An improved system for converting kinetic energy contained in naturally occurring horizontal flows occurring in fluids accumulated above ground into useful mechanical energy is specified in which at least one circulating element guided in a closed cycle, arranged substantially horizontal above the ground is provided, to which at least one buoyancy body having a crosssection of impact for the flow is attached by means of at least one guy rope or by means of a backstay chain, wherein in a section of the closed cycle, in which a propulsive power aligned in a direction of circulation acts on the buoyancy body via the flow impacting on the cross-section of impact, the distance between the circulating element and the buoyancy body is set greater than in a section of the closed cycle, in which via the flow impacting on the cross-section of impact no propulsive power aligned in the direction of circulation acts on the buoyancy body, further optimizing is carried out in that the buoyancy body is set during the cycle in its alignment to the flow such that with a flow acting transversely to the temporary direction of circulation of the circulating element it also experiences a force component acting in the direction of circulation and transfers to the circulating element via the at least one guy rope.