Choose the wrong anode and your prop and shaft pay for it. Zinc, aluminium or magnesium — here’s how to decide.

What an anode does

A sacrificial anode is a block of less-noble metal electrically bonded to the vessel’s underwater metalwork. In seawater, the anode corrodes preferentially — being consumed to protect the shaft, propeller, rudder and engine.

Zinc — the saltwater standard

Pure zinc anodes are the historical standard for seawater. They work. The downside is that they stop working in brackish or fresh water (the oxide layer insulates them). If your vessel moves between saltwater and freshwater, think about aluminium.

Aluminium — the modern all-rounder

Modern aluminium alloy anodes (containing indium) work in salt, brackish and fresh water. They last roughly 50% longer than zinc on an equivalent-mass basis. The main caution: don’t use aluminium anodes in a diesel engine’s heat exchanger unless the manufacturer approves it.

Magnesium — freshwater only

Magnesium anodes are strongly electronegative — too strong for seawater (they’ll alkaline-corrode aluminium hulls). Use them only in freshwater: lakes, rivers.

Sizing and placement

Shaft: one anode within 100mm of a bearing. Rudder: one on the trailing edge or stock. Hull: one or two bolted to a dedicated plate. Engine: one in the raw-water circuit. Replace at 50% consumption.

Checking they work

Half-cell reference voltage: for a steel/aluminium hull in seawater, read −800 to −1050 mV vs. silver/silver-chloride. For a GRP hull with bronze gear: −550 to −650 mV. More negative, over-protected (hydrogen on bronze alloy). Less negative, under-protected.

Need advice on your boat’s stern gear or propeller? Call Brian directly on 0413 771 090 or use the contact form — we respond the same day.