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Apparatus (10) for extracting energy from flowing water typically a tidal stream flow, the apparatus including a float assembly (41) having pair of spaced buoyant bodies (42) between which an elongated flow passage or tunnel (47) is located with an impeller assembly (61) in the flow passage (47) whi

Apparatus (10) for extracting energy from flowing water typically a tidal stream flow, the apparatus including a float assembly (41) having pair of spaced buoyant bodies (42) between which an elongated flow passage or tunnel (47) is located with an impeller assembly (61) in the flow passage (47) which can drive an electrical generator (76) on the float assembly (41). The bodies (42) include or define ballast tanks which can be filled or partially filled with water to submerge the flow passage or tunnel (47).

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1. An apparatus for extracting energy from flowing water, said apparatus comprising: a float assembly having a pair of spaced bodies, said bodies being at least partially buoyant;an elongated flow passage disposed between said bodies and extending longitudinally between said bodies, said flow passag

1. An apparatus for extracting energy from flowing water, said apparatus comprising: a float assembly having a pair of spaced bodies, said bodies being at least partially buoyant;an elongated flow passage disposed between said bodies and extending longitudinally between said bodies, said flow passage having a partially circular cross-section;a plurality of flow guiding vanes disposed within said flow passage and extending longitudinally for substantially the length of the flow passage and extending substantially parallel to a longitudinal axis of the flow passage;an impeller assembly disposed in said flow passage, said impeller assembly including: a central axle or axles;at least one pair of spaced-apart impellers mounted on said axle or axles for rotation therewith; anda support that supports said axle or axles centrally in said flow passage to extend said axle or axles longitudinally in said flow passage;said apparatus being at least partially submerged in a body of water and at least partially submerging said flow passage, said impeller assembly rotating upon flow of water through said passage; anda device disposed on said float assembly for extracting energy from said rotating impeller assembly. 2. The apparatus as claimed in claim 1, wherein said bodies are hollow ballast tanks or chambers adapted to be selectively filled or emptied of water; andsaid bodies have a variable buoyancy to enable the apparatus to be submerged or partly submerged in water. 3. The apparatus as claimed in claim 2, wherein said bodies of the float assembly include a pair of spaced floats, pontoons or hulls that define said hollow ballast tanks or chambers; andsaid floats, pontoons or hulls having inner opposing walls, said inner opposing walls of said spaced floats, pontoons or hulls defining at least part of the flow passage. 4. The apparatus as claimed in claim 3, wherein each said inner wall has a partially circular concave surface that defines at least part of the inner surface of the flow passage. 5. The apparatus as claimed in claim 3, wherein said floats, pontoons or hulls extend downwardly to be level with or extend below the lower side of the flow passage. 6. The apparatus as claimed in claim 1, wherein said flow passage is defined by an elongated flow passage-defining member supported between the floats, pontoons or hulls; andsaid flow passage defining member is a tubular or partially tubular member, said flow passage defining member being open on its lower side or fully enclosed around its periphery. 7. The apparatus as claimed in claim 1, wherein said float assembly has leading and trailing ends and wherein inner surfaces of the float assembly at said leading and trailing ends converge toward each other to define a throat which tapers in cross section toward the flow passage to direct water flow toward the flow passage. 8. The apparatus as claimed in claim 1, wherein said float assembly includes at least at its operative leading end, an end member that projects forwardly of the float assembly above the flow passage, said end member tapering forwardly to define a bow. 9. The apparatus as claimed in claim 8, wherein each end member is provided between opposing bodies and extends outwardly and longitudinally thereof. 10. The apparatus as claimed in claim 1, wherein a guard or shield is provided at the leading or trailing ends of the float assembly, or at the leading and trailing ends of the float assembly, to prevent debris or animals from passing into the flow passage. 11. The apparatus as claimed in claim 1, wherein said impeller assembly has a maximum diameter of up to 85% of the cross-section of the flow passage. 12. The apparatus as claimed in claim 1, wherein each said impeller includes a rotor having an inner hub, an outer ring and blades extending between the hub and ring. 13. The apparatus as claimed in claim 12, wherein each said impeller is mounted to said axle or axles via respective clutch assemblies, whereby rotation of one impeller is independent of the rotation of other impellers. 14. The apparatus as claimed in claim 12, wherein each said blade has a longitudinal axis and wherein each said blade is rotatable about said longitudinal axis, whereby the pitch of the blades is adjustable. 15. The apparatus as claimed in claim 1, wherein said impellers of said pair are located at opposite ends of the flow passage and include at least one intermediate therebetween, each intermediate impeller being of a smaller diameter than the opposite end impellers and wherein said flow passage is reduced in cross-section adjacent the intermediate impeller. 16. The apparatus as claimed in claim 1, wherein said axle or axles of the impeller assembly is coupled to an output shaft, said output shaft being adapted to drive the device disposed on said float assembly, the device disposed on said float assembly for extracting energy including an electrical generator, said generator being located on an upper side of the float assembly. 17. The apparatus as claimed in claim 1, in combination with a pile assembly, said pile assembly comprising an upstanding pile, a track extending along said pile, a carriage movable along said track and an assembly that couples said carriage to said apparatus. 18. An apparatus for extracting energy from flowing water, said apparatus comprising a float assembly having a pair of spaced buoyant bodies, an elongated flow passage disposed between said bodies and extending longitudinally between said bodies, said buoyant bodies extending below said flow passage whereby said flow passage is supported above a surface upon which said buoyant bodies rest, said flow passage having a circular cross-section, a plurality of flow guiding vanes disposed within said flow passage and extending longitudinally for substantially the length of the flow passage and extending substantially parallel to the longitudinal axis of the flow passage, an impeller assembly disposed in said flow passage, said impeller assembly including a central rotatable axle and at least one impeller mounted on said axle for rotation therewith, a support that supports said axle centrally in said flow passage to extend said axle longitudinally in said flow passage, an adjusting device that adjusts the buoyancy of said buoyant bodies to enable said flow passage to be submerged in water, said impeller assembly rotating upon flow of water through said passage, and a device disposed on said float assembly for extracting energy from said rotatable impeller assembly. 19. An apparatus for extracting energy from flowing water, said apparatus comprising a float assembly having a pair of spaced floats or hulls, said floats or hulls being at least partially buoyant, an elongated flow passage disposed between said floats or hulls and extending longitudinally thereof, a plurality of flow guiding vanes disposed within said flow passage and extending longitudinally for substantially the length of the flow passage and extending substantially parallel to the longitudinal axis of the flow passage, an impeller assembly disposed in said flow passage, said flow passage having an inner surface that has a substantially circular cross-section, each said float or hull having an inner wall, said inner walls of said floats or hulls opposing each other, each wall having a concave surface of a partially circular cross section that defines at least part of the inner surface of the flow passage, an adjusting device that adjusts the buoyancy of said floats or hulls to enable said flow passage to be submerged in a body of water, said impeller assembly rotating upon flow of water through said passage, and a device disposed on said float assembly for extracting energy from said rotatable impeller assembly.