Low-cost viscous drag reducing claddings.

Abstract

A low-cost viscous drag reducing cladding for a marine vessel hull with an airbag adapted to have a water repellent exterior surface. Each airbag includes a plenum that includes a substantially open space that does not impede airflow through the plenum. The airbag is inflated with compressed air to a pressure greater than the adjacent hydrostatic pressure. The airbag material includes a reinforcing fabric that is adapted to withstand forces exerted during operation and sealed with a sealant to be substantially impermeable to air. The outer water-repellent surface of each airbag is connected to the plenum by restriction holes. Air flows from the plenum through the restriction holes into the water-repellent layer.

Description

[Technical field]

[0001] The present invention relates to the reduction of viscous drag, and more particularly to the reduction of viscous drag on vessel hulls and other liquid contacting surfaces. [Background technology]

[0002] Viscous drag results from a phenomenon known in fluid mechanics as the "no slip" boundary condition at a solid / liquid interface. A layer of liquid in contact with a solid does not move relative to the solid. Shear forces occur between that layer and adjacent liquid layers. This condition is known to apply to ships moving liquids as well as to marine vessels. It also applies to liquids flowing through pipes, such as oil pumped through a pipeline or liquids passing through a heat exchanger. The viscosity of air is approximately 1 / 1000th of that of water. It has been known for many years that viscous drag in liquids can be effectively eliminated by inserting air between the liquid and the solid to completely separate the liquid from the solid.

[0003] Viscous drag consumes a large portion of the fuel required to propel large ships. As the shipping industry is a large global producer of greenhouse gases, any reduction in fuel consumption due to viscous drag would result in a significant reduction in environmental damage and significant economic savings.

[0004] Plants such as lotus leaves and certain insects exhibit the property of trapping air on their surfaces. This effect is typically produced by a hydrophobic surface coupled with a complex surface geometry, often with a hierarchical structure. This property results in a significant reduction in the wetted area. This non-wetting is commonly known as the "lotus effect". Many experimental materials have been tested to replicate this effect found in nature. Materials designed to mimic this effect may work in the short term when tested just below the surface, but have not been shown to perform long term under the hydrostatic pressures acting on the hulls of modern ships. Today's oil tankers typically have hulls with a depth of 20m. At this depth, the hull has a resistance of 0.2N / mm 2A hydrostatic pressure of 2 bar acts on the surface, which corresponds to a surface tension of 10 m. Moreover, waves 5 m high are common, and waves 10 m high are not uncommon. In comparison, the surface tension is very small. Water has a surface tension of 72 mN / m. The static buoyancy at a depth of 20 m corresponds to the surface tension of a completely hydrophobic material at each square millimeter of a length of almost 3 meters. The static buoyancy is therefore much greater than the surface tension that non-wetting materials depend on. By itself, this non-wetting behavior is limited only close to the water surface. A further limitation of the "lotus effect" is that trapped air will diffuse into the water over time, causing the surface to become saturated with water. Materials that reproduce this principle would not be useful for ships with intervals of more than one year between voyages and drydocking. It is clear that such materials cannot solve the problems associated with the viscous drag of a ship's hull.

[0005] US Patent No. 5,399,633 discloses a method of blowing air bubbles through the hull. However, because the holes themselves are large, the blown air forms large bubbles. Thus, the hull surface is kept wet. As a result, a physical phenomenon known as "bubble flow" occurs. This reduces the viscosity of the water and reduces the viscous drag. However, the bubbly flow also reduces the density of the water and reduces the buoyancy. The large power requirements required to generate a large amount of bubble flow tend to offset the power reduction due to the reduction in the viscous drag of the hull. Therefore, this system is not industrially supported.

[0006] US Patent No. 5,399,663 discloses a hydrophobic layer that traps air on the hull surface connected by a gas-permeable ply to a refillable reservoir made of porous media. The gas-permeable ply is 0.5 microns to 5 microns thick and lacks the robustness required to survive in the marine environment. The porous media of the refillable reservoir presents a significant barrier to air flow, resulting in large air pressure gradients as air passes through it. The objective of the invention is that when exposed to increased hydrostatic pressure from waves, the air in the air-retaining layer is not lost by being released into the sea, but instead is channeled from the air-retaining layer into the reservoir. However, collapse of the air layer occurs. No mechanism provides sufficient restoring force to rebuild the air layer, resulting in a permanent loss of the air layer. Subsequently, flooding of the air reservoir may occur shortly thereafter. This results in increased viscous drag. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] U.S. Patent No. 5,456,201 [Patent Document 2] U.S. Patent No. 9,630,373 Summary of the Invention [Problem to be solved by the invention]

[0008] Worldwide, there are approximately 100,000 ocean-going ships that generate up to 3% of the world's total carbon dioxide emissions and consume about 7% of the world's total crude oil production. An estimated 400,000 people die prematurely and an additional 14 million new cases of childhood asthma occur each year from shipping-related pollution. Thus, there is an urgent need for a solution to the problem of viscous drag on ships and other watercraft. [Means for solving the problem]

[0009] According to the present invention there is provided a hull cladding comprising at least one airbag including a plenum comprising a substantially open space which in use is pressurised with air, said airbag being formed from a material including a reinforcing fabric substantially sealed with a sealant, said airbag further comprising an outer surface layer adapted to have drainage properties, said outer surface layer being connected to the plenum via at least one restriction hole in the airbag through which air from the plenum can pass, to provide pressurised air within the drainage layer having a pressure level substantially equal to or greater than the adjacent hydrostatic pressure.

[0010] The airbag may be sealed with a vulcanized rubber, which may be selected from at least one of CSM, CR, EPDM, or silicone. Alternatively, the airbag may be sealed with a thermoplastic polymer, which may be selected from at least one of thermoplastic CSM or thermoplastic polyurethane.

[0011] The reinforcing fabric may include a woven fabric. The airbag may include at least two layers or sheets of reinforcing fabric and may include a drop stitch between the at least two layers or sheets of the reinforcing fabric. The reinforcing fabric may include a drop stitch between an outer layer and an inner layer of the reinforcing fabric. The reinforcing fabric may include yarns selected from at least one of polyester, para-aramid, meta-aramid, glass fiber, polyamide, polypropylene, PEEK, UHMPE, steel, carbon fiber. The material may include a laminate including at least one woven fabric reinforcement layer. The at least one restriction hole may be formed by mechanical drilling. The at least one restriction hole may be formed by laser drilling. The at least one restriction hole may be formed by feeding at least one hollow fiber through the airbag material such that one end of each hollow fiber terminates within the plenum and the other end of each hollow fiber terminates within the drainage outer layer. The restriction holes may be formed by securely attaching hollow fibers to the outer fabric reinforced facing layer such that one end of the fibers terminates within the plenum and the other end terminates within a drainage layer on the opposite side of the reinforced fabric. The outer facing layer may include means for forming tightly packed air pockets on said outer facing layer, each air pocket having a radius less than twice the capillary length of water, and each air pocket being connected to the plenum by at least one restriction hole.

[0012] The airbag may be adapted to be drainable by forming tightly packed air pockets on the outer layer, each air pocket having a radius less than twice the capillary length of water, and each air pocket may be connected to the plenum by at least one restriction hole.

[0013] The exterior layer may include loops of hydrophobic fibers woven into a reinforcing fabric of the airbag material. The exterior layer may be adapted to have drainage properties by weaving loops of hydrophobic fibers into the reinforcing fabric of the upper layer.

[0014] The outer layer may include a layer of hydrophobic loops attached to the outer side of the reinforcing fabric by adhesive bonding. The outer layer may be adapted to have drainage properties by attaching a layer of hydrophobic loops to the outer side of the reinforcing fabric by adhesive bonding. The layer of hydrophobic loops of fibers may be formed by one of weaving, knitting or electrospinning. The surface layer may be adapted to have drainage properties by forming hooks in the top layer, separately forming a layer of hydrophobic loops of fibers by weaving or knitting or electrospinning, and attaching the hydrophobic loops to the hooks by pressing them together so that they interlock and form a mechanical bond with each other. The hydrophobic loops may be made of a material selected from one of ePTFE, polyester, polyamide, polypropylene, polyethylene or other hydrophobic polymers. The hydrophobic loops may include hydrophilic or weakly hydrophobic fibers that have been treated with a coating to increase their hydrophobicity. The fibers can be selected from at least one of polyester, para-aramid, meta-aramid, fiberglass, polyamide, polypropylene, PEEK, UHMPE, steel, or carbon fibers. The coating can be selected from one of PTFE, PFA, or wax.

[0015] According to the invention there is further provided a cladding for a ship hull comprising at least one airbag comprising a material having an outer surface layer adapted to be drainage-resistant, and a plenum comprising a substantially open space pressurized with air, said material including a reinforcing fabric substantially sealed with a sealant so as to be impermeable to air and adapted to withstand pressure in said plenum, the air pressure in said plenum being greater than the hydrostatic pressure in the sea adjacent the airbag, the drainage-resistant outer surface layer being connected to the plenum via at least one restriction hole, and the pressure in the plenum being adapted to pressurize the air in the drainage layer to a level substantially equal to or greater than the adjacent hydrostatic pressure.

[0016] Thus, a marine cladding with an airbag is provided. According to one embodiment, the airbag is constructed of a fabric including an outer / top layer (distal to the hull) and a bottom / inner layer (proximal to the hull), which are connected to each other using drop stitches. The fibers of the drop stitch may be interwoven with both the top and bottom layers to provide structural strength to the airbag. The fabric may be sealed with vulcanized rubber. The outer surface may be adapted to have drainage properties. This may be achieved by forming a large number of tightly packed air pockets on the outer surface (herein referred to as the "outer surface") of the outer layer, the scale length of the air pockets may be less than twice the capillary length of water. The inlet of each air pocket may be supplied with pressurized air through a restriction, which ideally connects the air pocket to the airbag plenum. The cross-sectional area of ​​the restriction is preferably less than one hundredth, most preferably less than one thousandth, of the cross-sectional area of ​​the pocket's outlet. The inlet of each restriction may be connected to the airbag plenum. The plenum may include a space that allows unimpeded airflow therethrough so that air pressure is substantially uniform throughout the plenum. The plenum may be connected to at least one source of pressurized air. Alternatively, the outer surface may be adapted to have drainage properties by forming a fabric layer including loops of hydrophobic fibers on the outer surface, which fabric layer may be connected to the airbag plenum through restrictor holes. The air distribution system may be used to prevent bio-fouling of the cladding by occasionally passing an anti-fouling gas through the system instead of air. Suitable anti-fouling gases include ozone mixed with air in the appropriate concentration.

[0017] Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings. The viscous drag reducing properties of the present invention may be advantageously used in a variety of applications, including, but not limited to, ship and submarine hulls, torpedoes, oil and chemical pipelines.

[0018] The accompanying drawings, which are incorporated in and form a part of this disclosure, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. [Brief description of the drawings]

[0019] [Figure 1] 1 is an exploded isometric view with a cutaway section of one embodiment of a viscous drag reduction airbag suitable for use as a cladding on the surface of a ship hull in accordance with the present invention; FIG. [Diagram 2] FIG. 1 illustrates an embodiment of a reinforced fabric using drop stitches. [Diagram 3] FIG. 2 illustrates one embodiment of tightly packed pressurized air pockets suitable for placement on the outer surface of the outer layer of the airbag of the present invention. [Figure 4] FIG. 1 shows one embodiment of a restriction formed by exposing both ends of a hollow fiber. [Diagram 5] FIG. 2 illustrates one embodiment of a hydrophobic loop suitable for use on the exterior surface of the airbag of the present invention. [Figure 6] FIG. 1 illustrates a preferred embodiment of a ship's air distribution network. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] An embodiment of a cladding for a ship hull according to the present invention is shown in FIG. 1. It is useful to first explain some of the terminology used herein. An embodiment of the present invention is described below with respect to viscous drag reduction of the hull of a ship moving through water. However, the cladding of the present invention is also suitable for use with ships and objects such as submarines and missiles moving through fresh or salt water. It also applies to the field of oil, water, or other liquids being pumped along a pipeline. As used herein, the term ship is used to describe any ship, submarine, torpedo, pipeline, or any other solid object whose surface is normally subject to viscous drag due to relative movement with respect to the liquid. As used herein, the terms sea and water shall refer to the water, oil, or any other liquid where viscous drag occurs. The term ship hull shall refer to the surface of a ship, submarine, torpedo, pipeline, or any other solid object adjacent to water. Where the present invention is described with respect to a ship moving through a liquid, it should be understood that it also applies to liquids passing through fixed structures such as pipelines. The cladding surface adjacent to the water is referred to as the exterior surface. The opposite cladding surface adjacent the hull is referred to as the inner surface. References to the hydrostatic pressure of the sea apply equally to the hydrostatic pressure of fresh water, oil or other liquid adjacent the cladding.

[0021] Thus, the present invention is not limited to the reduction of viscous drag on the hull of a ship. The present invention is also directed to the reduction of viscous drag on the hull of a ship and on water-travelling or submerged objects such as submarines and torpedoes. The present invention is also directed to the reduction of viscous drag on the inner surfaces of pipes or ducts through which a liquid passes, such as in the chemical industry where a liquid such as oil is pumped along a pipeline.

[0022] The ship's hull operates in a harsh environment, and the present invention is designed to operate successfully in this environment. The ship's hull must be designed to withstand high levels of ultraviolet (UV) radiation and ozone, and large temperature fluctuations. Parts of the ship's hull are immersed in cold salt water when fully loaded, and exposed to intense sunlight when unloaded. Cladding, especially at the bow of the ship, may experience enormous impact forces from the slamming of breaking waves. The bottom of the ship may experience the abrasive action of water containing silt or sand. Wet areas of the ship are exposed to microbial growth, which, if left untreated, can lead to the growth of larger organisms such as mollusks.

[0023] The term "wet area" of a hull is typically used to describe the area of ​​the hull surface that is below the waterline. If all of the submerged area is wetted, then there is no need to distinguish between "wet area" and "submerged area". As used herein, the term "submerged area" refers to the area of ​​the hull surface that is below the waterline. The term "wet area" refers to only that portion of the "submerged area" that is wetted by water. "Non-wet area" refers to the area of ​​the hull surface below the waterline that is not wetted. As will be explained below with reference to the drawings, pocket interior areas are not included in this calculation. "Percentage non-wet area" is the ratio of "non-wet area" to "submerged area". "Percentage non-wet area" is a useful measure of the efficiency of the invention and correlates with the percentage reduction in viscous drag. When referring to cladding, it will be understood that the cladding can cover the entire submerged area of ​​the vessel hull or can be applied to only a portion of it. Although the invention is described in terms of cladding, it may usefully form part of the structural integrity of a watercraft, such as in the case of rigid inflatable boats (RIBs).

[0024] A material is said to be hydrophobic when a drop of water, at steady state, makes a contact angle of greater than 90° on the surface of the material. If the drop consists of oil, the material is said to be oleophobic. In general, a material is said to be non-wetting with any liquid when a drop of that liquid makes a contact angle of greater than 90° on the surface of the material. As used herein, the hydrophobic material property refers to the non-wetting property of a material by a liquid, regardless of what the liquid comprises. If the contact angle of water on the material is less than 90°, the material is said to be hydrophilic. The surfaces of modern ship hulls consist of steel, aluminum, fiberglass, copper, and copper-based paints, which are known to be hydrophilic.

[0025] In the present invention, the outer surface is preferably hydrophobic. This ensures that surface tension acts to resist water wetting the pressurized air pockets, as described below. Preferred hydrophobic surfaces are chlorosulfonated polyethylene (CSM), polychloroprene (CR), ethylene propylene diene monomer rubber (EPDM), thermoplastic polyurethane (TPU), perfluoroalkoxy copolymer resin (PFA), and polytetrafluoroethylene (PTFE). Many other fluoropolymers are also suitable, as are materials such as silicone rubber, fluoroelastomers, ePTFE, and wax. Both PTFE and PFA have the added advantage of being fouling resistant, which is also a desirable property in the present invention. CSM, TPU, and EPDM have the advantage that they are approved and widely used in marine environments. These hydrophobic sealants may be used in combination with each other. For example, CR may be used on the inner surface of the outer layer to provide good air tightness, and the more expensive CSM may be used on the outer surface of the outer layer to resist UV and ozone. Both EPDM and CSM are mechanically tough and have high resistance to both UV rays and ozone, making them the most preferred sealants for marine applications.

[0026] Any reference to air should be interpreted as any suitable gas and is not limited to air. Gas may also include germicidal gases such as ozone mixed in suitable concentrations with air in antifouling treatment of the hull. Such treatment may be performed from time to time as part of a fouling prevention program. If air is used, it must be dried to prevent condensation from forming on any part of the air pump system. Wet air should be dried by any suitable means as known to those skilled in the art. Air must also be filtered to ensure that no particulates that may block the restriction pass through the compressed air system. Pressurized air may be supplied from a compressor or a reservoir. In the case of a torpedo, the compressed air may be the product of a chemical reaction.

[0027] In fluid mechanics, the term "capillary length" is used to define the length scale at which the surface tension of a liquid becomes much greater than the force of gravity. It is defined as the square root of the surface tension of a liquid divided by its density multiplied by gravity. For water, this value is 2.7 mm. This indicates that a drop of water falling from air will be approximately spherical if its radius is less than 2.7 mm. It has been observed that if the dimensions of a single free-falling drop are more than doubled, it will tend to break up into smaller drops. This effect also applies to air bubbles in water. It should be noted that the capillary length of oil is less than 2 mm, which is shorter than the capillary length of water. This is mainly because the surface tension of water is considerably higher than that of oil. The present invention aims to be able to establish a substantially complete air phase within a pressurized air pocket, preventing the air from breaking up into separate bubbles, as would occur if the diameter of the pocket were significantly larger than the capillary length. To be most effective, the radius of the air pocket should be less than twice the capillary length. If the surface shape of the air pocket is not circular, radius is intended to mean the radius of an equivalent circle of the same area.

[0028] It should be noted that the drawings shown are not to scale. A ship is typically around 100m in size. The dimensions of the preferred airbag are in the range of 10m in length, 1m in width and 0.1m in thickness. The rubber coating is on the order of 1mm thick and the fibres are on the order of 0.1mm in diameter. The apertures are on the order of 0.01mm in diameter. If such features were kept to a constant scale there would be no way to represent these features, which differ in size by seven orders of magnitude, on the same drawing. The drawings are for illustrative purposes only.

[0029] The cladding of FIG. 1 includes at least one airbag. Pressurized air is supplied to the inlet of each airbag via the inlet 106, which is supplied by an air distribution system. The airbag comprises a substantially empty plenum, where the air pressure is substantially uniform during operation. The plenum is connected to the outer layer by a number of small holes, referred to herein as restrictions. The plenum presents only a small impediment to the airflow. This is a necessary condition for the proper functioning of the airbag. The small pressure fluctuations across the airbag plenum are substantially smaller than the pressure drop across the restrictions. If this were not the case, the pressure drop across the restrictions adjacent to the air inlet of the plenum would be higher than the pressure drop across these restrictions at the points furthest from the air inlet. In this situation, the airflow is only exhausted through the restrictions adjacent to the air inlet. Air does not pass through the restrictions furthest from the air inlet. Restrictions without airflow are blocked, the drainage of the outer surface is lost, and the viscous drag reduction effect of that area is lost. A tortuous air path, such as through a porous medium, causes a large pressure drop during operation and is not suitable for the present invention.

[0030] In airflow, restriction is a term used to describe a feature where the cross-sectional area of ​​the channel through which the air flows is reduced. It is instructive to compare the airflow rate through a channel plotted as a function of pressure drop for unrestricted flow compared to restricted flow. If the outlet is blocked, the outlet pressure will encounter sufficient supply pressure for both unrestricted and restricted flow. If the outlet is unblocked, the pressure drop from inlet to outlet is the same, but the flow rate for restricted flow is reduced. This reduction is highly proportional to the ratio of the cross-sectional area of ​​the restricted channel to the unrestricted channel. In the present invention, the combined cross-sectional area of ​​the outlets of the airbag can be reduced by forming a large number of restriction holes. The combined area of ​​these restriction holes must necessarily be small relative to the cross-sectional area of ​​the airbag itself. In designing the airbag of the present invention, it is useful to perform a simulation of the airflow using a software program such as ANSYS CFX or ANSYS Fluent, or a similar software package. In doing so, the power required to supply the compressed air consumed can be calculated, which can be usefully compared to the power saved by reducing the viscous drag on the vessel.

[0031] Referring to FIG. 1, the illustrated airbag is formed of a three-dimensional fabric including an upper layer 101, a lower layer 102, and drop stitches 103 connecting the upper and lower layers. The fabric layers are sealed with at least one layer of rubber 104 and 105 on the upper and lower surfaces, respectively. The edges are sealed with a rubber layer 107. The airbag is supplied with pressurized air via an inlet 106, which preferably incorporates a check valve. The airbag may be attached to the hull by a pressure-sensitive silicone adhesive layer 107. Drop stitches are woven into the upper and lower layers to support the structural load when the airbag is pressurized. The textile materials of the drop stitches and the upper and lower surfaces are selected to suit the application. If the airbag is designed to be exposed to lower pressures, the drop stitches, along with the upper and lower threads, typically comprise polyester fibers. If exposed to higher pressures, the outer surface may usefully comprise a multi-layer fabric, where one or more of the layers comprise aramid or polyamide fibers. Woven constructions are preferred because the mechanical load is distributed over all the yarns, but laminated constructions can also be used effectively. At least one layer of aramid fibers is suitable for military applications where the airbag is designed to resist cut and blast, as well as hydrostatic pressure. Hybrid yarns containing two or more fiber materials are also suitable.

[0032] For larger vessels exposed to high airbag pressures, airbag designs incorporating drop stitches are preferred. This has the desirable feature that the exterior surface of a vessel covered with such an airbag is the same as the exterior surface of the hull itself, but offset simply by the thickness of the airbag. In such a preferred design, as shown in FIG. 2, an upper surface layer 201 and a lower surface layer 202 are connected by drop stitches 203 and 204 woven into both the upper and lower layers. The two surfaces are typically parallel, but existing 3D weaving technology allows for a great deal of freedom in the design of both the upper and lower surfaces. The number of drop stitches per square meter is adapted to support the design load, typically tens of thousands per square meter. The diameter and material of the fiber used are selected similarly. Referring to FIG. 2, when the drop stitches 204 are substantially perpendicular to both the inner and outer layers, they provide a slight stiffness to the outer layer in the cross-section, i.e., the plane along the outer surface. If additional stiffness of the outer layer is required, a drop stitch is woven at an angle to the outer layer, as shown by drop stitch 203, which is tailored to provide the required stiffness to the woven reinforcement structure.

[0033] Thus, the illustrated three-dimensional woven structure provides the airbag of the present invention with the structural strength to support large structural loads, while also providing a plenum that offers virtually no resistance to airflow.

[0034] In smaller vessels, it may be acceptable for the vessel's operation to use an airbag that includes a substantially cylindrical surface. In this case, the outer surface layer is not exposed to any bending stresses, only hoop stresses. In such a case, the airbag may include only a reinforced surface layer and does not require a reinforcing drop stitch. The reinforcing fabric for a cylindrical airbag is inexpensively made by braiding or forming a sheet into a tube and joining its two edges. In the case of smaller vessels, the fabric reinforcing layer can be usefully battened and sealed onto the hull. This may be understood as an airbag whose bottom includes the hull of the vessel.

[0035] The exterior surface of each airbag is adapted to be water-repellent.

[0036] One embodiment of a drainage exterior surface is shown in FIG. 3. The exterior surface includes a number of closely packed air pockets 301. The exterior surface layer is shown as a laminate of pocket layer 302 and base layer 303, but may conveniently be formed as a single layer during the vulcanization process. Restriction holes 304 are formed in the base layer and connect each pocket to the airbag plenum. Pressurized air flows from the plenum through each restriction hole into the interior of each air pocket. The air pressure acts to keep water from entering the pocket and to expel any water that would otherwise enter the pocket. If the exterior surface of the pocket becomes blocked by water, the pressure in the pocket will rise quickly to the pressure of the plenum. This pressure is greater than the local hydrostatic pressure and clears the blockage, preventing the pocket from becoming flooded. If the pocket is cleared, the small diameter of the restriction limits the total air flow rate. Because the pocket diameter is smaller than the capillary length of the water, the pocket cannot become partially filled with water, as can happen with a large diameter pipe. Capillary forces act to ensure that each pocket is either filled with water or filled with air. If the plenum pressure is greater than the hydrostatic pressure, the pockets will remain filled with air. This is true even if the pocket surface has lost its hydrophobic properties. Thus, the surface is shown to be water repellent.

[0037] Because the viscosity of air is approximately 1000 times less than the viscosity of water, viscous drag on a pressurized air pocket filled with air is effectively eliminated. If multiple pressurized air pockets are on the submerged surface of the hull, the viscous drag on each pocket is effectively eliminated. If multiple air pockets are closely spaced, the viscous drag is greatly reduced over the area they cover. By covering substantially all of the submerged area of ​​the hull in pressurized air pockets, the overall viscous drag on the hull can be greatly reduced.

[0038] The production of compressed air requires high levels of energy consumption. The diameter of the orifices is a key determinant of an economical viscous drag reduction system. Two preferred methods for producing them are the known method of laser drilling and the known method of mechanical perforation. When producing orifices in thermoplastic polymers by mechanical perforation, it is useful to use hot needles. In FIG. 4, an embodiment of an orifice made by an alternative method is shown. The orifices are produced by securely stitching hollow fibers 401 into the top layer 402 of the airbag, and then cutting them so that one end is free in the plenum and the other end is securely located in the air evacuation layer. The fibers shown in FIG. 4 are first stitched between the top layer 402 and the bottom layer 403, forming fiber loops 404 in the top layer. A layer of adhesive is then preferably applied to the top layer to bond the fibers in place and seal the puncture holes in the rubber reinforcement fabric. The fiber loops 404 on the top surface are cut, exposing the inner hollow core. The fiber loops 405 on the bottom surface are then cut. The upper and lower layers are then separated by pulling them apart, and the ends of the cut fibers are pulled through the lower airbag surface, leaving them hanging freely within the airbag plenum. In this way, the hollow fibers are securely held in a position with one end exposed within the airbag plenum, and the other end held against the outer surface which, in operation, will be the drainage layer. The drawn fibers are preferably hydrophobic or treated to be hydrophobic. The hollow fibers are preferably of a material such as polyester, polyamide, polypropylene, polyethylene or other suitable polymers. Coated glass fibers are also suitable. Alternatively, this process can be achieved by weaving the fibers into the fabric during the weaving process, rather than stitching the fibers into a coated fabric. The coating process can be adapted accordingly.

[0039] In Fig. 5a, a further embodiment of the drainage layer is shown. The outer surface includes a network of hydrophobic loops 502. These are woven into the top layer of the airbag. Restrictions 503 connect the airbag plenum to the voids in the drainage layer. The drainage layer is adapted to correspond to form a large smooth surface at the interface with the water. The loop stiffness and number of loops per square millimeter are adapted so that the loop height above the base layer 501 is adapted to accommodate slight changes in the differential pressure between the air pressure in the drainage layer and the surrounding hydrostatic pressure. The loops may be usefully cut to form a single strand of fiber. Alternatively, the fiber may be stitched to the top surface of the fabric and its exposed end cut. Such a process is a known method in the textile industry. As used herein, the term "loop" is intended to include closed loops, cut loops and cut stitches. The loops 502 are then coated by known means, such as foam coating, to increase their hydrophobicity. Restriction holes 503 are formed in the base layer by the methods described above.

[0040] Known mechanical attachment systems of hooks and loops are also suitable and are illustrated in Figure 5b. The mushroom head hook layer 510 includes a number of mushroom-shaped projections 511 and a number of apertures 512. The mushroom head hook layer 510 is adhesively bonded to the base layer of the top layer. In an alternative embodiment, the loop layer is first woven into the top layer fabric. The loops are then cut to form the hooks and stems.

[0041] Another layer of loops 513 is then manufactured by known means such as knitting, weaving, or electrospinning. The loop layer is then mechanically attached to the hook layer by pressing it against the hook layer to form the drainage layer. The advantage of manufacturing the loop layer separately is that if any portion of the loop layer becomes fouled, it can be easily replaced in the drydock with minimal expense.

[0042] The preferred fibers for the loop material are ePTFE, as well as polyester, polyethylene, polypropylene, polyamide and aramid. Except in the case of ePTFE, it is preferred to apply a hydrophobic coating of PFA or PTFE.

[0043] As the wave crest increases the hydrostatic pressure, the drainage layer is compressed, increasing the air pressure in the void. The compressed air flows along the hull surface from high pressure areas to low pressure areas along the fiber layer. This air flow is continuous through both the wave crest and the wave trough. The hydrostatic pressure encountered by the vessel is greatest at the deepest part of the hull, shown as region 606 in FIG. 6, and drops toward the surface and toward the aft portion of the vessel, shown as region 607 in FIG. 6, when leaving port. The air flow in the drainage layer flows accordingly. As the hydrostatic pressure decreases, the corresponding drainage layer expands, increasing the air flow through it. The air pressure in the airbag plenum is adapted to be higher than the surrounding hydrostatic pressure throughout this cycle.

[0044] When retrofitting an existing vessel, it may not be possible or desirable to accommodate a pipe network internally throughout the entire submerged portion of the vessel. As shown in Figure 6, an additional framework is attached to the exterior of the vessel 601. The framework is connected to a pressurized air source at connection 604. The framework consists of a number of main channels 602 and several sub-channels 603 connected to each other.

[0045] The reinforcing fabric and rubber sealant of the present invention are known materials, and the 3D weaving and hydrophobic coating processes described above are known processes. They are used in the manufacture of consumer items such as high pressure lifting bags, and stand up paddle boards, and industrial items such as components for rigid inflatable boats (RIBs), inflatable life rafts, etc. The reinforcing fabric and rubber sealant are used in heavy duty items such as skirts for hovercraft, and inflatable fenders for pilot boats have been proven to withstand the marine environment for many years. This is a key advantage of the present invention that allows for rapid market adoption.

[0046] Knitted fabrics are less expensive and can be successfully used in place of woven fabrics in applications where the mechanical forces to be endured are less severe. Warp knitted fabrics are more flexible and open than woven fabrics. Weft knitted fabrics tend to be more open and flexible. In applications where the mechanical forces encountered by the fabric are low and stable or simply supported as hoop stresses, rubber sheets without any fabric reinforcement can be used. For applications where higher forces are encountered and where higher stiffness and dimensional stability are required, woven fabrics are preferred. Interlaced fibers impart higher strength and stiffness to the fabric. For further improvement of strength and stiffness, multiple layers of interlaced fibers are used.

[0047] The airbags of the present invention are preferably electrically insulated to greatly reduce anodic corrosion, which is a significant expense for metal vessels. A further advantage is that the airbag cladding reduces the transmission of noise from both the engines and propellers, and from the vessel's hull, into the ocean, where it is understood that noise from the shipping industry can disrupt marine life and inhibit fish reproduction. The airbag cladding reduces the power requirements from the engines and propellers, and attenuates noise from the vessel's hull. Those skilled in the art will recognize that the cladding provides protection against explosive attack, as well as protection from ground damage in shallow waters.

[0048] Pressure fluctuations occur due to sea conditions. As waves pass, the sea pressure at any given point on the wetted area of ​​the vessel increases. For example, a 10m wave will cause a pressure increase of 1 bar. The air pressure in the airbag is designed to always be higher than the sea pressure, even when exposed to the increasing load of a passing wave. By use of pressure sensors or check valves, the air supply pressure and flow rate, as well as the elasticity of the airbag, are easily adapted to achieve this. Safety pressure relief and drain valves are also used to minimize the risk of damage in the event of equipment failure. The air distribution network disclosed above provides controllable pressure and flow rate to the low profile cladding system over the submerged areas of the vessel's hull. This makes it highly suitable as a marine cladding system.

[0049] While the present invention allows for passive control through optimal design of channels, chambers and orifices, it is believed that known measurement devices such as pressure and flow sensors as well as regulators, control valves, etc. can be used to optimize the operation of the present invention, especially in larger installations.

[0050] Viscous drag accounts for a large portion of the power required to propel most current ships. Reducing viscous drag reduces the power required for a ship to an extent that facilitates the adoption of other environmentally friendly propulsion technologies to provide the remaining power requirements. The present invention facilitates the adoption of electric, sail and solar power to eliminate emissions of CO2 and other polluting gases in the transportation industry.

[0051] Thus, the present invention has been shown to provide a means for reducing the wetted area of ​​a vessel hull, thereby reducing the viscous drag encountered. While specific embodiments of the invention have been illustrated, they are not intended to be limiting. Modifications and variations will be apparent to those skilled in the art, and it is intended that the invention be limited only by the scope of the claims.

Claims

1. A ship hull cladding comprising: at least one airbag having a plenum including an open space which is pressurized with air in use; The airbag is formed from a material including a reinforcing fabric sealed with a sealant; The airbag further comprises an exterior layer adapted to have drainage properties, the exterior layer being connected to the plenum via at least one restriction hole in the airbag through which air from the plenum can pass, thereby providing pressurized air to the drainage layer having a pressure level substantially equal to or greater than the adjacent hydrostatic pressure.

2. The cladding of claim 1 , wherein the airbag is sealed with vulcanized rubber.

3. The cladding of claim 2 , wherein the vulcanized rubber is selected from at least one of CSM, CR, EPDM, polyurethane, or silicone.

4. The cladding of claim 1 , wherein the airbag is sealed with a thermoplastic polymer.

5. The cladding of claim 4 , wherein the thermoplastic polymer is selected from at least one of a thermoplastic CSM or a thermoplastic polyurethane.

6. The cladding of claim 1 , wherein the reinforcing fabric comprises a woven fabric.

7. The cladding of claim 6 , wherein the airbag comprises at least two layers of reinforcing fabric and includes a drop stitch between the at least two layers of reinforcing fabric.

8. The cladding of any one of claims 1 to 7, wherein the reinforcing fabric comprises fibers selected from at least one of polyester, para-aramid, meta-aramid, fiberglass, polyamide, polypropylene, PEEK, UHMPE, steel, or carbon fibers.

9. The cladding of any one of claims 1 to 8, wherein the material comprises a laminate including at least one woven fabric reinforcement layer.

10. The cladding according to any one of claims 1 to 9, wherein the at least one aperture hole is formed by mechanical drilling.

11. The cladding of any one of claims 1 to 9, wherein the at least one aperture hole is formed by laser drilling.

12. 10. The cladding of claim 1, wherein the at least one restriction hole is formed by feeding at least one hollow fiber through the material forming the airbag such that one end of each hollow fiber terminates within the plenum and the other end of each hollow fiber terminates within the outer surface layer.

13. The cladding of any one of claims 1 to 12, wherein the outer surface layer includes means for forming tightly packed air pockets on the outer surface layer, each air pocket having a radius less than twice the capillary length of water, and each air pocket being connected to the plenum by at least one restriction hole.

14. A cladding according to any one of claims 1 to 12, wherein the outer surface layer comprises loops of hydrophobic fibres woven into a reinforcing fabric of material from which the airbag is formed.

15. The cladding of any one of claims 1 to 12, wherein the outer surface layer comprises a layer of loops of hydrophobic fibre attached to the outside of the reinforcing fabric by adhesive bonding.

16. The cladding of claim 15 , wherein the layer of loops of hydrophobic fibers is formed by one of weaving, knitting, or electrospinning.

17. 13. The cladding of any one of claims 1 to 12, wherein the outer surface layer is adapted to have drainage properties by forming hooks in the outer surface layer and attaching the hydrophobic loops to the hooks by separately forming a layer of hydrophobic loops of fibres by weaving or knitting or electrospinning and pressing them together so that they interlock and form a mechanical bond with each other.

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