Blade for a turbine operating in water

1. A blade for use in water, the blade comprising an outer shell of fibre reinforced plastic defining a cavity, a substantial portion of the cavity being filled with a resin which adheres to the inner wall of the shell, wherein the cavity is divided into a number of sections, each section occupying less than the whole length of the span of the blade, arranged one next to the other in a direction of arrangement, the direction of arrangement being along the length of the span from the blade root toward the blade tip, with the density in one section being different from the density in an adjacent section such that the density of the blade varies along the span of the blade,

wherein a section of the cavity comprising the blade root comprises one or more orifices for the entry and exit of water.

2. A blade according to claim 1 , wherein the resin has a density of between 500 kg/m 3 to 2000 kg/m 3 .

3. A blade according to claim 2 , wherein the resin has a density of 600-1200 kg/m 3 .

4. A blade according to claim 1 , wherein the resin is selected from a range of thermoset resins such as polyester, vinylester, epoxy, phenolic, polyurethane and polypropylene.

5. A blade according to claim 1 , wherein the resin is a filled resin.

6. A blade according to claim 5 , wherein a filler of the filled resin is selected from a mineral such as Calcium Carbonate, Talc, Barium Sulphate, silica or solid glass spheres; a fibrous filler such as chopped or milled glass, chopped thermoplastic fibres or carbon fibre; or a hollow filler such as hollow glass microspheres, hollow alumino-silicate microspheres, hollow phenolic microspheres, macrospheres made from glass epoxy, carbon epoxy or thermoplastic resin.

7. A blade according to claim 5 , wherein a filler of the filled resin is present with a range of 0-60% by volume of the filled resin.

8. A blade according to claim 1 , wherein the resin contains 0-10% by weight of thixotrope.

9. A blade according to claim 1 , wherein a shear web running along the length of the blade is embedded in the resin.

10. A method of forming a blade for use in water, the method comprising:

forming two shell halves of fibre reinforced plastic, each half defining a cavity;

inserting bulkheads into the two halves of the shell to create a number of different sections, each section occupying less than the whole length of the span of the blade, arranged one next to the other in a direction of arrangement, the direction of arrangement being along the length of the span from the blade root toward the blade tip such that the density of the blade varies along the span of the blade, wherein a section of the cavity comprising the blade root comprises one or more orifices for the entry and exit of water;

filling a substantial portion of each cavity with a resin which adheres to a wall of the respective shell such that the sections have resins of different density;

applying adhesive to exposed surfaces of the resin of each cavity; and

adhering the two shell halves together.

11. A method according to claim 10 , wherein the resin of each cavity is provided in the respective cavity to a level below an edge of the respective shell to define a gap for the adhesive.

12. A method according to claim 10 , further comprising a step of inserting a shear web along the length of the span of one shell half before it is filled with resin;

inserting a dummy shear web coated with a release coating in the opposite shell half; and

removing the dummy shear web after the resin is cured.

13. A method for using the blade of claim 1 , the method comprising:

providing the blade according to claim 1 as a component of an underwater turbine; and

using the blade in water.

14. An apparatus comprising:

an underwater blade having an outer shell of fibre reinforced plastic defining a cavity, a substantial portion of the cavity being filled with a resin which adheres to the inner wall of the shell, wherein the cavity is divided into a number of sections, each section occupying less than the whole length of the span of the blade, arranged one next to the other in a direction of arrangement, the direction of arrangement being along the length of the span from the blade root toward the blade tip, with the density in one section being different from the density in an adjacent section such that the density of the blade varies along the span of the blade,

wherein a section of the underwater blade comprising the blade root comprises at least one orifice for the entry and exit of water.