A lot of unclear info in this thread.... just some points for clarification:
A zinc protects the under water
metal part(s) it is electrically connected to. So if it is connected to your prop and your prop is electrically connected to your shaft, both will be protected.
If you have a bonding system and the zinc(s) connect to that, all metal parts
will be protected. But a zinc can disappear fast if it is too small. You need to have a certain minimum surface area of zinc and the best way to find out how much is by checking how fast they disappear. If it's too fast, you need more surface area.
Whenever something changes around you, like you move your boat somewhere else, or another boat moves into the marina etc. the situation changes. Even if your zincs did just fine in the past, you must continue checking them.
Some zincs are expensive, like my big Maxprop zincs. We use a grouper zinc whenever we are at anchor
or moored for more than a couple of days. The grouper zinc has the same surface area as all the fixed zinc together. This means, that the other zincs will last twice as long. We have a bonding system, so we can connect the grouper zinc to any metal part. We use a mizzen capshroud.
Grouper zincs are expensive too. You can grab (used) zincs for free around every boatyard. Throw 'm in an old pot, melt them on a burner and cast them in a sand mold
with a piece of awg #10 wire. Strip the couple of inches that go in the zinc.
If you buy one grouper zinc, you can press it in the (moist) sand to create exact replica's. Make new ones before using your last one ;-)
Isolating yourself from the shore power ground-wire is the best thing you can do in a marina, but it needs to be done safely and that means an isolation transformer. A galvanic isolator uses diodes in an anti-parallel circuit so that is a 2-way circuit but it blocks the current
for very low voltages. It is unreliable and less safe than a transformer.
for Barnie: it doesn't matter if your AC and DC are separated. That does not protect your under water metals from an shore AC problem. It only helps if the shore AC is not connected to anything else than the outlets in the boat, so no ground plates or ground-to-metal-part connection anywhere. (which the ABYC does not recommend while it is the other way around in the EU). An isolation transformer is always the best option but cost more and weighs more.