"Nigel" wrote in message
...
I have a 24v electrical set up on my boat, but need a 12v supply for my
VHF
(amongst other things). Is there any reason why I can't just run a 12v
supply from just one of a pair of batteries, or do I need to use a 24v/12v
converter.

You need a converter.

I release this will draw charge from just one battery , but won't
the other just top it up and there by drain them both equally

No, because the other one is connected in series, not in parallel. So one
battery gets drained faster that the other and that is the success-formula
for wrecking that battery.

Meindert