If the above title strikes you as being evocative of a popular mystery series, that is by design.
This month’s subject is murder, although I have to confess that there is not much of a mystery here.
Dissimilar metals submerged in salt water invariably results in the untimely death of one of them. This occurs because two physically or electrically connected metals immersed in seawater create a battery. An electrical current flows unimpeded between the two metals. The electrons that make up this current are supplied by one of the metals losing bits of itself—in the form of metal ions—to the seawater. The name for this process is galvanic corrosion.
Unless preventive measures are taken, the consequence will be mayhem and eventually murder of the lesser (galvanically speaking) of the two metals. Demise can occur with surprising violence. In plastic sailboats, the most common victim of galvanic corrosion is a bronze propeller on a steel shaft.
Rudder fittings, metal struts, and bonded through-hulls are also at risk.
The only way to stop galvanic corrosion from occurring is to make sure all underwater metals are identical or to prevent electrical contact between dissimilar metals, but both of these measures can prove to be impractical. However, we can give corrosion an easy mark, deflecting its violence away from essential underwater components.
We do this by adding a third metal into the circuit, one that is quicker than the other two to give up its electrons. This third metal is usually zinc, but in some circumstances magnesium or a specialized aluminum alloy are substituted. These bolt-on or clamp-on castings are properly called sacrificial anodes
, but most sailors refer to them simply as zincs
Sacrificial anodes, properly installed, are 100 percent effective.
But the nature of their task means they have a limited life span, not unlike filters for oil and fuel.
If you wait too long to replace depleted zincs, it is your inaction that bears responsibility for bad things happening to your boat’s underwater metal components.
The amount of protection a sacrificial anode provides is directly related to its surface area. The anode surface area required varies with the kind of metal being protected and with the chemical makeup of the water, but you can use one percent of the surface area of the protected metal as a starting point. Check the protected metal frequently. If it shows signs of corrosion despite the zinc, you need more surface area.
|"There is an unfortunate misconception that a sacrificial anode can be mounted anywhere, even hung over the side on a string, and it will still perform its appointed duty."|
Replace zincs when about half of the anode has been lost to corrosion. If you haul out annually, then you would like for this not to occur in less than a year so you can replace the anodes with the boat out of the water. Anode lifespan is a function of the weight
of the anode, not surface area. If your anodes are not lasting a year, you need heavier anodes.
Of course, the need to compute anode sizes is far less common than simply replacing depleted zincs with new ones of the same size. You should check all zincs at least annually—more often until you have a sense of their rate of depletion. Replace anodes when they are half depleted. Unless you don’t mind changing anodes underwater, it is generally a wise investment to replace even moderately depleted zincs at every haulout.
There is an unfortunate misconception that a sacrificial anode can be mounted anywhere, even hung over the side on a string, and it will still perform its appointed duty. That is dead wrong! For an anode to provide any protection, it must be in electrical contact with the metal being protected. The conductivity of the water is not adequate. We need low-resistance, metal-to-metal contact—either by mounting the zinc directly to the metal being protected or by connecting the two with a wire. A hanging anode can provide protection if it is connected by a wire to the metal being protected.
Where an anode is mounted directly to the protected metal, such as a zinc collar clamped to the propeller shaft, it is essential to make sure the surface under the zinc is clean and bright before the anode is installed. This is to ensure good electrical contact. Likewise, when replacing zinc disks fastened to rudder fittings or other submerged metal surfaces, wet-sand the contact surfaces bare and bright before installing the anode to ensure good electrical contact.
If the through-hulls and other underwater fittings of your boat are bonded together electrically, they should also be connected to a sacrificial anode, usually in the form of one or more zinc plates bolted to the hull. The mounting bolts for these anodes should be connected by heavy-gauge electrical cable to the bonding circuit. For underwater components that can be isolated, no good can come from bonding them, but that is the subject for a different article. If your boat is bonded and you allow the protecting anodes to deplete or the electrical connection to deteriorate, the bonded components will begin to corrode.
Zinc plates are also fitted to metal boats to protect the hull. If you own a metal boat, this is a subject in which you should already have expertise. Never paint anodes. They cannot perform their function unless they are exposed. Putting bottom paint or any other coating on a zinc smothers it, rendering it useless.
All of your zincs are not on the hull and shaft. The internal surfaces of copper alloy heat exchangers are continually immersed, putting them at risk of galvanic corrosion. To combat this, most heat exchangers are fitted with a zinc “pencil” anode. You should find it under a threaded brass plug in the exchanger. If the pencil is half depleted—hard to determine unless you have a new one in hand—replace it by unscrewing it from the plug. Some engines have similar zinc pencils inside the cooling-water jacket to protect dissimilar metals in the engine. Check your engine manual. As with those below the waterline, engine and heat exchanger anodes need to be checked at least annually.
So back to our whodunit that isn’t, if you discover your prop or through-hulls or rudder gudgeons or heat-exchanger tubes full of holes, YOU know who figuratively pulled the trigger, don’t YOU?