Short answer
Match the alloy to the water you keep the boat in — different alloys are specified for full seawater, brackish and fresh water — and match the anode type to what the maker of your drive or engine specifies. Then read the wear rate: anodes that vanish in weeks indicate a stray-current or bonding problem, not an anode problem.
You will need: Correct anodes, spanners, wire brush
Anodes are consumed deliberately so that expensive metal is not. Getting the alloy or the fitment wrong quietly removes that protection.
Alloy follows the water
Different alloys are specified for full seawater, brackish and fresh water, and an alloy that works in one performs poorly in another. This matters more in Australia than owners expect, because so many boats live in rivers and estuaries where salinity swings with rainfall — the Swan in Perth, Pittwater and the Hawkesbury in Sydney, the Gold Coast rivers.
If your boat moves between brackish and coastal use, check anodes more often rather than assuming one specification covers both. See Perth and Sydney.
Fitment follows the maker
Saildrive legs, shafts, rudders, trim tabs and engine internal anodes (pencil anodes) all have specified parts. On an aluminium saildrive leg in particular, the anode, the coating system and the electrical bonding of the drive are a single protection system — mixing them up is expensive.
Read the wear rate
Anodes should waste steadily over a season. Two patterns are findings:
- Gone in weeks: stray current, a bonding fault, or something in the marina. Investigate the electrical cause rather than simply fitting more anodes.
- Barely touched after a season: the anode may not be making electrical contact, which means it is protecting nothing.
Electrical investigation of stray current is specialist work, and on shore-power systems it crosses into licensed territory.