I recently saw a boat hauled out that had hot-pink propellers and shaft nuts. There were no zincs left, and the pink was a sure sign of a “hot boat.” It reminded me of an incident we had years ago on the 37-foot Rybovich Brier Patch that was just as bad if not worse and led me down the path to learning about stray current and the different types of corrosion that can wreak havoc on your running gear and other metal items on board.
The boat had gone through a major refit that included removing all the old wiring, plumbing and all equipment, including the engines and generator. After the first couple of days, each time we washed the boat and touched the tower we felt the unmistakable tingle of electricity. Unable to find the gremlin, we called in Don Sloss, our electrical guru, to sort it out. We found a DC wire mistakenly connected to the bonding system, so when we were plugged into shore power, we were cooking with electric, so to speak.
There are several kinds of corrosion, but there are basically two types that underwater metal parts are susceptible to: galvanic corrosion and stray-current corrosion. Galvanic corrosion is an electrochemical reaction between different metals, for instance, stainless screws or bolts in aluminum. Oftentimes, if there is no paint barrier, powder coat, bedding or a product like Tef-Gel, the stainless bolts will corrode themselves to the aluminum and will not be able to be removed.
Stray-current corrosion is when an electrical current is added to the galvanic corrosion situation. This occurs when you have metal with electrical current flowing into it and it is submerged in water that is grounded. Current travels through the water to ground, making for quick corrosion of the metal where the current exits. This type of stray current has destroyed many an aluminum outdrive. Stray current can come from several sources. Some of the more common are outside sources, such as a short in the wiring system, improperly or poorly sealed connections, deteriorated bilge pump wires, a broken wire or a wire that has cracked insulation.
Stray current is most commonly associated with shore power. But with low-voltage DC currents flowing through the shore-power cord ground wire because the boat, dock and wire are connected, there can be DC present or leaking. Many boat systems have galvanic isolators that help prevent and control this situation. Galvanic isolators are tied into the green ground lead that runs through the boat, tying everything together to ground. The galvanic isolators work to filter out and block low-voltage DC currents yet still let the boat’s ground system do its job.
Sometimes you find yourself docked next to a boat that is leaking current that will eat away at your sacrificial zinc anodes quicker than normal. When you notice this, you need to check your boat first, then the dock and finally your neighbors. I’m always nervous about pulling into a marina with little-used, weekender live-aboards; they seem to have a higher instance of stray current in my experience.
Neither galvanic nor stray current are electrolysis. They deal with mixing water and metal along with an electrical current. Electrolysis is the breakdown of metal in salt water that has a current passing through it. This means there must be a source of DC, two types of metal to act as electrodes and seawater, which acts as an electrolyte.
Protecting your equipment from electrolysis requires the use of sacrificial zinc anodes. Typically found on your shafts, trim tabs and, in older applications, rudders, fresh zincs on your boat are key to helping keep your gear healthy. To work properly, the zincs need to be attached to a clean surface for a good bond, and sanding the shaft clean before installation is recommended. Many boats today have a comprehensive bonding system and may, in fact, have a place where zinc plates are bolted to the transom, which is in turn connected to a bonding plate on the inside of the boat that is tied together with the green wire bonding system.
Your green wire bonding system should be connected to the main engines, generators, AC raw water pumps, head pumps, watermaker pumps and everything electrical on your AC power side, and your DC systems as well. Periodic checks of the green wire bonding system should include not only the green wire insulation for cracking but also everywhere it is connected to the boat. Each piece of equipment should be clean and free of paint and debris and dry so that all connections are clean, allowing your ground system to work at its full potential.
Your main engines, raw water gear oil coolers, fuel coolers and other mechanical gear items also have zincs that need to be maintained on a regular basis to protect internal parts. It is important to check these and replace as needed as part of your regular maintenance program. When storing the boat for an extended period, having the ability to flush the engines and AC systems with fresh water also helps reduce the chances of a problem. However, fresh water that has elevated mineral levels is conductive, especially warm water. This is one reason why you have a higher risk of a problem in tropical water than in cooler water locations.
If you are having problems, you need to have a hull potential measurement performed. Your marine electrician can do this, or you can do it with a volt meter and a cable with a silver wire and silver end that you put in the water over the side of the boat, connecting the other end to the volt meter’s positive terminal. The negative lead should be connected to the boat’s battery ground. Setting the meter to the 2-volt DC position, you will get the boat’s hull potential, which should read between -550mV to -900mV. Lesser readings of -300mV to -400mV indicate the zinc anodes are eroded and not providing the protection needed.
The readings should stay the same with the shore power disconnected if the galvanic isolator is working. If the readings are more negative, it is possible you have a current leak through the shore cord. If less negative, there is most likely a stray DC current somewhere. If you notice definite reading changes with the engines running, it is possible there’s a stray current from an electrical device on the engine systems or the alternator.
Always consult a professional marine electrician. Not staying on top of zincs and not keeping the bonding system healthy can lead to a series of expensive equipment replacements. Semiannual inspections of the bonding system, sacrificial anode system, and regular maintenance on pumps and the like will keep your juice flowing smoothly and keep you from expensive repairs, or worse, an electrical jolt.