This is a parallel-flow impulse motor, the power being due almost entirely to the velocity of the water. The guide blades, in the vertical form of motor, may be closed by special vertical shutters worked by special gear, and the passages through the wheel are widened towards the outlet of the water. The efficiency of the Girard turbine may be highest on low powers. A suction tube cannot be used, as the wheel must be close to the level of the tail race.