The invention relates to a device for converting kinetic energy of a flowing water into kinetic energy of a rotatable rotor shaft, wherein the device is adapted to be effective in a first flow direction and a second flow direction substantially opposite to the first, wherein the device comprises two rotor blades each connected to a blade shaft coupled to the rotor shaft for transfer of a torque relative to the rotor shaft, wherein the blades are each rigidly connected to their blade shaft, the blade shafts are mounted for rotation about their own axis relative to the rotor shaft, the blade shafts are coupled to each other for rotation in the same rotation direction and the rotor blades each have an asymmetrical cross-section. There is hereby always flow against the asymmetrical rotor blades from the same direction relative to the blade during the energy conversion process, so that the profile of the rotor blades can be optimized for this flow direction.