Join Date: Sep 2006
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Think of those "hang over the side" style zincs, often shaped like fish. They are hung over the gunwhale and dipped in the drink. The other end is coupled electrically to your prop shaft, or motor, or whatever, and when the zinc slowly fizzes off, it will tend to hold the protected component negative relative to the zinc. Being negative will radically slow the corrosion reaction... that thrives on being positive.
When the zinc fizzes off ("sacrificial anode") you get another one.
It does not protect when you are on the move, as you have to lift the fish out, but most of us are stopped, most of the time.
In fresh water, some outboards use magnesium sacrificial anodes, as zinc cannot quite get enough sacrificing going (it is not as reactive as magnesium) but I don't think that you need them. Fresh water is FAR more forgiving to metals. Often they don't need any protection at all when in fresh water.