Abstract: A sail-type wind-driven prime mover is provided in which the area of the sails facing into the wind is adjustable. The prime mover comprises a drive shaft from which depend radially extending front and rear masts which support flexible sails. The area of the sails facing into the wind can be varied by moving the rear masts along and about the drive shaft while keeping the front masts fixed relative to the drive shaft. In a preferred embodiment in which the prime mover drives an electric generator, means are provided to automatically adjust the sails as a function of the output voltage of the generator.