View Full Version : Galvanic corrosion and zinc question

Jack Becker
11-09-2010, 10:25 PM
What is the current thinking regarding zincs on wooden boats? The debate, as I understand it, is that bonding (as recommended by ABYC) on a wooden boat, if not maintained scrupulously, can cause more damage than it prevents. That's one discussion point. The second one is, assuming an unbonded boat, what needs a zinc? Specifically, does an isolated bronze rudder (or any other isolated bronze fitting) need one? Or will that simply create a battery that will cause the white powder to form on the cathode (the bronze) which will then dissolve the wood around the rudder shaft (as I'm seeing on a friend's boat)? Bronze is a noble metal and by itself shouldn't corrode. And what about the stainless shaft/bronze prop combination? That is sort of a battery with the dissimilar metals, so I can see putting a zinc on it, but that also seems to create a more powerful battery than just the stainless and bronze, and also results in the white powder on the stuffing box (as I'm seeing on my boat). Opinions welcomed.

11-09-2010, 10:41 PM
The only zinc I use is on a Max Prop , and we cook one every 2 months , everything else looks good on a 5 year haul out schedule .


11-09-2010, 11:12 PM
The only zinc I use is on a Max Prop , and we cook one every 2 months , everything else looks good on a 5 year haul out schedule .

I don't have a folding/feathering prop, but right forward of my bronze prop on stainless shaft is the only zinc I have. In Maine waters (99% on a mooring - not at a dock), at the end of the 5-6 month season, the zinc has 80+% left. For the $8 I replace it each year. I've seen no powder or deterioration on my cutless bearing holder - but it's painted with bottom paint.

Not scientific by any means - so I'll be interested to see others chip in.

Ian McColgin
11-10-2010, 07:20 AM
If you have a simple electric system with no AC then a shaft zinc is likely all you need. Location matters. My shaft zinc looks good after six months cruising and on the mooring and is barely there at spring haul after six months at a dock surrounded by boats that are plugged in. When I had more stays than a catboat has, I used to for winter hang a zinc on a wire from the head stay, the upper shrouds of all three masts, and the back stay. That ensured the survival of the shaft zinc.

The only harm of "overzincing" is waste of money. If you put a zinc someplace and it wastes away, it's doing it's job. If it stays pristine year after year, it's functionless.

11-10-2010, 07:47 AM
Well, to say that "Bronze is a noble metal and by itself shouldn't corrode" isn't completely correct. Bronze is an alloy of various metals, and depending on the type of bronze it can be made up different metals that are either quite close together or quite far apart on the galvanic scale. One of the purposes of a zinc is to provide a metal that is lower on the galvanic scale than any of the others in the alloy, so that the zinc will corrode and not any of the elements in the alloy.

11-10-2010, 10:20 AM
The only harm of "overzincing" is waste of money.
OK, this question is beginning to get the better of me. I was just recently convinced that a woodie should not be bonded, after decades of believing bonding was necessary.

I have always heard that over zincing was worse than not zincing. Marine technicians have expensive meters to determine the proper level of zincing, and charge good money to use them for you. Is it really cool to ignore these guys and just start hanging zincs all over your boat until you see no deterioration of the sacrificial metal? This is a much simpler and cheaper solution.

Ian McColgin
11-10-2010, 11:53 AM
I should have qualified that the issue of overzincing is not a problem for an unbonded boat. On a bonded boat it's at least theoretically possible to add enough zincs in places that with the bonding you create currents where none existed before and then if for some reason amid all those extra zincs you left some key fittings of dissimilar metals or at the waterline able to make a through water or through soaked wood current path that does not involve a protective zinc, then trouble.

I think the at waterline problem is the more likely. Just the water slapping up and down can make a current between one part of a metal and another, as you see from copper bottom paints going at the waterline. So if you have a fitting at the waterline and one nearish below the waterline and an unfortunately located zinc, you could have a situtaion where the higher fitting wastes while the zinc and lower stay happy. It's pretty rare but worth thinking of.

I have also heard of overzincing being a cause of trouble in bronze fittings through ferrous fastened boats if the wood is really saturated, but I don't really understand the mechanism.