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The paper deals with the change brought about in hydroelectric practise by the introduction of the high-speed low-head propeller type of turbine. For a given head, up to about 70 ft., it is possible to run the units at a much higher speed, thus reducing materially the cost of the generator. The maximum efficiency of the unit is maintained and the part load efficiency greatly improved by the use of adjustable blade turbines, so that there is also a material gain where variable load or flow is to be handled. The fundamentals underlying the design of an adjustable blade propeller type turbine are given and an illustrative study made of a three-unit station, to show the gain in output obtainable from this type of unit.