Once you determine the correct shaft size, it's wise to check the zincs periodically. In my case, I put one new zinc on the shaft every 6 months. That's once when I haul the boat for its annual "look see" and once by the diver in the Fall. The zincs are about 50% OK, but I change them regardless. Not having a sacrificial zinc means the metal in your prop and shaft become the victims of electrolysis. Not good, as zincs are cheap compared to a replacement prop and shaft..