Thanks for your replies.
The battery is at one end of the boat and the socket I'm thinking about is at the other.
I'm trying to arrange a 12v supply for a DAB radio. I'm thinking about using a 12v to <whatever> volt supply from Maplins but these are designed for auto use and so come with a car cigarette lighter plug. I'm planning to chop this off and replace it with a 3(?) amp round pin plug to suit the 12v socket already there.
Thinking about it the obvious answer to my question is to set up the multimeter and test it on a, say, PP3 battery to check polarity.
The reason i asked the question was as there is a 50/50 chance with the polarity you can either get it 100% right or 100% wrong!
Changing the subject, any idea how much current a DAB radio might draw?
Thanks again for the help.