If ... it doesn't weigh too much, you can power ships, in particular, container ships. If the fuel is buoyant you can make a double hulled vehicle and store the

If ... it doesn't weigh too much, you can power ships, in particular, container ships. If the fuel is buoyant you can make a double hulled vehicle and store the fuel in it. (compressed air works very well here). Why would the fuel need to be buoyant? Ships use steel hulls, steel is denser than seawater, and, despite that, ships float. Some ships are even powered by lead-acid batteries (http://en.wikipedia.org/wiki/Submarine#The_snorkel http://en.wikipedia.org/wiki/Submarine#The_snorkel) -- though these batteries are recharged by onboard diesel generators -- and yet they float. Aircraft carriers and icebreakers are even powered by uranium -- 18 times the density of water. Pressurizing the space between the two hulls of a double-hulled boat would require thicker steel than normal. (For a single curve, the tensile PSI needed is the air-pressure in PSI multiplied by the radius in inches -- for a sphere, divide by two.) Since steel is denser than seawater, this would tend to make the vessel sink (so the fuel would hardly buoyant unless it were further being contained in-between the hulls in carbon-fiber cells -- but that would obviate the need for two hulls). It would also impact the shape of the boat, since containing pressure is easier with a smaller radius-of-curvature (e.g. the usual flat-sides of containter ships would be counter-efficient for this task of retaining air-pressure). An efficient design of such a container-ship might have two or three long, parallel tubes as hulls, with a container-platform mounted above on blade-pylons (to slice through the water). Optimally, the tubes would remain entirely submerged during cruising, so as not to interact with surface waves (which interaction normally causes efficiency losses).