Air lubrication of a hull of a vessel

Abstract

An air lubrication system (100) for a hull of a vessel comprises: a plurality of fluid outlets (22, 24) for mounting on the hull; at least one water inlet (32, 33); at least one ambient air inlet (52); and a controller (80). The air lubrication system (100) is configured to deliver an output fluid from the plurality of fluid outlets (22, 24), wherein the output fluid is an air and water mixture having an air to water ratio. The controller (80) is configured to control the air to water ratio of the output fluid delivered by the plurality of fluid outlets. The controller comprises an operating state in which the air to water ratio of the output fluid is non-uniform across the plurality of fluid outlets (22, 24).

Description

[0001] AIR LUBRICATION OF A HULL OF A VESSEL

[0002] BACKGROUND

[0003] A hull of a vessel experiences shear viscous drag forces as the hull is propelled through water. It is known to reduce such drag forces by interposing air bubbles between the exterior of the hull and the body of water that the vessel is moving through. Reducing drag forces on marine vessels can provide a reduction in energy requirements for propulsion of the vessels.

[0004] WO 2022 / 229054 A1 describes a system for reducing drag on a marine vessel. This system comprises a fluid ejector. At least one seawater inlet supplies seawater to the ejector. An ambient air inlet port supplies ambient air to the ejector. Ambient air is entrained into seawater flowing through the ejector. A mix of water and air bubbles flows from an outlet of the ejector.

[0005] SUMMARY OF THE INVENTION

[0006] There is provided an air lubrication system for a hull of a vessel comprising: a plurality of fluid outlets for mounting on the hull; at least one water inlet; at least one ambient air inlet; and a controller, wherein the air lubrication system is configured to deliver an output fluid from the plurality of fluid outlets, wherein the output fluid comprises an air and water mixture having an air to water ratio; and the controller is configured to control the air to water ratio of the output fluid delivered by the plurality of fluid outlets, the controller comprising an operating state in which the air to water ratio of the output fluid is non-uniform across the plurality of fluid outlets.

[0007] The operating state in which the air to water ratio of the output fluid is non-uniform across the plurality of fluid outlets may help to contain air bubbles adjacent the hull and reduce migration of air bubbles away from the hull.

[0008] The operating state in which the air to water ratio of the output fluid is non-uniform across the plurality of fluid outlets may be used as a course-keeping aid by providing different drag forces on different sides of the hull, such as a higher drag force on the port side compared to the starboard side to steer the vessel towards the port side, or a higher drag force on the starboard side compared to the port side to steer the vessel towards the starboard side.

[0009] Optionally, the controller is configured to cause the air lubrication system to: output fluid with a first air to water ratio from a first subset of the fluid outlets; and output fluid with a second air to water ratio from a second subset of the fluid outlets positioned laterally outwardly of the first subset of the outlets with respect to a direction of movement, wherein the second air to water ratio has a lower proportion of air compared to the first air to water ratio, or is only water.

[0010] This feature has the advantage that it may help to contain air bubbles adjacent the hull and reduce migration of air bubbles away from the hull.

[0011] Optionally, the controller is configured to cause the air lubrication system to: output fluid with a first air to water ratio from a first subset of the fluid outlets; and output fluid with a second air to water ratio from a second subset of the fluid outlets positioned laterally outwardly of the first subset of the outlets with respect to a direction of movement, wherein the second air to water ratio has a lower proportion of air compared to the first air to water ratio.

[0012] In some examples, the proportion of air in the second air to water ratio may be substantially zero.

[0013] That is, substantially no air from the at least one air inlet has been entrained in the water.

[0014] Optionally, the controller is configured to cause the air lubrication system to output fluid with a third air to water ratio from a third subset of fluid outlets positioned between the first subset of the fluid outlets and the second subset of the fluid outlets, wherein the third air to water ratio has a lower proportion of air compared to the first air to water ratio and a higher proportion of air compared to the second air to water ratio.

[0015] Optionally, the first subset of fluid outlets is positioned centrally on the hull, and the second subset of fluid outlets comprises at least one outlet on each lateral side of the first subset of fluid outlets.

[0016] Optionally, the air lubrication system comprises a plurality of fluid outlets distributed on sidewalls of the hull; and the controller is configured to cause the air lubrication system to: output fluid with a first sidewall air to water ratio from a first sidewall subset of the fluid outlets positioned lowermost on the sidewalls; and output fluid with a second sidewall air to water ratio from a second sidewall subset of the fluid outlets positioned above the first subset, wherein the second sidewall air to water ratio has a lower proportion of air compared to the first sidewall ratio.

[0017] In some examples, the proportion of air in the second sidewall air to water ratio may be substantially zero. That is, substantially no air from the at least one air inlet has been entrained in the water.

[0018] Optionally, the controller is configured to cause the air lubrication system to: output fluid with a first ratio of air and water from a first subset of the outlets positioned on a first side of the hull; and output fluid with a second ratio of air and water from a second subset of the outlets positioned on a second side of the hull, wherein the second ratio has a lower proportion of air compared to the first ratio, or is only water.

[0019] This feature has the advantage that it allows different drag forces on different sides of the hull. This feature may be used as a course-keeping aid.

[0020] Optionally, the controller is configured to control the output ratio of air and water according to at least one input indicative of: movement of the vessel; speed of the vessel; draft of the vessel; a requirement to course-keep the vessel.

[0021] Optionally, the controller is configured to store data indicative of the at least one input and corresponding outputs to control the air and water ratio at the plurality of fluid outlets.

[0022] Optionally, the controller comprises an operating state in which the air to water ratio of the output fluid is uniform across the plurality of fluid outlets and the controller is configured to select between: the operating state in which the air to water ratio of the output fluid is non-uniform across the plurality of fluid outlets; and the operating state in which the air to water ratio of the output fluid is uniform across the plurality of fluid outlets.

[0023] Optionally, the controller is configured to learn values of control signals to control the air to water ratio of the output fluid delivered by the plurality of fluid outlets.

[0024] Optionally, the system is configured to deliver an output fluid from at least one of the plurality of fluid outlets with an air and water mixture having an air to water ratio with less than 10% air, optionally less than 5% air. In some examples, the system is configured to deliver an output fluid from at least one of the plurality of fluid outlets which is only water.

[0025] Optionally, the air lubrication system comprises: an ejector having a first ejector inlet, a second ejector inlet and an ejector outlet, wherein the first ejector inlet is in fluid communication with the at least one water inlet, the second ejector inlet is in fluid communication with the at least one ambient air inlet and the ejector outlet is in fluid communication with at least one of the fluid outlets; a water supply section to supply water to the first ejector inlet; an air supply section to supply air to the second ejector inlet; and the controller is configured to control the water supply section and the air supply section to achieve a required air to water ratio.

[0026] Optionally, the water supply section comprises a controllable valve in a water flow path to the first ejector inlet and the controller is configured to control the controllable valve to achieve the required air to water ratio.

[0027] Optionally, the air supply section comprises a controllable valve in an air flow path to the second ejector inlet and the controller is configured to control the controllable valve to achieve the required air to water ratio.

[0028] Optionally, the water supply section comprises a water pump to deliver a flow of water to the first ejector inlet and the controller is configured to control the water pump to achieve the required air to water ratio.

[0029] Optionally, the water supply section comprises a first water supply path to deliver a passive flow of water to the first ejector inlet and a second water supply path to deliver an active flow of water to the first ejector inlet, wherein the water pump is in the second water supply path. Optionally, the air supply section comprises an air blower to selectively supply air to the second ejector inlet and the controller is configured to control the air blower to achieve the required air to water ratio.

[0030] Optionally, the air lubrication system is configured to deliver an output fluid with a propulsive force from at least one of the plurality of fluid outlets.

[0031] There is also provided a vessel comprising a hull and the air lubrication system as described or claimed.

[0032] There is also provided a controller for an air lubrication system for a hull of a vessel, wherein the air lubrication system comprises a plurality of fluid outlets for mounting on the hull, at least one water inlet and at least one ambient air inlet, the air lubrication system configured to deliver an output fluid from the plurality of fluid outlets, wherein the output fluid comprises an air and water mixture having an air to water ratio, wherein the controller is configured to: control the air to water ratio of the output fluid delivered by the plurality of fluid outlets with an operating state in which the air to water ratio of the output fluid is non-uniform across the plurality of fluid outlets.

[0033] Optionally, the controller is configured to perform any of the functionality which is described or claimed.

[0034] There is also provided a method of controlling an air lubrication system for a hull of a vessel, wherein the air lubrication system comprises a plurality of fluid outlets for mounting on the hull, at least one water inlet and at least one ambient air inlet, the air lubrication system configured to deliver an output fluid from the plurality of fluid outlets, wherein the output fluid comprises an air and water mixture having an air to water ratio, the method comprising: controlling the air to water ratio of the output fluid delivered by the plurality of fluid outlets with an operating state in which the air to water ratio of the output fluid is non-uniform across the plurality of fluid outlets.

[0035] Optionally, the method comprises causing the air lubrication system to: output fluid with a first air to water ratio from a first subset of the fluid outlets; and output fluid with a second air to water ratio from a second subset of the fluid outlets positioned laterally outwardly of the first subset of the outlets with respect to a direction of movement, wherein the second air to water ratio has a lower proportion of air compared to the first air to water ratio.

[0036] Optionally, the method comprises causing the air lubrication system to output fluid with a third air to water ratio from a third subset of fluid outlets positioned between the first subset of the fluid outlets and the second subset of the fluid outlets, wherein the third air to water ratio has a lower proportion of air compared to the first air to water ratio and a higher proportion of air compared to the second air to water ratio.

[0037] Optionally, the air lubrication system comprises a plurality of fluid outlets distributed on sidewalls of the hull; and the method comprises causing the air lubrication system to: output fluid with a first sidewall air to water ratio from a first sidewall subset of the fluid outlets positioned lowermost on the sidewalls; and output fluid with a second sidewall air to water ratio from a second sidewall subset of the fluid outlets positioned above the first subset, wherein the second sidewall air to water ratio has a lower proportion of air compared to the first sidewall ratio.

[0038] Optionally, the method comprises causing the air lubrication system to: output fluid with a first ratio of air and water from a first subset of the outlets positioned on a first side of the hull; and output fluid with a second ratio of air and water from a second subset of the outlets positioned on a second side of the hull, wherein the second ratio has a lower proportion of air compared to the first ratio, or is only water.

[0039] Optionally, the method comprises controlling the output ratio of air and water according to at least one input indicative of: movement of the vessel; speed of the vessel; draft of the vessel; a requirement to course-keep the vessel.

[0040] Optionally, the method comprises storing data indicative of the at least one input and corresponding outputs to control the air and water ratio at the plurality of fluid outlets.

[0041] Optionally, the method comprises selecting between: the operating state in which the air to water ratio of the output fluid is non-uniform across the plurality of fluid outlets; and an operating state in which the air to water ratio of the output fluid is uniform across the plurality of fluid outlets.

[0042] Optionally, the method comprises learning values of control signals to control the air to water ratio of the output fluid delivered by the plurality of fluid outlets.

[0043] There is also provided computer readable instructions that, when executed by a processor, cause the processor to perform the method as described or claimed.

[0044] The functionality described in this document can be implemented in hardware, software executed by a processing apparatus, or by a combination of hardware and software. The processing apparatus can comprise a computer, a processor, a state machine, a logic array or any other suitable processing apparatus. The processing apparatus may be a general- purpose processor which executes software to cause the general-purpose processor to perform the required tasks, or a plurality of general-purpose processors which collectively execute software to cause the processors to perform the required tasks. Alternatively, the processing apparatus can be dedicated to perform the required functions. Another aspect of the invention provides machine-readable instructions (software) which, when executed by a processor, perform any of the described methods. The machine-readable instructions may be stored on an electronic memory device, hard disk, optical disk or other machine-readable storage medium. The machine-readable medium can be a non-transitory machine-readable medium. The term “non-transitory machine-readable medium” comprises all machine- readable media except for a transitory, propagating signal. The machine-readable instructions can be downloaded to the storage medium via a network connection.

[0045] Within the scope of this application it is envisaged that the various aspects, embodiments, examples and alternatives, and in particular the individual features thereof, set out in the preceding paragraphs, in the claims and / or in the following description and drawings, may be taken independently or in any combination. For example, features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible.

[0046] BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying figures in which:

[0047] Figure 1 shows an example of a vessel with an air lubrication system;

[0048] Figure 2 shows an example of an operating state of the air lubrication system;

[0049] Figure 3 shows another example of an operating state of the air lubrication system; Figure 4 shows another example of an operating state of the air lubrication system; Figure 5 shows an ejector used in one implementation of the air lubrication system; Figure 6 shows part of the air lubrication system;

[0050] Figure 7 shows a controller for the air lubrication system;

[0051] Figure 8 shows a method performed by the controller;

[0052] Figure 9 shows a method performed by the controller;

[0053] Figure 10 shows an example of a computer system for implementing the controller.

[0054] DETAILED DESCRIPTION

[0055] Figure 1 schematically shows an example of a marine vessel 10 with a hull 12 and an air lubrication system 100. In this example the hull 12 has a flat bottom surface 16 and vertical sidewall surfaces 18. In other examples, the hull can have a different shape. The marine vessel 10 has a bow (front) 13 and a stern (rear) 14. The normal forward direction of vessel is shown by arrow 11. The hull 12 has a centreline 15. A part 2 of the hull 12 to the left of the centreline 15 will be called the port side of the hull. A part 3 of the hull 12 to the right of the centreline 15 will be called the starboard side of the hull.

[0056] An air lubrication system 100 for the vessel comprises a plurality of fluid outlets 22 positioned on the bottom surface 16 of the hull 12. The fluid outlets 22 are distributed (i.e. spaced apart) laterally across the width of the bottom surface 16 of the hull. In this example, the air lubrication system 100 also comprises a plurality of fluid outlets 24 positioned on the sidewall surfaces 18 of the hull 12. A set of fluid outlets are positioned on the left (port) and right (starboard) sidewall surfaces. The fluid outlets 24 are distributed (i.e. spaced apart) on the sidewall surface 18 of the hull. In some examples, the air lubrication system 100 may only comprise the plurality of fluid outlets 22 positioned on the bottom surface 16 of the hull 12. The number of fluid outlets 22, 24 is illustrative. The number of fluid outlets 22, 24 can be any suitable number.

[0057] Figure 1 (C) shows a cross-sectional view of the hull 12. Some terms of reference will be useful. The vessel has an unladen waterline 6 and a laden waterline 8. The unladen waterline 6 represents the position of the hull in the water when the vessel is unladen, or only laden with ballast. The laden waterline 8 represents the position of the hull in the water when the vessel is fully laden. The flat region 16 of the bottom surface of the hull is called flat of bottom. The sidewall surface 18 of the hull 12 comprises: a region 19 below the unladen waterline called vertical sides; a region 20 between the unladen waterline and the laden waterline called boot tops; and a region 21 above the laden waterline called topsides.

[0058] The fluid outlets 22, 24 are positioned on the portion of the hull which is normally submerged. In the example shown in Figure 1, the fluid outlets 24 are positioned in the vertical sides region 19 of the sidewall of the hull 12, below the unladen waterline 6. In this example, the fluid outlets 24 are offset from each other in the direction of the longitudinal (bow-stern) axis of the hull. Figure 1 shows a set of three fluid outlets 24. A first (highest) fluid outlet 24 is positioned nearest the bow 13. Other ones of the fluid outlets 24 are progressively offset from the first fluid outlet at increasing distances from the first fluid outlet. In other examples, the fluid outlets 24 can have a different configuration, such as all fluid outlets 24 aligned vertically.

[0059] In the example shown in Figure 1, the fluid outlets 22 are positioned in a general “V” shape on the bottom of the hull. Figure 1 shows a set of eight fluid outlets 22. A first pair of fluid outlets 22 is positioned about the centreline 15 of the hull, nearest the bow 13. Other fluid outlets 22, or pairs of fluid outlets 22, are positioned at increasing lateral distances outwardly from the centreline. In the example shown in Figure 1, the other fluid outlets 22, or pairs of fluid outlets 22, are positioned at increasing distances rearwardly from the first pair of fluid outlets 22. In the example shown in Figure 1 , the fluid outlets 22 are not positioned in the curved turn of bilge regions 17 on each side of the flat of bottom surface 16. In other examples, the fluid outlets 22 can have a different configuration.

[0060] The fluid outlets 22, 24 form part of an air lubrication system 100 for reducing drag on the hull. The air lubrication system is configured to deliver an output fluid from the plurality of fluid outlets. The output fluid is an air and water mixture. The air component of the air and water mixture is in the form of air bubbles. At least some of the air bubbles may be microbubbles or nanobubbles. In use, the air and water mixture is delivered, or injected, from the fluid outlets 22, 24 into a boundary layer which surrounds the hull of the vessel. The air and water mixture has an air to water ratio. The air to water ratio may be defined as a volumetric ratio. That is, the air to water ratio is ratio of a volume of air (Vair) to a volume of water (Vwater). The fluid outlets 22, 24 emit an air and water mixture which flows rearwardly along the bottom surface and sidewall surface of the hull 12. The term “void fraction” will be used to indicate the proportion of air in an air and water fluid mix. The void fraction increases with an increase in the proportion of air in the air and fluid mix. Stated another way, the void fraction increases with an increase in the air to water ratio.

[0061] A controller is configured to control the air to water ratio of the output fluid delivered by the plurality of fluid outlets 22, 24. In one operating state, the air to water ratio of the output fluid is the same (i.e. uniform) at each of the plurality of fluid outlets 22, 24. This operating state may be useful in calm conditions, where the vessel has a relatively small amount of roll.

[0062] Figure 2 shows an example of an operating state in which the air to water ratio of the output fluid is uniform across the plurality of fluid outlets 22, 24. This operating state may be used in calm conditions, such as sea conditions in which movement of the hull is less than threshold values of pitch and / or roll. The air to water ratio of the output fluid may be varied uniformly across the plurality of fluid outlets 22, or 22 and 24, to provide an air to water ratio with a higher ratio of air to water. For example, the air to water ratio of the output fluid may be increased for any operating conditions where the drag reduction benefit is beneficial. For example, the air to water ratio of the output fluid may be increased with speed of the vessel, or with movement of the vessel, or with any other operating conditions.

[0063] In another operating state, the air to water ratio of the output fluid is non-uniform across the plurality of fluid outlets 22, 24. Stated another way, the void fraction of the output fluid is non-uniform across the plurality of fluid outlets 22, 24. The controller is configured to control the air to water ratio of the output fluid delivered by each of, or by a subset of, the plurality of fluid outlets 22, 24.

[0064] Figure 3 shows an example of an operating state in which the air to water ratio of the output fluid is non-uniform across the plurality of fluid outlets 22, 24. Fluid outlets 22 positioned nearest the centreline 15 of the hull emit a fluid with a higher proportion of air (= higher void fraction) than fluid outlets positioned laterally outwardly of the centreline 15. The air to water ratio can be varied across the fluid outlets 22, with a highest proportion of air (highest void fraction) nearest the centreline 15 and a lowest proportion of air (lowest void fraction) at the laterally outermost fluid outlets 22. Reducing the proportion of air at the laterally outermost fluid outlets 22 has an effect of containing the fluid with a higher void fraction adjacent the bottom surface of the hull. The fluid with the lower proportion of air (= lower void fraction) at the laterally outermost fluid outlets 22 acts as a curtain to contain the fluid mixture with the higher proportion of air (= higher void fraction). This can help to reduce the migration of air bubbles from adjacent the bottom surface of the hull and thereby improve the amount of drag reduction. A secondary benefit to delivering fluid with the highest void fraction from the fluid outlets 22 nearest the centerline 15 of the hull is that the air bubbles produced on the centerline work ‘with’ the roll of the vessel in a sea way to ensure a more evenly distributed bubble carpet.

[0065] Some example values for the air to water ratios, measured within the vessel, are 1:2 (i.e. 1 part air to two parts water; void fraction = 33%) and 1:3 (i.e. 1 part air to three parts water; void fraction = 25%). Other possible values of the air-to-water ratio, measured within the vessel, for fluid outlets delivering the highest proportion of air are: 1:1 (void fraction = 50%); and A:B, where A>B, (void fraction greater than 50%). Other possible values of the air-to- water ratio (measured within the vessel) for fluid outlets delivering the lowest proportion of air are: 1:4 (i.e. 1 part air to four parts water; void fraction = 20%); and 1:5 (i.e. 1 part air to five parts water; void fraction = 17%). At an extreme, fluid outlets may deliver only water (void fraction substantially 0%), or a minimal proportion of air in the air and water mix, such as less than 10% air, or less than 5% air.

[0066] Figure 3 shows fluid outlets 24 on the sidewall surfaces of the hull 12. Fluid outlets 24 positioned uppermost on the sidewall surfaces emit an air to water ratio which has a lower proportion of air (= lower void fraction) than fluid outlets positioned beneath. The air to water ratio can be varied across the fluid outlets 24, with a lowest proportion of air (= lowest void fraction, optionally a substantially 0% void fraction) at the uppermost fluid outlets 24 and a highest proportion of air (= highest void fraction) at the lowest fluid outlets 24. Reducing the proportion of air at the uppermost fluid outlets 24 has an effect of containing the fluid with the higher proportion of air (= higher void fraction). The fluid with the lower proportion of air (= lower void fraction) at the uppermost fluid outlets 24 acts as a curtain to contain the fluid mixture with the higher proportion of air (= higher void fraction). This can reduce the migration of air bubbles from the sidewall surfaces of the hull and thereby improve the amount of drag reduction.

[0067] Figure 4 shows another example of an operating state in which the air to water ratio of the output fluid is non-uniform across the plurality of fluid outlets 22, 24. Firstly, the fluid outlets 22 on the bottom surface of the hull are considered. Fluid outlets 22 positioned on one side of the hull emit an air to water ratio which has a higher proportion of air than fluid outlets positioned on the opposite side of the hull. The sides of the hull are the regions of the bottom surface of the hull to each side of the centreline 15. There is a port side 2 of the hull and a starboard side 3 of the hull.

[0068] In this example, the fluid outlets 22 positioned on the port (left) side 2 of the hull emit an air to water ratio which has a higher proportion of air (= higher void fraction) than fluid outlets positioned on the starboard (right) side 3 of the hull. The fluid with the higher proportion of air (= higher void fraction) on the port-side 2 of the hull causes a lower drag on the port side of the hull compared to the drag on the starboard side 3 of the hull. This causes the vessel to move towards the starboard side. This operating state can be used as a steering or course-keeping aid. The steering / course-keeping effect of the air lubrication system can be used in addition to any normal steering / course-keeping action of the vessel, such as by rudder control. An advantage of using the air lubrication system to perform steering / course- keeping is reduced drag compared to a rudder. A rudder, when positioned off-centre, has a significant amount of appendage drag.

[0069] The air to water ratio across the fluid outlets 22 on the port side 2 of the vessel may be uniform. Alternatively, the profile of the air to water ratio can be varied across the fluid outlets 22 on the port side 2 of the vessel. For example, the air to water ratio can have a higher value (= higher void fraction) at the extreme port side and a lower air to water ratio (= lower void fraction) nearest the centreline 15.

[0070] Figure 4 shows an operating state in which the air to water ratio of the output fluid is non- uniform across the plurality of fluid outlets 22 on the bottom surface of the hull. When the vessel is fitted with sidewall fluid outlets 24, the air to water ratio at the sidewall fluid outlets 24 may be the same on the sidewall surface of the port side 2 and on the sidewall surface of the starboard side 3 of the vessel. Alternatively, the air to water ratio on the sidewall surface of the port side 2 may be different to the air to water ratio on the sidewall surface of the starboard side 3 of the vessel. In the example of Figure 4, the air to water ratio at the fluid outlets 24 on the sidewall surface of the port side 2 is higher (i.e. more air) than the air to water ratio at the fluid outlets 24 on the sidewall surface of the starboard side 3 of the vessel.

[0071] The fluid outlets 22, 24 of the air lubrication system deliver an air and water mixture along the hull. The air lubrication system may comprise ejector devices. Figure 5 shows an example of an ejector device 60 which can be used in the air lubrication system. Other terms for an ejector include an eductor or a venturi tube. The ejector 60 comprises a first fluid inlet 61, a second fluid inlet 62 and a fluid outlet 63.

[0072] In use, water 64 flows through the first fluid inlet 61 and through the body of the ejector 60. The flow of water through the body of the ejector 60 causes air 65 to be drawn into the second fluid inlet 62. An air and water mixture 66 flows out of the fluid outlet 63 of the ejector. The converging inlet section of each ejector 60 may be designed (proportioned) to maintain a pressure drop resulting in a net suction effect. The air is entrained as bubbles of air in the flow of water. Optionally, the pressure drop is arranged to be a pressure drop below atmospheric pressure in use.

[0073] The water 64 flowing into the first fluid inlet 61 may be a passive flow of water. The term “passive” means water which passively flows to the fluid inlet 61 of the ejector 60 without assistance, such as by pumping. For example, the forward motion of the vessel through water will passively deliver a flow of water to the first fluid inlet 61. This passive flow of water will vary according to forward speed of the vessel.

[0074] Alternatively, the water 64 flowing into the first fluid inlet 61 may be an active flow of water. The term “active” means water which is assisted by pumping (e.g. via an electrically- powered pump) to reach the first fluid inlet 61.

[0075] A passive flow of water is desirable as it reduces the power requirement of the system. However, under some operating conditions of the vessel, a higher flow of water may be needed than is provided by the passive flow.

[0076] The air 65 flowing into the second fluid inlet 62 may be a passive flow of air. The term “passive” means air which has not been forced (e.g. by a blower or compressor) to reach the second fluid inlet 62. Alternatively, the air 65 flowing into the second fluid inlet 62 may be an active flow of air. The term “active” means air which is forced (e.g. via an electrically- powered fan or blower) to reach the second fluid inlet 62.

[0077] The ratio of air to water in the output fluid 66 from the ejector 60 determines a void fraction in the output fluid. The void fraction is the fraction (percentage) of voids in the fluid. Figure 6 shows a more detailed example of part of an air lubrication system 100. One ejector 60 and one fluid outlet 22, 24 is shown. The system comprises a water supply section 30 and an air supply section 50. The water supply section 30 provides a supply of water to the first fluid inlet 61 of the ejector 60. The water supply section 30 can provide a passive flow of water or an active flow of water. The air supply section 50 provides a supply of air to the second fluid inlet 62 of the ejector 60. The air supply section 50 can provide a flow of air which is a passive flow of air or an active flow of air.

[0078] The water supply section 30 comprises water inlets 32, 33. The water inlets 32, 33 are provided on the hull 12 of the vessel. A valve 36 is provided upstream of the first fluid inlet 61 of the ejector 60. The valve 36 is controlled, via a control signal 81, to vary a flow rate of water to the first fluid inlet 61 of the ejector 60. Flow rate is measured by a flowmeter 72, with a signal 89 feeding back to the controller 80. The water supply section 30 comprises two main water supply paths W1, W2. A first water supply path W1 provides a flow path for a passive flow of water from water inlet 32 towards the first fluid inlet 61 of the ejector 60. A second water supply path W2 provides a flow path for an active flow of water from water inlet 33 towards the first fluid inlet 61 of the ejector 60. Valve 34 is controlled, via a control signal 84, to allow water to flow along the first water supply path W1. When valve 34 is opened, water is permitted to flow along the first water supply path W1. As the vessel moves forward through water, water flows into the water inlet 32 towards valve 36 and the first fluid inlet 61 of the ejector 60. When valve 34 is closed, water is blocked from flowing along the first water supply path W1 towards valve 36 and the first fluid inlet 61 of the ejector 60. Valve 42 is controlled, via a control signal 83, to allow water to flow along the second water supply path W2. When valve 42 is opened, water is permitted to flow along the second water supply path W2 from water inlet 33 towards valve 36 and the first fluid inlet 61 of the ejector 60. When valve 42 is closed, water is blocked from flowing along the second water supply path W2 towards valve 36 and the first fluid inlet 61 of the ejector 60. The second water supply path W2 comprises a pump 44. The pump 44 is controlled via a control signal 82. When a flow of water is required which is greater than a flow via the first water supply path W1, valve 34 is closed, valve 42 is opened and the pump 44 is turned on to deliver a flow of water. When the pump 44 is running, the flow of water to the first fluid inlet 61 of the ejector 60 is controlled by valve 36. Valve 36 is controlled by a signal 81 based on feedback from the flowmeter 72, and the signal 89. Flow of water through the first fluid inlet 61 of the ejector 60 is controlled by one or more of: valve 36; valves 34, 42; pump 44. One of the water supply paths W1, W2 is used at a time. When valve 34 is opened, valve 42 is closed. Similarly, when valve 42 is opened, valve 34 is closed.

[0079] The air supply section 50 comprises at least one air inlet 52. The air inlet 52 is provided on a part of the vessel which can access ambient air, such as an external part of the vessel or an interior part of the vessel which has a flow path to ambient through a bulkhead or a deckplate of the vessel. A valve 56 is provided upstream of the second fluid inlet 62 of the ejector 60. The valve 56 is controlled, via a control signal 87, to vary a flow rate of air to the second fluid inlet 62 of the ejector 60. Flow rate is measured by a flowmeter 73, with a signal 90 feeding back to the controller 80. The air supply section 50 comprises two main air supply paths A1, A2. A first air supply path A1 provides a flow path for a passive flow of air towards the second fluid inlet 62 of the ejector 60. A second air supply path A2 provides a flow path for an active flow of air towards the second fluid inlet 62 of the ejector 60. Valve 54 is controlled, via a control signal 85, to allow air to flow along the first air supply path A1. The second air supply path A2 comprises at least one air blower or compressor 58. The blower 58 is controlled via a control signal 86. A plurality of blowers 58 may be provided. Providing more than one blower 58 allows the number of active blowers 58 to be scaled to the air requirements of the air lubrication system 100. For example, when the requirement for air is low, the passive flow of air via air supply path A1 may fully meet the requirement and the blower (or blowers) 58 can be turned off to save energy. When a flow of air is required which is greater than a flow received via the first air supply path A1 , one or more of the blowers 58 is operated at a speed to deliver a required flow of air. Providing at least two blowers 58 allows some redundancy, with a first blower in operation and a second blower as standby.

[0080] As described above in connection with Figure 5, flow of water through the ejector 60 draws air into the second fluid inlet 62. Therefore, for a particular value of water flow rate through the ejector (due to a passive flow of water, or an active (pumped) flow of water) there is a corresponding value of air flow rate and a corresponding air to water ratio (= void fraction). It is possible to vary the amount of air that would naturally be drawn into the ejector 60 and hence vary the air to water ratio by controlling air supply to the ejector 60. For example, the air supply section 50 can push more air into the ejector 60 than would naturally be drawn into the ejector by a flow of water through the ejector. This can increase the air to water ratio above the normally expected value. Conversely, the air supply section 50 can limit, or throttle, the amount of air into the ejector (via valve 56) from an amount that would naturally be drawn into the ejector by a flow of water through the ejector 60. This can reduce the air to water ratio below the normally expected value.

[0081] An outlet valve 71 prevents ingress of unwanted fluid to the system. Valve 71 is controlled by a control signal 88. When the air lubrication system 100 is operating, valve 71 is open.

[0082] In the apparatus shown in Figure 6, a flow meter 72 measures a flow of water into the ejector 60 and a flow meter 73 measures a flow of air into the ejector 60. The measurements made by the flow meters 72, 73 gives an in-board (i.e. within the vessel) indication of the air to water ratio of the air and water mixture. The air and water mixture flows out of fluid outlets 22, 24 into a boundary layer of water adjacent the hull of the vessel. Forces acting on the air bubbles may vary the air to water ratio of the air and water mixture within the boundary layer, i.e. outside the vessel.

[0083] In the apparatus shown in Figure 6, pump 44 is turned on / off and flow of water to the first inlet 61 of the ejector 60 is controlled by valve 36. In another example apparatus, a speed of pump 44 is controlled to vary flow of water along water supply path W2.

[0084] The air lubrication system 100 may comprise a plurality of units or “pods” 95, each comprising a fluid inlet 32, a water supply section 30, an ejector 60 and a fluid outlet 22, 24. The plurality of the pods 95 are distributed across the hull of the vessel 10. This approach has an advantage of minimising ducting within the vessel to distribute water to ejectors 60. It also reduces losses in ducting as each source of water is positioned near to the ejector 60 that it serves. One air supply section 50 may serve all of the pods 95, or a subset of the pods 95. The air supply section 50 may comprise a plenum chamber 55 which receives air via the air supply paths A1, A2 and outputs air to each pod 95. Alternatively, a dedicated air supply section 50 may be provided per pod 95.

[0085] In the apparatus shown in Figure 6, the air lubrication system comprises fluid inlets 32, 33 per pod. In other examples, the air lubrication system may comprise a smaller number of fluid inlets (and water supply sections 50) than fluid outlets 22, 24. For example, a single water inlet 32 and a single water supply section 30 may provide water for a set of ejectors 60 and fluid outlets 22, 24.

[0086] The water inlets 32, 33 to the air lubrication system may be provided as openings formed in the hull of the vessel, with the water inlets dedicated to delivering water to the air lubrication system 100. In other examples, one or more of the water inlets to the air lubrication system may be located within the hull of the vessel, such as an outlet from a sea chest within the vessel.

[0087] While an example of the air lubrication system has been described above, other apparatus is possible. For example, the air lubrication system may omit the ejectors 60 and use water pumps and air blowers or compressors to generate an air and water mixture.

[0088] Figure 7 schematically shows the controller 80. The controller 80 forms part of a control system for the air lubrication system 100. The controller 80 is configured to receive various input signals 92. The input signals 92 may comprise an input signal indicative of movement of the vessel, such as a value of pitch and / or roll of the vessel. This is indicative of sea conditions. The input signals 92 may comprise an input signal indicative of draft of the vessel. The input signals 92 may comprise an input signal indicative of a course-keeping requirement. The input signals 92 may comprise one or more input signal indicative of: speed of the vessel; engine shaft power; fuel consumption of the vessel. The input signals 92 may comprise an input signal 89 indicative of water flow and an input signal 90 indicative of air flow within the air lubrication system. In an air lubrication system with a plurality of “pods”, the controller 80 may receive signals 89, 90 from each of the pods. The input signals 89, 90 are a measure of the current air to water ratio of the air and water mix flowing to each of the fluid outlets of the air lubrication system. The controller 80 may be configured to receive other input signals.

[0089] The controller 80 is configured to output control signals 81-88. Output control signals 81-84 control operation of the water supply section 30; output control signals 85-87 control operation of the air supply section 50; and output control signal 88 controls operation of the outlet valve 71. The controller 80 is configured to use one or more of the input signals 92, and stored data, to generate output signals 81-88.

[0090] The controller is configured to vary the output ratio of air and water according to at least one of: movement of the vessel (e.g. in rough seas reduce the air to water ratio at outer outlets 22, or reduce the air to water ratio at outer outlets 22 and upper outlets 24, to retain fluid with a higher void fraction alongside the hull); speed of the vessel (e.g. increasing speed of the vessel requires a higher flow rate of water and air to maintain a particular void fraction in the boundary layer); draft of the vessel (e.g. an increase in draft pushes fluid outlets to a lower depth and may require a higher proportion of air to maintain a required void fraction); a requirement to course-keep the vessel and a course-keeping direction that is required (e.g. a requirement to course-keep or steer towards the starboard side may be achieved by increasing the void fraction on the port side of the vessel.)

[0091] Figure 8 shows an example of a method of operating the air lubrication system 100. The method 200 may be performed by the controller 80.

[0092] At block 202 the controller 80 is configured to receive at least one input signal. As described above, the input signals may be indicative of one or more of: movement of the vessel; speed; draft; requirement for course keeping and a required direction.

[0093] At block 204 the controller 80 is configured to determine an air to water ratio for the fluid outlets of the air lubrication system based on the input signals. One possible output is to operate in a first operating state 206 in which the air lubrication system delivers a uniform air to water ratio across the fluid outlets. The air to water ratio may be varied across a range of values, uniformly across the fluid outlets. Another possible output is to operate in a second operating state 208 in which the air lubrication system delivers a non-uniform air to water ratio across the fluid outlets. The need for the second operating state may be due to sea conditions (e.g. rough seas) and a need to contain air lubrication alongside the hull or due to a requirement to course keep the vessel. The controller 80 controls the air to water ratio at each of the fluid outlets.

[0094] Figure 9 shows an example of a method of operating the air lubrication system 100. The method 250 may be performed by the controller 80, or by a different controller. The controller 80 can be configured to learn an optimum set of values for operating the air lubrication system 100. The controller 80 can be configured to determine the effectiveness of a trial set of output values and modify the output values to determine if the result is better or worse.

[0095] At block 252 the controller 80 is configured to select a first trial set of values for the control signals 81-84 applied to the water supply section 30 and for the control signals 85-87 applied to the air supply section 50 at particular values of vessel speed and sea conditions (pitch, roll). At block 254 the controller 80 is configured to determine an effectiveness of the trial set. This may be measured in terms of fuel consumption over a period of time, or power requirement to achieve a vessel speed, or some other criterion or criteria.

[0096] At block 256 the controller 80 is configured to store the results. The controller 80 is configured to store the trial set of values (of signals 81-87) and the data related to performance of that set of control signals.

[0097] Optionally, the controller 80 is configured to modify 258 the set of values for the control signals. For example, the controller may be configured to select a second, different, trial set of values for the control signals 81-87 at the same values of vessel speed and sea conditions and determine if the second set of trial values is more, or less, effective. For example, if the second trial set of values offers a lower fuel consumption for the same speed and sea conditions, the second set is determined to be more effective than the first trial set. Conversely, if the second trial set of values offers a higher fuel consumption for the same speed and sea conditions, the second set is determined to be less effective than the first trial set.

[0098] The controller may be configured to test a large number of trial sets of values over a period of time to determine an optimum set of values which reduce drag on the hull for different conditions. The particular set of values may vary based on factors such as: the type of vessel (e.g. hull shape), the number of fluid outlets and their position on the hull; operating conditions (speed, sea conditions).

[0099] The controller of a vessel may learn an optimum set of control signal values for itself using the method shown in Figure 9. The set of learned data may be used by the controller as a look-up table for selecting values of control signals for controlling the air lubrication system. The look-up table is used to select optimum values of control signals. In another example, the controller of a vessel may use a set of learned data from another vessel (e.g. the same, or a similar, type of vessel), or use a set of learned data from another vessel as a starting point, and modify the data over a period of time.

[0100] Figure 10 shows an example of a processing apparatus 300 which may implement at least part of the controller, such as the method of Figures 8 and / or 9. Processing apparatus 300 comprises one or more processors 301 which may be any type of processor for executing instructions to control the operation of the device. The processor 301 is connected to other components of the device via one or more buses 306. Processor-executable instructions 303 may be provided using any data storage device or computer-readable media, such as memory 302. The processor-executable instructions 303 comprise instructions for implementing the functionality of the described methods. The memory 302 is of any suitable type such as non-volatile memory, a magnetic or optical storage device. The processing apparatus 300 comprises input / output (I / O) interfaces 307. The I / O interfaces 307 can receive signals from other apparatus, such as: flow meters in the flow paths of the water supply section 30 and air supply section 50; inputs from a control system of the vessel indicative of movement, speed, draft; inputs from a control system of the vessel indicative of a requirement to course keep (e.g. steer to port / starboard). The I / O interfaces 307 may output signals to other apparatus, such as the apparatus shown in Figure 6.

[0101] The processing apparatus 300 connects to a user interface 308. Memory 302, or a separate memory, stores data used by the processor. This can include: a store of data 311 which comprises values of parameters such as movement, speed, draft and corresponding values of control signals 81-87 for the air lubrication system.

[0102] The main purpose of the air lubrication system 100 is to provide a quantity of air bubbles into the boundary layer adjacent the hull of the vessel to reduce drag on the hull. The air lubrication system 100 may also provide a propulsive force to the vessel. That is, the flow of air and water from at least one of the fluid outlets 22, 24 may assist the forward movement of the vessel by providing a propulsive force.

[0103] The air lubrication system 100 may be adapted to make use of wind energy. For example, the air supply section (50, Figure 6) may harness at least some of the energy of air blown into the air inlet 52 to assist movement of the air through the air supply section 50.

[0104] The system and method in accordance with the present invention may be applied to any water borne vessel, such as merchant shipping, ferries, leisure craft, inland waterway vessels or any other vessel adapted to move through water.

[0105] Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, means “including but not limited to”, and is not intended to (and does not) exclude other components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Claims

22CLAIMS:

1. An air lubrication system for a hull of a vessel comprising: a plurality of fluid outlets for mounting on the hull; at least one water inlet; at least one ambient air inlet; and a controller, wherein the air lubrication system is configured to deliver an output fluid from the plurality of fluid outlets, wherein the output fluid comprises an air and water mixture having an air to water ratio; and the controller is configured to control the air to water ratio of the output fluid delivered by the plurality of fluid outlets, the controller comprising an operating state in which the air to water ratio of the output fluid is non-uniform across the plurality of fluid outlets.

2. The system of claim 1 wherein the controller is configured to cause the air lubrication system to: output fluid with a first air to water ratio from a first subset of the fluid outlets; and output fluid with a second air to water ratio from a second subset of the fluid outlets positioned laterally outwardly of the first subset of the outlets with respect to a direction of movement, wherein the second air to water ratio has a lower proportion of air compared to the first air to water ratio, or is only water.

3. The system of claim 2 wherein the controller is configured to cause the air lubrication system to output fluid with a third air to water ratio from a third subset of fluid outlets positioned between the first subset of the fluid outlets and the second subset of the fluid outlets, wherein the third air to water ratio has a lower proportion of air compared to the first air to water ratio and a higher proportion of air compared to the second air to water ratio.

4. The system of claim 2 or 3 wherein the first subset of fluid outlets is positioned centrally on the hull, and the second subset of fluid outlets comprises at least one outlet on each lateral side of the first subset of fluid outlets.

5. The system of any one of the preceding claims wherein the air lubrication system comprises a plurality of fluid outlets distributed on sidewalls of the hull, and the controller is configured to cause the air lubrication system to:output fluid with a first sidewall air to water ratio from a first sidewall subset of the fluid outlets positioned lowermost on the sidewalls; and output fluid with a second sidewall air to water ratio from a second sidewall subset of the fluid outlets positioned above the first subset, wherein the second sidewall air to water ratio has a lower proportion of air compared to the first sidewall ratio.

6. The system of any one of the preceding claims wherein the controller is configured to cause the air lubrication system to: output fluid with a first ratio of air and water from a first subset of the outlets positioned on a first side of the hull; and output fluid with a second ratio of air and water from a second subset of the outlets positioned on a second side of the hull, wherein the second ratio has a lower proportion of air compared to the first ratio, or is only water.

7. The system of any one of the preceding claims wherein the controller is configured to control the output ratio of air and water according to at least one input indicative of: movement of the vessel; speed of the vessel; draft of the vessel; a requirement to course-keep the vessel.

8. The system of claim 7 wherein the controller is configured to store data indicative of the at least one input and corresponding outputs to control the air and water ratio at the plurality of fluid outlets.

9. The system of any one of the preceding claims wherein the controller comprises an operating state in which the air to water ratio of the output fluid is uniform across the plurality of fluid outlets and the controller is configured to select between: the operating state in which the air to water ratio of the output fluid is non-uniform across the plurality of fluid outlets; and the operating state in which the air to water ratio of the output fluid is uniform across the plurality of fluid outlets.

10. The system of any one of the preceding claims wherein the controller is configured to learn values of control signals to control the air to water ratio of the output fluid delivered by the plurality of fluid outlets.

11. The system of any one of the preceding claims which is configured to deliver an output fluid from at least one of the plurality of fluid outlets with an air and water mixture having an air to water ratio with less than 10% air, optionally less than 5% air.

12. The system of any one of the preceding claims wherein the air lubrication system comprises: an ejector having a first ejector inlet, a second ejector inlet and an ejector outlet, wherein the first ejector inlet is in fluid communication with the at least one water inlet, the second ejector inlet is in fluid communication with the at least one ambient air inlet and the ejector outlet is in fluid communication with at least one of the fluid outlets; a water supply section to supply water to the first ejector inlet; an air supply section to supply air to the second ejector inlet; and the controller is configured to control the water supply section and the air supply section to achieve a required air to water ratio.

13. The system of claim 12 wherein the water supply section comprises a controllable valve in a water flow path to the first ejector inlet and the controller is configured to control the controllable valve to achieve the required air to water ratio.

14. The system of claim 12 or 13 wherein the air supply section comprises a controllable valve in an air flow path to the second ejector inlet and the controller is configured to control the controllable valve to achieve the required air to water ratio.

15. The system of any one of claims 12 to 14 wherein the water supply section comprises a water pump to deliver a flow of water to the first ejector inlet and the controller is configured to control the water pump to achieve the required air to water ratio.

16. The system of claim 15 wherein the water supply section comprises a first water supply path to deliver a passive flow of water to the first ejector inlet and a second water supply path to deliver an active flow of water to the first ejector inlet, wherein the water pump is in the second water supply path.

17. The system of any one of claims 12 to 16 wherein the air supply section comprises an air blower to selectively supply air to the second ejector inlet and the controller is configured to control the air blower to achieve the required air to water ratio.2518. The system of any one of the preceding claims wherein the air lubrication system is configured to deliver an output fluid with a propulsive force from at least one of the plurality of fluid outlets.

19. A vessel comprising a hull and the air lubrication system of any one of the preceding claims.

20. A controller for an air lubrication system for a hull of a vessel, wherein the air lubrication system comprises a plurality of fluid outlets for mounting on the hull, at least one water inlet and at least one ambient air inlet, the air lubrication system configured to deliver an output fluid from the plurality of fluid outlets, wherein the output fluid comprises an air and water mixture having an air to water ratio, wherein the controller is configured to: control the air to water ratio of the output fluid delivered by the plurality of fluid outlets with an operating state in which the air to water ratio of the output fluid is non-uniform across the plurality of fluid outlets.

21. A method of controlling an air lubrication system for a hull of a vessel, wherein the air lubrication system comprises a plurality of fluid outlets for mounting on the hull, at least one water inlet and at least one ambient air inlet, the air lubrication system configured to deliver an output fluid from the plurality of fluid outlets, wherein the output fluid comprises an air and water mixture having an air to water ratio, the method comprising: controlling the air to water ratio of the output fluid delivered by the plurality of fluid outlets with an operating state in which the air to water ratio of the output fluid is non-uniform across the plurality of fluid outlets.

22. Computer readable instructions that, when executed by a processor, cause the processor to perform the method of claim 21.

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