KR20130120656A - Apparatus for redusing air resistance for vessel - Google Patents

Abstract

An apparatus for reducing the air resistance of a vessel is disclosed. According to an embodiment of the present invention, an apparatus for reducing the air resistance of a vessel comprises a first container row which is aligned in the width direction of a vessel; a second container row which is separated from the first container row with a gap in the longitudinal direction of the vessel; and, possibly, a gap removing module which suppresses flow separation and stagnation occurring when the flow passes the gap.

Description

Air resistance reduction device of ship {APPARATUS FOR REDUSING AIR RESISTANCE FOR VESSEL}

The present invention relates to a device for reducing the air resistance of a ship, and to a device for reducing the air resistance of a ship used in container ships, cargo ships and the like.

Vessels can be broadly classified into merchant ships, warships, fishing vessels and special working vessels according to their purpose of use.

Merchant ships are vessels intended to carry passengers or cargo to earn freight revenue, which are further divided into cargo ships, passenger ships and passenger ships.

In general, a ship may have a shape above the surface that is susceptible to wind pressure. For example, when a private ship, a container ship, or a tanker receives wind from the front, not only the ship speed is lowered by the direct influence of the wind, but also the underwater resistance due to the change of attitude of the ship due to the wind (tilted navigation). This increase is known to affect the running performance.

In particular, container ships carry cargo such as container boxes (hereinafter referred to as 'containers') on the deck as well as inside the hull.

When such a container ship is winded obliquely from the left or right front of the hull, a lateral force is generated from the starboard or the port of the ship, and a yawing moment may be generated to impart a clockwise rotational force to the hull.

At this time, in order to maintain the heading in the original direction of the ship against the yawing moment, it is necessary to adjust the rudder, and the force of the water acting on the rudder may become a resistance.

Since the ship is subjected to the lateral force in this manner, the ship is moved obliquely and the water resistance acting on the hull can also be increased.

As a background art of the invention, a patent document discloses a vessel for reducing the underwater resistance caused by the lateral force of the hull.

However, the vessels of the patent document form a hull in a linear form as a dedicated line for automobiles, or provide a housing for covering the ventilation unit, and resists against wind that is inclined to the side or the side through the gap between the containers. While reducing the increase, there is a means for reducing the air resistance due to the gap formed between the plurality of container loading portions.

That is, referring to Figure 1, when looking at the main features of the upper region of the container ship there is a lashing (lashing) portion for binding the container, which is due to this between one container and the other container or container loading portion (1, 2) There is a gap corresponding to a relatively large area, that is, a gap G, and there are about 15 or more such gaps G in the longitudinal direction of the ship.

Here, the gap G may be a lashing space between one container and the other container or between the container loading parts 1 and 2.

Looking at the effect of the gap (G) on the speed performance, if the wind blows from the front it is free-stream (similar to the resistance increase or decrease).

However, when the wind (W) is blown in the lateral direction or the inclined direction of the side, the high pressure acts on the end (P) of the container loading portion (1, 2) and at the same time the flow entering through the gap (G) from the rear of the container As the fall off, large peeling occurs and stagnant flow (Q) is generated, resulting in an increase in resistance.

Published Patent Publication No. 10-2006-0011771

The embodiment of the present invention can minimize the air resistance by controlling the air flow in the gap between the first container row and the second container row corresponding to the lashing space for fixing the container, to achieve improved fuel economy in the actual operating state It is intended to provide a device for reducing the air resistance of a ship.

According to an aspect of the present invention, in the air resistance reduction apparatus of a ship having a first container row arranged in the hull width direction and a second container row spaced apart from the gap in the longitudinal direction of the hull, the flow entering through the gap An air resistance reduction apparatus of a ship may be provided that includes a gap elimination module that prevents separation and stagnation flow generated when the flow passes through the gap.

At this time, the gap removal module may be installed on the deck of the vessel at the entry point of the gap on the basis of the width direction of the vessel.

The gap removing module may include a pair of pillars standing upright at a port or starboard entry portion of the gap with a separation distance corresponding to a width of the gap; And a cover part coupled between the pillar parts.

The gap removing module may further include: a pair of guide rails installed in parallel to each other along both edges between the rows of containers corresponding to the gaps; A shutter box disposed between the guide rails; And a shutter door provided in the shutter box to open or close a gap by moving upward or downward along the guide rail from a rotating drum driven by a motor.

In addition, the gap removal module, the gap is disposed on the bottom of the gap, the roller portion for winding or keeping the curtain wound around the bobbin by the motor driving force to provide; A pulley portion rotatably supported at an upper end of the container above the roller portion; A winch part spaced horizontally from the roller part; And a wire part connected to a plurality of ends of the curtain and extending upwardly and downwardly from the roller part to be connected to the winch part.

The gap elimination module may include a pair of bottom rails disposed along a width direction of the ship at a bottom surface of the gap; A hinge portion corresponding to an extension direction of the bottom rail and installed on a bottom surface of a port or a starboard entry point of the gap; A first plate rotatably coupled to the hinge portion and having a size corresponding to a width of the gap and a maximum container loading height; And a joint end portion disposed between the first plate and the bottom rail and hinged to both sides of the rear surface of the first plate at one end thereof, and the wheel driving device for driving or stopping the wheel along the bottom rail. It may include a pair of supports provided at the end.

In addition, the gap elimination module, the hinge portion is provided on the bottom surface of the entry point of the port or starboard of the gap; A second plate rotatably coupled to the hinge portion and having a size corresponding to a gap width and a maximum container loading height; And a plurality of actuators connected between the bottom surface of the gap and the second plate.

In addition, the gap elimination module, the hinge portion is provided on the bottom surface of the entry point of the port or starboard of the gap; A third plate rotatably coupled to the hinge portion, the third plate having a size corresponding to a gap width and a maximum container loading height, and having a driven gear on a plate leg corresponding to the rotation axis of the hinge portion; The third plate may include a gear driving unit for driving the drive gear engaged with the driven gear by a motor so that the third plate stands or lays down by rotation based on the hinge portion.

An embodiment of the present invention is to provide a vessel air resistance reduction apparatus that can minimize the air resistance by closing the container gap or control the opening and closing of the gap with the gap removal module.

In addition, the embodiment of the present invention by the gap removal module to suppress the separation and stagnation flow of the flow coming through the gap between the first container row and the second container row, the ship having the gap removal module in actual operation There is an advantage that can achieve improved fuel economy in the state.

1 is a block diagram illustrating a gap between a container and a container according to the prior art.
2 is a view comparing the configuration between the vessel equipped with a device for reducing the air resistance of the vessel according to an embodiment of the present invention and the unloaded vessel.
3 is a graph showing the relationship between the wind load coefficient (Cx) and the wind direction angle (AOA) according to the computational fluid analysis (CFD).
4 is a perspective view of the first gap removing module of the air resistance reduction apparatus of the ship shown in FIG.
5 is a perspective view of a second gap removing module of the air resistance reduction apparatus of the ship shown in FIG.
6 is a perspective view of the third gap removing module of the air resistance reduction apparatus of the ship shown in FIG.
7 is a perspective view of a fourth gap removing module of the air resistance reduction apparatus of the ship shown in FIG.
8 is an operational state diagram of the fourth gap removing module illustrated in FIG. 7.
9 is a perspective view of a fifth gap removing module of the air resistance reduction apparatus of the ship shown in FIG.
FIG. 10 is an operational state diagram of the fifth gap removing module illustrated in FIG. 9.
FIG. 11 is a perspective view of a sixth gap removing module of the air resistance reduction apparatus of the ship shown in FIG. 2.
FIG. 12 is an operational state diagram of the sixth gap removing module illustrated in FIG. 10.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description of the present invention, detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.

2 is a view comparing the configuration between the vessel equipped with a device for reducing the air resistance of the vessel according to an embodiment of the present invention and the unloaded vessel.

Referring to FIG. 2, the present embodiment may provide a gap removing module 30, which is an air resistance reduction device mounted on a container ship.

2 (a) and 2 (c) show a case of a general container ship, and a second container spaced apart from the first container row 10 arranged in the hull width direction and the gap G in the longitudinal direction of the hull. There is a column 20.

The first container row 10 or the second container row 20 may be arranged in plurality in the longitudinal direction of the hull.

The gap G may be between the first container row 10 and the second container row 20, as well as between other container rows through which the wind passes to generate delamination and stagnation flow. .

In this case, (a) is a case where the container ship loaded 100% of the container, and (c) is a case where 50% is loaded.

2 (b), (d) shows a container ship equipped with a device for reducing air resistance of the ship according to the present embodiment.

Here, the gap removing module 30 may be formed between the first container row 10 and the second container row 21, which are 100% stacked, or the first container row 11 and the second container row 21, which are 50% stacked. As disposed between, it may correspond to the flow interference.

Here, the flow interference portion may be formed of any one or a combination of the gap removal module 30 or the first to sixth gap removal module to be described below to suppress the separation and stagnation flow of the flow coming through the gap. .

In addition, the gap removing module 30 corresponds to the port and starboard ends of the gap G between the adjacent first container rows 10 and 11 and the second container rows 20 and 21 based on the width direction of the ship. Installed in any one or more of the deck of the ship can play a role of suppressing the separation and stagnation flow (flow) through the gap (G).

For example, the gap removing module 30 may be a windbreak means such as a cover part, a shutter door, a curtain, a plate, or the like, which prevents the entry portion of the port G and the starboard (for example, a gap inlet or an outlet) of the gap G, or a material to be described in detail below. It may be a first to sixth gap removing module.

As a result of performing simulation, that is, computational fluid analysis (CFD) of the ship having the gap elimination module 30 as described above, it is possible to confirm the effect of reducing air resistance as shown in FIG. 3.

3 is a graph showing the relationship between the wind load coefficient (Cx) and the wind direction angle (AOA) according to the computational fluid analysis.

According to the performance evaluation using the computational fluid analysis of Figure 3, when the container 100% loading as shown in Figure 3 (e) it was shown that there is an effect of reducing the air resistance of about 40%.

In addition, as shown in (f) of FIG. 3, when the container is loaded at 50%, the air resistance is reduced by about 35%.

That is, referring to FIG. 2 and FIG. 3 simultaneously, as a result of comparing the wind load coefficient (Cx) according to the presence or absence of a gap, the wind direction angle (AOA) identified as an important area in the actual operation state of the container ship Air resistance reduction device of the ship of the present embodiment so that there is no gap (G) than when there is a conventional gap (G) in the range of 0 to 60 degrees (e.g. w / o plate, w / o means without) That is, when the gap removal module 30 is installed or mounted (for example, w / plate, where w / means with), air resistance is reduced by about 35% to 40%.

The reason for this phenomenon is that as the gap G is removed by the gap removing module 30 or the first to sixth gap removing modules having various configurations to be described below, the pressure acting in the lateral direction decreases. At the same time, it can be understood that the air resistance decreases due to the reduction of stagnant flow accompanied by large separation.

In addition, ships without a gap G (w / o Gap) by various gap removal modules 30 described in this embodiment are shown in Table 1 below (100% container loading) and Table 2 ( It can be seen that there is an improvement in fuel economy under actual operating conditions, such as 50% loading containers. Here, the evaluation of the fuel efficiency improvement effect (eg, the reduction rate in the actual operation state) in the actual operating state may be determined by multiplying the operation rate by the fuel reduction rate due to the decrease in air resistance for each BF.

Here, BF means a viewport wind scale and may mean sea roughness or wind speed intensity. For example, in the case of BF 0, wind speed means a sea roughness of 1 knot or less corresponding to wind speed, and in the case of BF 8, wind speed 34-40 knots or a corresponding sea roughness as fresh gale. Can be.

Hereinafter, the first gap removing module to the sixth gap removing module having the same effect as the aforementioned gap removing module 30 and having a detailed structure or driving method will be described.

4 is a perspective view of the first gap removing module of the air resistance reduction apparatus of the ship shown in FIG.

Referring to FIG. 4, a gap may exist between the first container row 10 and the second container row 20 arranged in the width direction of the ship.

The first gap removing module 100 may be a flow interference part, and has a separation distance corresponding to the width of the gap between one container and the other container or between the first container row 10 and the second container row 20. It may include a pair of pillar portions 101 and 102 standing up at the port and starboard entry portion of the gap, and a cover portion 103 coupled between the pillar portions 101 and 102.

Here, since the pillars 101 and 102 may have a height corresponding to the maximum container loading height according to the type of container ship to be installed, the height of the pillars 101 and 102 may not be limited.

The pillars 101 and 102 may have a simple post shape or may be manufactured in a structure capable of varying the length in a telescopic structure.

Port and starboard entry portion of the gap may refer to the top surface of the deck of the ship located at the end of the port and starboard relative to the width direction of the vessel in the gap.

The cover part 103 may be a windproof panel made of a lightweight material such as a cloth for windproof material for reducing weight, an aluminum alloy, or the like.

The cover 103 may be installed and fixed to the pillars 101 and 102 by using a suitable fixing means for the material.

The first gap removing module 100 is a fixed structure, easy to configure and install the device, it is possible to minimize the weight increase according to the installation of the device compared to the drive device, there is an advantage that easy maintenance.

5 is a perspective view of a second gap removing module of the air resistance reduction apparatus of the ship shown in FIG.

Referring to FIG. 5, the second gap removing module 200 may be configured to adjust the opening or closing height of the gap according to the container stacking height so as to open or close the gap, such as a wind pressure shutter device. .

For example, the second gap removing module 200 includes a pair of guide rails 210 installed in a vertical direction parallel to each other along both edges between the first container row 10 and the second container row 20 corresponding to the gap. , 220, and a shutter box 230 disposed below the guide rails 210 and 220, and a guide rail from a rotating drum 231 provided in the shutter box 230 and driven by a motor 232. The shutter door 240 may move upward or downward along the 210 and 220 to open and close the gap.

The shutter door 240 includes a plurality of folding-type slats for soundproofing, dustproofing, and heat dissipation, and face each other so that the slats are not separated from the guide rails 210 and 220 so as to overcome the wind pressure. It may be coupled to the sliding groove of the guide rails (210, 220) extending to.

The second gap removal module 200 may adjust the height of the upper end of the shutter door 240 up and down according to the container loading height.

6 is a perspective view of the third gap removing module of the air resistance reduction apparatus of the ship shown in FIG.

Referring to FIG. 6, the third gap removing module 300 is a semi-driven device, and may be manufactured as a collapsible curtain type device.

To this end, the third gap removing module 300 is disposed at the bottom of the gap, the roller portion 310 to store or release the curtain 320 to the rotating drum by the motor driving force, and the upper portion of the roller 310 A plurality of pulleys 340 rotatably supported at the top of the container, a winch 350 disposed horizontally spaced from the roller 310, and a plurality of ends of the curtain 320 and extending upwardly. Then it may include a wire portion 330, 331 connected to the winch unit 350 by extending downward from the roller unit 310 as a starting point.

Here, both ends of the pulley portion 340 includes a pair of clamping devices that can be mounted on the container or the lashing structure around the container, a shaft rotatably coupled to the clamping device, and a rotating wheel coupled to the shaft. Can be configured.

In addition, the third gap removing module 300 may further include a stop sensor and a driving synchronization controller (not shown).

When the drive synchronization controller rotates the rotating drum from the roller portion 310 toward the unwinding of the curtain 320, the drive synchronization controller operates the winch portion 350 toward the winding of the wire portion 330 connected to the curtain 320. ) May be expanded upward from the bottom of the gap between the container rows.

Conversely, the drive synchronization controller rotates the rotating drum from the roller portion 310 to the winding of the curtain 320 so that the curtain 320 is moved downward and received, the wire portion 330 connected to the curtain 320. ) Can control the operation of the winch unit 350 to release.

The third gap removal module 300 does not need to install a weight structure such as a guide rail as compared to the second gap removal module described above, and can adjust the unfolding height of the curtain according to the container loading height while increasing the weight of the ship. The burden may be less.

7 is a perspective view of a fourth gap removing module of the air resistance reduction apparatus of the ship shown in Figure 2, Figure 8 is an operating state diagram of the fourth gap removing module shown in FIG.

Referring to FIG. 7 or 8, the fourth gap removing module 400 may minimize or reduce interference effects when unloading or loading containers by storing or standing up the first plate in the width direction of the vessel.

The fourth gap removing module 400 coincides with an extension direction of the bottom rail 410 and the pair of bottom rails 410 disposed along the width direction of the ship at the bottom surface (eg, deck) of the gap, and the gap. The hinge portion 421 installed on the bottom surface of the port and starboard entry point of the first port 420, rotatably coupled to the hinge portion 421, the first plate 420 having a size corresponding to the width of the gap and the maximum container loading height And a joint end portion 431 disposed between the first plate 420 and the bottom rail 410 and hinged to both sides of the rear surface of the first plate 420, the bottom rail 410 is provided. It may include a pair of supports 430 having a wheel driving device 440 at the other end to drive or stop the wheel along.

Here, the bottom rail 410 may have a groove structure rail structure formed on the bottom surface of the gap or a rail bar type projecting on the bottom surface.

In addition, the wheel driving device 440 may further have a device (not shown) for driving and controlling the wheel.

In addition, in the storage state as shown in FIG. 8, the first plate 420 may be in a down state.

In this case, when the wheel driving device 440 moves toward the first plate 420, a force for pushing up the first plate 420 by the support 430 connected to the wheel driving device 440 may act. As a result, the first plate 420 may stand as shown in the use state of FIG. 7 while rotating based on the hinge portion 421.

The wheel driving device 440 or the bottom rail 410 is further provided with a stop sensor, an operation controller, and the like, which are not shown, and the operation controller is a time point at which the first plate 420 is fully standing vertically at the bottom of the gap. The wheel driving device 440 may be controlled by the signal of the stop sensor input when the first plate 420 lies down toward the bottom surface of the gap.

On the contrary, when the wheel driving device 440 moves to the opposite side of the first plate 420, a force for pulling down the first plate 420 by the support 430 connected to the wheel driving device 440 may be applied. As a result, the first plate 420 may be returned to its original lying state while rotating backward with respect to the hinge portion 421.

The fourth gap removing module 400 may be used to store or stand at the bottom of the gap by rotating the entire first plate 420, and guide the second gap removing module that may generate interference when unloading or loading the container. There is an advantage that does not require a rail or the like.

9 is a perspective view of a fifth gap removing module of the air resistance reduction apparatus of the ship shown in Figure 2, Figure 10 is an operating state diagram of the fifth gap removing module shown in FIG.

9 or 10, the fifth gap removing module 500 may be characterized by using a hydraulic system when the second plate is laid down or standing up in the width direction of the ship.

That is, the fifth gap removing module 500 is rotatably coupled to the hinge portion 521 and the hinge portion 521 provided on the bottom surface of the port and the starboard starting position of the gap, and the width of the gap and the maximum container loading. A plurality of actuators 510 connected to the second plate 520 having a size corresponding to the height, a hydraulic source or a hydraulic circuit provided in the container ship, and connected between the bottom surface of the gap and the second plate 520. ) May be included.

One end of each actuator 510 is connected to the joint of the bottom surface of the gap, the other end of each actuator 510 is connected to the joints on both sides of the rear surface of the second plate 520, in the same principle as the hydraulic door Since it can be configured and operated, the operation relationship of the fifth gap removing module 500 can be omitted.

FIG. 11 is a perspective view of a sixth gap removing module of the air resistance reduction apparatus of the ship illustrated in FIG. 2, and FIG. 12 is an operating state diagram of the sixth gap removing module illustrated in FIG. 10.

Referring to FIG. 11 or FIG. 12, the sixth gap removing module 600 may use a gear system when the third plate is laid down in the width direction of the ship or used in a standing position.

That is, the sixth gap removing module 600 is rotatably coupled to the hinge portion 624 and the hinge portion 624 installed on the bottom surface of the port and the starboard starting position of the gap, the gap width and maximum container loading A third plate 610 having a size corresponding to the height and having a driven gear 623 on a plate leg corresponding to the rotation axis 625 of the hinge portion 624, and the third plate 610 is a hinge portion ( It may include a gear driving unit 620 for driving the drive gear 622 engaged with the driven gear 623 by the motor 621 to stand or lay down by rotation based on the 624.

Here, the engaged drive gear 622 and the driven gear 623 may be configured as a worm gear set, or when the power transmission direction is properly designed, various types of gear sets may be used, and thus may not be limited thereto.

In addition, the sixth gap removing module 600 also operates to control the operation of the motor 621 by a stop sensor that detects an operating state of the driven gear 623 or the third plate 610 and an input signal of the stop sensor. It may further include a controller.

The operation controller of the sixth gap removing module 600 also stops input when the third plate 610 is fully standing vertically at the bottom of the gap or when the third plate 610 is laid down toward the bottom of the gap. The motor 621 can be stopped by the signal of the sensor.

The sixth gap removal module 600 may also minimize the interference effect when unloading or loading the container by storing or standing up the third plate 610 in the width direction of the vessel.

In particular, the gear driving unit 620 is disposed below the third plate 610, so that it does not require a relatively large installation area compared to the above-described floor rail or actuator, it is possible to maximize the space utilization.

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, You will understand. For example, a person skilled in the art can change the material, size and the like of each constituent element depending on the application field or can combine or substitute the embodiments in a form not clearly disclosed in the embodiment of the present invention, Of the range. Therefore, it should be understood that the above-described embodiments are to be considered in all respects as illustrative and not restrictive, and that such modified embodiments are included in the technical idea described in the claims of the present invention.

30, 100 ~ 600: gap removal module 101, 102: column
103: cover portion 210, 220: guide rail
230: shutter box 240: shutter door
310: roller portion 320: curtain
340: pulley portion 350: winch portion
410: floor rail 420: first plate
430: support 440: wheel drive device
510: actuator 520: second plate
610: third plate 620: gear drive unit

Claims (8)

In the ship's air resistance reduction device having a first container row arranged in the hull width direction and a second container row spaced apart from the gap in the longitudinal direction of the hull,
It includes a gap removal module for blocking the flow flowing through the gap to suppress the separation and stagnation flow generated when the flow passes through the gap;
Air drag reduction device for ships.
The method of claim 1,
The gap removal module,
An apparatus for reducing air resistance of a vessel installed on the deck of the vessel at the entry point of the gap with respect to the width direction of the vessel.
3. The method of claim 2,
The gap removal module,
A pair of pillars standing upright at a port or starboard entry portion of the gap with a separation distance corresponding to the gap width; And
It includes a cover portion coupled between the pillar portion
Air drag reduction device for ships.
3. The method of claim 2,
The gap removal module,
A pair of guide rails installed in a vertical direction parallel to each other along both edges between the first and second container rows corresponding to the gap;
A shutter box disposed between the guide rails; And
It is provided in the shutter box, comprising a shutter door for opening and closing the gap by moving up or down along the guide rail from the rotating drum driven by a motor
Air drag reduction device for ships.
3. The method of claim 2,
The gap removal module,
A roller unit disposed at the bottom of the gap and configured to wind or store the curtain around the bobbin by a motor driving force;
A pulley portion rotatably supported at an upper end of the container above the roller portion;
A winch part spaced horizontally from the roller part; And
A plurality of wires connected to an end of the curtain and extending upwardly and extending downwardly from the roller to the winch;
Air drag reduction device for ships.
3. The method of claim 2,
The gap removal module,
A pair of bottom rails disposed along the width direction of the ship at the bottom surface of the gap;
A hinge portion corresponding to an extension direction of the bottom rail and installed on a bottom surface of a port or starboard position of the gap;
A first plate rotatably coupled to the hinge portion and having a size corresponding to a width of the gap and a maximum container loading height; And
Disposed between the first plate and the bottom rail, and having a joint end portion hinged to both sides of the rear surface of the first plate at one end, the other end of the wheel driving device for driving or stopping the wheel along the bottom rail Includes a pair of supports provided in
Air drag reduction device for ships.
3. The method of claim 2,
The gap removal module,
A hinge portion provided on the bottom surface of the port or starboard entry position of the gap;
A second plate rotatably coupled to the hinge portion and having a size corresponding to a gap width and a maximum container loading height; And
A plurality of actuators connected between the bottom surface of the gap and the second plate
Air drag reduction device for ships.
3. The method of claim 2,
The gap removal module,
A hinge portion provided on the bottom surface of the port or starboard entry position of the gap;
A third plate rotatably coupled to the hinge portion, the third plate having a size corresponding to a gap width and a maximum container loading height, and having a driven gear on a plate leg corresponding to the rotation axis of the hinge portion;
A gear driving part for driving the drive gear engaged with the driven gear by a motor so that the third plate stands or lays down by rotation with respect to the hinge part.
Air drag reduction device for ships.

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