The rotors are not as effective compared to other low-cost windmills. For example, "The data from Bodek and Simonds' experimental S-rotor in the West Indies shows that the useful energy from a 12 mph wind... means that one can pump 75 Imperial gallons/hour up to 30' above the water level (341 liters/hour up to 9.14 m). In an 8 mph wind... only 25 Imperial gallons/hour (104 liters/hour) can be pumped to the same height."
This is not quite as good as the 5.4 m Cretan sail windmill, which is reported to pump as much as 15 times this volume of water in an 8 mph wind. Low-cost sail or bamboo mat windmills in Thailand also appear to be considerably more productive.
Original content from A.T. Wiki