Allen Judy of Whortonsville, North Carolina, asks:
"My boatyard technician tells me that there is such a thing as being “over-zinc’d” and that there is an optimum amount of zinc anode to install on any a given boat. He says this has something to do with an electrical field imbalance and that it is best is to have just enough imbalance to sacrifice the zinc that is in place. I always thought more zinc is better. Can you enlighten me on this point?"
Nigel Calder replies:
This is a fairly complex and technical subject. When you put metals in salt water and place what is called a reference electrode of silver/silver chloride in the same body of water and then measure between the two with a sensitive voltmeter, you will see a negative voltage that varies from one metal to another. Zinc has the highest negative voltage of all common boat metals. When it is wired to another metal in place of the reference electrode, the zinc drops the voltage on the other metal and this results in the zinc corroding while the other metal is protected. The more zinc, the greater the drop in voltage.
What you want to see is about a 200- millivolt negative shift. Less than this usually means that your protection is inadequate. If it is more than this you are wasting zinc. On a wooden boat you might also be destroying the wood around the metal fixtures. My Boatowner’s Mechanical and Electrical Manual has quite a lot of detail on measuring these voltages and determining how much zinc you do need.