We have four zinc plates on the transom with shaft and rudder zincs. Last summer our diver said our zincs were inactive. Our electrician took readings and determined we were over zinced. We pulled the shaft zincs and saw some activity. Just was in the boat yard and left the shaft zincs off and plan to monitor, may need to remove rudder zincs too? Our dock is not near another dock with a in the water boat. Never experienced having to remove zincs.