0.350 and over you have no protection(nl)0.250 to 0350 : under protected(nl)0.100 to 0.250: adequately protected(nl)0.000 to 0.100: adequate but high range(nl)More than 0.000: over protected. You can take a silver tea pot or some other silver based item and hook up a wire to your didgital volt meter and drop it over the side. Then use the positive lead and test every item that protected by your bonding system and write them down. Using the numbers above, you can tell how your system is doing.
Put your meter on the milliamp range. We can see that you have no protection on the rudder using these generally accepted numbers. I'm thinking it's a bad wire or no wire at all. Do you have some kind of a plastic bushing separating your rudder from the rest of the boat? Did you take your readings with a new zink. You might be covered correctly with a new one. How long did it take to wipe out the zink? Did something change causing it to be depleted sooner than what was a historical norm?