Photo courtesy of DockTester.com
This prop shows clear signs of pitting due to galvanic corrosion. Chances are there was no zinc, it dissolved, or the connection between it and the prop or propshaft was poor.
Turn the multimeter’s selector knob to the smallest voltage range and touch the probe to your main zinc. Your meter should read around one volt. (On my boat it was 1.059 volts.) This is your base voltage. Now touch the probe to any part of your bonding strip. If the reading is not identical, the strip is not connected to the zinc.
Assuming the strip-to-zinc connection is solid, touch the probe to each metal object on your boat that has contact with seawater, such as through-hulls, raw-water strainers, and prop and rudder shafts, while keeping the reference electrode in the water. If each is properly connected to the bonding circuit, you should get a number fairly close to your base reading. On my boat, for example, touching the probe to the main seacock for the engine-cooling system produced 0.944.
Unfortunately (or maybe not), I have no underwater metallic components aboard that are not connected to my bonding circuit. But in researching this article, I found reports of unbonded metals generating voltage ranging from 0.3 to 0.4. In any case, the exact voltage is not as important as how it compares to the other voltages and, of course, your base reading. You’re looking for an obvious anomaly, which indicates a problem that needs to be addressed.
Photo courtesy of DockTester.com
Large sacrificial doughnuts and a sacrificial transom block should both be connected directly to the boat’s general bonding system.
Fortunately, once you’ve located the problem, correcting it is simple. You can pick up a bronze bonding strip at any marine supply store. Connect it to your existing bonding system with sheet-metal screws or rivets—anything that ensures tight metal-to-metal contact. Bonding strips can be, and often are, painted once they’re in the boat, but you must have bright metal-to-bright metal contact between the strip and another piece of strip or a component. You can attach the strip to the component using either bolts or screws; most surveyors like to see the strip attached to a mounting bolt if possible.
Regardless of how you attach it remember: the object is to connect the strip to every component that is in contact with seawater, so beware of things that impede solid metal-to-metal contact like caulking, paint, and non-metallic washers and backing plates. After you’ve connected the strip and component, double-check its electrical continuity with your multimeter and reference electrode, just to make sure you’ve got a solid contact. If you get something close to your reference reading, you should be good to go.
For most boats, the process of checking on bonding system continuity should be a once-a-year job. But beware: Anything can happen during the season when your boat’s in the water and you can’t see what’s going on below. A stray current from your boat, another boat, or even from your marina’s shore power system can drastically accelerate the degradation of your sacrificial anodes and underater components. In mere weeks your boat could be left unprotected and damaged. So it’s a good idea to visually inspect your sacrificial anodes from time to time during the season, even if it requires donning a mask and fins. (This is one good reason why you should hire a dive service to clean and inspect your hull.) After all, you wouldn’t want to be halfway to the Bahamas and have a through-hull fall off.
This article originally appeared in the March 2009 issue of Power & Motoryacht magazine.