I'm confused by what you mean by the "plate" zinc....are you talking about the rectangular plate on the side of the rudder?..if so that's not a zinc, its the ground plate for the SSB radio.
If your rudder zincs are fine, but the prop zinc isn't you may have a bonding issue...the rudder zincs should be decaying....in fact you shouldn't need the prop zinc if your bonding system is all good.
