The present invention provides a prime mover (2) for harnessing energy from a flow of fluid, the prime mover (2) comprising a shaft (4) having a rotational axis, arranged to be rotatably mounted to a substructure, the shaft (4) comprising at leas one arm (6) extending radially from the shaft (4), the or each arm (6) comprising at least one blade (8) wherein the or each blade (8) is oriented such that flow action on the blade (8) effects rotation of the shaft (4), characterized in that the or each blade (8) is movably mounted on an arm (6) and wherein each blade (8) is movable from a first position, having a first drag, to a second position, having a second drag, wherein the first drag is higher than the second drag. The prime mover (2) of the invention provides substantially reduced drag in a flow of fluid, and an increased torque output, compared to prior art prime movers.