Your boats bonding system may be the cause of your galvanic problems. As stray currents travel through the water, the trough hull farthest away from your zinc will have a greater potential difference. You have connected this through hull with whatever your zinc is connected to via a bonding wire, creating a battery.
There are two schools of thought here; to bond and not to bond. On my boat I have nothing bonded together. Every through hull is independent and the prop and shaft is isolated by a coupler. I do not run any electricity or potential electricity through the prop, the through hulls or any other metal in contact with the water.
The exception I would make is lightning protection to the keel, but i haven't done it yet.
The standard by ABYC says to ground your electrical system to the shaft and prop. I just don't see a benefit and haven't heard a good enough argument to convince me yet.
