This system has been used for many years, the fitting of zinc plates in way
of bronze propellers and other immersed fittings being common practice.
Initially results with zinc anodes were not always very effective owing to the
use of unsuitable zinc alloys. Modern anodes are based on alloys of zinc,
aluminium, or magnesium which have undergone many tests to examine
their suitability; high purity zinc anodes are also used. The cost, with various
other practical considerations, may decide which type is to be fitted