You might also consider purchasing a power brick for your laptop that runs
directly from 12 volts. I have two of these from Lind Electronics, one
for
a Dell and one for a Panasonic. Each one is specifically designed for the
particular laptop. Handy to have as I can recharge the laptop from the
car's cigarette lighter socket or run the laptop in the car to use
Microsoft
Streets and Trips.

The only downside to using a 12v powersupply is having to bring along
another one for on-shore use with AC. But we've now got a pair of 12v
adapters, one for hers and one for my laptop. Saves from having to fire up
the generator. Also means we leave 'em on the boat so don't have to worry
that the adapter didn't get brought along. For the on-shore use I got a
110v to 12v lighter socket adapter. Works well enough in a pinch.

-Bill Kearney