The 50% Rule (and how to choose the right anode material)
Anodes are your Tohatsu’s sacrificial corrosion defence. Learn the 50% rule, where to check anodes on your outboard, and which anode material to use in salt, brackish, or fresh water.
Sacrificial anodes protect your Tohatsu outboard from galvanic corrosion by “giving themselves up” first. The 50% rule is simple: replace an anode when it is around half consumed. Waiting until it is nearly gone often means the remaining metal cannot protect properly, and corrosion may start on the gearcase, bracket, or fasteners.
Where to check on a Tohatsu outboard: most setups have anodes on the gearcase/lower unit, the transom bracket, and often a trim tab anode (depending on model). Check them regularly, especially if you moor in the water.
Which anode material to use:
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Zinc: best in saltwater (traditional choice).
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Aluminium: excellent in salt and brackish and a strong all-rounder for many boaters.
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Magnesium: for freshwater only. Do not use it in saltwater as it can disappear very quickly.
Key tips for best protection: keep anodes clean and unpainted (paint stops them working), use the correct alloy for your water type, and avoid mixing random anode types on the same engine. If anodes are vanishing unusually fast, investigate stray current and check shore power and nearby wiring. As a routine, inspect at least monthly when in use and at every service.
