Marine anode selection: zinc, aluminum, or magnesium

Sacrificial anodes protect submerged metals from galvanic corrosion. Choose by the water you sit in. (1) SALT WATER: zinc anodes work but are being phased out in some jurisdictions for environmental reasons. Aluminum-zinc-indium alloy (often called "aluminum" anodes, e.g. Navalloy or Alcoa alloys) is now the marine standard, same protection as zinc, longer life, less water pollution, works in both salt and brackish. (2) BRACKISH: aluminum only. Zinc goes passive (stops working) in low-salinity water. (3) FRESH WATER: magnesium anodes only. Zinc and aluminum both go passive in fresh water and don't protect. Anode installation: bond the anode to the item it's protecting, for a bronze rudder, the anode is bolted directly to the rudder post; for a shaft, it's a shaft anode clamped to the shaft. Anode consumption 50 percent = replace. Anodes lasting more than 3 seasons in salt water = check bonding (probably not connected). Anodes gone in 6 months = stray current (nearby boat with shore power fault, or your own boat backfeeding). Never mix anode types on the same boat, magnesium anodes on a bonded system in salt water will consume violently and can cause paint blistering.