As I understand it you shouldn't need the zinc on the propellor as the C-drive is connected to the bonding system.
I had that zinc for a while and it degraded at about the same rate as the rudder zincs. Last haulout I took it off and replaced it with the plastic cap.
Maybe you need to check the connectivity between your c-drive and the rudder zincs?...or your propellor and the rudder zincs.
If the resistance is very low you should have no problem with the plastic cap.
If the resistance is more than an ohm or two then you have an issue with the bonding connections somewhere.