RE: What's this?
While you are on the hard, pull that transon zinc off and clean the mounting thoroughly. Sand paper and a mini S.S. wire brush for the bolt and nuts and clean the mounting area on the zinc itself. I've found even new zincs need a cleaning of the mounting area. They look bright but the contact area cannot be trusted.
I learned the hard way that not checking and ensuring the contact area is clean is a sure way to a non effective zinc, which you don't want to find later.
Check the connections on the boat inside where the mounting bolts come through the transom. If these connections are in poor shape the zinc will not be effective.