Any time you have two different metals that are physically or electrically connected and immersed in seawater, they become a battery. Some amount of current flows between the two metals. The electrons that make up that current are supplied by one of the metals giving up bits of itself-in the form of metal ions to the seawater. This is called galvanic corrosion and, left unchecked, it quickly destroys underwater metals. The most common casualty of galvanic corrosion is a bronze propeller on a stainless steel shaft, but metal struts, and rudders are also at risk. The way we counteract galvanic corrosion is to add a third metal into the circuit, one that gives up its electrons quicker than the other two. This piece of metal is called a sacrificial anode, and most often it is zinc. In fact, most boaters refer to sacrificial anodes simply as zincs. It would be hard to overstate the importance of maintaining the anodes on your boat. When an anode is gone, the metal component it was installed to protect begins to dissolve. The amount of protection an anode provides depends on its surface area. The anode surface area needed varies with the kind of metal being protected and with the chemical make-up of the water, but you can use 1% of the surface area of the protected metal as a starting point. If it shows signs of corrosion despite the anode, you need more surface area. Anodes should be replaced when about half of the anode has been lost to corrosion. Ideally you want that to occur not more frequently than annually. The longevity of a sacrificial anode is a function of its weight. When an anode lasts less than a year, you need one with more weight. 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. You need low-resistance, metal-to-metal contact either by mounting the anode 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. Anodes cannot perform their function unless they are exposed. Putting paint on an anode smothers it, rendering it useless. Never coat anodes with bottom paint, or anything else. Propellers are normally protected by an anode collar fashioned in two pieces and bolted together around the shaft forward of the propeller. Metal rudders and struts are most easily protected with anode disks bolted directly to the metal. Bonding is a different subject altogether. Boats with all underwater fittings bonded together electrically are typically fitted with one or more anode plates bolted to the hull. The mounting bolts for these anodes are connected by heavy-gauge electrical cable to the bonding circuit. If these anodes are allowed to deplete or if the electrical connection deteriorates, other underwater metal, such as bronze through-hull fittings, will begin to corrode. Anode hull plates are also fitted to metal boats to protect the hull. During my cleanings, I will inspect the condition of your anodes. If your anodes are dissolving faster than normal your boat may have an electrical problem. The insulation on one of your boats electrical wires may be damaged or your galvanic isolator may need repair. I will let you know if I notice a change during my cleanings.