Air release unit with diverging side walls

Abstract

The invention relates to a vessel (1) comprising an air lubrication system (16) for providing an air lubrication layer between a bottom (6) and water flowing along the bottom when the vessel is moving through the water. The system comprises at least one cavity (33) defined by side walls (18, 18'), a top wall (19) and an interface plane (20) in which water and air mix due to the Kelvin Helmholtz effect. The side walls (18, 18') extend from a dagger-shaped nose portion (26) at the front end (22) of the cavity (33) to the rear end (28) of the cavity (33) and diverge so that the distance between the side walls increases along the length of the cavity when travelling in the rearward direction. The diverging cavity provides a stable air lubrication layer covering a relatively large area of ​​the bottom, so that the number of cavities can be reduced. The alignment of the diverging cavity with the streamlines has relatively large tolerances, so that the construction is simplified and can be carried out in a cost-effective manner. The diverging side walls result in reduced vortex formation and reduced drag.

Description

Technical Field

[0001] The invention relates to an air lubricated boat and an air cavity, also called an "air release unit", wherein the boat comprises a hull having a bottom and an air lubrication system for providing an air lubrication layer between the bottom and water flowing along the bottom when the boat is moving in the water. The air lubrication system comprises at least one cavity defined by side walls, a top wall and an interface plane, the interface plane being located at a distance from the top wall and extending transversely to the side walls substantially at the level of the bottom, the cavity having a front end, a rear end and a length Lc extending in the length direction of the cavity, with an air inlet spaced apart from the opening of the cavity. Background Art

[0002] Such a system and a vessel are known from WO2015 / 133899 filed in the name of the applicant. In this publication, it is described that an effective air lubrication of the flat bottom of a vessel is achieved by providing an open cavity of relatively small size and injecting air into the cavity at approximately hydrostatic pressure, thereby forming a substantially flat water-air interface at the height of the bottom. At this interface, due to the occurrence of Kelvin Helmholtz instabilities, air mixes into the water and a stream of bubbles escapes from the rear of the cavity. It was found that such a cavity provides a stable and effective way to provide a layer of bubbles contained in a boundary layer along the bottom, reducing frictional drag so that the energy gain due to the reduction in friction during propulsion far outweighs the additional energy required to inject air into the cavity at hydrostatic pressure. Using an air lubrication system, a reduction in fuel consumption of up to 10% can be achieved.

[0003] To facilitate evacuation of the cavity during startup, one or more baffles extending transversely across the cavity may be used. The baffles reduce turbulence within the cavity and keep air in the cavity longer, requiring a reduced capacity compressor for injecting less powerful air during startup.

[0004] It is an object of the present invention to provide an air lubrication system and an air release unit of the above type which have improved efficiency and utilize a relatively small amount of energy to lubricate the hull. It is a further object of the present invention to improve the stability of the air lubrication and the release of air from the cavity. It is a further object of the present invention to provide an air lubrication system in which the installation of the cavity in the bottom of the hull can be simplified. Summary of the invention

[0005] To this end, the air chamber boat according to the present invention comprises at least one cavity defined by side walls, a top wall and an interface plane, the interface plane being located at a distance Hc from the top wall and extending transversely to the side walls substantially at the level of the bottom, the cavity having a front end, a rear end and having a width Wr at its rear end, wherein the ratio Lc / Hc is in the range of 6:1 to 15:1. The ratio Wr / Hc is in the range of 1.3:1 to 5:1 and the ratio Lc / Wr is in the range of 3.5:1 to 7:1, the length Lc is between 2m and 10m, preferably between 4m and 8m, wherein the side walls extend from a nose portion at the front end where the side walls meet to a transition zone located at a distance Lt from the front end, the side walls between the rear end and the transition zone extending at an angle relative to the length direction of the cavity such that the distance between the side walls increases along the length of the cavity when travelling in the rearward direction.

[0006] As defined herein, with respect to a "transition zone," the boundary refers to the length along the cavity between the converging sidewalls of the nasal portion and the sidewall components of the cavity extending toward the rear end of the cavity.

[0007] By means of the diverging side walls of the relatively small size of the air lubrication chamber according to the invention, the gas bubbles will leave the chamber with a wider tail, which results in a more effective covering of the bottom of the hull with gas bubbles. Thus, a smaller number of air lubrication chambers, for example 12 instead of 14, can be used to lubricate the bottom. Compared to the known air lubrication system described in WO2015 / 133899, in which the side walls extending from the transition zone to the rear end are placed in parallel, using the chamber according to the invention, a reduction of energy consumption for air lubrication of about 15% can be achieved.

[0008] By means of the diverging sidewalls according to the invention, it has emerged that the length direction of the cavity can deviate from the direction of the streamlines at the location of the cavity, while still achieving a stable bubble release that is less sensitive to the flow direction. This allows greater freedom in mounting the air cavity and angling the length direction of the air cavity relative to the streamlines. Since the cavity according to the invention does not need to be perfectly aligned with the streamlines, this enables the mounting of the cavity parallel to the ship structure components (such as longitudinal beams, bulkheads and reinforcement structures). This greatly simplifies the process of building new ships and / or remodeling existing ships by cutting holes in the bottom for accommodating the cavity units therein.

[0009] It has also been found that the diverging side walls of the air cavity according to the present invention reduce the vortex formed at the rear end of the cavity, thereby reducing drag and improving the efficiency of lubrication.

[0010] Preferably, the nose tip is substantially V-shaped or dagger-shaped. A dagger-shaped nose has the advantage of preventing the formation of waves in the cavity at the free surface.

[0011] It should be noted that a large-sized V-shaped air cavity in a high-speed surface effect boat is known from European patent application EP 0 667 282. The known cavity extends over more than half the length of the boat, all the way to the stern. The known air cavity is not suitable for controlled release of gas bubbles that are formed by Kelvin-Helmholtz mixing of air and water at the cavity interface plane and that adhere to bottom downstream components after leaving the cavity.

[0012] The transition position Lt may be located at a distance between 1 m and 2 m from the front end, preferably at a distance between 1.2 m and 1.8 m.

[0013] In an embodiment of the vessel according to the invention, an air inlet is provided in the top wall at a length position substantially corresponding to the transition position Lt. This was found to produce an optimal air distribution within the cavity, resulting in a minimum pressure difference within the cavity and minimum disturbance of the air-water interface.

[0014] In an embodiment of the ship according to the invention, the distance Wr between the side walls near the rear end is between 70 cm and 150 cm, preferably between 80 cm and 120 cm, and wherein the distance Wt between the side walls at the transition zone is between 40 cm and 70 cm, preferably between 45 cm and 60 cm.

[0015] The compact size of the air chamber according to the invention provides stable and effective air lubrication.

[0016] Preferably, the rear wall of the cavity slopes downwardly from the top surface to be tangential to the interface plane over a rear wall length Lr of between 50 cm and 2 m, preferably between 1 m and 1.5 m.

[0017] The inclined rear wall terminates in the interface plane and forms a wedge-shaped relief section from which the air / water mixture can leave the cavity in a controlled manner to be evenly spread over the downstream bottom part. Preferably, the bottom section downstream of the cavity is substantially flat so that air bubbles adhere to the bottom and do not have the possibility to move upwards towards the water surface.

[0018] The deflector may be located substantially parallel to the interface plane at a distance between 1 cm and 50 cm from the interface plane, the deflector having a side wall substantially parallel to the side wall of the cavity and extending at a distance between 1 cm and 10 cm from the side wall. By means of the deflector, the cavity may be quickly filled with air at start-up. In use, the deflector prevents waves from entering the cavity, prevents the air in the cavity from being emptied, thereby improving the stability of the cavity. The deflector may be arranged according to WO2013 / 125951, WO2015 / 133899 or WO2018 / 044163 filed in the name of the applicant.

[0019] In a preferred embodiment according to the invention, the vessel includes streamlines along the bottom of the vessel and has two or more cavities at different width positions across the bottom on each side of the centerline, each cavity being positioned so that the length direction of the cavity makes an angle of between 0.01° and 7°, preferably between 0.01° and 5°, with the streamlines at the position of the cavity.

[0020] The design of the cavity according to the invention including diverging side walls makes the cavity less sensitive to the direction of the water flow. Therefore, the cavity according to the invention can be placed not in precise alignment with the streamlines along the bottom of the ship, but rather the cavity is aligned with the internal reinforcement structure of the ship, such as bulkheads and beams. Therefore, the design and arrangement of the cavity is simplified and the construction of the cavity is made more cost-effective.

[0021] The vessel according to the invention may comprise two cavities in length on each side of the centre line, wherein the distance between the front end of the forward cavity placed closest to the bow of the vessel and the front end of the cavity placed closer to the stern, when viewed in length, is between 4m and 10m.

[0022] The cavities can be spaced relatively far apart in the length direction of the ship while still covering the entire (preferably flat) bottom with bubbles. In addition, the tails of the bubbles released from the cavities are relatively wide, thereby achieving large area coverage of the bubble layer using a reduced amount of air. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Some embodiments of the ship according to the invention will now be described in detail by way of non-limiting examples with reference to the accompanying drawings. In the drawings:

[0024] Figure 1 shows a schematic side view of a vessel comprising an air lubrication system according to the invention;

[0025] Figure 2 shows a perspective view of an air release unit or cavity unit including diverging side walls;

[0026] Figure 3 shows a side view of a cavity unit according to the invention;

[0027] Figure 4 shows a top view of a cavity unit according to the invention;

[0028] Figure 5 A plan view showing the bottom of a boat including a plurality of air lubrication cavities according to the invention; and

[0029] Figure 6 The position of the cavity relative to the streamlines is schematically shown. DETAILED DESCRIPTION

[0030] Figure 1A ship 1 is shown with a length Lv between 20m and 500m and a width between 5m and 75m. The ship 1 may have a displacement of at least 10,000 tons, preferably at least 50,000 tons, and is a sea-going ship. The ship 1 has a hull 4 with a bow 2, a stern 3, a side 5, a substantially flat bottom 6 and a propeller 10. Air lubrication cavities 7, 8 open in the plane of the bottom 6 are distributed along the bottom 6 to create a layer of bubbles 9 in a boundary layer that travel along the flat bottom 6 towards the stern 3. A compressor 11, 12 is connected to each cavity 7, 8 for supplying air at least at the hydrostatic pressure inside each cavity at the main draft level of the ship. The compressors 11, 12 have an air outlet duct 14 connected to the cavities 7, 8 and have an air inlet duct 13 for sucking in ambient air. The compressors 11, 12 are controlled by a controller 15 for regulating the air supply according to the sailing speed, the sea state and during starting and stopping.

[0031] Figure 2 An air lubrication system 16 is shown which is constructed as an integral module forming a cavity 33 which can be fitted into the bottom 6 of the hull 4 of the boat 1. The system 16 includes side walls 18, 18' extending in length from the front end 22 to the rear end 28 along the centerline 51. A top wall 19 extends across the side walls at their upper edges, thereby defining the cavity 33. The side walls 18, 18' are supported by a lower edge 29 on a flange 17 which can be welded into the flat bottom 6 of the boat 1. The side walls 18, 18' define an opening 20 which is substantially flush with the flat bottom surface of the boat. In use, the opening 20 forms a smooth air-water interface plane at which the air mixes into the water due to the Kelvin Helmholtz mixing effect. The air bubbles mixed with the water at the interface plane leave the cavity along the rear edge 21 to smoothly transition from the cavity onto the downstream flat bottom portion and travel unrestricted in the boundary layer along the flat bottom 6 in the direction of the stern 3. A concavely curved, downwardly inclined rear wall member 27 extends a length Lr of approximately 1.5 m from the top wall 19 to the rear edge 21 to guide the air and water in the cavity in a smooth flow pattern to an exit point located along the lower rear edge 21.

[0032] Near the front end 22 of the cavity 33, a dagger-shaped nose portion 24 is provided, which extends along a length Lt from the front end 22 to a transition zone 26. The side walls 18, 18' diverge in a rearward direction from the transition zone 26 to a rear end 28. An air inlet 23 is located in the top wall 19. The air inlet 23 can be connected to one of the air outlet ducts 14 of the compressors 11, 12.

[0033] In the cavity 33, the baffle 25 extends across the largest part of the opening 20 at a small distance between 1 cm and a few cm from the side walls 18, 18'. The cavity 33 may have a length Lc of about 4 m, a length Lt of about 1 m, a width Wr of about 1 m at the rear, Wt of about 50 cm at the transition zone 26, and a height Hc of about 50 cm. The side walls 18, 18' may have a thickness of 16 mm, while the flange 17 and the top wall 19 may have a thickness of 20 mm.

[0034] The deflector 25 in the cavity 33 stabilizes the water flow in the cavity. This is important because the deflector enables the cavity to be filled with air during boat start-up and prevents the cavity from filling with water, for example due to waves. Secondly, when the air lubrication system 16 is turned off (no air output), the deflector 25 minimizes the resistance of the cavity.

[0035] The deflector 25 is positioned above the interface plane of the cavity for obtaining an undisturbed flow of water through the cavity during the ship's sailing speed. When the cavity 33 is filled with air, the deflector 25 is spaced from the water surface. The deflector helps to keep the Kelvin-Helmholtz interface stable during the rolling motion of the ship.

[0036] The slope at the rear wall 27 of the cavity 33 ensures smooth release of the bubbles into the ship's boundary layer and is designed to inject the bubbles formed by Kelvin Helmholtz mixing into the ship's surface boundary layer, thereby minimizing vertical dispersion and optimizing drag reduction.

[0037] The shape of the nose portion 24 of the cavity 33 controls the water flow through the Kelvin Helmholtz effect, minimizing wave instabilities at the air / water interface and improving consistent air mixing entering the boundary layer.

[0038] The length Lc of the cavity 33 is chosen to be long enough, for example about 4 m, to produce a stable Kelvin Helmholtz air mixing effect for constant bubble generation and bubble flow into the boundary layer.

[0039] The relative positioning of the plurality of cavities 33 across the bottom 6 is important to maximise the air-lubricated surface area of ​​the hull.

[0040] The size of the cavity determines the air volume required for stable bubble generation and the air volume required for cavity recovery after cavitation collapse. Optimizing the cavity size determines the overall lubrication efficiency and the efficiency of the total air lubrication.

[0041] Figure 3The air lubrication system 16 is shown in a side view. The height Hc of the cavity from the interface plane 20 to the top wall 19 may be about 45 cm. The height Hd of the guide plate 25 from the interface plane 20 may be, for example, 5 cm. The length Lc of the cavity from the front end 22 to the rear edge 21 may be 4 m. The distance Lt of the transition zone 26 from the front end 22 may be about 1 m. The inlet 23 is located near the transition zone 26.

[0042] Figure 4 A top view of the air lubrication system is shown, wherein the edge of the deflector 25 is indicated by a dotted line. The side walls 18, 18' extend from the transition zone 26 in the rearward direction at a diverging angle α of, for example, about 5° to the centerline 51 of the cavity 33 and any line parallel to the centerline 51. Depending on the hull structure, the angle α may be in the range of 2°-10°, preferably 2°-8°, more preferably 4°-7°. The rear width Wr of the cavity at the end of the side walls 18, 18' may be about 1 m. The width Wt of the cavity at the transition zone 26 may be about 50 cm.

[0043] The diverging side walls 18, 18' placed at an angle α of up to about 10° result in a diverging air blanket across the bottom, thereby providing the possibility of reducing the number of air lubrication systems and adequately covering the hull bottom with air bubbles using a reduced amount of air. In a given hull configuration, the cavity is designed so that the angle α is as large as possible to obtain optimal air lubrication.

[0044] The trailing edges of the diverging side walls 18, 18' at the trailing end 28 prevent the formation of vortices, thereby improving air lubrication stability and reducing frictional drag.

[0045] Figure 5 The bottom 6 of the ship 1 and the streamlines 40, 41 along which the water flows from the bow 2 to the stern are shown. A plurality of cavities 42-47 extend at different positions along the width Wv of the ship (which may be up to, for example, 20 m), in a V-shaped arrangement fanning out laterally from the centerline 50 of the ship. The centerlines of the cavities do not need to be precisely aligned along the streamlines 40, 41. Due to the diverging sidewalls, the distance Li between two adjacent cavities 45, 46 when viewed in the length direction of the ship may be relatively large, such as 1-3 m longer than the cavity length, for example 4 m-7 m or more.

[0046] Figure 6 The cavity 16 is shown disposed with its centerline 51 at an angle β between 0.01° and 7° relative to the streamlines 41. In hull design, the angle β is less than the angle α and is preferably selected so that streamlines intersecting the cavity at the forward end exit the cavity at the stern, preferably between the sidewalls 18, 18' along the aft end 28.

Claims

1. A vessel (1) comprising a hull (4) having a bottom (6) and an air lubrication system for providing an air lubrication layer (9) between the bottom and water flowing along the bottom when the vessel is moving in the water, the air lubrication system comprising at least one cavity defined by side walls (18, 18'), a top wall (19) and an interface plane (20), the interface plane (20) being located at a first distance from the top wall (19) and extending transversely to the side walls (18, 18') substantially at the level of the bottom, the cavity having a front end (22), a rear end, a first length extending in a length direction of the cavity and a first width at the rear end of the cavity, wherein The first length / first distance is in the range of 6:1 to 15:1, the first width / first distance is in the range of 1.3:1 to 5:1, and the first length / first width is in the range of 3.5:1 to 7:1, the first length is between 2m and 10m, wherein the side walls (18, 18') diverge from a nose portion (24) at the front end (22) where the side walls contact to a transition zone (26) located at a second distance from the front end (22), and the side walls (18, 18') between the rear end and the transition zone extend at an angle (α) between 2° and 10° relative to the length direction of the cavity, so that when traveling in a rearward direction, the distance between the side walls (18, 18') increases along the length of the cavity from the transition zone to the rear end.

2. The vessel (1) according to claim 1, wherein: The first length is between 4m and 8m.

3. The vessel (1) according to claim 1, wherein: The transition zone (26) is located at a second distance between 1 m and 2 m from the front end (22).

4. The vessel (1) according to claim 3, wherein: The transition zone (26) is located at a second distance between 1.2 m and 1.8 m from the front end (22).

5. The vessel (1) according to claim 1 or 2, wherein: The nose portion (24) is substantially V-shaped or dagger-shaped.

6. The vessel (1) according to any one of claims 1 to 3, wherein: The bottom (6) is substantially flat.

7. The vessel (1) according to any one of claims 1 to 4, wherein: An air inlet (23) is provided in the top wall (19) at a length position substantially corresponding to the length position of the transition zone (26).

8. The vessel (1) according to any one of claims 1 to 4, wherein: The distance between the side walls (18, 18') near the rear end is between 70 cm and 150 cm, and wherein the distance between the side walls (18, 18') at the transition zone (26) is between 40 cm and 70 cm.

9. A vessel (1) according to claim 8, wherein: The distance between the side walls (18, 18') near the rear end is between 80 cm and 120 cm.

10. The vessel (1) according to claim 8, wherein: The distance between the side walls (18, 18') at the transition zone (26) is between 45 cm and 60 cm.

11. Vessel (1) according to any one of claims 1-4, the rear wall (27) of the cavity being inclined downwardly from the top wall (19) to be tangential to the interface plane (20) over a rear wall length between 50 cm and 2 m.

12. Vessel (1) according to claim 11, the rear wall (27) of the cavity being inclined downwardly from the top wall (19) to be tangential to the interface plane (20) over a rear wall length of between 1 m and 1.5 m.

13. The vessel (1) according to any one of claims 1 to 4, wherein: The guide plate (25) is located substantially parallel to the interface plane (20) at a distance between 1 cm and 50 cm from the interface plane, the guide plate (25) having a side wall substantially parallel to the side wall (18, 18') of the cavity and extending at a distance between 1 cm and 10 cm from the side wall (18, 18') of the cavity.

14. A vessel (1) according to any one of claims 1 to 4, comprising streamlines (40, 41) along a bottom (6) of the vessel and comprising two or more cavities on each side of a centre line (50) at different width positions across the bottom, each cavity being positioned such that the length direction of the cavity makes an angle (β) of between 0.01° and 7° with the streamlines (40, 41) at the location of the cavity.

15. Vessel (1) according to claim 14, each cavity being positioned such that the length direction of the cavity makes an angle (β) between 0.01° and 5° with the streamline (40, 41) at the location of the cavity.

16. A vessel (1) according to any one of claims 1 to 4, comprising two cavities at different length positions of the bottom (6), wherein: When viewed in the length direction, the distance between the front end of the forward cavity located closest to the bow (2) of the ship and the front end of the adjacent cavity located closer to the stern is greater than between 1 and 5 times the first length of the cavity.

17. A cavity unit for use in a ship (1), wherein: The ship (1) comprises a hull (4) having a bottom (6) and an air lubrication system for providing an air lubrication layer (9) between the bottom and water flowing along the bottom when the ship is moving in the water, the air lubrication system comprising the cavity unit, wherein the cavity unit comprises: two opposite side walls (18, 18') having an upper edge and a lower edge (29); a top wall (19) connected to the upper edges of the side walls; and an interface plane (20) at or near the lower edges (29) of the side walls at a distance from the top wall, the cavity unit having a front end (22), a rear end, a first length extending in a length direction of the cavity unit, an air inlet (23) spaced apart from the interface plane (20), a first distance of the top wall (19) from the interface plane (20) and a distance between the top wall (19) and the interface plane (20). a first width at a rear end of the cavity unit, wherein the first length / first distance is in the range of 7:1 to 13:1, the first width / first distance is in the range of 1.3:1 to 2.5:1, and the first length / first width is in the range of 3.5:1 to 7:1, the first length is between 2m and 10m, wherein the side walls (18, 18') diverge from a nose portion (24) at the front end (22) where the side walls contact to a transition zone (26) located at a second distance from the front end (22), and the side walls (18, 18') between the rear end and the transition zone (26) extend at an angle (α) between 2° and 10° relative to the length direction, so that when traveling in a rearward direction, the distance between the side walls (18, 18') increases along the length of the cavity unit from the transition zone to the rear end.

18. The cavity unit according to claim 17, wherein: The first length is between 4m and 8m.

19. The cavity unit according to claim 17, wherein: The transition zone (26) is located at a second distance between 1 m and 2 m from the front end (22).

20. The cavity unit according to claim 19, wherein: The transition zone (26) is located at a second distance from the front end (22) between 1.2 m and 1.8 m.

21. The cavity unit according to claim 17 or 19, wherein: The nose portion (24) is substantially V-shaped or dagger-shaped.

22. The cavity unit according to any one of claims 17 to 20, wherein: An air inlet (23) is provided in the top wall (19) at a length position substantially corresponding to the length position of the transition zone (26).

23. The cavity unit according to any one of claims 17 to 20, wherein: The distance between the side walls (18, 18') near the rear end is between 70 cm and 150 cm, and wherein the distance between the side walls (18, 18') at the transition zone (26) is between 40 cm and 70 cm.

24. The cavity unit according to claim 23, wherein: The distance between the side walls (18, 18') near the rear end is between 80 cm and 120 cm.

25. The cavity unit according to claim 23, wherein: The distance between the side walls (18, 18') at the transition zone (26) is between 45 cm and 60 cm.

26. The cavity unit according to any one of claims 17-20, the rear wall (27) of the cavity unit being inclined downward from the top wall (19) to be tangential to the interface plane (20) at a rear wall distance between 50 cm and 2 m.

27. The cavity unit according to claim 26, the rear wall (27) of the cavity unit being inclined downward from the top wall (19) to be tangential to the interface plane (20) at a rear wall distance between 1 m and 1.5 m.

28. The cavity unit according to any one of claims 17 to 20, wherein: The guide plate (25) is located substantially parallel to the interface plane (20) at a distance between 1 cm and 50 cm from the interface plane (20), the guide plate (25) having a side wall substantially parallel to the side wall (18, 18') of the cavity unit and extending at a distance between 1 cm and 10 cm from the side wall (18, 18') of the cavity unit.

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