A trawler owner recently contacted us to schedule a routine haul-out and bottom-paint job. He told us that he had some concerns about past discoloration of the bottom paint around through-hull fittings. As it turns out, the paint discoloration pointed toward a much more serious condition, a life-threatening one at that. This example also serves as a reminder to pay attention to your through-hull fittings each time you haul your boat.
Through-hull fittings get no respect. Hidden below the waterline, slathered inside and out with bottom paint, and often fouled with marine growth, these fittings do their jobs in obscurity. When you haul your boat, the through-hulls usually receive little more than some scraping and more bottom paint. But bronze through-hull fittings offer a surprising and important window into the condition of your boat’s electrical system. Like the canary in the coal mine, through-hulls can warn you about potentially life-threatening AC electrical leaks, damaging stray current corrosion, bonding issues, and sacrificial anode problems.
In this particular case the problem and the solution involved multiple issues. Before we dive into the dangerous condition that discolored the bottom paint, we need a bit of background information.
Making the Connection
You might be wondering what an underwater pipe fitting has to do with your electrical system. On most fiberglass powerboats all underwater hardware connects to your electrical system through the bonding circuit. Bonding refers to connecting all underwater hardware with a low-resistance, reliable electrical connection. Bonding achieves two goals: first, it minimizes corrosion due to dissimilar metals (galvanic corrosion); second, it reduces the risk of an electrical shock from a faulty shorepower system.
We’ll save the detailed discussion about galvanic corrosion for another column. Let’s clarify some terminology. We often use the terms “zincs” and “sacrificial anodes” interchangeably. Zincs, however, are one type of sacrificial anode, but aluminum and magnesium also serve as sacrificial anodes in different applications. For now, it suffices to say that if you bond all underwater metal, and if you have a sacrificial anode connected to that bonding circuit, the anode will protect all of the bonded hardware from galvanic corrosion. This bonding circuit also connects to the boat’s DC ground wire, and that gives us a connection between your through-hulls and your DC electrical system.
Next, let’s look into the life-saving function of bonding. Bringing 110-volt current (or 220) from land onto a floating boat creates risk. Alternating current always returns to its source—in this case, the power plant located somewhere on shore. It comes into the boat through the cord and returns to its source by the same means. If you have an electrical short on, say, your battery charger, the case can become hot. The current wants to return to its source and can safely do so through the green wire attached to that case, and then through your shorepower cord.
What if that circuit has a fault somewhere on your boat, on the dock, or ashore? The AC current still needs a path back to land and if you touch the case while standing barefoot or in socks against a damp hull, you become the path as the current passes through your body, into the water, and back to shore. And you would likely die in the process (along with any swimmers in that path, if they happen to be in fresh or brackish water).
To minimize this risk, boatbuilders and boatyards provide an alternative path by connecting the green wire to all of your seacocks and through-hulls. If the shorepower safety circuit is compromised, the current has an alternative path to the water via the through-hulls. The good electrical connection through the green bonding wire provides a preferred path for the current, favoring the low-resistance electrical circuit to the meager choice your body offers (swimmers are still at risk). This safety circuit works so well you probably won’t even know that you have a killer aboard, unless your through-hulls tell you.
Reading Your THROUGH-hulls
Armed with an understanding of the connection between through-hulls and your electrical system, let’s assume your boat has been hauled, providing an opportunity to inspect the hardware for signs of trouble. You will be looking for three conditions: color of the metal, pitting or erosion of the metal, and discoloration of the bottom paint around the through-hull.
Let’s begin with the color of the through-hull fittings. Healthy bronze through-hulls exhibit a uniform golden hue. When properly protected from corrosion they will retain this color for many years. We have maintained a particular sailboat for the past 40 years, and she still has her original through-hulls (57 years and counting). Any pinkish spots or patches indicate deterioration of the metal alloy, a process known as dezincification. The departure of zinc from the bronze alloy leaves behind a porous copper structure in a weakened condition. While the thick lip of the through-hull might have sufficient healthy metal remaining, the threads which hold it in place are certainly at risk. Once the gold hue turns pinkish the fitting should be removed and replaced. This condition points to an ineffective bonding circuit or inadequate sacrificial anodes. When the boat is launched, the bonding system and sacrificial anode protection must be checked—more about that shortly.
Also look for pitting or loss of metal. Stray current from your boat’s DC electrical system can destroy underwater hardware very quickly. AC stray current can kill a person, but rarely attacks underwater hardware. Pitting almost always points to stray DC current—usually a poor connection in a wet location, such as bilge pump wiring. If you see pitting or loss of metal, the fitting must be replaced and further testing must be performed.
In addition, have a look at the bottom paint surrounding each through-hull. If you see a circular area (a halo) of discolored paint you might have too much sacrificial anode protection or you might have stray AC current. In this particular scenario, the through-hull fitting will not be affected (no change in color or pitting). When it comes to sacrificial anodes, too much of a good thing can be a problem. Remember, for a bonded boat with a common anode, the anode affects the voltage of all bonded hardware. For bronze hardware, 750 millivolts (mV) provides optimal protection. Bottom paints containing cuprous oxide lose their effectiveness when exposed to voltages above 750 mV. When the quantity of sacrificial anodes raises the voltage above 750 mV, ions flow from the through-hull into the bottom paint, creating discoloration and reduced anti-fouling properties.
After Launching
If you discovered discoloration, pitting, or halos, further checks should be completed as soon as possible. Four different tests can be performed.
- Bonding system integrity:
This test would be indicated by pinkish coloration on the underwater bronze, especially if the problem is limited to some but not all of the fittings. Using a multimeter set on resistance, a marine electrician can check the integrity of your bonding system (maximum of 1 ohm of resistance between any two points). This test must be done while the boat is on land. - Proper cathodic protection from sacrificial anodes:
This test would also be indicated by pinkish coloration on the underwater bronze, or if you found halos in the bottom paint. This procedure requires a silver/silver chloride reference cell that is dropped into the water and connected to one side of a meter. The other side connects to your underwater hardware inside the boat. If properly bonded, all metal fittings will rest at the same voltage with respect to the reference cell. For bronze hardware, that voltage should be approximately 750 mV. If too low, the hardware will not be properly protected. Too high and you might impact the bottom paint around the through-hulls. - DC Stray Current:
This test would be done if you found pitting (loss of metal) on the through-hulls. Same test as #2, but now you have to methodically go through the onboard circuits to find out which one causes a change in the voltage of the underwater hardware. - AC Stray Current:
This test would be used if you found halos in the paint around the through-hulls. Working with the shore power circuits is dangerous and should be done by a professional. As a starting point, an electrician would use a high-quality special clamp-type ammeter placed around the shore power cord. We want to know if all of the amperage coming into the boat is returning to shore. A few milliamps would be acceptable, but anything above 30 milliamps presents a risk.
Let’s return to that trawler with the halos of discolored bottom paint. Before hauling the boat, we plugged in the shore power and clamped a specialized ammeter around the cord. The meter showed 1.8 amps, or 1,800 milliamps. As we said earlier, 30 milliamps would be too much. We now had a smoking gun—an AC stray current leak into the water. The electrician found a connection between the neutral and grounding wire on a washer/dryer. In addition, the generator lacked a proper grounding wire and relied on its connection to the DC ground circuit for a connection. These connections, and lack of connections, allowed current from the neutral wire to return to ground through the through-hulls and seawater. This flow created the halos in the bottom paint.
Next time you have your boat out of the water, take some time before painting the bottom to inspect your through-hull fittings. If covered with bottom paint, use a scraper and some sandpaper to expose the metal on a few of them. This simple practice provides a valuable look into the health of your electrical system, bonding, and effectiveness of the sacrificial anode. If you do have a weakened fitting you can replace it now, with the boat already on land. And if the inspection points to a stray-current problem, you may have saved a life.
