An apparatus and process for capturing mechanical work from the kinetic energy of a moving vessel driven by buoyancy and gravitational forces. The apparatus comprises a moving vessel, gas compression means, force transfer means, a conduit, and a force recovery means. In use, the submerged moving vessel receives compressed gas, generating a buoyancy force in the moving vessel. As the moving vessel rises in water and approaches the top of its upstroke, the moving vessel releases the gas, causing the moving vessel to sink. A first component of the buoyancy force is transmitted from the moving vessel, through the force transfer means, to the gas compression means, where it compresses gas. The compressed gas is channeled through the conduit into the charging valve while the moving vessel sinks. The charging valve releases the compressed gas from the conduit into the moving vessel, and the moving vessel's vertical oscillation cycle repeats.