TimJM

This is one of the things I particularly checked, as I had no recollection, either of making that link when the boat was still fitting out ashore for a considerable period, or of removing it when the boat went afloat. I find that the link is definitely not in place, and I am sure now it never was, ie the boat may have been potentially unsafe when ashore through lack of earthing, but is correctly wired now. Tim.

Thanks for working away at this, Guys. None of the blisters were burst, ie the coating was intact and not damaged, other than "something" is lifting it away from the bare steel in small blisters in fairly well defined areas. The pressure washer did not take the tops off the blisters. I had to scrape quite hard to get inside any of them by removing the very tough epoxy coating. The vast majority were left intact after the dry docking. Tim.

Thanks for weighing in, Tony. Just to confirm, all my continuity tests were carried out with all cables connected as per Victron. It seemed very clear that the shore side earth is completely isolated. The hull, boat side earth and transformer case were all firmly connected to each other. No doubts here, unless my ordinary small cheap multimeter is somehow not up to the job.

Just responding to the last two posts: Mark99: Nearest railway about half a mile away. No other similar sources of DC in the vicinity. Yes, the baseplate was sprayed with the two pack epoxy - all except the two patches where the boat was resting on its supports. I painted those bits when craning, but only with vinyl tar (Comastic). The boat was to sit in a drying tidal berth for some time, so it seemed good to protect the baseplate where possible. No paint on baseplate since. Roger: Well, Hallingbury marina is indeed where we are. However, a) we are not in an alongside berth and the isolation transformer (see my post that started the topic) ought to prevent this sort of trouble. But interesting to note and will bear it in mind, thanks. Tim.

Quick catch up on last couple of posts - thanks everyone, it's all grist to the mill! Correct no short once ammeter removed, but given large areas of steel involved I presume the PD could cause a small current to flow through the water. Yes, professionally blasted and sprayed two pack epoxy. Total blistered area a small fraction of total surface.

Re microbial possibility: We did consider this but rejected it as no sign of slime in the blisters. Plus assumption (perhaps wrong) that the pits would only be bright metal through electrical activity. Re anodes: We had aluminium anodes when the boat lived in salt/brackish water. Now we are definitely in fresh water (river Stort). Anodes have become heavily pitted in three years. The "different steels" possibility has also been raised and might make sense, but 1) you might expect some protection close to the anodes - they seem to make no difference. 2) How do I measure potential difference under water? Presumably even if we slipped the boat again, the absence of the water would stop any activity?

No, no physical contact. A plastic fender hangs between the hull and the wooden edging on the walkway. Disbonding - interesting possibility. How could I prove or disprove this? The "shiny but matt" appearance of the steel when blisters were scraped off seemed to indicate electrical activity?

Thanks for this prompt response. Yes, there are six large magnesium anodes, fitted three years ago at a previous dry docking (before that we had aluminium anodes as the boat was originally moored in tidal water). The anodes are heavily pitted - look a bit like a sponge, but the vast majority of the metal is still there. The parts of the hull closest to the anodes are not any less affected than those farther away. Following my previous post it occurred to me to replace the voltmeter between our hull and neighbouring boats/steel walkway with a milliameter. I found that a current of around 100mA flowed in the link (effectively a short).

On a recent dry docking of our steel narrowboat I discovered blistering of the two part epoxy hull coating, mainly in a band running below the under water rubbing strip. The steel under the blisters was shiny, not dull or rusted. The boat is in a marina, with boats either side and a steel framed walkway. We connect to shore power via an isolation transformer and I have checked that this is correctly wired and does indeed isolate the shore side earth from the boat side earth, the case of the transformer and the hull as a whole. I have discovered that there is a potential difference of nearly half a volt between the hull and each of the neighbouring boats, also the walkway. Our hull is at the lower potential. I would be grateful for any clues as to what may be going on here and what can be done about it! Many thanks.