US Patent 7927163 Recreational flotation device

A flotation recreational device consists of a buoyant vessel having a semispherical exterior surface and a sufficient internal area adapted to accommodate an operator. The vessel is configured to have a geometrical center which is a metacenter of a spherical buoyant body serving as a base of the partially-spherical exterior surface. The center of sphere is spaced from and located above the center of gravity of the operator, so the radius of sphere is greater than the height of said center of gravity. The stability of a vessel-operator system is determined by a stability of a distance between the metacenter and the center of gravity of the operator. The vessel can be in the form of a circular shell, an elliptical shell, or a barrel.