Lahey and his chief designer, an Englishman named John Ramsay, were certain they could do it, especially since the sub was so simple: It needed only two seats. That allowed Ramsay to make the design compact, just 15 feet long, 6.2 feet wide, and 12 feet high, about the size of two stacked minivans. Keeping a sub small lowers costs and, more important, weight. You have to make up for every pound you add with buoyancy, and whatever provides that lift (air, gas, foam) takes up space. That doesn’t matter for giant military craft that carry wee nuclear power plants and rarely go deeper than 500 meters. But it matters a lot for submersibles, a class of vessel toted on surface ships and lifted into and out of the water by deck cranes.