TidalStream’s website (TidalStream, 2008) explains that its concept has two 20 m rotors, is rated at 1-2 MW depending on current speed, and operates in 30-50 m water depths (Figure 4.25 (right)). Each rotor runs in clean water upstream of its support arm with a gravity base and the swinging arm ball joint. The swinging arm is hinged at its upper end to the main spar buoy so that it can be stowed for installation or removal. During operation it is held in place by a cranked strut. No performance data were available at the time of publication.