Rotor sail

Abstract

To provide a method for manufacturing a rotor body that forms a part of a rotor sail.SOLUTION: There is provided a method of manufacturing a rotor body that forms a part of a rotor sail, comprising the steps of: winding first fibers around a mandrel to form a tubular first skin forming a rotor tube having a tube axis; forming a plurality of strips from second fibers; and attaching the strips to a surface of the first skin such that at least a part of the second fibers extend axially along the rotor body.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a rotor sail for wind-assisted propulsion of an object, particularly but not exclusively to a rotor sail for wind-assisted propulsion of a ship, and also to a method of forming such a rotor sail and a ship incorporating the rotor sail. [Background technology]

[0002] This type of rotor sail is also known as a fretted narrow rotor. Known such rotor sails are typically made as a cylindrical sleeve that forms the rotor. This sleeve is adapted to rotate on a static tower. An upper bearing and a lower bearing locate the rotor on the tower. Typically, the rotor is subjected to a wind load, which is seen as a reduction in air pressure on one side of the rotor, known as the suction side. The air pressure distribution has two main effects on the structure: Bending moment on the rotor as a whole (i.e. acting as a beam of circular hollow section subjected to a distributed load). This gives rise to stresses in the plane of the rotor skin, mainly tension and compression forces in the rotor axial direction, with some in-plane shear forces caused by incidental shear forces. Localized bending moments on the rotor skin, due to uneven distribution of air pressure around the section, which tend to distort the circular cross section. This gives rise to tensile and compressive stresses, mainly circumferentially around the rotor.

[0003] During the life of a rotor sail, the large stresses on the rotor fluctuate or even reverse with each revolution. The number of revolutions during the rotor's life is very large, on the order of billions of revolutions. This means that known sail rotors are made from materials that are resistant to fatigue failure. Laminated composites of continuous glass or carbon fibres in a polymer resin are suitable for this application.

[0004] To provide strength to the rotor, the fibers of the composite material used to make the cylindrical sleeve must be aligned with the principal stresses on the sleeve during use of the rotor, as the composite fibers provide the greatest strength to the entire rotor.

[0005] In known rotor sails, about 50% of the total strength is required to be longitudinal, in other words aligned with the rotor axis, and about 30% of the total strength is required to come from fibres that are approximately circumferentially oriented, i.e. perpendicular to the rotor axis.

[0006] The remaining material provides resistance to in-plane shear stresses, but the in-plane shear stresses of the rotor sail are relatively small due to the inherent shear and torsional resistance of the large diameter tubes of the rotor sail type forming portion.

[0007] A further requirement is that the circumferentially oriented fibers should be as far away as possible from the mid-plane or neutral axis of the laminate material forming the cylindrical sleeve. Such an arrangement provides optimal bending strength in the circumferential direction. This helps to resist localized bending moments that tend to distort the circular cross-section.

[0008] Circumferential bending stiffness is also beneficial for the cylinder to resist buckling.

[0009] A cylindrical sleeve is more susceptible to buckling in the circumferential direction than in the axial direction because a cylinder is necessarily more resistant to axial buckling due to the curvature of the cylinder's surface.

[0010] In known rotor sails, to achieve the proper spacing of the circumferential fibers from the neutral axis of the cylinder-forming laminate, it is known to add a foam core to the laminate material forming the cylindrical sleeve, such a foam core being added in the center of the laminate to form a sandwich-type construction.

[0011] Both the in-plane axial stresses and the circumferential bending stresses on the laminations forming known rotors vary with each revolution of the rotor. Large rotor sails typically rotate at up to 250 rpm. This means that over the life of such a rotor sail, which is typically 20 to 25 years, the laminations forming the rotor may be subjected to on the order of 1 to 2 billion fatigue cycles.

[0012] For a fiberglass / epoxy laminate, the fatigue strength at 1 billion cycles is only about 15% of the static stress. For a carbon fiber / epoxy laminate, the fatigue strength may be about 25% of the static strength.

[0013] The requirements for rotor sail laminations are therefore significantly more onerous than those for other large composite structures such as ship hulls, wind turbine blades, aircraft wings, pressure vessels, pipes or water tanks.

[0014] The requirements for laminates when used to make rotor sails are significantly different, and more challenging, than those for other large composite structures, and therefore known methods of making suitable composite structures may not be suitable for rotor sails.

[0015] One known method of making composite materials is known as pultrusion. This method is suitable for making straight tubes of circular or any other hollow or solid cross section in a single operation. Pultrusion is an automated and low-cost process because the raw materials are in their simplest form (liquid polymer resin and glass or carbon fiber tows) used directly from a bobbin on which the tows are wound.

[0016] Tension is used to pull the profile through the die, which means the fibres are straight, maximising the compressive strength of the hardened material forming a straight tube.

[0017] For many applications, compressive strength is often the primary consideration when designing structural parts from composite materials, and therefore pultrusion is often the appropriate method to use. However, pultrusion is most practical when the cross section is small or the product is long. Pultrusion is more expensive for larger diameter tubes due to the need to use larger dies and exert larger tensile forces.

[0018] Furthermore, pultrusion is not suitable for forming composites that must withstand circumferential stresses unless off-axis fiber incorporation is used, which adds cost to the process.

[0019] Another known method involves wrapping pre-impregnated tape or fibers ("prepreg") around a mandrel. This process can be automated to minimize labor costs, but is still expensive due to the high cost of the prepreg material and the high temperatures required to cure the material.

[0020] Another known method for forming composite materials is resin infusion, also known as Vacuum Assisted Resin Transfer Moulding (VARTM), which is a method that has been used by the wind turbine blade industry to reduce material costs and is quite economical.

[0021] However, the process of layering material into a mold and applying vacuum consumables is labor intensive and difficult to automate, and the vacuum consumables typically cannot be reused or recycled, representing a wasted cost.

[0022] In addition, resin infusion is not suitable for forming finished tubular structures. For this reason, tubular structures formed using resin infusion are typically formed from two or more parts. These parts then need to be bonded together after the material has hardened. The bonding operation adds a process step, thus increasing the time and cost required.

[0023] Another known method for forming hollow tubes from composite materials is filament winding. This method is economical, especially since it can be almost completely automated, thus reducing labor costs. In addition, similar to the pultrusion method, the materials used are in their simplest form: the materials are potentially in the form of liquid polymer resins, and glass or carbon fiber tows are used directly from bobbins that house such fibers.

[0024] Filament winding is not suitable for producing composite structures in which the majority of the fibers are axially oriented along the tube formed from the winding process.

[0025] For these reasons, it is known to manufacture rotor sails using a resin infusion process. A typical known rotor sail made using resin infusion uses composite materials having a sandwich structure with a foam core in the middle of the composite structure. The foam core separates the outer layers to provide the required bending strength in the circumferential direction. Disadvantages of this method include: Resin infusion methods limit the scope for automation (because fabric must be manually laid into molds) or the reduction in material costs (because fibers must first be woven or sewn into fabric and then cut to size before molding). In order for the resin to be easily infused, it must have a very low viscosity, which limits the molecular weight of the resin and therefore its strength. Other processes can use longer polymer molecules, offering improved mechanical performance. Resin Infusion does not apply any tension to keep the fibers straight while the resin cures. This means that more material is needed than if the fibers were held straight, reducing the compressive strength of the material. Foam cores are relatively expensive, both in material and labor costs, and absorb a significant amount of resin, which increases the weight of the part and adds further cost.

[0026] For rotor sails, typical operating strains must be kept below about 0.15% in both the axial and circumferential directions to achieve adequate fatigue life.

[0027] Therefore, a need exists for an economical method of forming composite materials for forming rotor sails having the required axial and circumferential strength. Summary of the Invention

[0028] According to a first aspect of the invention, there is provided a method of manufacturing a rotor body forming part of a rotor sail, the method comprising the steps of winding a first fibre around a mandrel to form a rotor tube to form a tubular first skin having a tube axis, forming a plurality of strips from a second fibre, and attaching the strips to a surface of the first skin such that at least a portion of the second fibre extends axially along the rotor body.

[0029] The tubular first skin forms the outer cylindrical sleeve of the rotor sail. Because a filament winding method is used, the cylindrical sleeve can be manufactured in one piece around the circumference. This means that there is no need to use an additional step to join separate parts together to form the tubular shape.

[0030] Filament winding is, among other things, economical because it can be automated.

[0031] The method includes the additional step of forming a plurality of strips made from a second fiber, the fibers in the plurality of strips extending axially along the rotor body.

[0032] The strip is attached to a surface of the first skin.

[0033] This strip therefore provides the required axial strength for the rotor sail.

[0034] The inventors have realised that by forming a first skin using a filament winding process and then attaching a strip to a surface of the first skin, the strip is formed from fibres, at least a portion of which extends axially along the rotor body, providing both circumferential and axial strength to the rotor body.

[0035] Thus, embodiments of the present invention allow a cylindrical sleeve for a rotor body of a rotor sail to be produced at relatively low cost.

[0036] In an embodiment of the invention, the method includes the further step of impregnating the fibers with resin before wrapping the fibers around the mandrel.

[0037] In an embodiment of the invention, the first fibers are wrapped around the mandrel such that the orientation of the first fibers is between 45° and 90° to the tube axis. In another embodiment of the invention, the fibers are wrapped such that their orientation is between 50° and 80° to the tube axis.

[0038] Because the first skin is formed by wrapping a fiber or material around a mandrel, the first skin can be made to have any desired thickness.

[0039] In another embodiment of the invention, winding the first fiber around the mandrel includes winding a fabric formed from the first fiber around the mandrel.

[0040] In such an embodiment of the invention, rather than using a filament winding method where a tow of fiber is wrapped around a mandrel, a material formed from warp and weft yarns, with the warp and weft yarns appropriately oriented, may be wrapped around a mandrel.

[0041] In an embodiment of the invention, the first skin has a thickness of 2 mm to 6 mm and a diameter of 3 mm to 6 mm.

[0042] The strip may be formed by any desired method and embodiment of the present invention, wherein the strip is formed using a pultrusion process.

[0043] In such an embodiment of the invention, the rotor body may be made particularly efficiently because the filament winding and pultrusion processes may be automated.

[0044] As is well known in the art, in pultrusion, the material is pulled through a die as it is extruded through the die. This has the advantage of orienting the fibers axially along the strip. Strips formed from such processes are known as pultruded products.

[0045] Using the pultrusion process, the strip may be formed to have any desired dimensions, and in some embodiments of the invention, the strip has a thickness of from 1 mm to 10 mm.

[0046] Both the first skin and the pultrusion can be made to any desired length.

[0047] In an embodiment of the invention, the strip is attached to the outer surface of the first skin.

[0048] The strip may be attached by any convenient method, in an embodiment of the invention the strip is attached to the outer surface of the first skin by pressing the strip against the first fibres before the resin around the first fibres hardens.

[0049] In such an embodiment of the invention, uncured resin flows around the strip and, when cured, bonds the strip to the outer surface of the first skin.

[0050] In some embodiments of the invention, the strips may be attached to the outer surface of the first skin such that they are spaced apart from one another, while in other embodiments of the invention, the strips may be disposed on the outer surface of the first skin such that, when in place, adjacent strips axially abut one another. The first fibers may comprise glass fibers and the second fibers may include glass fibers or carbon fibers.

[0051] In an embodiment of the invention, the method includes the further step of forming a second skin by wrapping a third fiber around the strip.

[0052] In such an embodiment of the invention, the first and second skins together form a rotor tube.

[0053] In an embodiment of the invention, a filament winding process is used to wrap the third fiber around the strip in a manner similar to how the first fiber is wrapped around a mandrel to form the first skin.

[0054] In an embodiment of the invention, the third fibers are wrapped around the strip such that the orientation of the third fibers is between 45° and 90° to the tube axis. In other embodiments of the invention, at least some of the third fibers are wrapped such that their orientation is between 50° and 80° to the tube axis. In some embodiments of the invention, one or more outer layers of the third fibers are wrapped such that the fibers are oriented at about 90°, preferably between 88° and 90° to the tube axis, which provides a smoother surface and greater consolidation pressure on the fibers while the resin cures.

[0055] In an embodiment of the invention, the method comprises the further step of impregnating the third fibre with resin before wrapping it around the strip.

[0056] In an embodiment of the invention, the method includes the further step of holding the strip in place until a third fiber is wrapped around the strip.

[0057] In some embodiments of the invention, holding the strip in place comprises applying one or more straps around the strip, the one or more straps being unwound as the third fiber is wrapped around the strip.

[0058] In such an embodiment of the invention, the strip may be attached to an outer surface of a first skin and an inner surface of a second skin to form a composite material in which the strip is sandwiched between the first and second skins.

[0059] In some embodiments of the invention, the strip is attached to both the first and second skins by applying pressure to the first and / or second skins before the skins have hardened.

[0060] In an embodiment of the invention, the resin flows around the strip and, when hardened, attaches the strip to the first and second skins.

[0061] The third fiber may be formed from any convenient material, and in an embodiment of the invention, the third fiber comprises glass fiber.

[0062] In an embodiment of the invention, forming the strip includes forming a hollow strip, which allows the skins to be spaced further apart to increase the thickness of the strip without increasing weight.

[0063] In other embodiments of the invention, the method may include attaching a strip to an inner surface of the first skin.

[0064] In such an embodiment of the invention, the rotor body comprises only the first skin and the strip, and the rotor body does not comprise the second skin.

[0065] An advantage of such an embodiment of the present invention is that fewer steps may be required to create the rotor body.

[0066] In such an embodiment of the invention, the number of strips attached to the inner surface of the first skin may vary axially, in other words there may be more or fewer strips along a portion of the rotor body, allowing the rotor strength to be adapted to the varying bending moments along the length of the rotor body.

[0067] In an embodiment of the invention, the rotor body may have a length between 18m and 48m, but may have any desired length.

[0068] In such an embodiment, the rotor body may be formed from a plurality of rotor tubes, each of which may have a length of between 6m and 12m.

[0069] In those embodiments of the invention in which the rotor body comprises a plurality of rotor tubes, the rotor tubes may be joined together by any desired method to provide a rotor body having a desired length.

[0070] In some embodiments of the invention, the strip may be the same length as the rotor tube, but in other embodiments of the invention, such as those in which the strip is attached to the inner surface of a skin, there is no second skin and the strip may be longer than the rotor tube. In such embodiments of the invention, the strip spans the joint between adjacent rotor tubes and thus adds strength to the rotor body.

[0071] According to a second aspect of the invention, there is provided a rotor body forming part of a rotor sail, the rotor body comprising a tubular first skin forming a rotor tube and having a tube axis, and a plurality of strips extending axially along a surface of the skin, the first skin being integrally formed from a first fibrous material formed from first fibres and the strips being formed from a second fibrous material formed from second fibres, at least some of the second fibres extending axially along the rotor body.

[0072] In an embodiment of the invention, the first fibers are oriented at 45° to 90° to the tube axis, preferably 50° to 80° to the tube axis.

[0073] In an embodiment of the invention, the first skin has a thickness of 2 mm to 6 mm and a diameter of 3 mm to 6 mm.

[0074] In an embodiment of the invention, the strip may have a thickness of from 1 mm to 10 mm.

[0075] In an embodiment of the invention, the strip extends along the outer surface of the first skin.

[0076] In an embodiment of the invention, the first and second fibers are glass fibers.

[0077] In other embodiments of the invention, the first fiber may be a glass fiber and the second fiber may be a carbon fiber.

[0078] In an embodiment of the invention, the rotor body comprises a second integrally formed skin formed from a third fibrous material, at least a portion of the third fibers being oriented at between 45 and 90 degrees to the tube axis, optionally between 50 and 80 degrees to the tube axis, and the first and second skins together forming the rotor tube.

[0079] In an embodiment of the invention, the third fibers in the one or more outer layers of third fibers are oriented at about 90 degrees to the tube axis, optionally between 88 degrees and 90 degrees to the tube axis.

[0080] In an embodiment of the invention, the third fiber may include glass fiber.

[0081] In an embodiment of the invention, the strip comprises a hollow strip, which allows the skins to be spaced further apart to increase the thickness of the strip without increasing weight.

[0082] In an embodiment of the invention, the strip may extend along an inner surface of the first skin, in such an embodiment of the invention, the rotor body comprises only the first skin and the strip, and the rotor body does not comprise the second skin.

[0083] In an embodiment of the invention, the number of strips may vary along the length of the rotor body, which allows the rotor strength deformation to be adapted to the variation of bending moments along the length of the rotor body.

[0084] In an embodiment of the invention, the rotor body comprises a plurality of rotor tubes, which are joined together to form the rotor body.

[0085] In such embodiments, the rotor body can be made in any desired length by joining together an appropriate number of rotor tubes.

[0086] In embodiments of the invention in which the strip extends along the inner surface of the first skin and no second skin is present, at least a portion of the strip may span the joint between adjacent rotor tubes.

[0087] According to a third aspect of the present invention there is provided a rotor body according to an embodiment of the second aspect of the present invention formed using a method according to an embodiment of the first aspect of the present invention.

[0088] According to a fourth aspect of the present invention there is provided a marine vessel comprising a rotor sail attached to a portion of the vessel, the rotor sail comprising a rotor body according to an embodiment of the first and third aspects of the present invention. [Brief description of the drawings]

[0089] The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Figure 1] 4 is a schematic representation of a rotor sail comprising a rotor body according to an embodiment of the second aspect of the present invention. [Diagram 2] 4 is a schematic representation of a first skin being formed by method steps according to an embodiment of the first aspect of the present invention; [Diagram 3] 4 is a schematic representation of a plurality of strips attached to a first skin according to an embodiment of the first aspect of the present invention. [Figure 4] 1 is a schematic representation of a plurality of strips attached to a first skin according to an embodiment of the first aspect of the present invention. [Diagram 5] 4 is a schematic representation of a second skin being formed by method steps according to an embodiment of the first aspect of the present invention. [Figure 6] 6 is a schematic representation of a rotor body formed by the method shown in FIG. 5. [Figure 7] 4 is a schematic representation of a rotor body according to an embodiment of the second aspect of the invention, comprising a solid strip; [Figure 8] 4 is a schematic representation of a rotor body according to an embodiment of the second aspect of the invention, comprising a solid strip; [Figure 9] 4 is a schematic representation of a rotor body according to an embodiment of the second aspect of the invention, comprising a solid strip; [Figure 10] 4 is a schematic representation of a rotor body according to an embodiment of the second aspect of the invention, comprising a solid strip; [Figure 11] 4 is a schematic representation of a means for joining two rotor bodies according to an embodiment of the second aspect of the invention, the means being provided with tapered edges. [Figure 12]4 is a schematic representation of a means for joining two rotor bodies according to an embodiment of the second aspect of the invention, the means being provided with tapered edges. [Figure 13] 4 is a schematic representation of a means for joining two rotor bodies according to an embodiment of the second aspect of the invention, the means being provided with tapered edges. [Figure 14] 14 is a schematic representation of a method of forming the tapered edges shown in FIGS. 11-13. [Figure 15] 14 is a schematic representation of a method of forming the tapered edges shown in FIGS. 11-13. [Figure 16] 14 is a schematic representation of a plurality of rotor bodies according to an embodiment of the second aspect of the invention joined together by means such as those shown in FIGS. 11-13; [Figure 17] 4 is a schematic representation of a further means for joining two rotor body parts according to an embodiment of the second aspect of the invention, comprising tapered edges; [Figure 18] 4 is a schematic representation of a further means for joining two rotor body parts according to an embodiment of the second aspect of the invention, comprising tapered edges; [Figure 19] 19 is a schematic representation of a plurality of rotor bodies according to an embodiment of the second aspect of the invention joined together by means such as those shown in Figures 17 and 18; [Figure 20] 4 is a schematic representation of a rotor body according to an embodiment of the second aspect of the invention, comprising hollow strips. [Figure 21] 4 is a schematic representation of a rotor body according to an embodiment of the second aspect of the invention, comprising hollow strips. [Figure 22] 4 is a schematic representation of a rotor body according to an embodiment of the second aspect of the invention, comprising hollow strips. [Figure 23] 23 is a schematic representation of a means for joining two of the rotor bodies shown in FIGS. 20-22. [Figure 24]4 is a schematic representation of a rotor body according to an embodiment of the second aspect of the invention, comprising spaced apart strips. [Diagram 25] 25 is a schematic representation of a means for joining two of the rotor bodies shown in FIG. 24. [Figure 26] 4 is a schematic representation of a further means of joining two rotor bodies according to an embodiment of the second aspect of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0090] Referring initially to Figure 1, an embodiment of a rotor sail or fretted narrow rotor is shown, the rotor sail being generally defined by the reference numeral 4. The rotor sail comprises a rotor body 3 rotatably mounted to a static cylinder 6 via an upper bearing 8 and a lower bearing 10. The rotor body 3 comprises a plurality of circumferential ribs 5 which reinforce the rotor body 3 to provide bending strength and stiffness in the circumferential direction. In this embodiment, the circumferential ribs 5 are mounted on the interior of the rotor body 3, although the circumferential ribs could also be mounted on the exterior of the rotor body.

[0091] 2 illustrates wrapping or filament winding a first fiber around a mandrel 16 to form a tubular first skin 12 that forms a rotor tube 2 having a tube axis 18. The rotor body can be formed of a single rotor tube 2 or of multiple rotor tubes 2 coaxially joined together.

[0092] In this embodiment of the invention, the first fibers are bundled to form a first fiber tow 14, which is distributed from a first fiber tow pool 15, passed through a resin bath 21 to be coated with resin 20, and then wrapped around a mandrel 16. As the first fiber tow 14 is wrapped around the mandrel, it is guided up and down along the length of the mandrel 16 parallel to the tube axis 18, thereby gradually forming a tubular first skin while stacking layers of first fibers on top of each other. The winding process may continue until the first skin is formed to a desired thickness, for example 2-4 mm. To reduce the time required to form the first skin 12, the distribution, coating, and winding of multiple first fiber tows 14 are performed simultaneously.

[0093] The first fibers may be any suitable material, such as, for example, carbon, aramid, basalt, E-glass, S-glass, or ECR-glass. Similarly, the resin may be any suitable type of resin, such as, for example, epoxy resin, vinyl ester resin, polyester resin, polyurethane resin, or acrylic resin. The resin may be thermosetting or thermoplastic and may be cured at ambient or elevated temperatures to suit the required process speed and the final strength and temperature resistance required for the rotor body in use forming part of the rotor sail. For example, the resin may be an epoxy resin that cures to become solid after a few minutes or hours at ambient temperature, allowing the winding process to be conveniently carried out at ambient temperature and then removing the rotor tube from the mandrel after the resin has solidified. The resin and any adhesives used in the manufacture of the rotor body may then be further cured (post-cured) by increasing the temperature of the finished rotor body to increase the cure of the resin and further increase its strength and temperature resistance.

[0094] The orientation of the first fibers of the first skin 12 is determined by the orientation at which the first fiber tows 14 are wrapped around the mandrel 16 relative to the tube axis 18. This orientation can be varied by varying the speed at which the first fiber tows 14 are guided up and down the length of the mandrel 16 relative to the speed at which the mandrel 16 rotates to wrap the first fiber tows 14 around it.

[0095] The mandrel 16 may be tapered to facilitate removal of the first skin 12 from the mandrel 16 after the resin 20 has cured. Thus, the first skin 12 may be frustum-shaped rather than cylindrical.

[0096] In an embodiment of the present invention, the first fiber tows 14 are oriented at 45 to 90 degrees relative to the tube axis 18. For example, the first fiber tows 14 can be oriented in a mixed orientation, such as partially wrapped at ±45 degrees and partially wrapped at about 90 degrees (e.g., ±88 degrees).

[0097] In an embodiment of the invention, the first fiber tows are oriented at 50-80 degrees. For example, all of the first fiber tows 14 can be wrapped at about ±70 degrees. This allows the first fiber tows 14 to be guided up and down the length of the mandrel 16 at a constant speed throughout the process of forming the first skin 12 and facilitates a greater degree of automation, and also allows the mandrel 16 to rotate faster than if the fiber tows 14 were wrapped at ±45 degrees.

[0098] 3 illustrates the attachment of a plurality of strips 22 to the first skin 12. The strips 22 are formed from a second fiber and are attached to the first skin 12 such that at least a portion of the second fiber extends axially along the first skin 12, i.e., parallel to the tube axis 18. The second fiber may be any suitable material, such as carbon, aramid, basalt, E-glass, S-glass, or ECR glass. Each strip 22 may be formed using a pultrusion process.

[0099] In this embodiment of the invention, the strips 22 are positioned on the exterior surface of the first skin 12 while the resin that forms part of the first skin 12 is still curing so that the strips 22 may be bonded to the first skin 12 as the resin cures. To ensure that the multiple strips 22 remain in place until attached to the first skin 12, a temporary strap assembly 24 is used to maintain contact with the multiple strips 22 as the first skin 12 rotates around the mandrel 16 to allow additional strips 22 to be positioned.

[0100] In other embodiments of the invention, multiple strips may be temporarily bonded to a backing scrim 26, as shown in Figure 4. Thus, multiple strips 22 may be attached to the first skin 12 as a single assembly, rather than as individual strips. This is similar to mosaic tiles in bathrooms and kitchens, for example, which are attached to a backing layer so that the tiles can be applied to a wall in large sheets rather than individual tiles.

[0101] The use of backing scrim 26 may simplify attachment of strips 22 to first skin 12. However, the process of temporarily bonding strips 22 to backing scrim 26 requires additional process steps that are not required in the method shown in FIG.

[0102] In a further embodiment of the invention, the strips may be held in place on the first skin by a rigid or flexible fixture adapted to ensure the desired spacing of the strips around the circumference of the first skin (on either its inner or outer surface).

[0103] 5, a second skin 32 is formed by wrapping a third fiber around the strips 22 once they are all positioned on the first skin 12. The second skin 32 forms the rotor tube 2 together with the first skin 12. Similar to the first fibers, the third fibers are bundled together to form a third fiber tow 34 that is dispensed from a third fiber tow pool 35, passed through a resin bath 21 to be coated with resin 20, and then wrapped around the strips 22. The third fiber can be any suitable material, such as carbon, aramid, basalt, E-glass, S-glass, or ECR glass.

[0104] Initially, a temporary strap assembly (such as temporary strap assembly 24 shown in FIG. 3) may hold strip 22 in place until a sufficient amount of second skin 32 is formed to hold strip 22 without requiring additional support. If multiple straps are used along the length of the body of the temporary strap assembly, the straps may be removed in sequence as second skin 32 is formed.

[0105] The wrapping process may be continued until the second skin 32 is formed to a desired thickness, for example, 2-4 mm. To reduce the time required to form the second skin 32, multiple third fiber tows 34 may be simultaneously distributed, coated, and wrapped, similar to the first fiber tows 14 (shown in FIG. 2).

[0106] Although not included in this embodiment of the invention, in other embodiments of the invention, external circumferential ribs may be incorporated into the second skin 32 by wrapping a third bundle of fibers at 90 degrees to the tube axis 18 and at intervals along the length of the rotor tube 2. In further embodiments of the invention, the circumferential ribs may be formed separately to the rotor tube 2 and bonded to the first skin 12 or the second skin 32.

[0107] To provide a smooth outer surface to the second skin 32 to maximize the compaction pressure on the layers of fiber below, an outermost layer of a third fiber may be formed (e.g., forming the outermost 0.5 mm second skin) with the third fiber tows 34 oriented at approximately 90 degrees (e.g., ±88 degrees) relative to the tube axis 18.

[0108] The resin 20 coating the third fiber tows 34 also penetrates the strips 22 as it cures, thereby bonding the second skin 32 to the plurality of strips 22 as well as the first skin 12 .

[0109] Once the resin 20 has cured throughout the distinct layers of the first, second, and third fibers, the rotor tube 2 may be removed from the mandrel 16 .

[0110] Referring now to Figure 6, there is shown a rotor tube 2 comprising a first skin 12 formed as shown in Figure 2, a layer of strips 22 attached to the first skin 12 as shown in Figure 3, and a second skin 32 formed as shown in Figure 5. The rotor tube 2 forms part of a rotor body according to an embodiment of the second aspect of the invention, which in turn may form part of a rotor sail, such as the rotor sail shown in Figure 1.

[0111] The first skin 12 and the second skin 32 provide the rotor tubes 2 with circumferential / hoop strength perpendicular to the tube axis, with the first and third fibres oriented at 45-90 degrees to the tube axis, so that the rotor body comprising one or more of the rotor tubes 2 can maintain its circular cross-sectional shape when forming part of a rotor sail during use. On the other hand, the strips 22, at least some of which have second fibres oriented parallel to the tube axis, provide the rotor tubes 2 with axial strength parallel to the tube axis, so that the rotor body comprising the rotor tubes 2 can withstand bending forces caused by wind pressure during use.

[0112] 7, the rotor tube 2 is shown in cross section. Each strip 22 is substantially rectangular in cross section with an edge abutting the edge of an adjacent strip 22. Resin 20 fills any gaps between adjacent strips 22 and between the first skin 12, the plurality of strips 22, and the second skin 32, and acts to bond the first skin 12, the plurality of strips 22, and the second skin 32 to one another.

[0113] Resin 20 bonds the various components of the rotor tube together, and reducing the amount of resin used may advantageously reduce material costs and the weight of the resulting rotor tube. Thus, it may be preferable to avoid using more resin than is necessary to bond the layers of the rotor tube together.

[0114] 8 thus illustrates a rotor tube 102 similar to the rotor tube 2 shown in FIG. 7, except that each of the plurality of strips 122 has an arcuate cross-sectional shape so that the strips 122 may conform more closely to the first skin 12 and the second skin 32. Also, the edges of the strips 122 are angled so that each strip may conform more closely to adjacent strips. Thus, because the space between adjacent strips 122 and between the first skin 12, the plurality of strips 122, and the second skin 32 is reduced compared to the rotor tube 2 shown in FIG. 7, less resin 20 may be required to fill the space and bond the parts together. Thus, the rotor tube 102 may be manufactured at lower material costs and with less weight.

[0115] The strips may also be shaped to improve their ease of attachment to the first skin 12 and retention therein while the second skin is formed. For example, in FIG. 9, a rotor tube 202 from which a portion of a rotor body may be formed according to another embodiment of the second aspect of the invention is shown comprising a plurality of strips 222. Each strip 222 has a contoured edge 229 that is shaped to nest against the contoured edge 229 of an adjacent strip 222. Each strip 222 may thereby encourage its adjacent strip 222 to stay in place and reduce strain on the temporary straps 24 (shown in FIG. 3) or second skin 32 (shown in FIG. 5) to hold the strips 222 in place.

[0116] Similarly, in FIG. 10, a rotor tube 302 is shown having a plurality of strips 322 with contoured edges 329 that interlock with the contoured edges 329 of adjacent strips 322 .

[0117] A rotor sail requires its rotor body to be 18-48m long, whilst the winding process may be limited to forming rotor tubes of 6-15m in length. Thus, in an embodiment of the invention, two or more rotor tubes may be joined together coaxially to form a single rotor body suitable for forming part of a rotor sail.

[0118] The two rotor tubes, which may form part of the rotor body according to an embodiment of the second aspect of the invention, may be joined by any suitable means. For example, in Fig. 11, an end of a first rotor tube 402a and an end of a second rotor tube 402b, the diameter of which is equal to the end of the first rotor tube 402a, are abutted against each other. The abutting edges of the first rotor tube 402a and the second rotor tube 402b are joined with a resin or adhesive 20.

[0119] To reduce stress concentrations in the resin 20 connecting the parts together, the ends of the rotor tubes 402a, 402b are tapered in lamination thickness, which is the combined thickness of the first skin, the layer of strip, and the second skin. In this embodiment of the invention, each rotor tube 402a, 402b includes a tapered edge 440, and the lamination thickness of each rotor tube 402a, 402b tapers from the outer second skin 32 to the inner first skin 12. A wedge-shaped first joining part 442a fills the groove formed by the tapered edge 440, while a flat second joining part 442b covers the joining and adjacent parts of the first skin 12 of each rotor tube 402a, 402b.

[0120] The interface components 442a, 442b may be laminated and cured directly onto the rotor tubes 402a, 402b which are bonded separately or laminated and cured before being bonded onto the rotor tubes 402a, 402b with a structural adhesive.

[0121] In Fig. 12, two rotor tubes 502a, 502b are joined with rotor tubes 402a, 402b in a similar manner as shown in Fig. 11, except that the rotor tubes 502a, 502b have tapered edges 540, and the lamination thickness of each rotor tube 502a, 502b tapers from the inner first skin 12 to the outer second skin 32. Accordingly, the joining parts 442a, 442b are inverted such that the wedge-shaped first joining part 442a is positioned on the inner surface of the rotor tubes 502a, 502b and the flat second joining part 442b is positioned on the outer surface of the rotor tubes 502a, 502b.

[0122] In Figure 13, the first rotor tube 502a shown in Figure 12 is joined to the second rotor tube 402a shown in Figure 11. Thus, the tapered edges 440, 540 abut against each other, eliminating the need for a wedge-shaped joining piece and allowing the use of two flat joining pieces 642.

[0123] Tapering the lamination thickness from the outer surface to form the tapered edge 440 shown in Figure 11 can be accomplished by grinding the rotor tube 402 to a taper after curing, but while still internally supported by the mandrel 16, as shown in Figure 14. The outer section 444 is then removed from the rotor tube 402.

[0124] If an internal taper is required, for example to form the tapered edge 540 shown in Figure 12, this can be achieved by wrapping the rotor tube 502 over a wedge shaped piece 46 on a mandrel 16, as shown in Figure 15. The rotor tube 502 can then be ground from the outside to remove the outer section 544 and leave the tapered edge 540.

[0125] In Figure 16, a rotor body 603 suitable for forming part of a rotor sail comprises six rotor tubes 402, 502 joined together as shown in Figures 11-13. Each rotor tube 402, 502 is frustum shaped due to the taper of the mandrel from which they are each formed. To ensure equal diameter ends are joined to facilitate the joining procedure shown in Figures 11-13, some of the rotor tubes 402, 502 are oriented with an opposite taper to the taper of the other rotor tubes 402, 502.

[0126] However, in other embodiments of the present invention, the rotor tubes may be joined together so that they overlap one another.

[0127] In Figure 17, the end of a first rotor tube 402a is received within the end of a second rotor tube 402b, which has a slightly larger diameter than the end of the first rotor tube 402a. The ends are joined with resin or adhesive 20, similar to the joined ends of Figures 11-13, and a first joining part 742a and a second joining part 742b cover the inner and outer surfaces of the joint.

[0128] In Figure 18, a first rotor tube 502a is joined to a second rotor tube 402b in a similar manner as shown in Figure 17, except that the first rotor body includes a tapered edge 540 that is tapered to avoid there being a large space between the first rotor tube 502a and the second rotor tube 402b that would need to be filled with resin or adhesive 20. Thereby, the rotor tubes 502a, 402b may be joined at a lower material cost and less additional weight may be required to be added.

[0129] The rotor tubes 402, 502 joined as shown in FIGS. 17 and 18 may form a rotor body 703 as shown in FIG.

[0130] 20, a rotor tube 802, which may form part of a rotor body according to another embodiment of the second aspect of the present invention, comprises a plurality of strips 822. The rotor tube 802 is similar to the rotor tube 2 shown in FIG. 7, except that each strip 822 is a hollow strip with an air gap 28.

[0131] Because of the voids in each strip, the strips can be formed with a larger cross-sectional area while using the same amount of material and therefore having the same weight. Thus, the first skin 12 and the second skin 32 can be spaced further apart in the rotor tube 802 compared to the skins of the rotor tube 2 shown in FIG. 7. The increased spacing of the first skin 12 and the second skin 32 can increase the circumferential bending strength and stiffness of the rotor tube 802 without increasing the parasitic cost or weight of the foam core or the material cost or weight of the strips. This increased bending strength and stiffness can avoid the need for circumferential ribs.

[0132] 21, rotor tube 902 is similar to rotor tube 802, except that it includes a number of strips 922 that are trapezoidal in cross section. In other words, the edges of the strips 922 that abut against the edges of other strips are angled such that the spacing between the strips can be reduced and the amount of resin 20 required to fill the space is also reduced.

[0133] In Figure 22, rotor tube 1002 is similar to rotor tubes 802 and 902 shown in Figures 20 and 21, except that each strip is wider and includes multiple voids 28. This further increases the efficiency of the material required to form strips 1022 without sacrificing the shear strength of strips 1022. Each strip may be formed with angled edges similar to strips 922 shown in Figure 21, and may also be formed with arcuate cross sections 122 similar to those shown in Figure 8, thereby reducing the spacing present in rotor tube 1002 and reducing the amount of resin 20 required.

[0134] Two rotor tubes with hollow strips as shown in Figures 20, 21 and 22 may be joined in a similar manner to the method shown in Figures 11, 12 and 13 for rotor tubes with solid strips. However, when joining rotor tubes with hollow strips it may be preferable for the air gap to be sealed to ensure stability of the rotor body with the joined rotor tubes when forming part of the rotor sail in use.

[0135] In FIG. 23, two rotor tubes 802 are joined (although similar means can be used to join rotor tubes 902 and 1002). Due to the increased stack thickness of the rotor tubes 802, it would be necessary to lengthen the taper that extends substantially from the first skin 12 to the second skin 32, and would require the removal of a large amount of material. Rather than doing this, each rotor tube is provided with two tapered edges 840, one tapered from the first skin 12 and one tapered from the second skin 32. To ensure that the voids can be sealed with a minimal amount of resin 20, none of the tapered edges 840 extend into the voids 28. Wedge-shaped first and second joining parts 842a and 842b similar to those shown in FIGS. 11 and 12 are applied on each side of the void.

[0136] 24, there is shown a rotor tube 1102 which may form part of a rotor body according to another embodiment of the second aspect of the invention. The rotor tube 1102 comprises a first skin 12 and no second skin.

[0137] In this embodiment of the invention, the first skin 12 may be made according to the method shown in Figure 2 and removed from the mandrel. A number of strips 1122 are later added to the inside surface of the first skin 12, rather than the outside surface as in previously described embodiments of the invention. The strips 1122 are bonded in place with a structural adhesive 20, since the resin in the first skin 16 needs to be cured in order to be removed from the mandrel.

[0138] Advantageously, the wrapping can be done in a single operation, eliminating the need to hold the strip in place before the second skin is wrapped around it. Furthermore, the amount of axial material can be more easily varied along the length of the rotor (by adding more strip locally) to accommodate variations in bending moment, thereby minimizing the total weight and cost of axial material.

[0139] Also, more expensive carbon fiber strips of actual thickness can be used cost-effectively instead of glass fiber because they can be dispersed rather than abutted in a continuous layer. In a continuous layer, only about 1-2 mm of carbon fiber thickness is required on a rotor body of 5 m diameter, which means that the previously described embodiment of the invention has little benefit from a pultrusion to separate the first and second skins for good bending strength. In this embodiment of the invention, narrower strips can be used, having dimensions of, for example, 50 mm width, 5 mm thickness, and 150 mm gap. Carbon fiber is advantageous because it is stronger and lighter, specifically, carbon fiber has better fatigue resistance than glass fiber. The strength advantage of carbon fiber is particularly noticeable when pultruded because the straightness of the fiber is beneficial. Thus, using carbon fiber in the axial direction of the rotor sail according to an embodiment of the invention can be more cost-effective than using glass fiber, even though the carbon fiber material per kg is more expensive.

[0140] Additionally, filament winding machines typically have a maximum mandrel length that is shorter than the desired length of the rotor body that is used to form part of the rotor sail. Thus, several rotor bodies according to an embodiment of the second aspect of the invention may need to be joined together and the joints between them may need to carry the entire axial load. In the embodiment of the invention shown in FIG. 24, multiple rotor tubes 1102 can be joined before the strips 1122 are joined. Thus, the strips 1122 are continuous throughout each joint, as shown in FIG. 25, providing the required axial strength, while the first skin 12 and joint parts 1142 applied throughout the joints only need to transmit relatively low shear forces that are easily accommodated by a 2-3 mm thick biaxial (±45 degrees) material.

[0141] However, without the pultrusion in the middle of the two skins, the required thickness of the first skin 12 for circumferential bending strength would have to be constructed using more first fibers, i.e., the first skin 12 would have to be thicker. Furthermore, a second step of bonding the pultrusion to the first skin 12 would be required, requiring a large amount of adhesive 20, which adds cost and weight.

[0142] Referring now to Figure 26, a further means of joining two rotor tubes is shown. This joining means can be applied to any of the tubes shown in Figures 6-25, but the rotor tube 2 shown in Figure 7 is used as an example. A joining piece 1242 is joined to each of the rotor tubes 2 to be joined using an adhesive (not shown). Each joining piece 1242 comprises a radial surface 1243 adapted to abut against the radial surface of the other joining piece 1242. The two joining pieces 1242 are then bolted together using a bolt assembly 1248.

[0143] An advantage of this joining means is that it allows the rotor body to be disassembled for transport and it can be adapted to incorporate circumferential ribs such as the circumferential rib 5 shown in FIG.

[0144] Preferences and options for any given aspect, feature or parameter of the invention should be considered as disclosed in combination with all preferences and options for all other aspects, features and parameters of the invention, unless the context dictates otherwise.

Claims

1. 1. A method of manufacturing a rotor body forming part of a rotor sail, the method comprising the steps of: wrapping the first fiber around a mandrel to form a tubular first skin forming a rotor tube having a tube axis; forming a plurality of strips from a second fiber; and attaching the strip to a surface of the first skin such that at least a portion of the second fibers extend axially along the rotor body.

2. 2. The method of claim 1, wherein the step of winding a first fiber comprises winding the first fiber such that the first fiber is oriented at 45 to 90 degrees, preferably 50 to 80 degrees, relative to the tube axis.

3. The method of claim 1 or 2, comprising the further step of impregnating the first fiber with a resin prior to wrapping the fiber around the mandrel.

4. 10. The method of any one of the preceding claims, wherein the step of wrapping a first fiber comprises wrapping a material formed from the first fiber around the mandrel.

5. 10. The method of any one of the preceding claims, wherein the strip is formed using a pultrusion process.

6. 10. A method according to any one of the preceding claims, wherein the step of attaching the strip comprises attaching the strip to an outer surface of the first skin.

7. The method of claim 6 , wherein the step of attaching the strip comprises pressing the strip against the first fibers before the resin between the first fibers cures.

8. 10. A method according to any one of the preceding claims, comprising the further step of forming a second skin by wrapping a third fibre around the strip, the first and second skins together forming the rotor tube.

9. 9. The method of claim 8, wherein the step of wrapping a third fiber comprises wrapping at least a portion of the third fiber such that the third fiber is oriented at 45-90 degrees to the tube axis, optionally at 50-80 degrees to the tube axis.

10. 10. The method of claim 8 or 9, wherein the step of wrapping a third fiber comprises wrapping one or more outer layers of a third fiber such that the third fiber is oriented at about 90 degrees to the tube axis, preferably between 88 degrees and 90 degrees to the tube axis.

11. A method according to any one of claims 8 to 10, comprising the further step of impregnating the third fibre with a resin before wrapping the fibre around the strip.

12. The method of any one of claims 8 to 11, wherein the step of wrapping a third fiber comprises wrapping a fabric formed from the third fiber around the mandrel.

13. The step of attaching the strip includes applying pressure to both the first and second skins before the resin impregnating the first and third fibers cures. The method of claim 11 .

14. A method according to any one of claims 8 to 13, wherein the step of forming a plurality of strips comprises forming a plurality of hollow strips.

15. The method of any one of claims 1 to 5, wherein the step of attaching the strip comprises attaching the strip to an inner surface of the first skin.

16. 10. A method according to any one of the preceding claims, comprising forming a plurality of rotor tubes and joining adjacent rotor tubes together to form the rotor body.

17. 17. A method according to claim 16 when dependent on claim 15, including forming the strip so that it spans joints between adjacent rotor tubes.

18. 16. A method according to claim 15 or any claim dependent thereon, comprising the further step of axially varying a number of the strips attached to the inner surface of the first skin.

19. A rotor body forming a part of a rotor sail, a tubular first skin forming a rotor tube and having a tube axis; a plurality of strips extending axially along a surface of the skin; A rotor body, wherein the first skin is integrally formed from a first fibrous material formed from first fibers and the strip is formed from a second fibrous material formed from second fibers, at least a portion of the second fibers extending axially along the rotor body.

20. The first fibers are at an angle of 45-90 degrees to the tube axis, and optionally at an angle of 100 degrees to the tube axis. The rotor body of claim 19 oriented at 50 to 80 degrees.

21. The first skin has a thickness of 2 mm to 6 mm and a diameter of 3 m to 6 m.

21. A rotor body as described in 19 or 20.

22. A rotor body according to any one of claims 19 to 21, wherein the strip has a thickness of between 1 mm and 10 mm.

23. A rotor body according to any one of claims 19 to 22, wherein the strip extends along an outer surface of the first skin.

24. A rotor body according to any one of claims 19 to 23, wherein the first and second fibers are glass fibers.

25. A rotor body according to any one of claims 19 to 23, wherein the first fibres are glass fibres and the second fibres are carbon fibres.

26. 26. A rotor body according to any one of claims 19 to 25, wherein the rotor body comprises a second integrally formed skin formed from a third fibrous material formed from third fibres, at least some of the third fibres being oriented at 45-90 degrees to the tube axis, optionally 50-80 degrees to the tube axis, the first and second skins together forming the rotor tube.

27. 27. A rotor body as claimed in claim 26, wherein third fibres in one or more outer layers of third fibres are oriented at approximately 90 degrees to the tube axis, optionally between 88 degrees and 90 degrees to the tube axis.

28. A rotor body according to any one of claims 18 to 26, wherein the third fibres are glass fibres.

29. A rotor body according to any one of claims 26 to 28, wherein the strip comprises a hollow strip.

30. A rotor body according to any one of claims 19 to 22, wherein the strip extends along an inner surface of the first skin.

31. A rotor body according to any one of claims 19 to 30, wherein the rotor body comprises a plurality of rotor tubes, the rotor tubes being joined together to form the rotor body.

32. A rotor body according to claim 31 when dependent on claim 30, wherein at least a portion of the strip spans a joint between adjacent rotor tubes.

33. A rotor body according to claim 30 or any claim dependent thereon, wherein the number of strips varies along the tube axis.

34. A rotor body as claimed in any one of claims 19 to 33, formed using a method as claimed in any one of claims 1 to 18.

35. A marine vessel comprising a rotor sail attached to a portion of the vessel, the rotor sail comprising a rotor body according to any one of claims 19 to 33.

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