I would like to add to that post that the problem was solved last October (2009). An astute mechanic made the accurate determination that the old (original) battery charger was bleeding off AC current over to the the DC side and, hence, into the bonding system. It got so bad that all under hull zincs were fried in 30 days along with the rudder posts. Expensive damage! Now all zincs are lasting about year. I Just replaced engine zincs over the weekend and some really did not need replacing - though I did anyway.
Regardz.