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Old 22-08-2011, 11:17   #1
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Join Date: Oct 2010
Location: The boat - New Bern, NC, USA; Us - Kingsport, TN, USA
Boat: 1988 Pacific Seacraft 34
Posts: 1,246
Galvanic Corrosion of Chain Galvanizing

How do I teach the galvanic corrosion gremlins to attack the zinc anodes on my boat rather than the zinc galvanizing on my anchor and chain? The anodes are thick, relatively cheap, and easy to replace. The chain galvanizing is thin, expensive, and something of a bother to replace.

I have a 34’ Pacific Seacraft sailboat. There are 100 feet of 5/16 inch BBB chain on the main anchor which is a 44 pound Bruce. I seldom anchor in water deep enough to require me to use the nylon line that backs up the chain. The galvanizing on the anchor chain lasted for three years during which we were anchored perhaps a total of 10,000 hours before it needed to be re-galvanized. The boat has a zinc anode on the propeller hub, a teardrop anode on the hull, a teardrop zinc on the rudder gudgeon, and a pencil zinc in the sea water cooler for the refrigeration. The zinc anodes generally last six months before they need replacement. With the exception of the rudder gudgeon and its anode, all of the anodes are connected to the boat grounding system as are all the underwater bits of lead, bronze, and copper. The anchor chain is also connected to the grounding system. The chain touches the grounded stemhead fitting as it sags between the anchor roller and the Muir electric windlass, and it also touches the grounded windlass both on the chain wheel and on edges of the hawse hole in the windlass body.

With the exception of the gudgeon and its zinc, all of the zinc is electrically corrected to all of the underwater metal. The zinc on the chain is farther from the other underwater metal than the anodes and there is probably significant electrical resistance in the points of contact from one link of chain to the next. On the other hand the anodes only have an area of a few square inches of surface area while the chain has an area of 15 or 20 square feet.

Should I be making an effort to separate my chain from the boat grounding system? Do others do that? Would separating the chain from the grounding system be a mistake if the boat were struck by lightning?

I would really like to teach the galvanic corrosion gremlins to eat the cheap zinc and leave the expensive zinc alone.

Bill Murdoch
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