Moominpapa, I'm sure he will not mind me saying, is a very very clever person. He programmes big computers and he understands eckeltricity. He explained to Jim that electrical devices (and possibly some other sorts) work because of magic smoke. All is well as long as the magic smoke is contained within the device. But sometimes there is a bang or a phut and you can see the magic smoke escape. Then the thing doesn't work any more. That's the kind of electricity I understand.
I bought a 12v car charger for my beloved phone. I tried it on the boat, and it didn't work. Oh dear, I thought, obviously duff, shouldn't have bought a cheap one. Although it was proper Nokia I bought it cheap on the internet, deriving some unjustified comfort from the fact that the supplier was based in Eastbourne, i.e. local, despite the fact that I kept no record of their address or contact details. So when I was in Maplins last week, I bought another one; a universal one this time that will charge everyone's phones. This time we tested it in the car first and it was fine. Got to the boat, plugged it in, went phut and the magic smoke escaped - saw it happen that time. That had me worried of course, that there was something wrong with our 12v system (although the vacuum cleaner worked fine, at least as fine as you'd expect a 12v one to work, and so did a little LED light we have).
So I asked Moominpapa. He said, maybe the polarity is the wrong way round. That immediately made sense, because for all these things we've been using an adapter, from our three-pin sockets to a car-type one, and of course the three pin plug was wired the wrong way round. MP demonstrated this by poking it with Jim's multimeter, and I then put it right. Next time I visit Maplins I will buy a third 12v charger and hope for the best.
Jim, who has missed his vocation as a careers adviser, is determined that Moominpapa should retire from crunching the human genome and become a boat electrician.