Zincs and Protection from Galvanic Corrosion Use zincs to protect against the galvanic currents that are set up by dissimilar metals on your boat that are immersed and that are in electric contact with one another. The best example is your bronze propeller on a stainless shaft. The best protection is to put a zinc right on the shaft next to the propeller, or a zinc on the propeller nut . An isolated bronze thru-hull doesn't need protection because it is not in electrical contact with another immersed dissimilar metal. Conductors running from the external keel or ground plate to the mast, stays and to the metal fuel tank will protect against a lighting strike, and there will be no DC connections to the engine or to the electrical system.
Stainless steel is a special case.
Generally, it is a bad idea to use stainless steel underwater, because it can pit. The electrochemistry of this assertion is compelling enough to recommend that you protect a stainless steel rudder shaft with a zinc. For the reasons described above, ensure that your metal rudder shaft is not electrically connected to anything else. Your stainless steel propeller shaft will be protected from itself, by the same shaft zinc that protects the propeller from the stainless steel shaft. In both cases the pits, if they appear, will appear where the stainless steel is not exposed to the water.