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?
Author:
Updated:
Original:

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.

NigelCalder

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.

Related

Beneyteau-Excess12

Boat Review: Excess 12

Groupe Beneteau, builder of Lagoon catamarans, has introduced a new multihull line called Excess. The first of the boats to reach U.S. shores at the Annapolis boat show was the Excess 12, a 38ft 6in design based on the popular Lagoon 40. The thought process behind this new boat ...read more

Spindriftracing

Extreme Sailing: No Piece of Cake

It can be easy to take for granted the incredible performance of today’s most cutting-edge grand prix racing boats. The latest crop of full-foiling 75ft America’s Cup monohulls, for example, were all up on their foils and even successfully tacking within hours of their first ...read more

Solar-Dinghy-pump-photo

Gear: Solar Powered Dinghy Pump

Tired of forever finding your dinghy or open daysailer filled with water when you arrive to go sailing? For years, sailors and engineers have sought a solution to this seemingly eternal problem, and now it appears the folks at Sea Joule Marine may have finally found it in their ...read more

BestBoatPromo-03

Best Boats 2020

How’s this for a thought experiment: imagine setting a diminutive Tiwal 2 inflatable dinghy alongside a Catalina 545 cruiser? It would be hard to imagine two more different watercraft, and yet they are both still very much sailboats. They are also both winners in this year’s ...read more

Hanse-675

Video Tour: Hanse 675

This past fall at the Annapolis Sailboat show, we had a chance to catch up with Hanse’s  Maxim Neumann, who kindly provided us a tour of the company’s new flagship, the Hanse 675. An impressive, well-built production yacht that boldly ventures into maxi-yacht territory, the ...read more

TOTW_PromoSite

SAIL's Tip of the Week

Presented by Vetus-Maxwell. Got a tip? Send it to sailmail@sailmagazine.com Be thrifty with propane  If you like to cook on board, the propane tanks supplied as standard with many modern yachts won’t get you far. Whether we bake bread or not, the one thing we all do is boil ...read more