Vacuum Ship

You're right, people have often pondered a vacuum ship, though it offers relatively little benefit. Helium (atomic weight 4, double molecule) and especially hydrogen (atomic weight 1, double molecule) are so much less dense than air (mostly nitrogen, atomic weight 14, double molecule - or even heavier oxygen) that you don't get a colossal boost in lift - roughly 7 or 40 per cent, compared to H or He. Plus you laugh at pressure height, of course. So it's useful, but not such as to justify the awful difficulties of making a huge, ultralightweight vacuum bottle or internally braced vac-sac. A convex rigid skin would seem likelier, though, as it would provide some bracing of itself. But consider what big pressure bottles look like with our materials tech: ships and submarines and space stations, much too heavy to float on their own buoyancy in 1-bar gas.

I think there was an Iain M Banks book where the airships were full of nanothin carbon vacuum spheres, super light but super empty. As in the book, though, by the time you can actually build a working vacuum airship you're probably so advanced that you would only do so for for a laugh. For everyday travel you'd just hop into your artificial-intelligence antigrav starship or whatever.

Right. Must do some work now.