I have two notebooks that id like to carry permanently on a thunderbird
26,
mainly for navigation use, so they'll be on most of the time and id like
them to last at least a couple of years if possible....

Get a couple of the exact same model, used even. That way you can do the
install once and then copy the drives. Even keeping a backup of the drive
image to ease reloading it later should it fail. For navigation it's not
like you need much in the way of computing power. Something with a 1gHz
processor would be about right for the task. Trying to make it pull
double-duty as a DVD player is probably a bad idea. Considering how cheaply
portable DVD players are these days that's usually a better solution.

I had the idea of
separating the screen and the motherboard/HDD, encasing most of it in a
waterproof container, nolting the screen to a bulkhead and using an
external
keyboard.... a bit over the top?

Bad idea. The wiring that connects from the motherboard to the displays is
not designed to be extended. Sure, it "might" be possible but then you'd
have to deal with the *Extremely* fragile connectors on each end. Don't
bother. If anything, just use the VGA output from the PC and feed it into a
regular monitor.

Is anyone shockproofing their laptop, or just sitting them on a table?

The drives in most laptops are more that capable of dealing with typical
use. It might be worth considering use of the sort of hold-down straps and
clips you'd use on other gear. It really depends on if you expect to use it
in rough conditions. If that's the case then you might be better off use a
fixed-mount monitor. That way the laptop's screen isn't going to be
'flapping around' when going gets rough. It might then be handy to use a
waterproof mouse and keyboard. The laptop then becomes somewhat akin to a
plain old desktop box, but one that can be easily disconnected from the
cables and used above decks in good weather or onshore.

-Bill Kearney