EP3055146A1

EP3055146A1 - Aquatic vehicle

Info

Publication number: EP3055146A1
Application number: EP14851535.6A
Authority: EP
Inventors: Kari NEULANIEMI; Kai JÄRVIMÄKI; Kai Peltonen
Original assignee: Patria Land Systems Oy
Current assignee: Patria Land Systems Oy
Priority date: 2013-10-11
Filing date: 2014-10-07
Publication date: 2016-08-17
Also published as: FI125295B; FI20136016A; EP3055146A4; WO2015052378A1

Abstract

The invention relates to an aquatic vehicle, and a method of balancing a vehicle in water. The aquatic vehicle is pro- vided with a body and a bow wave plate arranged in connection with the body. The bow wave plate is turnable towards the body and away from the body. Between the bow wave plate and the body, a bag is arranged. The bag has a wall made of a material which is water-and air-tight as well as flexible.Further, a lower part of the bag is provided with an opening.

Description

Aquatic vehicle

Background of the invention

[0001] The invention relates to an aquatic vehicle.

[0002] The invention further relates to a method of balancing a vehicle in water.

[0003] Properties that are important to an aquatic vehicle include those of floating and moving. Particularly in higher waves and when the travel speed is to be increased, it becomes more and more challenging to make these properties sufficient. It is quite challenging to make these properties sufficient particularly in vehicles that are viable both on land and in water. In armoured amphibious vehicles, the weight and shape of the vehicle and, consequently, distribution of the weight, present special challenges to the properties of such vehicles to float and move in water.

Brief description of the invention

[0004] An object of the present invention is to provide a novel aquatic vehicle, and a method of balancing a vehicle in water.

[0005] The solution according to the invention is characterized by what is disclosed in the independent claims.

[0006] Some embodiments of the invention are disclosed in the dependent claims.

[0007] In the disclosed solution, the aquatic vehicle is provided with a body and a bow wave plate arranged in connection with the body. The bow wave plate is turnable towards the body and away from the body. Between the bow wave plate and the body, a bag is arranged. The bag has a wall made of a material which is water- and air-tight as well as flexible. Further, a lower part of the bag is provided with an opening. Such a solution enables, when desired, the bow wave plate to be turned so as to prevent water from flowing onto the body while the bag enables water to be prevented from collecting behind the bow wave plate, onto the body. Consequently, the swimming position of the vehicle can be kept as desired. The solution enables the vehicle to be used in quite high waves as well as at quite a high travel speed. The bow wave plate is quick to be turned both open, i.e. away from the body, and close, i.e. towards the body.

[0008] According to an embodiment, the vehicle may be used such that when driving into water, the bow wave plate is turned away from the body, in which case the bow wave plate straightens up the bag. When driving in water, the opening in the bottom of the bag settles below the surface of water, in which case the pressure of the water inflates the bag, i.e. when travelling in water, the bag is inflated and thus works efficiently, improving the swimming properties. Upon driving out of the water, the bow wave plate can be quickly turned towards the body as the bag is emptied through the opening in the bottom.

[0009] The idea of an embodiment is that the bag is narrower than the bow wave plate. In such a case, the structure of the disclosed solution can be implemented in a simple manner. The bow wave plate may for instance be provided with a middle plate behind which the bag is arranged, and both sides of the middle plate may be provided with a side protection, no bag being thus provided behind the side protections. For example in such a solution, water is allowed to flow off the body at points behind the side protections, also increasing the flow of water away from behind the middle plate, off the body.

[0010] The idea of an embodiment is that the opening in the bottom is arranged in a middle part of the bag, in which case when the vehicle travels in water, the opening may in its entirety be kept under water as well as possible, in which case a pressure to keep the bag inflated remains inside the bag. According to an embodiment, support means, such as support tubes, are arranged in connection with the bag. The support means enable the bag to be supported reliably and, at the same time, it is possible to ensure that the bag has its basic shape, which makes the operation of the bag reliable, as desired.

[0011] According to an embodiment, the vehicle is also viable on land.

[0012] According to an embodiment, the vehicle is an armoured vehicle.

[0013] According to an embodiment, a front part of the body of the vehicle is tapered, and the bow wave plate is arranged in the front part of the body.

Brief description of the figures

[0014] The invention is now described in closer detail in connection with preferred embodiments and with reference to the accompanying drawings, in which:

Figure 1 is a schematic side view of a vehicle. Figure 2 is a schematic side view of a detail of the vehicle according to Figure 1 .

Figure 3 is a schematic top view of the solution according to Figure

2.

Figure 4 is a schematic top view of a second vehicle.

Figure 5 shows a detail of the solution according to Figure 4 when viewed from outside the vehicle.

Figure 6 shows the detail according to Figure 5 when viewed from a direction of a body of the vehicle.

Figures 7a and 7b are schematic side views of a bow wave plate moving mechanism.

Detailed description of the invention

[0015] Figure 1 shows a vehicle which, in this case, is an armoured infantry tank. For the sake of clarity, Figure 1 shows the structure of the vehicle in a simplified manner. Figure 1 thus only shows a basic form of a body 1 . In contrast, Figure 1 shows no armature, access hatches, control and observation means nor any other corresponding details of the vehicle, either.

[0016] The shape of the body 1 is such that its front part, i.e. the right-hand end of Figure 1 , has a tapered cross-section when viewed from the side. Owing to the tapered shape, the front part of the body 1 of the vehicle is provided with bevelled surfaces, making the body 1 more resistant against shots fired at it. Further, the tapered front part means that the area in the vehicle exposed to impacts is small. As can be seen in Figure 3, the front part of the body 1 of the vehicle is tapered also when viewed from above. On the other hand, the body 1 of the vehicle is made to be at its strongest in the armour of the front part. Thus, in the front part of the body 1 the volume of the vehicle over the rest of the body is smaller and, on the other hand, its weight at the front part is heavier. Consequently, in water the weight distribution of such a vehicle is such that the front part tends to tilt downwards. When moving forward, this becomes a problem since the front part tends descend under the surface.

[0017] The vehicle is also provided with means for moving it in water. Such means may comprise for instance propellers and a rudder and corresponding means. For the sake of clarity, of such means Figure 1 only shows a propeller 2. [0018] The vehicle of Figure 1 is also viable on land. For moving on land, the vehicle of Figure 1 is provided with wheels 3. The means for moving on land may also be crawlers or other corresponding means. For the sake of clarity, Figure 1 shows no motors nor power transmission nor other corresponding means necessary for moving the vehicle on-land or offshore.

[0019] The front part of the body 1 of the vehicle is provided with a bow wave plate 4. The bow wave plate 4 may also be called a bow plate, a trim vane and/or a water blocking plate. The bow wave plate 4 is connected with the body 1 through a hinge 5. This enables the bow wave plate 4 to be turned towards the body 1 and away from the body 1 . In Figure 1 , the bow wave plate 4 is turned against the body 1 while in Figure 2 the bow wave plate 4 is turned away from the body 1 .

[0020] When the vehicle according to Figure 1 is driven on land, the bow wave plate 4 is typically turned against the body 1 . In such a case, the driver of the vehicle has as unobstructed a view forward as possible.

[0021] When driving the vehicle in water, the bow wave plate 4 is turned into a position indicated in Figure 2. In such a case, the bow wave plate 4 is typically at the same or almost the same angle as a lower part 6 of the front part of the body 1 . According to an embodiment, an angle a between the opened bow wave plate 4 and the lower part 6 of the front part of the body 1 is less than ±30°, according to a second embodiment less than ±20°, and according to a third embodiment less than ±10°.

[0022] The bow wave plate 4 prevents water from entering on top of an upper part 7 of the front part of the body. Thus, upon moving forward, the bow wave plate 4 enables the front part of the body to be less susceptible to start diving under the surface of water.

[0023] The bow wave plate 4 is moved by hydraulic cylinders or some other appropriate actuator 8. In connection with the actuator 8 is provided a supporting arm mechanism 10 equipped with a locking toggle joint 9. When the bow wave plate 4 is turned into a position for moving in water shown in Figure 2, the supporting arm mechanism 10 is straightened such that the toggle joint 9 is turned over the pivot, into a locked position. In such a case, the supporting arm mechanism 10 at least mainly receives a force caused by water pressing the bow wave plate 4 towards the body 1 .

[0024] The embodiment of Figures 7a and 7b also shows a solution in which a bow wave plate turning mechanism formed by the actuator 8 and the supporting arm mechanism 10 has been formed such that when the bow wave plate 4 is in the position for moving in water, the supporting arm mechanism 10 receives a force caused by water pressing the bow wave plate towards the body 1 greater than that applied to the actuator 8.

[0025] In Figure 7a, the supporting arm mechanism 10 is shown in a position in which the bow wave plate 4 has been turned into the position for moving in water while in Figure 7b the supporting arm mechanism 10 is shown in a position in which the bow wave plate 4 has been turned against the body 1 .

[0026] The supporting arm mechanism 10 comprises a lower part 21 a and an upper part 21 b. A bottom end of the lower part 21 a is fastened to the body 1 through a lower joint 22. The lower joint 22 is provided in a support part 23 which is fixedly fastened to the body 1 . The lower part 21 a is allowed to turn about the lower joint 22.

[0027] A top end of the upper part 21 b is in a corresponding manner fastened to the bow wave plate 4 through an upper joint 24. The upper part 21 b thus turns through the upper joint 24 with respect to the bow wave plate 4.

[0028] The lower part 21 a and the upper part 21 b are connected by a middle joint 25. The lower part 21 a and the upper part 21 b are thus allowed to turn with respect to one another through the middle joint 25.

[0029] In connection with the lower part 21 a, a lower toothed arc 26 is further arranged. Further, in connection with the upper part 21 b, an upper toothed arc 27 is arranged. The lower toothed arc 26 and the upper toothed arc 27 are arranged to mesh as illustrated in Figures 7a and 7b.

[0030] The lower toothed arc 26 is arranged in connection with the lower part 21 a through a turning joint 28 of the lower toothed arc. The lower toothed arc 26 is further provided with an end joint 29 which is arranged at an end of a shaft 30 of the lower toothed arc. The shaft 30 of the lower toothed arc is arranged between the turning joint 28 and the end joint 29 of the lower toothed arc.

[0031] The actuator 8 is at its bottom end fastened to the lower joint 22. Consequently, the actuator 8 is capable of turning about the lower joint 22. At its upper end the actuator 8 is fastened to the end joint 29. Moving a shank of the actuator 8 in a direction of arrow C makes the lower toothed arc 26 turn about the turning joint 28 of the lower toothed arc. The upper toothed arc 27 moves due to the influence of the lower toothed arc 26. The upper toothed arc 27 is arranged fixedly in the upper part 21 b. Consequently, when the actuator 8 is used for turning the lower toothed arc 26, the upper part 21 b turns with respect to the lower part 21 a about the middle joint 25.

[0032] When the bow wave plate is turned in the position for moving in water, water presses the bow wave plate towards the body 1 in a direction illustrated by arrow A. Consequently, the force effect in question acts in a direction of a straight line defined by the upper joint 24, the middle joint 25, and the lower joint 22. In order for the bow wave plate to turn towards the body, the lower toothed arc 26 would have to turn in a direction of arrow B. Since a force in the direction of arrow A causes no turning in the direction of arrow B, no force effect acts on the actuator 8, either. Consequently, the force effect acts on the supporting arm mechanism 10, not being substantially directed at the actuator 8. The direction of influence of the movement of the actuator 8 is direction C between the lower joint 22 and the end joint 29, which thus differs from the direction between the lower joint 22 and the middle joint 25. In the embodiment of Figures 7a and 7b, the turning joint 28 of the lower toothed arc resides on the straight line between the lower joint 22 and the middle joint 25. In such a case, the structure is simple and easy to make strong and reliable.

[0033] In the embodiment according to Figures 7a and 7b, the actuator 8 is arranged underneath the supporting arm mechanism 10. This enables the actuator 8 to be prevented from being damaged. The actuator 8 may be arranged for instance partly inside the lower part of the supporting arm mechanism 10 by forming the lower part 21 a such that its middle part is provided with a slot to receive the actuator 8. In such a case, the lower part 21 a may be shaped for instance such that at least part of its cross-section has the shape of a letter U turned upside down.

[0034] This description has thus disclosed an arrangement for turning a bow wave plate, the arrangement comprising an actuator 8 and a supporting arm mechanism 10. Further, in the arrangement the supporting arm mechanism 10 is arranged to at least mainly receive a force caused by water pressing the bow wave plate towards the body when the bow wave plate has been turned into the position for moving in water.

[0035] Between the bow wave plate 4 and the body 1 , a bag 1 1 is arranged. The bag 1 1 is fastened, on one hand, to the bow wave plate 4 and, on the other hand, to the body 1 . Consequently, turning the bow wave plate 4 into the position shown in Figure 2 straightens up the bag 1 1 . When the bow wave plate 4 is turned into the position shown in Figure 1 , the bag 1 1 is folded between the bow wave plate 4 and the body.

[0036] The bag 1 1 has a wall made of a material which is water- and air-tight as well as flexible. Further, the wall should be made of quite a strong material. An example of such a material is a fabric used for making diving or floating suits. The wall may for instance be provided with a layer of rubber and a layer of fabric. The wall may be made for instance of a trilaminate coated with a fabric available under the trade name Cordura.

[0037] Support tubes 12 are arranged in connection with the bag 1 1 . The support tubes 12 are arranged to rotate about the hinge 5. The bag 1 1 is fastened to the support tubes 12, in which case the support tubes 12 give the bag 1 1 its basic form and support it. Figure 2 shows two support tubes 12, but if such support tubes are used, one or two or three or even more support tubes may be provided. If two or more support tubes are used, they are arranged to extend over a different distance from the hinge 5. In such a case, when the bow wave plate 4 is turned towards the body 1 , they do not settle on top of one another, if the support tubes 12 are differently sized in the width direction of the vehicle as well. This enables the bow wave plate 4 to be turned close to the upper part 7 of the body. Further, the support tubes 12 help to fold the bag 1 1 when the bow wave plate 4 is being turned towards the body 1 . Instead of the support tubes 12, support bars or some other appropriate supporting means may also be used as means for supporting the bag.

[0038] A lower part of the bag 1 1 is provided with an opening 13. The size of the opening 13 is, on one hand, arranged to be sufficiently small in order to be able to keep the opening 13 in its entirety under water when the vehicle is in water. On the other hand, the opening 13 should be large enough for the bag to become emptied of the air and possible water therein sufficiently quickly and efficiently when the bow wave plate 4 is being turned towards the body 1 .

[0039] The size of the bag 1 1 is adapted on the basis of the size and necessary utility properties of the vehicle. According to an example, in connection with a vehicle having the width of about 2.8 m, a bag may be used whose width is about 1 .4 m, whose depth, i.e. the measure of the bag in a direction of travel of the vehicle, is about 0.6 m, and whose height is about 0.6 m. Further, in such a bag, the size of the opening 13 may be for instance 0.1 m x 0.2 m. The volume of the bag 1 1 may vary for instance from 200 litres to several thousands of litres. The shape of the opening 13 may be that of a circle or a square or a rectangle or any other polygon, or another shape. According to an embodiment, the smallest diameter or otherwise the smallest measure of the opening 13 is at least 50 mm. Further, according to an embodiment, the size of the opening 13 is less than 80% of the largest cross-sectional area of the bag when viewed from above. Further, according to an embodiment, the size of the opening 13 is less than 50% of said cross-sectional area and, according to yet another embodiment, the size of the opening 13 is less than 20% of said cross-sectional area. According to still another embodiment, the largest diameter or another corresponding measure of the opening 13 is less than 1 m. According to a second embodiment, the largest diameter or another corresponding measure is less than 0.5 m and, according to a third embodiment, the diameter or another corresponding measure is less than 0.3 m.

[0040] The opening 13 is thus arranged in the lower part of the bag 1 1 . In Figure 3, the opening 13 is illustrated in broken lines. When the vehicle is in water, the opening 13 in the bottom of the bag 1 1 settles under the surface of water. This means that water tries to enter the bag 1 1 through the opening 13. The pressure of water presses the air in the bag 1 1 . The pressurized air in the bag 1 1 prevents additional water from flowing into the bag 1 1 . When the opening 13 in its entirety remains under water, air pressure higher than that of the external environment remains inside the bag 1 1 and the bag 1 1 remains inflated.

[0041] The bag 1 1 prevents water from collecting behind the bow wave plate 4 on top of the upper part 7 of the body 1 . The bag 1 1 and the air therein also make the vehicle float. Preferably, when dimensioning the floating properties of the vehicle, the floating capacity of the bag 1 1 is not, however, taken into account but the properties of the vehicle to float are dimensioned such that they are sufficient even if the bag 1 1 did not improve the floating properties in any way. All in all, however, the bag 1 1 enables the properties of the vehicle to float and move in water to be improved in any case. The vehicle is for instance capable of moving in higher waves and at a higher speed than a corresponding vehicle with no bag.

[0042] As stated above, the opening 13 is large enough for the bag 1 1 to be efficiently emptied of the air and possible water therein when the bow wave plate 4 is being turned towards the body 1 . This enables the bow wave plate 4 to be quickly turned against the body 1 . The turning of the bow wave plate 4 against the body 1 may be performed in seconds.

[0043] According to an embodiment, the opening 13 is arranged in a middle part of the bottom of the bag 1 1 . In such a case, when the vehicle travels in water, it is possible to keep the opening 13 in its entirety under water as well as possible. A pressure then remains inside the bag 1 1 to keep the bag 1 1 inflated.

[0044] As shown in Figure 3, the width of the bag 1 1 is smaller than the width of the bow wave plate 4. In such a case, the bow wave plate 4 turning means, for instance the hydraulic cylinders 8 and the supporting arms 10 in connection therewith, can be arranged between the bow wave plate 4 and the body 1 but at the same time outside the bag 1 1 . The structure of the solution in its entirety can then be implemented in a simple manner.

[0045] According to an embodiment, the bow wave plate 4 turning means may be arranged outside the bow wave plate 4. In such a case, the bag 1 1 between the bow wave plate 4 and the body 1 may equal the bow wave plate 4 in width also without any complex arrangements.

[0046] As shown in Figure 4, the bow wave plate 4 may consist for instance of a middle plate 14 and a side protection arranged on both sides thereof, such as a side plate 15. The bag 1 1 is arranged behind the middle plate 14, and no bag is provided behind the side protections provided on both sides of the middle plate. For example in such a solution, water is allowed to flow off the upper part 7 of the body 1 at points behind the side protections, at the same time increasing the flow of water away from behind the middle plate 14, off the upper part 7 of the body 1 .

[0047] Figure 4 also shows a cable 16 fastened at its ends to the edges of the body. Further, the cable runs through fastening rings 17 provided in the middle plate 14. When the bow wave plate 4 is turned into an open position, i.e. for instance into the position shown in Figures 2 and 4, the cable is tight, simultaneously providing additional support to the structure of the bow wave plate.

[0048] According to Figure 4, the side protections may be installed bevelled with respect to the middle plate 14. In such a case, the side plates 15 serving as the side protection may be formed for instance according to the structure illustrated in Figures 5 and 6. The side plate 15 may be formed from an inner part 15a and an outer part 15b. The inner part 15a is fastened by a hinge 18 to the middle plate 14. The outer part 15b, again, is fastened to the body 1 of the vehicle by a hinge 19. Thus, the hinge 18 is vertical while the hinge 19 is horizontal. The inner part 15a and the outer part 15b are allowed to move with respect to one another. A slot 20 is arranged in the outer part 15b to ensure that the inner part 15a and the outer part 15b stay together upon turning the bow wave plate 4 towards the body. Since the middle plate 14 and the side plates 15 are at an inclined angle with respect to one another, an edge of the outer part 15b facing the middle plate 14 is made bevelled, in which case the outer part 15b and the middle plate 14 do not settle on top of one another even when the bow wave plate 4 resides against the body 1 . The inner part 15a, again, is shaped like a sector, in which case it is allowed to turn by sliding with respect to the outer part 15b.

[0049] In the solution illustrated in Figures 5 and 6, the side plate 15 is formed from two parts. When desired, the side plate 15 may also be formed of one part, or from more than two parts.

[0050] Further, instead of the stiff plate shown in Figures 5 and 6, the side protection may also be formed of some other material. The side protection may be for instance made of fabric or flexible plastic or rubber or some other corresponding flexible material. When the side protection is made of a flexible material, it can be folded easily when the bow wave plate is turned against the body. The fabric to be used in the side protection may be for instance the same fabric as that used as the wall of the bag. The edges and/or middle part of a side protection with a flexible wall may be provided with stiff reinforcements, for instance.

[0051] The above-disclosed solution and all different embodiments and combinations thereof may be applied to any aquatic vehicle. The aquatic vehicle may thus be a boat or a barge or a ship, for instance. In such a case, the bow wave plate is turned away from the body when necessary, i.e. for instance when thus required by high waves or when thus required by an increase in the travel speed.

[0052] According to an embodiment, the vehicle is also viable on land. In such a case, typically, the bow wave plate is held against the body when the vehicle travels on land, and the bow wave plate is turned away from the body when the vehicle moves from land into water. The bow wave plate is opened before driving into water. [0053] According to an embodiment, the vehicle is an armoured vehicle. In such a case, it is not, at least not so much, necessary to take into account in the armour of the armoured vehicle the compromise-necessitating properties of the vehicle to float and/or move in water.

[0054] According to an embodiment, the front part of the body of the vehicle is tapered, and the bow wave plate is arranged in the front part of the body. In such a case, the front part of the body of the vehicle can be shaped as desired, and it may, for instance when thus required by armour, also be heavy but, nevertheless, the bow wave plate and the bag enable the properties of the vehicle to float and/or move in water to be made extremely good.

[0055] The vehicle may be provided with one or more bow wave plates, and also such that the vehicle is provided with a bow wave plate but no bag. On the other hand, when using a greater number of bow wave plates, a bag may be provided in connection with some bow wave plate while some bow wave plate has no bag. The basic structure of the bow wave plate may be any one of those disclosed above, i.e. if for instance a vehicle is provided with a plurality of bow wave plates, they may be either similar in structure or the bow wave plates may be mutually different. For example, the front part of the vehicle may be provided with a bow wave plate formed from a plurality of plates which may further be formed from several parts, while the side of the vehicle may be provided with simple bow wave plates.

[0056] It will be apparent to a person skilled in the art that as technology advances, the basic idea of the invention may be implemented in many different ways. The invention and its embodiments are thus not restricted to the examples described above but may vary within the scope of the claims.

Claims

1. An aquatic vehicle comprising a body, a bow wave plate arranged in connection with the body and turnable towards the body and away from the body, and a bag arranged between the bow wave plate and the body, the bag having a wall made of a material which is water- and air-tight as well as flexible, and a lower part of the bag is provided with an opening.

2. A vehicle as claimed in claim 1, characterized in that the width of the bag is smaller than the width of the bow wave plate.

3. A vehicle as claimed in claim 1 or 2, characterized in that the bow wave plate is provided with a middle plate and a side protection on both sides thereof, and that the bag is arranged behind the middle plate.

4. A vehicle as claimed in claim 3, characterized in that the side protection is a side plate formed from two or more parts.

5. A vehicle as claimed in any one of the preceding claims, characterized in that the size of the opening in the bottom of the bag is less than 50% of a maximum cross-section of the bag when viewed from above.

6. A vehicle as claimed in any one of the preceding claims, characterized in that the smallest diameter or another corresponding measure of the opening in the bottom of the bag is at least 50 mm.

7. A vehicle as claimed in any one of the preceding claims, characterized in that the opening is arranged in a middle part of the bottom of the bag.

8. A vehicle as claimed in any one of the preceding claims, characterized in that support means are arranged in connection with the bag.

9. A vehicle as claimed in any one of the preceding claims, characterized in that the vehicle is also viable on land.

10. A vehicle as claimed in any one of the preceding claims, characterized in that the vehicle is an armoured vehicle.

11. A vehicle as claimed in any one of the preceding claims, characterized in that a front part of the body of the vehicle is tapered, and the bow wave plate is arranged in the front part of the body.

12. A vehicle as claimed in any one of the preceding claims, characterized in that an angle between the bow wave plate turned away from the body and a lower part of the front part of the body is less than ±30°.

13. A method of balancing a vehicle in water, the vehicle comprising a body, a bow wave plate arranged in connection with the body, and a bag arranged between the bow wave plate and the body, a lower part of the bag being provided with an opening, wherein the bag, on one hand, is fastened to the body and, on the other hand, to the bow wave plate, and turning the bow wave plate away from the body straightens the bag and, while driving in water, the opening in the bottom of the bag is arranged under the surface of water, the pressure of water inflating the bag.

14. A method as claimed in claim 13, characterized in that after coming out of water, the bow wave plate is turned towards the body and the bag is emptied of air and possible water through the opening in the bottom.

15. A method as claimed in claim 13 or 14, characterized in that the bow wave plate is turned away from the body by a distance so as to make an angle between the bow wave plate and a lower part of the front part of the body less than ±30°.