It's well known  that Savonius rotor creates high torque and is self-starting even at low wind speeds, but is relatively low in efficiency rating.
(2005), on the performance analysis of Savonius rotor with twisted blades, Renewable Energy, 31, pp.
In order to improve sharp bending streamlines occurring in overlap area of Savonius rotor with two blades, the novel rotor is composed of three blades in order to adjust angle of accepting airflow of the oriented blade.
Abid, "Experimental investigation of helical Savonius rotor with a twist of 180[degrees]," Renewable Energy, vol.
Four bladed Savonius rotor
of 1[m.sup.2] projected areas with blade size of 0.92 x 0.46 m fixed on 0.6 m tripod stand was developed in workshop of Mechanical Engineering Department of Quaid-e-Awam University Nawabshah, Sindh, Pakistan.
Because their starting torque is low, they also require an external power source or additional Savonius rotor
to start turning.
A simple vertical axis Savonius rotor with two straight blades, semi circular in cross-section is shown in figure 1.
 Benesh, a.h., 1988., "wind turbine system using a vertical axis Savonius rotor, "U.S.
Both of these companies produce small helical or fluted bladed machines, where the drums of the Savonius rotor have evolved into spiral-formed vanes.
Several vertical axis designs are currently available, among them a number of build it yourself kits for conventional Savonius rotor wind turbines.
The early current meters used a spinning turbinelike device called a Savonius rotor
(developed in Finland in the 1930s for power generation) to sense the current speed.
A combined aerorotor was designed, consisting of a Savonius rotor
and a Darrieus turbine which ensure its operation at low wind speeds of 4-5 m/s.