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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), th

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.

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What is claimed is: 1. A prime mover for harnessing energy from flow of a fluid, the prime mover comprising a shaft having a rotational axis, arranged to be rotatably mounted to a substructure, the shaft comprising at least one arm extending radially from the shaft, the at least one arm comprising

What is claimed is: 1. A prime mover for harnessing energy from flow of a fluid, the prime mover comprising a shaft having a rotational axis, arranged to be rotatably mounted to a substructure, the shaft comprising at least one arm extending radially from the shaft, the at least one arm comprising at least one blade, wherein the at least one blade is oriented such that fluid flow acting on the at least one blade effects rotation of the shaft, wherein the at least one blade is movably mounted on an arm and wherein each blade is movable from a first position, wherein the each blade is substantially perpendicular to the flow of fluid and has a first drag, to a second position, wherein the each blade is substantially parallel with the flow of fluid and has a second drag, wherein the first drag is higher than the second drag, and wherein the at least one blade is provided with a trim tab, attached to or integral with the blade and partly or completely spanning blade tip or the trailing edge of the at least one blade wherein each arm and/or blade comprises means to prevent each blade moving between the first and second position until the shaft has rotated a prescribed distance. 2. A prime mover for harnessing energy from flow of a fluid, the prime mover comprising a shaft having a rotational axis, arranged to be rotatably mounted to a substructure, the shaft comprising at least one arm extending radially from the shaft, the at least one arm comprising at least one blade, wherein the at least one blade is oriented such that fluid flow acting on the at least one blade effects rotation of the shaft, wherein the at least one blade is movably mounted on an arm and wherein each blade 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, and wherein the at least one blade is provided with a trim tab, attached to or integral with the blade and partly or completely spanning blade tip or the trailing edge of the at least one blade, wherein each arm and/or blade comprises means to prevent each blade moving between the first and second positions until the shaft has rotated a prescribed distance. 3. A prime mover as claimed in claim 2, wherein the at least one blade is airfoil-shaped. 4. A prime mover as claimed in claim 2, wherein the flow of fluid effects movement of the at least one blade from the first to the second position as the shaft is rotated. 5. A prime mover as claimed in claim 2, wherein the surface area of the at least one blade impinged on by the fluid is greater in the first position than in the second position. 6. A prime mover as claimed in claim 2, wherein the flow of fluid effects movement of the at least one blade from the second position back to the first position as the shaft completes a fill rotation. 7. A prime mover as claimed in claim 2, wherein the prime mover comprises at least two arms, and the at least one blade is in the second position when at least one other blade is in the first position. 8. A prime mover as claimed in claim 2, wherein each blade and/or arm comprises means to limit movement of each blade on an arm to a prescribed amount. 9. A prime mover as claimed in claim 2, wherein each blade is rotatably mounted to an arm such that the blade may be rotated from the first to the second position. 10. A prime mover as claimed in claim 2, wherein each arm and/or blade comprises means to limit movement of each blade on an arm to a prescribed angular displacement of the blade with reference to the corresponding arm. 11. A prime mover as claimed in claim 2, wherein each arm comprises more than one blade, each blade being movably mounted on the arm. 12. A prime mover as claimed in claim 2, wherein the prime mover is arranged to be rotatably mounted to any suitable sub-structure, such as an energy generating apparatus, wherein said suitable sub-structure comprises an energy generating apparatus. 13. A method of generating energy from flow of a fluid, the method comprising mounting the prime mover as claimed in claim 2, to a suitable energy-generating device, and locating the resultant apparatus in the flow of fluid. 14. A method as claimed in claim 13, wherein the prime mover is oriented in the flow of fluid such that the rotational axis of the shaft is oriented substantially vertically with respect to ground level and the arms extend radially substantially horizontally with respect to ground level. 15. A method as claimed in claim 13, wherein the prime mover mounted to the energy generating apparatus is located in the flow of any fluid, air or water. 16. A prime mover as claimed in claim 2, wherein said at least one blade is substantially parallel with said flow of fluid in said second position. 17. An energy-generating device comprising a prime mover for harnessing energy from flow of a fluid, the prime mover comprising a shaft having a rotational axis, arranged to be rotatably mounted to a substructure, the shaft comprising at least one arm extending radially from the shaft, the at least one arm comprising at least one blade, wherein the at least one blade is oriented such that fluid flow acting on the at least one blade effects rotation of the shaft, wherein the at least one blade is movably mounted on an arm and wherein each blade 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, and wherein the at least one blade is provided with a trim tab, attached to or integral with the blade and partly or completely spanning blade tip or the trailing edge of the at least one blade connected to an energy generating apparatus, wherein each arm and/or blade comprises means to prevent each blade moving between the first and second positions until the shaft has rotated a prescribed distance.