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Zincs-

Zinc anodes are placed on hulls, propeller shafts, rudders, trim tabs,
outboard engines, stern drives, and in the cooling system of most
inboard engines to protect their metal parts from galvanic corrosion.

Galvanic corrosion occurs when dissimilar metals, such as an aluminum
stern drive and a bronze through-hull, are placed in an electrolyte
solution such as seawater. Dissimilar metals are those metals which
have different corrosion potential (measured in volts) as determined by
the Galvanic Series, and the most common scenario in boating is
current passing from the more noble bronze to the less noble aluminum.
This transfer of current results in deterioration of the less noble metal.
The addition of a zinc actually increases current activity between the
metals, but because zinc is the least noble of the three metals in this
example, corrosion damage is primarily confined to the zinc itself when
properly installed.

Zincs must be impurity-free to work properly.

Zincs should be replaced when they have been reduced to about one
half their original size or weight.