Magnesium is the ideal anode material for boats that operate entirely in freshwater. Magnesium anodes have a higher oxidation potential than zinc or aluminum anodes, making them particularly well-suited to the less electrolyteconcentrated environment of freshwater. Does not build up oxide layer in freshwater like zinc or aluminum Not for use in salt or brackish water - except for rare, brief exposures, a total of 14 days per year Complete units include propeller nut (right hand) and magnesium anode Replacement only units are just the magnesium anode