Method of reducing frictional resistance of a hull, and frictional resistance reducing vessel

Abstract

In the present invention, perpendicular flows (Fa) along a running direction of a hull (10) are induced accompanying the operation of a vessel, and vortex structures in the vicinity of a submerged surface (11) of the hull are varied by the perpendicular flows. As a result, energy consumption during operation of the vessel can effectively be conserved by reducing the frictional resistance with a low level of energy consumption.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method of reducing the frictional resistance of a hull, and a frictional resistance reducing vessel.

[0003] 2. Description of the Prior Art

[0004] Conventionally, many technologies for reducing the frictional resistance between the hull and water have been proposed for the purpose of conserving energy consumed during the operation of vessels. For example, technologies for feeding gas into the water and generating a large number of bubbles in the vicinity of the wall surface (submerged surface) of the hull shell plate in order to reduce the frictional resistance using the bubbles have been proposed by the present applicant and others as disclosed in Japanese Unexamined Patent Application, First Publication No. Sho 50-83992, Japanese Unexamined Patent Application, First Publication No. Sho 53-136289, Japanese Unexamined Patent Application, First Publication No. Sho 60-139586, Japanese Unexamined Patent Application, First Publication No. Sho 61-71290, Japanese Utility Model Application, First Publication No. Sho 61-39691 and Japanese Utility Model Application, First Publication No. Sho 61-128185.

[0005] In these technologies, air pressurized by a pump, blower, or other pressurization apparatus is blown into the water from a plurality of holes or a porous plate provided in the hull.

[0006] However, in these methods of reducing the frictional resistance using bubbles, since energy is required to operate the pressurizing apparatus, the amount of energy conserved as a result of reduction of friction using the bubbles ends up being lost. At locations of comparatively large water depths, such as the bottom of the vessel, in particular, it is necessary to pressurize the gas to a high pressure corresponding to the hydrostatic pressure when blowing the gas into the water, thereby resulting in the consumption of a large amount of energy. In addition, for the installation of the pressurizing apparatus in the hull, huge costs are incurred, such as equipment costs and installation costs.

SUMMARY OF THE INVENTION

[0007] In the method of reducing the frictional resistance of a hull of the present invention, to achieve the above objects, perpendicular flows are induced along the running direction of the hull accompanying the operation of the vessel, and vortex structures are varied in the vicinity of a submerged surface of the hull by the perpendicular flows.

[0008] According to this method, since the operation of an apparatus for reducing the frictional resistance is not required, energy consumption during the operation can effectively be conserved by reducing the frictional resistance to a low level.

[0009] In the method of reducing the frictional resistance of a hull, the perpendicular flows are induced by vortices which are generated by an object which is provided on the submerged surface, or the vortex structures are varied by vortices which are generated by an object which is provided on the submerged surface, for example.

[0010] In addition to the aforementioned method of reducing the frictional resistance of a hull, a method comprising a step for forming a negative pressure region at a low pressure relative to a gaseous space in the water accompanying the operation of the vessel, and inducing a gas being led from the gaseous space to the negative pressure region, can be performed for the purpose of reducing the frictional resistance by releasing bubbles onto a submerged surface of the hull.

[0011] According to this method, by employing a pressure gradient formed around the hull, the gas can be introduced into the water with an energy consumption less than for the case of pressurizing the gas, and therefore, the frictional resistance of the hull can be effectively reduced.

[0012] In the method of reducing the frictional resistance of a hull, the negative pressure region is developed by a cyclic flow which is generated by a wing which is provided on the submerged surface, and the perpendicular flows are generated by vortices which are generated by the cyclic flow, for example.

[0013] Furthermore, in the method of reducing the frictional resistance of a hull, an upward lift which acts on the hull may be generated by the wing.

[0014] The present invention also relates to a frictional resistance reducing vessel for reducing the frictional resistance between a submerged surface of a hull and water. The frictional resistance reducing vessel comprises a vortex generator which generates vortices for inducing perpendicular flows along a running direction of the hull accompanying the operation of the vessel.

[0015] Furthermore, the frictional resistance reducing vessel may comprise a negative pressure forming portion which is provided on the submerged surface for forming a negative pressure region at a low pressure relative to a gaseous space in the water; an outlet (air outlet) which is provided behind the negative pressure forming portion; a flow path of which one end opens to a gaseous space and the other end opens to the water via the outlet; and an object which generates vortices for inducing perpendicular flows along a running direction of the hull accompanying the operation of the vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIG. 1 is a drawing that explains one embodiment of a method of reducing the frictional resistance of a hull, and a frictional resistance reducing vessel, according to the present invention.

[0017] FIG. 2A is a side view showing one embodiment of a vortex generator according to the present invention.

[0018] FIG. 2B is a plan view showing one embodiment of a vortex generator according to the present invention.

[0019] FIG. 3A is a side view showing another embodiment of a vortex generator according to the present invention.

[0020] FIG. 3B is a side view showing another embodiment of a vortex generator according to the present invention.

[0021] FIG. 4 is a drawing that explains another embodiment of a method of reducing the frictional resistance of the hull and a frictional resistance reducing vessel, according to the present invention.

[0022] FIG. 5 is a drawing of a large ship in which the present invention is applied FIG. 6 is a bottom view of the large ship taken along an arrow A in FIG. 5.

[0023] FIG. 7A is a drawing that explains another embodiment of a method of reducing the frictional resistance of a hull, and a frictional resistance reducing vessel, according to the present invention.

[0024] FIG. 7B is a drawing that explains another embodiment of a method of reducing the frictional resistance of a hull, and a frictional resistance reducing vessel, according to the present invention.

[0025] FIG. 8A is a perpendicular cross-sectional view showing one embodiment of a bubble generator according to the present invention.

[0026] FIG. 8B is a plan view showing one embodiment of a bubble generator according to the present invention.

[0027] FIG. 9A is a side view showing another embodiment of a bubble generator according to the present invention.

[0028] FIG. 9B is a plan view showing another embodiment of a bubble generator according to the present invention.

[0029] FIG. 10 is a drawing of a large ship in which the present invention is applied FIG. 11 is a bottom view of the large ship taken along an arrow A shown in FIG. 10.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] Preferred embodiments of the method of reducing the frictional resistance of a hull, and the frictional resistance reducing vessel of the present invention will be explained in the following.

[0031] FIG. 1 is a drawing schematically showing a flow of water in the vicinity of a submerged surface (bottom) 11 of a hull. In the method of reducing the frictional resistance of the present invention, when a vessel operates, flows Fa, which are perpendicular to a traveling direction Dv of a hull 10, are induced, and these perpendicular flows Fa change the structure of vortices in the vicinity of the submerged surface of the hull.

[0032] The frictional resistance of the hull is greatly influenced by the structure of the vortices in the vicinity of the submerged surface (wall surface) of the hull, especially, in the turbulent boundary layer. In the vortices in the turbulence, perpendicular vortices such as hair-pin vortices (horseshoe vortices) are considered to have a great influence on the frictional resistance in the turbulence. In the present invention, a reduction of the frictional resistance is accomplished by changing the structure of the vortices, especially changing the structure of the perpendicular vortices in the vicinity of the wall surface of the hull by inducing the aforementioned perpendicular flows Fa.

[0033] The aforementioned perpendicular flows Fa are induced by providing an object on the submerged surface of the hull to generate new vortices (derivative vortices) SC in the water, for example. A vortex generator 20 shown in FIG. 1 is provided for generating such vortices, and it is preferable that vortices are generated which extend from the bow to the stem of the vessel and which have axes along the longitudinal direction of the vessel, for example.

[0034] The vortex generator 20 is formed of a combination of a plurality of parts having predetermined shapes. Furthermore, the shape of the vortex generator 20 is designed using various kinds of analysis such as computational fluid dynamics (CFD), so as to minimize the resistance (drag) against the flow of the water which is generated by the operation of the vessel and so as to generate vortices which have the aforementioned desirable structures by the flow of the water.

[0035] For example, as shown in FIGS. 2A and 2B, the vortex generator 20 is composed of a first member 21 which is provided approximately parallel to the submerged surface 11 of the hull separated by a space, and a second member 22 which is provided between the first member 21 and the submerged surface 11 of the hull. In this embodiment, the first member 21 is a plate-shaped member which has a circular or an oval outline and a predetermined thickness, and the second member 22 is a post-shaped member which has an approximately semicircular cross-section. Furthermore, the second member 22 is provided so that a curved surface 22a thereof faces the bow (front side in the traveling direction). In addition, a material, such as a corrosion-resistant metal or plastic that has surface corrosion resistance primarily with respect to sea water and is also resistant to adherence to the surface by marine organisms is preferably used for the material of the first and second members 21 and 22.

[0036] Returning to FIG. 1, the vortex generator 20 generates vortices in the water by varying the flow of the water Fw along the submerged surface (wall surface) 11 of the hull. That is, the water flows along the curved surface 22a, and vortices SC are generated as the water passes the curved surface 22a. The vortices SC extend along the stern so as to rotate around the axes parallel to the longitudinal direction of the vessel (or the axes gradually separate from the wall surface as they approach the stern).

[0037] In this case, the vortices SC generated by the vortex generator 20 induce the flows Fa which are perpendicular to the traveling direction Dv of the hull 10, and with the vanishing of the perpendicular vortices such as the hair-pin vortices, in the boundary layer adjacent to the wall surface, and with the carrying of the momentum of the perpendicular vortices to the outside of the boundary layer, these perpendicular flows Fa change the structures of the vortices which influence the frictional resistance. Consequently, the momentum of the water in the boundary layer is decreased, and the frictional resistance of the hull is reduced.

[0038] In the method of reducing the resistance of a hull which utilizes the momentum of water only, since the disadvantage resulting from the water depth is unlikely to occur as it does in the method by dispersing gas in water, an effective reduction of the frictional resistance can be easily performed. In addition, the directions and axes of the vortices which are newly generated by the vortex generator are not limited to those shown in FIG. 1, and other directions and axes may be employed as long as they can effectively change the structures of the vortices which influence the frictional resistance in the boundary layer. Therefore, the shape, position, and number of vortex generators are suitably determined in compliance with conditions such as the shape of the hull, the standard cruising velocity of the vessel, and the like.

[0039] FIGS. 3A and 3B show another embodiment of the vortex generator.

[0040] In the vortex generator 30, the aforementioned first and second members are formed as wing forms. That is, the vortex generator 30 is composed of a first wing 31 which is provided approximately parallel to the submerged surface 11 of the hull separated by a space, and second wings 32, 33, which are provided between the first wing 31 and the submerged surface 11 of the hull for supporting the first wing 31.

[0041] An NACA wing form, an Ozibal wing form, or various other wing forms can be applied as the shapes of these wings 31, 32 and 33, and are determined in compliance with the shape and the standard cruising velocity of the vessel. Furthermore, the wings 31, 32 and 33 are provided so that their front and back ends face the traveling direction Dv of the hull 10, and the first wing 31 is provided so that its surfaces face the upper and lower directions.

[0042] In the vortex generator 30, vortices (derivative vortices) SC are generated by cyclic flow &Ggr; which are generated around the wings 31, 32 and 33 as shown in FIG. 4. For example, a cyclic flow &Ggr;(1) which flows toward the stem along the surface of the wing 31 that faces a waterway surrounded by the wings 31, 32 and 33 and which flows toward the bow along the opposite (outer) surface of the wing 31, is generated around the wing 31. Similarly, cyclic flows &Ggr;(2) and &Ggr;(3) which flow toward the bow along the surface of the wing 31 that faces the waterway and which flow toward the stem along the opposite (outer) surface of the wing 31, are generated around the wings 32, 33. Therefore, a plurality of vorices SC are generated in the water by these cyclic flows &Ggr;(1), &Ggr;(2) and &Ggr;(3). Furthermore, in the vortex generator 30, since the vortices are generated by the wings, the resistance (drag) against the flow of water is reduced, and an effective reduction of the frictional resistance can be easily performed.

[0043] The aforementioned vortex generator can be applied to a large ship in comparison with other types of vessels in which the surface area of the bottom is formed to be large relative to the side of the vessel. In FIG. 5, reference symbol M indicates a frictional resistance reducing vessel; 40 is a hull; 42 is a hull shell plate (submerged surface); 43 is a propeller; 44 is a rudder; and 45 is a water surface (waterline). The frictional resistance reducing vessel M is a large ship in the manner of, for example, a Very Large Crude-oil Carrier (VLCC). Furthermore, the aforementioned vortex generators 30 (or the vortex generators 20) are provided on the bottom of the hull in the vicinity of the bow of the vessel.

[0044] FIG. 6 shows the bottom of the hull 40 in the vicinity of the bow of the vessel in which the vortex generators 30 (or the vortex generators 20) are provided. A plurality (here, two) of vortex generators 30 are arranged in a row along the width of the bottom of the vessel, and as a result, vortices can be generated in a wider area.

[0045] Next, another embodiment of the method of reducing the frictional resistance of a hull and the frictional resistance reducing vessel of the present invention will be explained in the following.

[0046] FIGS. 7A and 7B schematically show the flow of water in the vicinity of the submerged surface (bottom) 11 of the vessel 10. FIG. 7A is a cross-sectional view of the hull 10 as shown from the side of the hull, and FIG. 7B is a cross-sectional view of the bottom of the hull 10 as shown from the water. In the method of reducing the frictional resistance of the hull of the present embodiment, the method employs a “negative pressure method”, in which a negative pressure region 12 which is at a low pressure relative to the atmosphere, in the water during the operation in order to release gas into the water without using a pressure device.

[0047] The negative pressure region 12 can be formed by varying the relative flow of the water during the operation of the vessel resulting from the occurrence of a separation region or cavitation formed by means of an indentation 13 or an edge member 14 on the submerged surface 11 of the hull 10. At this time, by opening one end of a flow path 16 (air outlet 17) to the negative pressure region 12 and the other end (air inlet 18) to the atmosphere, the force of a pressure gradient Pf (pressure in the air outlet 17 <pressure in the air inlet 18) acts on the fluid in the flow path 16. In this embodiment, air which has flowed in the flow path 16 from the atmosphere through the air inlet 18 is fed into the water through the air outlet 17 in the form of microbubbles 120 by using the force of the pressure gradient Pf.

[0048] Here, when bubbles having a volume Qv are released at a location at a depth h (m) from a liquid surface in a static liquid of density &rgr; (the density of the bubbles is assumed to be zero), the energy required for their release is represented by the following equation:

E=(P−Pa)Qv  (1)

[0049] where Pa is the pressure of the gaseous space (atmospheric pressure), and P is the pressure at the location where the bubbles are released (=&rgr;gh, where g is gravitational acceleration).

[0050] At this time, if the flow rate in the vicinity of the air outlet 17 on the bottom is taken to be V1, then the pressure P at that location is represented by the following equation:

P=Pa−&rgr;(V12−V2)/2+&rgr;gh  (2)

[0051] Furthermore, although the flow rate V1 changes according to the release of bubbles into the boundary layer, this change is ignored here.

[0052] As is clear from Equation (1), for the case when the pressure P at the location where bubbles are released is low in comparison with the atmospheric pressure Pa, namely when P<Pa, the energy becomes negative (E<0), and additional energy is not required to move air to the bottom. That is, in the method of reducing the frictional resistance of the hull of the present invention which employs the negative pressure method, bubbles can be generated in the water using less energy than conventional pressurization methods. Furthermore, in addition to the bubbles that move from the atmosphere to the bottom via the flow path 16, the bubbles in the vicinity of the air outlet 17 also include bubbles generated when the pressure of the negative pressure region 12 becomes low in comparison with the saturated vapor pressure as a result of cavitation and separation that occur results from the pressure of the indentation 13 or edge member 14.

[0053] FIGS. 8A and 8B show an embodiment of a bubble generator for generating bubbles in the water. A bubble generator 50 comprises a recess 51 which is formed on the submerged surface 11 of the bottom of the hull, a flow path 16 in which one end opens to the atmosphere and the other end opens to the recess 51, and a hood 52 which is provided on the submerged surface 11 so as to cover at least a part of the recess 51 from the underwater side.

[0054] In this embodiment, the recess 51 is formed by connecting a member 53 having a crooked plate shape with a rectangular-shaped opening bored through the submerged surface 11 of the hull, and the recess 51 is formed so that the depth from the submerged surface 11 gradually becomes shallower in the direction of the stem. In addition, the flow path 16 of this embodiment is provided as an inner space of an air induction pipe (AIP) 54 which is connected with the plate shaped member 53 for forming the recess 51. That is, an opening 53a for introducing a gas into the water is bored through the plate-shaped member 53, and the tubular air induction pipe (AIP) 54 is connected with the opening 53a. Furthermore, the hood 52 is a member having a crooked plate shape, and an inclined surface 52a is formed thereon so that the amount projecting from the submerged surface 11 is gradually increased in the direction of the stem, and a part of the inclined surface 52a covers the recess 51 from the underwater side. Moreover, the bow end of the hood 52 is connected with the submerged surface 11, and the other end (stem end) of the hood 52 is positioned at a predetermined height from the submerged surface 11 of the hull.

[0055] In this bubble generator 50, the relative flow of the water during the operation of the vessel is varied by the inclined surface 52a of the hood 52 and the recess 51. For example, at the inclined surface 52a of the stem side edge member 14a and at the indentation 13 on the tip end of the recess 51, cavitation and separation occur due to the sharp angle thereof, and hydrostatic pressure decreases. Furthermore, the inclined surface 52a of the hood 52 facilitates a decrease in hydrostatic pressure, as a result of narrowing of the water flow path and the increase in the water flow rate, since the amount of the inclined surface 52a projecting from the submerged surface 11 is gradually increased in the direction of the stem. Consequently, the aforementioned negative pressure region is formed in the water. Moreover, some of the bubbles which are released from the flow path 16 enter into the recess 51 and are diffused along the width of the vessel, and therefore, a large area of the submerged surface 11 of the hull is covered by concentrated bubbles. In addition, as a result of cavitation and separation occurring at the inclined surface 52a of the stem side edge member 14a and the indentation 13 on the tip end of the recess 51 in the negative pressure region, the gas and water is actively mixed at the interface thereof, facilitating the release of bubbles from the interface. Furthermore, in the bubble generator 50, since the recess 51 and the hood 52 are manufactured using a crooked plate shape member, a plurality of recesses 51 and hoods 52 can be provided on the submerged surface 11 of the hull without reducing the strength thereof.

[0056] Returning to FIG. 7, in this embodiment, a wing 25, which has a surface approximately parallel to the submerged surface 11 of the hull, generates the cyclic flow &Ggr;(1), which develops the negative pressure region 12. That is, the cyclic flow &Ggr;(1), which flows toward the stem (rear) along the surface of the wing 25 that faces the submerged surface 11 and which flows toward the bow (traveling direction Dv) along the opposite surface of the wing 25, is generated around the wing 25. At this time, in the waterway between the submerged surface 11 of the hull and the wing 25, the flow rate thereof is increased because the momentum of the water caused by the cyclic flow &Ggr;(1) is added to the flow 15 of the water along the submerged surface 11, and therefore, the pressure in the negative pressure region 12 is further decreased. Consequently, the negative pressure region 12 is further developed, the force of the pressure gradient Pf is increased, a large quantity of air bubbles 120 are released into the water, and the frictional resistance of the hull is effectively reduced by the presence of these bubbles 120 on the submerged surface 11.

[0057] Furthermore, in this embodiment, due to the circulating flow &Ggr;(1) that occurs around the wing 25 (because the flow rate over the upward facing wing surface is greater than that over the downward facing wing surface), a pressure difference occurs above and below the wing 25, and an upward lift Lf acts on the hull 10 from the wing 25. Consequently, the bow of the hull 10, in particular, is raised up due to this lift, the submerged surface area of the hull 10 decreases, and the frictional resistance of the hull 10 is further reduced.

[0058] Moreover, in this embodiment, similar to the embodiment shown in FIG. 1, the flows Fa, which are perpendicular to the traveling direction Dv of the hull 10, are induced, and the perpendicular flows Fa change the structure of the vortices in the vicinity of the submerged surface 11 in order to reduce the frictional resistance of the hull 10 as shown in FIG. 7B.

[0059] That is, in this embodiment, the perpendicular flows Fa are induced by providing objects 26, 27 on the submerged surface of the hull for generating new vortices (derivative vortices) SC in the water, and the perpendicular flows Fa change the structure of the vortices related to the frictional resistance, such as the perpendicular vortices in the vicinity of the submerged surface 11, in order to reduce the frictional resistance of the hull 10, for example.

[0060] The vortices extending from the bow to the stern of the vessel and having axes along the longitudinal direction of the vessel, for example, are preferably provided as the vortices SC, similar to those of the embodiment shown in FIG. 1. Furthermore, the vortices SC can be easily generated by generating the circulating flow &Ggr;(2), &Ggr;(3) around the objects 26, 27. The vortices SC extend along the stem and change so as to rotate around the axes parallel to the longitudinal direction of the vessel (or axes gradually separate from the wall surface as they approach the stem), and induce the flows Fa which are perpendicular to the traveling direction Dv of the hull 10. These perpendicular flows Fa change the structures of the vortices related to the frictional resistance, similar to those of the embodiment shown in FIG. 1. Consequently, the momentum of the water in the boundary layer is decreased, and the frictional resistance of the hull is reduced.

[0061] Moreover, in this embodiment, the vortices (derivative voritices) SC generated by the objects 26, 27 are developed by mixing the aforementioned bubbles 120 into the vortices SC. That is, when the bubbles are mixed with the vortices SC, flows Fd of the fluid are generated along the direction of separation from the wall surface of the hull by means of the momentum of the bubbles caused by their buoyancy. Therefore, a plurality of bubbles are formed since the momentum of the original vortices are diffused, the momentum of the vortices related to the frictional resistance in the boundary layer is decreased, and the frictional resistance of the hull is reduced. In this case, it is preferable that the objects 26, 27 for generating the bubbles are provided in front of the aforementioned air outlet 17 on the submerged surface 11 so as to separate on both sides of the air inlet 17 along the width of the vessel, in order to mix the bubbles 120 into the vortices SC effectively.

[0062] FIGS. 9A and 9B show an embodiment of a wing body 60 which has the aforementioned wing 25 and struts 26, 27 as the aforementioned objects. One end of each of the struts 26, 27 is connected with the submerged surface 11 of the hull 10 and the other end of each of the struts 26, 27 is connected with each end of the wing 25 along the width thereof. An NACA wing form, an Ozibal wing form or various other wing forms can be applied for the shapes of the wing 25 and struts 26, 27, and are determined in compliance with the shape and the standard cruising velocity of the vessel. Furthermore, the wing 25 and struts 26, 27 are provided so that their front and back ends face the traveling direction Dv of the hull 10, and the wing 25 is provided so that its convex surface faces upward. In addition, a material, such as a corrosion-resistant metal or plastic that has surface corrosion resistance primarily with respect to sea water and which is also resistant to adherence to the surface by marine organisms, is preferably used for the material of the wing 25 and struts 26, 27. Moreover, the directions and axes of the vortices SC which are newly generated by the wing body 60 are not limited to this, and they may also be of another form as long as they can effectively change the structures of the vortex concerning the frictional resistance in the boundary layer. Furthermore, the vortices SC can be generated with low resistance (drag) against the flow of water by employing the circulating flow generated by the wing.

[0063] The aforementioned bubble generator 50 and wing body 60 can also be applied to a large ship as shown in FIG. 10 similar to the vortex generators 20, 30, for example. In addition, FIG. 11 shows the bottom of a hull 40 in the vicinity of the bow of the vessel in which the bubble generator 50 and the wing body 60 are provided. A plurality (here, two) of pairs of the bubble generator 50 and the wing body 60 are arranged along the width of the bottom of the vessel. Since the pairs are arranged along the width, a wider area on the bottom can be covered by the vortices and the frictional resistance of the hull can be reduced more effectively. Furthermore, the size, number and location of the bubble generators and wing bodies are suitably determined in compliance with conditions such as the shape of the hull and the standard cruising velocity of the vessel. Moreover, the shape of the bubble generator and wing body are suitably determined based on the results of flow field analysis, operational testing, etc. obtained by computational fluid dynamics (CFD), so that the resistance (drag) against the flow of water is minimized, and the flow of water is in the desired state during operation.

[0064] In this embodiment, the frictional resistance of the hull is reduced more effectively as a result of the generation of circulating flows &Ggr;(1), &Ggr;(2) and &Ggr;(3) by the wing body 60. Since adequate circulating flows are generated even during low-speed operation (for example, at about 10 knots), this embodiment can be applied over a wide range of operating velocities.

[0065] Furthermore, since bubbles which are mixed into the water are formed at an internal pressure lower than the hydrostatic pressure corresponding to the water depth, when the above bubbles move at a constant water depth (for example, when the bubbles are moving along the vessel bottom), a higher water pressure acts on the bubbles the farther they move away from the negative pressure region, thereby causing the size of the bubbles to gradually become smaller. According to research conducted thus far by the present applicants, comparatively small bubbles are preferable for reducing the frictional resistance of hull. Thus, bubbles generated by the negative pressure also act advantageously for reducing the frictional resistance with respect to this point as well.

[0066] In addition, the various shapes and combinations, etc. of each composite member shown in the embodiment described above refer to only a single example, and can be altered in various ways based on design requirements and the like within a range that does not deviate from the purpose of the present invention. Furthermore, since the present invention is not considered to be susceptible to disadvantages caused by water depth, the frictional resistance reducing vessel of the present invention is also advantageous for application to the aforementioned large ships. However, the type of vessel to which the present invention is applied is not limited to large ships, and the hull may also be of another form such as that of a high-speed vessel or a fishing vessel.

Claims

1. A method of reducing frictional resistance of a hull comprising a step for inducing perpendicular flows along a running direction of the hull accompanying operation of a vessel, and a step for varying vortex structures in the vicinity of a submerged surface of the hull by the perpendicular flows.

2. A method of reducing frictional resistance of a hull according to claim 1, wherein said perpendicular flows are induced by vortices which are generated by an object which is provided on said submerged surface.

3. A method of reducing frictional resistance of a hull according to claim 1, wherein said vortex structures are varied by vortices which are generated by an object which is provided on said submerged surface.

4. A frictional resistance reducing vessel for reducing frictional resistance between a submerged surface of a hull and water, comprising a vortex generator which generates vortices for inducing perpendicular flows along a running direction of the hull accompanying operation of the vessel.

5. A method of reducing frictional resistance of a hull by releasing bubbles onto a submerged surface of the hull comprising: a step for forming a negative pressure region at a low pressure relative to a gaseous space in the water accompanying operation of a vessel, and inducing a gas being led from the gaseous space to the negative pressure region; and a step for inducing perpendicular flows along a running direction of the hull and varying vortex structures in the vicinity of the submerged surface by the perpendicular flows.

6. A method of reducing frictional resistance of a hull according to claim 5, wherein said negative pressure region is developed by a cyclic flow which is generated by a wing which is provided on said submerged surface, and said perpendicular flows are generated by vortices which are generated by the cyclic flow.

7. A method of reducing frictional resistance of a hull according to claim 6, wherein an upward lift which acts on the hull is generated by said wing.

8. A frictional resistance reducing vessel for reducing frictional resistance between a submerged surface of a hull and water, comprising: a negative pressure forming portion which is provided on the submerged surface and which forms a negative pressure region at a low pressure relative to a gaseous space in the water; an outlet which is provided behind the negative pressure forming portion; a flow path of which one end opens to a gaseous space and the other end opens to the water via said outlet; and an object which generates vortices for inducing perpendicular flows along a running direction of the hull accompanying operation of the vessel.