PS Advisor: 01/15/02
When I got my Cabo Rico 38, with nine bronze through-hulls, all bonded, she lived in the brackish waters of Tampa Bay. But since taking her into the salty ocean an apparent electrolysis problem has arisen, Big Time. I first noticed one hull zinc, but not the other, disappearing at an alarming rate. The bonding wire makes them both part of a circuit. Then the through-hull fittings began to turn green inboard. Now, upon hauling out, I find 3-foot diameter donuts around each of the through-hulls where the paint—Pettit Trinidad—is eaten away. We live mostly on the hook, nowhere near stray current. Should I cut the bonding wires or just attach light bulbs and enjoy the free volts?
Just because you don’t live in a marina doesn't mean you're immune from stray current corrosion. In order for this type of corrosion to occur, you need two electrically connected metals immersed in the same electrolyte. In the case of a boat, two or more through-hulls in seawater satisfies the criterion. Now, add electrical current from an external source. If one of the metals is less noble (higher on the galvanic series) than the other, it becomes anodic and begins sacrificing itself while the other metal is protected. If one is your zinc anode, it’s just doing it's job. But if there are no zincs, or they've wasted away, then the next least noble metal starts to go.
So what is a likely external source of electrical current?
If you have an inverter aboard (an electronic device that converts 12-volt DC to 120-volt AC) for AC power away from the dock, a short somewhere in the AC electrical system could be leaking current to one of your through-hulls. We had this problem once when a partially chafed AC wire inside the boat lay across a through-hull. Interestingly, the through-hull was sound, but the bronze rudder shoe was punky and nearly fell off during the next haul-out. We found the wire (buried beneath the refrigerator) by checking AC outlets with a voltmeter. Sure enough, one outlet measured considerably less than the others—something like 90 volts, which was still enough to power an AC light bulb or kitchen utensil, so its diminished output wasn't noticed earlier.
Stray current corrosion also can be caused by leaking DC. Common sources are terminal strips in the bilge or shorted bilge pumps. To identify a possible source, you’ll need to check all electrical equipment and wiring in contact with underwater metals, which would include not only through-hulls but the propeller shaft and ground plate.
To bond or not to bond, that is the question. Theoretically, if you bond everything, and strategically place a number of zinc anodes (such as on the prop shaft), all voltage differentials are averaged and the zincs protect all underwater metal masses. Plus, you've got a bigger ground for lightning protection and SSB radio. But if the system is overwhelmed or breaks down, as may have happened on your Cabo Rico, then you’ve got problems.
The other approach is to unbond and electrically isolate all underwater metals. This means putting a non-conductive spacer in your prop shaft coupling.
To learn more about this fascinating and potentially devastating phenomenon, hit the books. Try Charlie Wing's Boatowner's Illustrated Handbook of Wiring, or Everett Collier's Boatowner's Guide to Corrosion, both from International Marine/McGraw-Hill. As Charlie Wing says, "In general, bonding of immersed metal components prevents corrosion due to stray currents inside the hull, but it causes corrosion due to stray currents outside the hull."
This suggests that the source is not a wet DC terminal strip or AC short, unless, as hinted at above, the bonding system has broken down. You might not have good electrical connections between through-hulls.
In any case, you’ve got some detective work to do. Good luck.