An anode (often referred to as "Zincs") is a sacrificial block of either zinc or aluminum alloy that is mounted on, or connected to, the underwater metal parts of your boat E.G. stern drives, shafts, propellers, rudders, trim tabs and skin fittings. Anodes come in a variety of shapes and sizes. They're designed to sacrifice themselves by depleting first, rather than expensive parts of your drive or underwater metal fittings. Stern drives, running gear and hull fittings are made up of two or more types of metal. Galvanic corrosion is an electrochemical reaction between two or more different (or dissimilar) metals submerged in seawater. Anodes connected to these metals are more galvanically active and will corrode before the vital parts of your vessel.