rotor sail

Abstract

A method of manufacturing a rotor body forming part of a rotor sail, the method comprising the steps of: winding a 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 strips to a surface of the first skin so that at least a portion of the second fiber extends axially along the rotor body.

Description

[Technical Field]

[0001] The present invention relates to rotor sails for wind-assisted propulsion of objects, particularly but not exclusively to rotor sails for wind-assisted propulsion of ships, and also to methods of forming such rotor sails and ships incorporating rotor sails. [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. Upper and lower bearings position the rotor on the tower. Typically, wind loads act on the rotor, 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 moments acting on the rotor as a whole (i.e. acting as a beam of circular hollow section subjected to a distributed load). This causes stresses in the plane of the rotor skin, primarily tension and compression forces in the rotor axial direction, with some in-plane shear forces resulting from incidental shear forces. Local bending moments on the rotor skin due to the uneven distribution of air pressure around the section, which tend to distort the circular cross section, causing 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 rotation. The number of rotations over 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 fibers 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 this is what provides 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 fibers 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 in the type forming portion of the rotor sail.

[0007] A further requirement is that the circumferentially oriented fibers should be as far 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 likely to buckle circumferentially than axially because a cylinder is naturally 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 rotor revolution. Large rotor sails typically rotate at speeds of up to 250 rpm. This means that over the lifetime of such a rotor sail, which is typically 20 to 25 years, the laminations forming the rotor may experience on the order of 1 to 2 billion fatigue cycles.

[0012] For glass fiber / epoxy laminates, the fatigue strength at 1 billion cycles is only about 15% of the static stress. For carbon fiber / epoxy laminates, the fatigue strength can 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 laminations when used to make rotor sails are significantly different and more onerous than those for other large composite structures, and therefore known methods for making suitable composite structures may not be appropriate for rotor sails.

[0015] One known method of making composite materials is known as pultrusion. This method is suitable for making straight tubes of round 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 tow) used directly from tow-wound bobbins.

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

[0017] For many applications, compressive strength is often a primary consideration when designing structural components 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 greater tensile forces.

[0018] Furthermore, pultrusion is not suitable for forming composites that must withstand hoop 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 VARTM (Vacuum Assisted Resin Transfer Moulding), which 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] Additionally, 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 particularly economical because 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: potentially in the form of liquid polymer resin, with glass or carbon fiber tows directly from bobbins containing 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 a composite material with a sandwich structure, with a foam core in the center 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 of material costs (because fibers must first be woven or sewn into fabric and then cut to size before molding). For the resin to be easily infused, it must have a very low viscosity, which limits the resin's molecular weight 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 more material is required than if the fibers were held straight, which reduces the compressive strength of the material. Foam cores are relatively expensive, both in material and labor costs, and absorb a significant weight of resin, which increases the weight of the part and adds further cost.

[0026] For rotor sails, typical operating strains in both the axial and circumferential directions must be kept below about 0.15% 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 present 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 fiber 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 fiber; and attaching the strips to a surface of the first skin so that at least a portion of the second fiber extends axially along the rotor body.

[0029] The tubular first skin forms the outer cylindrical sleeve of the rotor sail. Because a filament winding process 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 sections together to form the tubular shape.

[0030] Filament winding is economical, especially since it can be automated.

[0031] The method includes the additional step of forming a plurality of strips made from the second fibers, 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 axial strength required for the rotor sail.

[0034] The inventors have realised that by using a filament winding method to form a first skin and then attaching a strip to the 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 a relatively low cost.

[0036] In an embodiment of the invention, the method comprises 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° relative to the tube axis. In another embodiment of the invention, the fibers are wrapped such that their orientation is between 50° and 80° relative 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 in which tows of fiber are wrapped around a mandrel, a material formed from warp and weft yarns, with the warp and weft yarns appropriately oriented, can be wrapped around a mandrel.

[0041] In an embodiment of the present 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, material is pulled through a die as it is extruded through the die, which has the advantage of orienting the fibers axially along the strip. Strips formed from such processes are known as pultruded products.

[0045] Using pultrusion, the strip can 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 strips may be attached by any convenient method. In an embodiment of the invention, the strips are attached to the outer surface of the first skin by pressing the strips against the first fibers before the resin around the first fibers has cured.

[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 abut one another axially. 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 the 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 embodiments of the invention, the third fibers are wrapped around the strip such that the orientation of the third fibers is between 45° and 90° relative 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° relative 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°, relative to the tube axis, which provides a smoother surface and greater compaction pressure on the fibers during resin curing.

[0055] In an embodiment of the invention, the method comprises the further step of impregnating the third fiber 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 the outer surface of the first skin and the inner surface of the 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 fibers may be formed from any convenient material, and in an embodiment of the present invention, the third fibers comprise glass fibers.

[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 interior 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. This allows the rotor strength to adapt to changes in bending moments along the length of the rotor body.

[0067] In an embodiment of the present invention, the rotor body may have a length of 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 6 m and 12 m.

[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 in any desired manner 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 inside surface of the 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, thus adding strength to the rotor body.

[0071] According to a second aspect of the present 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 present invention, the first fibers are oriented at an angle of 45° to 90° to the tube axis, preferably 50° to 80° to the tube axis.

[0073] In an embodiment of the present 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 present invention, the strip may have a thickness of 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 present 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 45 to 90 degrees to the tube axis, optionally 50 to 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 present 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 the inner surface of the first skin, and 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 strength deformation of the rotor to be adapted to changes in bending moment 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 an embodiment, the rotor body can be made to 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 explanation of the drawings]

[0089] The present 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; [Figure 2] 3 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; [Figure 3] 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; [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; [Figure 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 7-10] 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 11-13] 5 is a schematic representation of a means for joining two rotor body parts according to an embodiment of the second aspect of the invention, comprising tapered edges; [Figure 14-15] 14 is a schematic representation of a method for 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 to 13; [Figure 17-18] 10 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 FIGS. 17 and 18; [Figure 20-22] 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; [Figure 25] 25 is a schematic representation of a means for joining two of the rotor bodies shown in FIG. 24. [Figure 26] 5 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 INVENTION

[0090] Referring initially to Figure 1, an embodiment of a rotor sail or fretted narrow rotor is shown, the rotor sail 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 circumferential bending strength and stiffness. In this embodiment, the circumferential ribs 5 are mounted on the interior of the rotor body 3, although 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 multiple rotor tubes 2 joined together coaxially.

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

[0093] The first fiber can be any suitable material, such as, for example, carbon, aramid, basalt, E-glass, S-glass, or ECR-glass. Similarly, the resin can be any suitable type of resin, such as, for example, epoxy resin, vinyl ester resin, polyester resin, polyurethane resin, or acrylic resin. The resin can be thermosetting or thermoplastic and can be cured at ambient or elevated temperatures to suit the required process speed and the final strength and temperature resistance required for the in-use rotor body that forms part of the rotor sail. For example, the resin can 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 manufacturing the rotor body can 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 wound on 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 the mandrel 16.

[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 being partially wound at ±45 degrees and partially wound at approximately 90 degrees (e.g., ±88 degrees).

[0097] In an embodiment of the present invention, the first fiber tows are oriented at 50-80 degrees. For example, all of the first fiber tows 14 can be wrapped at approximately ±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, facilitating a higher 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 second fibers and are attached to the first skin 12 such that at least a portion of the second fibers extend axially along the first skin 12, i.e., parallel to the tube axis 18. The second fibers 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 can 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, allowing additional strips 22 to be positioned.

[0100] In another embodiment 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 the wall in large sheets rather than individual tiles.

[0101] The use of backing scrim 26 may simplify the 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 periphery of the first skin (either on its inner or outer surface).

[0103] 5, the second skin 32 is formed by wrapping a third fiber around the strips 22 once all of the strips 22 are positioned on the first skin 12. The second skin 32, together with the first skin 12, forms the rotor tube 2. Like the first fibers, the third fibers are bundled to form a third fiber tow 34, which is dispensed from a third fiber tow spool 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 can 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 can be simultaneously dispensed, 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 third fiber bundles 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 from 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 and maximize the compaction pressure on the underlying fiber layers, a third outermost layer of fibers (e.g., forming the outermost 0.5 mm second skin) may be formed, 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 tow 34 also penetrates the strips 22 as it cures, thereby bonding the second skin 32 to the plurality of strips 22 in the same manner as the first skin 12 .

[0109] Once the resin 20 has cured throughout the different 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 and may in turn form part of a rotor sail such as the rotor sail shown in Figure 1.

[0111] The first skin 12 and second skin 32 provide the rotor tubes 2 with strength in the circumferential / hoop direction perpendicular to the tube axis, with the first and third fibers 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 fibers oriented parallel to the tube axis, provide the rotor tubes 2 with strength in the axial direction, 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 has a substantially rectangular cross-sectional shape, 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 can advantageously reduce material costs and the weight of the resulting rotor tube. Therefore, it may be preferable to avoid using more resin than is necessary to bond the layers of the rotor tube together.

[0114] 8 shows 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 can fit more closely to the first skin 12 and the second skin 32. Additionally, the edges of the strips 122 are angled so that each strip can fit more closely to an adjacent strip. Therefore, 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 components together. Therefore, the rotor tube 102 may be manufactured with lower material costs and 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, FIG. 9 shows 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 present invention, 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 help its adjacent strip 222 stay in place and reduce strain on 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 18-48 m long, while the winding process may be limited to forming rotor tubes of 6-15 m in length. Thus, in an embodiment of the present 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] Two rotor tubes that may form part of the rotor body according to one embodiment of the second aspect of the present invention may be joined by any suitable means. For example, in Figure 11, an end of a first rotor tube 402a and an end of a second rotor tube 402b, which has a diameter equal to that of 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 components together, the ends of the rotor tubes 402 a, 402 b are tapered in lamination thickness, which is the combined thickness of the first skin, the strip layer, and the second skin. In this embodiment of the invention, each rotor tube 402 a, 402 b includes a tapered edge 440, and the lamination thickness of each rotor tube 402 a, 402 b tapers from the outer second skin 32 to the inner first skin 12. A wedge-shaped first joining piece 442 a fills the groove formed by the tapered edge 440, while a flat second joining piece 442 b covers the joining and adjacent portions of the first skin 12 of each rotor tube 402 a, 402 b.

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

[0121] 12, two rotor tubes 502a, 502b are joined with rotor tubes 402a, 402b in the same manner as shown in FIG. 11, except that the rotor tubes 502a, 502b include tapered edges 540, and the laminate thickness of each rotor tube 502a, 502b tapers from the inner first skin 12 to the outer second skin 32. Accordingly, the joining pieces 442a, 442b are inverted so that the wedge-shaped first joining piece 442a is positioned on the inner surface of the rotor tubes 502a, 502b and the flat second joining piece 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 achieved 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 a taper in the opposite direction 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 a resin or adhesive 20, similar to the joining ends of Figures 11-13, and a first joining piece 742a and a second joining piece 742b cover the inner and outer surfaces of the joint.

[0128] 18, the first rotor tube 502a is joined to the second rotor tube 402b in the same manner as shown in FIG. 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. This allows the rotor tubes 502a, 402b to be joined at a lower material cost and requires less additional weight 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 that 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 within 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 plurality of strips 922 that are trapezoidal in cross section. In other words, the edges of strips 922 that abut against the edges of other strips are angled so that the spacing between strips can be reduced and the amount of resin 20 required to fill the space is also reduced.

[0133] 22, rotor tube 1002 is similar to rotor tubes 802 and 902 shown in FIGS. 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 FIG. 21 and may further be formed with arcuate cross sections 122 similar to those shown in FIG. 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 such as those 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 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 could be used to join rotor tubes 902 and 1002). The increased laminate thickness of rotor tube 802 necessitates lengthening the tapered section that extends substantially from first skin 12 to second skin 32, and requires the removal of a large amount of material. Rather than doing this, each rotor tube includes two tapered edges 840, one tapering from first skin 12 and one tapering from second skin 32. To ensure that the voids can be sealed with a minimal amount of resin 20, neither of tapered edges 840 extends into void 28. Wedge-shaped first and second joining pieces 842 a and 842 b, similar to those shown in FIGS. 11 and 12 , are applied to each side of the joint.

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

[0137] In this embodiment of the invention, first skin 12 can be made and removed from the mandrel according to the method shown in Figure 2. A plurality of strips 1122 are later added to the interior surface of first skin 12, rather than the exterior surface as in previously described embodiments of the invention. Because the resin in first skin 16 must be cured before it can be removed from the mandrel, strips 1122 are bonded in place with structural adhesive 20.

[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] Additionally, more expensive carbon fiber strips of actual thickness can be cost-effectively substituted for 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 is required on a 5-m diameter rotor body, meaning that the previously described embodiment of the present invention does not benefit much from pultrusion to separate the first and second skins for good bending strength. This embodiment of the present invention allows for the use of narrower strips, e.g., 50 mm wide, 5 mm thick, and 150 mm gap. Carbon fiber is advantageous because it is stronger and lighter; specifically, it has better fatigue resistance than glass fiber. The strength advantage of carbon fiber is particularly pronounced when pultruded, due to the beneficial straightness of the fiber. Therefore, using carbon fiber axially in the rotor sail according to an embodiment of the present invention can be more cost-effective than using glass fiber, even though the carbon fiber material is more expensive per kg.

[0140] Additionally, filament winding machines typically have a maximum mandrel length that is shorter than the desired length of the rotor body used to form a portion of the rotor sail. Accordingly, several rotor bodies according to embodiments of the second aspect of the present 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 present 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 components 1142 applied across the joints only need to transmit relatively low shear forces that are easily accommodated by biaxial (±45°) material 2-3 millimeters thick.

[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 require the use of 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 for 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 has 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 bolt assemblies 1248.

[0143] An advantage of this joining means is that it allows the rotor body to be disassembled for transport and can be adapted to incorporate circumferential ribs such as the circumferential rib 5 shown in Figure 1 .

[0144] Preferences and alternatives for any given aspect, feature or parameter of the invention should be considered as disclosed in combination with any and all preferences and alternatives 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, said method comprising: wrapping a first fiber around a mandrel to form a tubular first skin along an axial direction of the mandrel, the first skin forming a rotor tube; forming a plurality of strips from the second fiber; attaching the strip to a surface of the first skin along an axial direction of the rotor tube such that at least a portion of the second fibers extend along an axial direction of the rotor body; The method comprising the further step of impregnating the first fiber with a resin before wrapping the first fiber around the mandrel.

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

3. 3. 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 50 to 80 degrees relative to the tube axis.

4. The method of claim 1 or 2, wherein the step of winding a first fiber comprises winding a fabric formed from the first fiber around the mandrel.

5. The method of claim 1 or 2, wherein the strip is formed using a pultrusion process.

6. The method of claim 1 or 2, wherein the step of attaching the strip comprises attaching the strip to an outer surface of the first skin.

7. The method of claim 1 , wherein the step of attaching the strip comprises pressing the strip against the first fibers before the resin between the first fibers cures.

8. 3. A method according to claim 1 or 2, including the further step of wrapping a third fibre around the strip, said third fibre forming a second skin.

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 to 90 degrees relative to the tube axis.

10. 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 50 to 80 degrees relative to the tube axis.

11. 9. The method of claim 8, wherein the step of wrapping a third fiber comprises wrapping one or more layers of third fiber such that the third fiber is oriented at 88 degrees to 90 degrees relative to the tube axis.

12. 9. The method of claim 8, including the further step of impregnating said third fiber with a resin before wrapping said third fiber around said strip.

13. 13. The method of claim 12, wherein 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 first and second skins together forming a rotor tube.

14. The method of claim 8 , wherein forming a plurality of strips comprises forming a plurality of hollow strips.

15. The method of claim 1 or 2, wherein the step of attaching the strip comprises attaching the strip to an interior surface of the first skin.

16. 3. The method of claim 1, further comprising forming a plurality of rotor tubes and joining adjacent rotor tubes together to form the rotor body.

17. 3. The method of claim 1 or 2, including forming the strip so that it spans a joint between adjacent rotor tubes.

18. 16. The method of claim 15, including the further step of axially varying the number of strips attached to the inner surface of the first skin.

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

20. The rotor body of claim 19, wherein the first fibers are oriented at 45 to 90 degrees relative to the tube axis.

21. The rotor body of claim 19, wherein the first fibers are oriented at 50 to 80 degrees relative to the tube axis.

22. A rotor body according to claim 19 or 20, wherein the first skin has a thickness of between 2 mm and 6 mm and a diameter of between 3 mm and 6 mm.

23. A rotor body according to claim 19 or 20, wherein the strip has a thickness of between 1 mm and 10 mm.

24. A rotor body according to claim 19 or 20, wherein the strip extends along an outer surface of the first skin.

25. 21. A rotor body according to claim 19 or 20, wherein the first and second fibres are glass fibres.

26. 21. A rotor body according to claim 19 or 20, wherein the first fibres are glass fibres and the second fibres are carbon fibres.

27. 21. A rotor body according to claim 19 or 20, wherein the rotor body comprises a second integrally formed skin formed from a third fibrous material formed from third fibers, at least some of the third fibers being oriented at 45 to 90 degrees relative to the tube axis, the first and second skins together forming the rotor tube.

28. 21. A rotor body according to claim 19 or 20, wherein the rotor body comprises a second integrally formed skin formed from a third fibrous material formed from third fibers, at least some of the third fibers being oriented at 50 to 80 degrees relative to the tube axis, the first and second skins together forming the rotor tube.

29. 28. The rotor body of claim 27, wherein the third fibers in one or more layers of third fibers are oriented at between 88 degrees and 90 degrees relative to the tube axis.

30. 28. The rotor body of claim 27, wherein the third fibers are glass fibers.

31. The rotor body of claim 27 , wherein the strip comprises a hollow strip.

32. A rotor body according to claim 19 or 20, wherein the strip extends along an inner surface of the first skin.

33. 21. A rotor body according to claim 19 or 20, wherein the rotor body comprises a plurality of rotor tubes, the rotor tubes being joined together to form the rotor body.

34. The rotor body of claim 33 , wherein at least a portion of the strip spans a joint between adjacent rotor tubes.

35. 33. The rotor body of claim 32, wherein the number of strips varies along the tube axis.

36. 21. A marine vessel comprising a rotor sail attached to a portion of the vessel, the rotor sail comprising a rotor body according to claim 19 or 20.

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