Anyway, Garmins seem to me to have the most extensive support in the
freeware community, for some reason, but I don't really know what you
want to do to your chartplotter with your pc.

I'm not entirely sure either. Up to now, I have only used my Raymarine
chartplotter on board (with a Psion PDA which can backup and transfer
waypoints), and this only for nav around the Mediterranean.

This system will be too expensive to go further afield, as the Raymarine
charts are hugely expensive. I am therefore looking for a more versatile
system which can use swapped and downloaded electronic charts from lots of
different sources.

Ozie-explorer on a laptop may be a good possibility. However, it works on
Windoze which is notoriously unreliable, slow, energy-greedy and crashprone,
which is a downer. I'd ideally like a separate chartplotter which consumes
less electricity and is more reliable, but which connects to a PC allowing
me to squirt charts over to it. Also, I could then put the chartplotter by
the wheel and keep the laptop indoors on the nav-table. That's the way I'm
thinking at the moment, though input welcome.

I'd agree from what I have researched so far that Garmins seem to be the way
to go for the moment.