Craig Thomson

Found this article here https://www.corroconsult.com/blog-news/ac-corrosion-demystified/9/3/2018 which details how AC currents are transformed into DC corrosion at coating defects on pipelines, via the coating defects themselves acting as diodes. The article does note that the dangers are most prescient to pipes running parallel for over 2km to HV lines and that they result in concentrated corrosion around small areas of coating defects. Whether any of this is relevant to a 40ft long boat I don't know, but it does cross my mind that having a new coat of blacking on everything except the few strips where she was resting on the slipway bearers perhaps puts these remaining unprotected areas at even greater risk of electrolytic corrosion than when the whole bottom was rust!

Thanks for all the replies so far, it's certainly an interesting topic. My main point of interest though is less the effects on human health (quite probably negative, but I also hypothesised that by living inside a big metal box one might actually be quite well protected by a Faraday cage effect ..?) and moreso the electrolytic effect such a setup might have on the outside of a steel hull. I have read about corrosion effects on pipelines in parallel with HV lines which seems to be something to do with long parallel conductors acting as a massive capacitance loop (please correct me if you have a better understanding!) and the resultant voltages leaking into the earth and corroding the pipes in the process. Is it thus probable that a similar (if much smaller) effect is acting on a boat hull earthing into the canal water? (itself also running fairly parallel to the power lines for several hundred metres) If so, can anything actively be done about it? There are a dozen other boats here in a similar situation who might also like to know the answer to this ... Does a galvanic isolator / isolation transformer have any effect here or is the point moot as it is independent of whether the boat is connected to shore power or not?

The boat is indeed new to us, we've been living aboard since April last year. Just slowly getting to grips with all the various aspects of maintenance. The amount of pitting on the hull, though noted in the previous boat report from 2019, was a bit shocking to see first hand. From what I gather some boats just seem to rust like hell and others somehow manage to get on fine for a hundred years and nobody quite knows why. Environmental factors surely have something to do with it, as do things like mill scale being left on new boats etc. Bad and extensive pitting though, seems to me to be likely down to electrical issues somewhere, about which we currently have little expertise. The boat is fitted with a galvanic isolator, but no idea whether it is in serviceable nick or not. Anodes are wearing ok and providing some localised protection to the bow and stern but not seemingly anywhere much else. As for the power lines, it is my gut feeling is that they must certainly be inducing some kind of voltage/current in the hull of the boat, but whether or not it is enough to do substantial damage is a bit of a mystery. We were considering moving the boat soon anyway to be closer to my other half's workplace so it shouldn't be a big problem for us to relocate. It seems to be a difficult situation to find any solid information on as it is so specific to canal boating (I can't imagine a lot of seagoing vessels are likely to encounter such an eventuality!)

Just back from blacking our boat for the first time (40ft live-aboard, 2002 build) and slightly paranoid about electrical corrosion after seeing first hand the amount of pitting on her sides and bottom. There is widespread pitting of 3 or more mm across the baseplate. We cleaned and blacked it all (well except for where she was resting on the bearers), but are now looking to what we can do to prevent further corrosion worsening the state of the hull. We intend to get an isolating transformer to deal with any stray current from the shore line (we are permanently attached at our home mooring) but then I also got to thinking and realised that our mooring is directly beneath overhead power lines (see picture below ...) So what I want to ask is - is it possible/likely that stray current from high voltage power lines could cause electrolytic corrosion to a steel boat hull? If so, how would one measure such a stray voltage and what could be done to insulate the boat from it? (Aside from moving to another mooring, which we are likely going to do anyway as a precautionary measure!) Any insight from the electrically minded much appreciated. CT