Why a foil boat can float on a gas
A boat floats when it pushes aside enough fluid to make an upward buoyant force equal to its weight. That fluid does not have to be water: gases exert pressure, too, and pressure increases with depth. The slightly greater pressure beneath an object than above it creates a net upward push.
Sulfur hexafluoride (SF₆) is much denser than ordinary air. In a demonstration vessel, it can settle into a layer under the surrounding air, even though the gas itself is invisible. A hollow foil boat placed on that layer pushes some SF₆ downward and out of the space it occupies. The displaced gas’s weight provides the buoyant force.
The boat’s shape does the work
A flat sheet of foil would not displace much gas before sinking. Folded into a shallow, hollow hull, the foil encloses a larger volume. That lets the boat push aside more of the dense gas while adding little weight of its own. If the displaced gas weighs at least as much as the boat, the boat can be supported.
The effect is the same basic principle as floating in water, but the gas is far less dense than water, so the boat must displace a comparatively large volume. The striking part is that the supporting “surface” is not a hard boundary at all: it is a layer of invisible gas, held in place beneath lighter air by its greater density.
