Bonding circuits tie all the underwater metals together and hold everything at the same potential, and thus reduce the amount of galvanic corrosion. Often the bonding circuit—which should be wired in green cable—connects a zinc anode to through-hulls, (usually) the prop shaft, the keel and any other metals that come in contact with the sea. If everything works as intended, then the anode will dissolve during the course of the year in preference to the expensive bits of metal—hence the term “sacrificial” anode.
Bonding is often misunderstood. In a correctly set up system where both the AC and DC circuits are well installed and maintained, the bonding circuit should not conduct electricity except for a few millivolts generated from the zinc anode wasting away. But things can and do go awry, and in most cases bonding circuit problems come down to poor connections. For bonding to work properly, all connections must be of the highest order with no corrosion. Connections that are corroded have increased resistance and will not work as designed. Bonding is something that deserves further discussion, but for now the best advice I can give you is to check all bonding connections and make sure that they are in good order and not like the example shown above.
Lifelong boat addict and marine surveyor Mark Corke can be reached at surveymyboat.com