I have been suffering a long bout of galvanic corrosion issues on my C&C 40. I've had these problems ever since I have owned the boat. In general, I burn one to two 1&1/8" shaft zincs every month. Since I scuba dive the boat every two weeks or so to keep her race ready it's easy for me to change out zincs but the problem continues to confound me.
I have a very nice Xantrex Truecharge 40 battery charger system that incorporates galvanic isolation on every input and output. I also have a dedicated galvanic isolator on the boat right as the shore power enters. All A/C circuts test perfect and all D/C circuts test perfect. The galvanic isolator tests perfect too. I keep the A/C and D/C grounds separate as well. Also, the boats around me don't appear to have any zinc problems at all. All marina wiring tests out perfectly too. I've even gone so far as to test for stray current in the water all around my end of the marina and everything comes up perfect every time.
I just had her hauled for the season on Wednesday and I noticed that the bottom paint was coming off in wierd patterns on the lead part of the centerboard trunk ( keel ). I used good lead metal primer too. It almost looks like a galvanic pattern you might see on an old prop that was being chewed up by galvanic corrosion. The exposed lead is, of course, fine underneath but it got me thinking. Could a combination of aluminum mast step, aluminum spacers, stainless steel nuts and bolts and a big chunk of lead in a saltwater solution create a battery capable of eating away at a shaft anode through the water? ( there is no connection between the boats grounding system and the keel/mast/rigging combination that I am aware of )
Here's a picture when she was fresh out of the water...
Please let me know if anyone has any ideas.
George Jones
C&C 40 "Delphinus"