Boat Owner Tool
Anodes are meant to disappear — that is the whole job. The useful question is when to replace them, and whether yours are going faster than the water alone would explain.
This estimates service life from the three things that actually drive it: how much anode metal you have, how much bare underwater metal it is protecting, and how hard the water is working. The arithmetic is printed underneath so you can check it against your own numbers.
Preferred for Florida salt and brackish water. Protects well in both.
Total of all anodes protecting the same bonded metal. A common shaft anode is about 1 lb.
Square metres of exposed bronze and stainless. Painted hull is not counted.
Replace by
—months
How this is calculated
current demand = area × current density
life = (mass × fraction consumed × capacity) ÷ demand
This is a planning estimate, not a substitute for looking. Anode wear varies with bonding quality, water chemistry, and what the boats around you are doing. Have the anodes checked at every cleaning and note how fast they are actually going — wear that runs well ahead of this estimate is a stray-current signal worth chasing. Method follows Under the Waterline, Chapter 5.
Common questions
At roughly fifty percent consumed. A half-gone anode is not a problem — it is an anode doing exactly what you paid for. Waiting until it is nearly gone is what leaves the prop and shaft exposed before anyone notices.
Aluminum has become the preferred all-around choice for many boats here, because it protects well in both salt and brackish water and a great deal of Florida boating happens in that river-and-bay transition zone. Zinc is the traditional saltwater choice and still works well in salt, but it passivates and stops protecting in fresh water. Magnesium is fresh water only.
Stray current is the prime suspect — a wiring fault on your own boat, a neighbouring vessel leaking through the water, or a shore-power problem. It is faster and more damaging than ordinary galvanic corrosion, and the fix is an electrical check rather than bigger anodes. More on galvanic corrosion.
No. Protection depends on clean metal-to-metal contact. A loose anode is electrically disconnected and protects nothing, however new it looks. We check tightness on every visit as part of anode and prop service.