on the three small engine zincs which were being very quickly dined on by the evil Mr. Galvanic Electricity, along with an order of fava beans I suspect.

If you look closely at the picture, the installation of the zinc looks to be somewhat haphazardly installed. This does not appear to be original equipment, and it's somewhat larger than what I would expect to find on a boat of this size. Further, you can see the zinc on the right side is actually laying on top of one of the aluminum tabs for the motor mount structure. This looks like a case of measure once, drill the holes, and then go Doh!

There is another small item that caught my eye. This is the first time I have seen a zinc with two bonding wires attached. It won't hurt anything, and both went to the same block, I just haven't seen it before. There is some green patina occurring around the nut and connector, but I think this has been caused by some leakage around the bolts when they got loosen up with the erosion of the zinc away from them.

My best logic goes as follows. The motor is the original unit sold with the boat. The boat has survived 12 years now, and the majority of the damage has happened in the past 9 months. The vessel has a mishmash of metals including a bracket of 6061 aluminum, various cast aluminum's in the engine, stainless steel, bronze, and others. The combination makes a decent battery when the vessel is in the water. My numbers were a bit on the high side, but not alarmingly high. The answer must be the unchanged depleted zinc, in the water, with Mr. Electricity.

I believe the boat must have had some issues with corrosion from the very beginning, which resulted in the larger zinc being installed. The hole in the motor bracket is being temporarily patched with some epoxy, a new zinc is being installed, along with a galvanic isolator, and the boat will go home for two weeks, and will then come back for a check. If it's okay, we will repeat the two week cycle another time, and if it's still good, the badly pitted areas of the mount will be filled by welding. If things are not better, I will have to scratch my head and look again. I think I'm right in a grayish sort of way.

I'm not going to do a long windy dissertation on galvanic corrosion here using words like cathode, but you can go the the website Corrosionist to get all of the info you can stand on the subject, along with a lot of resources, and a forum on the subject.

If I have screwed up, and it is something else, I will let you know.