If you notice on the rudder there is a silver zinc and also on the bottom of the keel.
The reason that boat owners replace the zincs when they haul out is because electrolysis eats away at metal under the water line on a boat but it eats away at the zincs first so the rest of the metal goes untouched by the electrolysis. If the boat owner keeps up on replacing the zincs when they get eaten away the other metal below the boat will stay in better shape over time.
Rudder and Prop on The Holy Mackerel, originally uploaded by captjoe06.
Good Morning Gloucester 