As far as I am aware, Blue Opal was kept in Conwy from when she was launched until I bought her. Conwy happens to be at the mouth of a river, and the marina gets a fair bit of fresh water flowing into it – I’m not saying it’s fresh water, but it’s almost certainly got a lower salt concentration than the sea. The hypothesis from the local yard engineer was that the stern gear wasn’t bonded to the anode, and given the complete isolation from the steel prop shaft, it turned into a battery.
Well, he was right about the lack of bonding wire. I’ve reviewed the survey that was conducted pre-purchase, and it indicates that there was full conductivity between the anode and stern gear, so perhaps the gland had conductive contact with the stern gear and shaft simultaneously; when the gland was replaced, the conductivity was removed. Something to check in to. The other interesting thing, and this may be the human mind trying to find things that don’t exist, is that a pre-purchase photo shows a bit of pinking where the top bolt cap crumbled – but the survey didn’t raise this as an issue.