Ask Sail: Too Much Protection

How can I stop the galvanic action I suspect is causing the delignification of the surrounding wood? More zincs, or no zincs?
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Peter Karczmar, of Providence, Rhode Island, asks:

I have a wooden Mariner ketch with an Indel refrigerator that has a through-hull heat exchanger. I am seeing delignification of the wood around the through-hull, suggesting galvanic action. I’m having a similar problem with the wood around the bronze prop shaft. The boat spends nine months of the year on a mooring with only a handful of other boats in the area. Winter is spent in a slip at a marina plugged into shorepower.

I have a galvanic isolator, and the boat is not bonded. There are zincs on the prop, the through-hull for the heat exchanger, and on the keel. How can I stop the galvanic action I suspect is causing the delignification of the surrounding wood? More zincs, or no zincs?

Nigel Calder replies:

It sounds like the wood is suffering from alkali attack as a result of there being too much galvanic protection for your heat exchanger and prop shaft. This is surprising, given that you don’t seem to have much zinc deployed.

If you can lay your hands on a silver chloride half cell and a good multimeter, you can test the level of protection. Put the meter in DC volts mode, plug the half cell into one side of the multimeter and hang it over the side in the water. Then put the other meter probe on the fitting to be tested (e.g., the heat exchanger or prop shaft). You will see a reading in millivolts (negative or positive, depending on how the probes are plugged into the meter). On a wooden boat this should not be above 550-600 millivolts. If it is higher, you have too much protection.

I would say in general a bronze prop shaft does not need a zinc, nor does the fridge heat exchanger if it is bronze, so try getting rid of those two zincs. However, the heat exchanger does need to be grounded inside the boat to battery negative, because of the copper refrigeration lines running into it. Without a grounding wire, if some fault in the fridge results in a short to the copper lines, the fault current could find a way back to the battery via the water, which would rapidly destroy the heat exchanger and could sink the boat.

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