WO2012098922A1

WO2012098922A1 - Hull having changeable outer surface shape

Info

Publication number: WO2012098922A1
Application number: PCT/JP2012/050004
Authority: WO
Inventors: 石井昭良
Original assignee: ISHII Teruyoshi
Priority date: 2011-01-18
Filing date: 2012-01-04
Publication date: 2012-07-26
Also published as: JP5578621B2; JP2012148627A

Abstract

[Problem] The present invention is designed to solve the problem of minimizing the wave-making resistance of a hull under two navigation conditions different from each other in response to changes in the wave-making characteristic of a bow bulb under both the conditions in a displacement-type ship. [Solution] To solve the problem, this bow bulb device is characterized in that the bulb side surface is made movable in order to enable the formation of two outer surface shapes corresponding to different Cp curves, a smooth curved surface with no difference in level is maintained by disposing a surface that connects a hull and the movable bulb side surface, and the size and the outer shape of a bow bulb can be changed.

Description

Hull with external shape changeable

The present invention relates to a hull equipped with a bow valve for reducing wave-making resistance acting on the hull.

The bow valve is widely used to reduce the wave resistance acting on the hull. The size and shape of the bow valve that is optimal for reducing wave resistance varies depending on the shape and speed of the main hull, but it is difficult to change the size and shape of a fixed bow valve. It is common.

The shape-changeable hull is intended to reduce wave-making resistance in response to changes in ship speed by making it possible to change the size and shape of the bow valve. By using a hull that can be changed in shape, the wave resistance can be reduced according to the navigation conditions and the loading state, and the energy consumption of the ship can be reduced.

Patent Document 1 proposes a movable bow valve that can change the outer surface shape by moving a rigid body and can change the wave-making characteristics at different ship speeds.

Japanese Patent No. 4598854

The problem to be solved by the invention is to reduce hull resistance under two navigation conditions by a hull with a bow valve that can be changed to two types of outer surface shapes suitable for the flow field around the hull corresponding to two different navigation conditions. It is something to try.

When producing products that satisfy the above basic patents, it is possible to produce a smooth outer surface shape that satisfies the above conditions, and to store a split surface that is used only when forming one outer surface shape in the hull. It is intended to solve the problem of making the structure maintainable.

The present invention employs the following means in order to solve the above problems. The shape-changeable hull according to claim 1 is capable of forming two types of outer surface shapes corresponding to two types of Cp curves having wave-making characteristics in which the magnitude of wave-making resistance is reversed at a certain fluid number. The first outer surface shape is composed of a fixed outer plate surface, a movable outer plate surface, and a connecting surface between the fixed outer plate surface and the movable outer plate surface, and the second outer surface shape is the same as the first outer surface shape. The fixed outer plate surface, the movable outer plate surface used in common with the first outer surface shape, and the connecting surface between the fixed outer plate surface and the movable outer plate surface different from the first outer surface shape, and the first outer surface. It has a movable outer plate that can be repositioned to change from the shape to the second outer surface and its reverse direction, and a drive unit and link mechanism that move the connecting surface between the fixed outer plate and the movable outer plate. The outer surface formed with a connecting surface between the fixed outer plate surface and the movable outer plate surface that is not required when forming the outer shape of one side The curvature of the water line at both ends in the longitudinal direction of the movable outer plate that changes in the shape of the two outer surfaces is stored inside the shape, and the length of the connecting surface between the two fixed outer plates and the corresponding movable outer plate. By making it the same as the curvature of the water line at the end in the direction, a smooth curved surface that does not cause a step when the outer surface shape is changed is formed.

According to the invention described in claim 1, by changing the shape of the outer surface of the bow valve, it is possible to change to the optimum valve size for reducing wave resistance under two different kinds of navigation conditions, which works on the hull. The effect of reducing wave resistance can be enhanced. At the same time, a smooth outer surface shape can be manufactured, a split surface used only when forming one outer surface shape can be stored in the hull, and a structure capable of maintaining the strength of the bow valve can be obtained.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. 1 and 2 relate to an embodiment of the invention of claim 1, FIG. 1 is a plan view of a hull skin, and FIG. 2 is a side view of a hull skin corresponding to FIG.

FIG. 3 shows a part of a hull diagram according to an embodiment of the invention of claim 1 and shows a water line having a depth having a maximum valve width.

FIG. 4 is a diagram showing a wave-making resistance curve in two states when the valve size is changed and when the valve size is changed according to the change in the fluid number.

FIG. 5 is a diagram showing Cp curves in the same planned draft state in two states at the normal valve size and when the valve size is changed.

FIG. 6 is a diagram showing a wave-making resistance curve corresponding to a change in draft at a constant ship speed in two states, a normal valve size and a valve size change.

FIGS. 7 and 8 relate to another embodiment of claim 1. FIG. 7 is a plan view of the hull skin and FIG. 8 is a side view of the hull skin corresponding to FIG.

The configuration of the invention of claim 1 will be described with reference to FIG. FIG. 1 is a plan view of the outer shape of the hull viewed from the bottom of the ship. FIG. 1 shows two types of outer surface shapes, (1) showing a normal valve size and (2) showing a valve size change.

The first outer surface shape shown in (1) of FIG. 1 is a normal valve size and is composed of one fixed outer plate surface, 2R and 2L movable outer plate surfaces, and 3R and 3L connecting surfaces. Is done.

The second outer surface shape shown in (2) of FIG. 1 is a valve size change, 1 of the fixed outer plate surface common to the first outer surface shape, 2R and 2L common to the first outer surface shape. The position of the movable outer plate surface is changed from (1) and the connecting surface of 4R and 4L.

The difference in the outer shape of the hull between the first outer shape and the second outer shape is limited to the side of the ship between Xa and Xf in the longitudinal direction of the hull.

On the movable outer plate surfaces 2R and 2B, a water line having the maximum valve width is (1) in the longitudinal direction between X1 and X2 and in the width direction between Y1 and Y2, and (2) in the longitudinal direction from X3 to X4. Between Y3 and Y4 in the width direction. The valve size can be changed by changing the position of the movable outer plate surface from (1) to (2) or from (2) to (1).

3R and 3L are connecting surfaces for connecting the fixed outer plate surface and the movable outer plate surface with a smooth curved surface at the normal valve size. 4R and 4L are connecting surfaces for connecting the fixed outer plate surface and the movable outer plate surface with a smooth curved surface when the valve size is changed. The connecting surface is different between (1) and (2), and the other connecting surface that is not required when forming one outer shape is housed in the hull.

By changing the external shape from (1) in FIG. 1 to (2) in FIG. 1 or from (2) in FIG. 1 to (1) in FIG. 1, the Cp curve of the bow valve portion is changed. As a result, the wave-making characteristics of the bow valve can be changed. *

2 is a side view corresponding to FIG. 1. As in FIG. 1, (1) is the first outer surface shape when the normal valve size is used, (2) is the second outer surface shape when the valve size is changed. Shows things.

When the first outer surface shape is formed in (1) of FIG. 2, the fixed outer plate surface 1 and the movable outer plate surface 2R are selected by appropriately selecting the arrangement of the movable outer plate surface 2R and the shape of the connecting surface 3R. In addition, a discontinuous curvature does not occur in the water line and the buttocks line formed by the connecting surface 3R. *

When the second outer surface shape is formed in (2) of FIG. 2, similarly, the arrangement of the movable outer plate surface 2R and the shape of the connecting surface 4R are appropriately selected, so that one fixed outer plate surface 2R The water line and the buttocks line composed of the movable outer plate surface and the 4R connecting surface have no discontinuous curvature.

As shown in FIG. 2, the difference between the outer shapes of (1) and (2) is limited to the ship side between Xa and Xf in the longitudinal direction of the hull and between Zt and Zb in the depth direction. As a result, the outer surface can be fixed from the keel to the stem of the hull, and the internal structure can be reinforced in the same way as a conventional bow valve without moving parts, and it has sufficient strength. become.

FIG. 3 is a view showing a water line corresponding to FIG. 1. As in FIG. 1, (1) is a first outer shape and is a normal valve size, and (2) is a second outer shape. Shown when the valve size is changed.

As shown in FIG. 3, the difference between the outer shapes of (1) and (2) is limited to between Xa and Xf in the longitudinal direction of the hull. 41 and 42 are water lines on the fixed outer plate surface, and there is no difference between (1) and (2).

The water lines on the movable outer plate surface correspond to 43 in (1) and 46 in (2), respectively. In (1), the curvature of the water line 44 of the connecting surface and the water line 41 of the fixed outer plate surface on the rear side is made the same by Xa, and the connection between both water lines is made smooth. Similarly, the curvature of the end of the water line connecting 44 and 43 in X1, 43 and 45 in X2, and 45 and 41 in Xf is made the same, so that a series of water lines are connected smoothly.

Similarly in (2), the curvature of the end of the waterline connecting 41 and 47 for Xa, 47 and 46 for X3, 46 and 48 for X4, and 48 and 42 for Xf is the same, and a series of waterlines The connection is smooth.

Although FIG. 3 shows a method for realizing smoothness at both ends of the connecting surface between the fixed outer plate surface and the movable outer plate surface for a typical water line, each water line connects the curvatures at both ends of the connecting surface. By combining with the curvature of the part, the water line composed of the fixed outer plate surface, the movable outer plate surface and the connecting surface when the outer surface shape is changed does not have a discontinuous curvature, and a smooth three-dimensional curved surface is formed. The

The wave resistance curve shown in FIG. 4 shows the wave resistance coefficient based on the Froude number. 31 corresponds to the first outer surface shape shown in (1) of FIG. 1, and 32 corresponds to the second outer surface shape shown in (2) of FIG. Both curves intersect at the Froude number Fn1, and the magnitude relationship is reversed before and after that.

According to the present invention, the first outer surface shape is used when navigating with a fluid number lower than Fn1, and the second outer surface shape is used when navigating with a fluid number higher than Fn1, thereby providing a bow valve with only one outer surface shape. Compared to a ship that performs, the wave resistance when navigating at two or more speed conditions can be reduced.

In the Cp curve shown in FIG. 5, 33 corresponds to the first outer surface shape shown in (1) of FIG. 1, and 34 corresponds to the second outer surface shape shown in (2) of FIG. Both curves have a difference in size and inclination in the region corresponding to the bow valve.

The characteristics of the wave resistance curve shown in FIG. 4 are based on the difference in the Cp curve shown in FIG. The difference in the outer shape shown in FIG. 1 reflects the difference in the Cp curve shown in FIG.

The wave resistance curve shown in FIG. 6 depicts the wave resistance coefficient based on the draft. 35 corresponds to the first outer surface shape shown in (1) of FIG. 1, and 36 corresponds to the second outer surface shape shown in (2) of FIG. Both curves intersect at the draft d1, and the magnitude relationship is reversed before and after that.

According to the present invention, a ship equipped with a bow valve having only one outer surface shape by adopting a first outer surface shape when navigating with a draft larger than d1 and a second outer surface shape when navigating with a draft smaller than d1. In comparison with this, it is possible to reduce the wave resistance when sailing with two or more drafts.

As described above, in the embodiment shown in FIGS. 1 and 2, the hull having a bow valve equipped on the hull has two types of Cp curves in which the magnitude of the wave resistance is reversed around a certain fluid number. The outer surface shape can be formed.

7 and 8 show another embodiment of the present invention. FIG. 7 is a plan view of the outer shape of the hull as seen from the bottom of the ship, and FIG. 8 is a side view of the outer shape of the hull. FIG. 7 and FIG. 8 show two types of outer surface shapes. (1) is a normal valve size, and (2) is a valve size change.

(1) The fixed outer plate surface 11 is the same in (2). Although the movable outer plate surface 12R of (1) and the movable outer plate surface 22R of (2) are common, the arrangement is different.

7 and 8 is characterized in that the connecting surface is divided into four parts on the starboard side and the port side, respectively. On the starboard side of (1), the rear connecting surface 13R, the front connecting surface 14R, the lower side The entire connecting surface is constituted by the side connecting surface 15R and the upper connecting surface 16R. Similarly, on the port side of (1), the rear connecting surface 13L, the front connecting surface 14L, the lower connecting surface 15L, and the upper connecting surface 16L constitute the entire connecting surface.

On the starboard side of (2), the rear connecting surface 23R, the front connecting surface 24R, the lower connecting surface 25R, and the upper connecting surface 26R constitute the entire connecting surface. Similarly, on the port side of (2), the entire connecting surface is constituted by the rear connecting surface 23L, the front connecting surface 24L, the lower connecting surface 25L, and the upper connecting surface 26L.

When the first outer surface shape is formed in (1) of FIG. 8, the arrangement of the movable outer plate surface 12R, the rear connecting surface 13R, the front connecting surface 14R, the lower connecting surface 15R, and the upper connecting surface 16R. By appropriately selecting the shape, a water line composed of a fixed outer plate surface 11, a movable outer plate surface 12R, a rear side connecting surface 13R, a front side connecting surface 14R, a lower side connecting surface 15R and an upper side connecting surface 16R. In addition, there is no discontinuous curvature in the buttocks line. *

Similarly, when forming the second outer surface shape in (2) of FIG. 8, the arrangement of the movable outer plate surface 22R, the rear side connecting surface 23R, the front side connecting surface 24R, the lower side connecting surface 25R and the upper side as well. By appropriately selecting the shape of the connecting surface 26R, the fixed outer plate surface 11, the movable outer plate surface 22R, the rear connecting surface 23R, the front connecting surface 24R, the lower connecting surface 25R, and the upper connecting surface 26R are configured. Discontinuous curvature does not occur in the water line and buttocks line.

By dividing the connecting surface into four parts, when moving the connecting surface to change the outer surface shape, interference with other connecting surfaces and movable outer plate surfaces can be avoided. Further, when storing unnecessary connecting surfaces, it is possible to avoid interference with the fixed outer plate surface and the movable outer plate surface.

The features of the present invention will be further described with reference to a horizontal sectional view of the bow valve shown in FIG. Concerning the external components commonly used in the first outer surface shape (1) and the second outer surface shape (2), there are a fixed outer plate surface 51 and a movable outer plate surface 52. The movable outer plate surface 52 is moved to a predetermined position by a link mechanism portion 52S and a drive portion 52K for horizontally moving the 52, and a pulley and drive portion 52P for rotating and moving the 52, a wire and a link portion 52W. It becomes possible.

The rear side connecting surface 53 and the front side connecting surface 54 used only in the first outer surface shape (1) can be moved by 53S and 54S as a link mechanism portion and 53K and 54K as a drive portion. When forming the second outer surface shape, 53 and 54 are stored inside the second outer surface shape as shown in (2).

Similarly, the rear side connecting surface 55 and the front side connecting surface 56 used only in the second outer surface shape (2) can be moved by 55S and 56S as the link mechanism portion and 55K and 56K as the drive portion. When forming the first outer surface shape, 55 and 56 are stored inside the first outer surface shape as shown in (1).

FIG. 10 is a vertical sectional view in the width direction of the bow valve. The external components commonly used in the first outer surface shape (1) and the second outer surface shape (2) are the fixed outer plate surface 61 and the movable outer plate surface 62. These are the same when the fixed outer plate surface 51 and the movable outer plate surface 52 in FIG.

In FIG. 10, the arrangement and storage of the lower connecting surface 63 and the upper connecting surface 64 of the first outer surface shape (1), and the lower connecting surface 65 and the upper connecting surface 66 of the second outer surface shape (2). It shows a state.

The lower connecting surface 63 and the upper connecting surface 64 used only in the first outer surface shape (1) can be moved by 63S and 64S as a link mechanism portion and 63K and 64K as a drive portion. When forming the second outer surface shape, 63 and 64 are stored inside the second outer surface shape as shown in (2).

Similarly, the lower connecting surface 65 and the upper connecting surface 66 used only in the second outer surface shape (2) can be moved by 65S and 66S as the link mechanism portion and 65K and 66K as the driving portion. When forming the first outer surface shape, 65 and 66 are stored inside the first outer surface shape as shown in (1).

Since the port-side link mechanism and the drive unit are arranged on a cross-section different from the cross-section shown in FIG. 10 in order to avoid interference between the starboard-side link mechanism and the drive unit, FIG. Not.

9 and 10 show one embodiment of the fixed outer plate surface, the movable outer plate surface and the connecting surface, and the link mechanism unit and the driving unit, and two types of outer surface shapes are formed and unnecessary connection is made. It shows that the surface can be stored in the hull.

In the embodiment of FIG. 9, an example is shown in which a pulley and a wire are used to change the angle formed with the center line of the movable outer plate surface. Although not shown, the angle changing function can also be realized by using a plurality of combinations of link mechanism units and drive units each having a rotatable gimbal as another embodiment.

In the embodiment of FIGS. 9 and 10, an example of a link mechanism unit and a drive unit that are linearly moved to move the connecting surface is shown. Although not shown, the movement of the connecting surface can also be realized by adopting a rotational movement method by combining a link mechanism portion having a hinge portion and a drive portion as another embodiment.

1 is a plan view of an outer shape of a hull showing an embodiment of the invention of claim 1. (1) corresponds to the outer shape of the normal valve size, and (2) corresponds to the outer shape when the valve size is reduced. It is a side view of the hull outline which shows one embodiment of the invention of Claim 1. (1) corresponds to the outer shape of the normal valve size, and (2) corresponds to the outer shape when the valve size is reduced. FIG. 3 is a part of a hull diagram showing an embodiment of the invention of claim 1. (1) corresponds to the water line of the normal valve size, and (2) corresponds to the water line when the valve size is reduced. It is a figure which shows the wave-making resistance curve of the ship corresponding to (1) and (2) of FIG. 1 according to the change of the fluid number. It is a figure which shows the Cp curve of the ship corresponding to (1) and (2) of FIG. It is a figure which shows the wave-making resistance curve of the ship corresponding to (1) and (2) of FIG. 1 according to the change of draft at constant ship speed. It is a top view of the hull outline which shows another embodiment of the invention of Claim 1. (1) corresponds to the outer shape of the normal valve size, and (2) corresponds to the outer shape when the valve size is reduced. It is a side view of the hull outer shape which shows another example of embodiment of invention of Claim 1. (1) corresponds to the outer shape of the normal valve size, and (2) corresponds to the outer shape when the valve size is reduced. It is a horizontal sectional view of the bow valve in another embodiment of the invention of claim 1. (1) corresponds to a cross-sectional view of the normal valve size, and (2) corresponds to a cross-sectional view when the valve size is reduced. It is a vertical sectional view of the width direction of the bow valve in another embodiment of the invention of claim 1. (1) corresponds to a cross-sectional view of the normal valve size, and (2) corresponds to a cross-sectional view when the valve size is reduced.

1: Main hull fixed outer surface 2R: Starboard side movable outer plate surface 2L: Port side movable outer plate surface 3R: Connecting surface 3L between starboard side fixed outer surface and movable outer plate surface at normal valve size: Normal 4R connecting port side fixed outer surface and movable outer plate surface when the valve size is changed: Starboard side connecting outer surface and fixed outer surface when the valve size is changed 4L: port side when changing the valve size Connecting surface 11 of the fixed outer surface and movable outer plate surface 11: Main hull fixed outer surface 12R: Starboard side movable outer plate surface 12L: Port side movable outer plate surface 13R: Starboard side fixed outer shape at normal valve size A rear connecting surface 13L between the surface and the movable outer plate surface: a fixed outer surface on the port side at the normal valve size and a rear connecting surface 14R between the movable outer plate surface and a fixed outer surface on the starboard side at the normal valve size Front connecting surface 14L with movable outer plate surface: Normal valve size 15R on the front side of the fixed outer surface on the starboard side and the movable outer plate surface: Downward connecting surface 15L between the fixed outer surface on the starboard side and the movable outer plate surface at the normal valve size: Port on the normal valve size Lower connecting surface 16R between the fixed outer surface on the side and the movable outer plate surface: Upper connecting surface 22R between the fixed outer surface on the starboard side and the movable outer plate surface at the normal valve size: Starboard side when the valve size is changed Movable outer plate surface 22L: Port side movable outer plate surface 23R when the valve size is changed: Starboard side connecting surface 23L: Port side fixed outer surface when the valve size is changed and the movable outer plate surface Port side when the valve size is changed Rear connecting surface 24R between the fixed outer surface on the side and the movable outer plate surface: Front connecting surface 24L between the fixed outer surface on the starboard side when the valve size is changed and the movable outer plate surface: Port side when the valve size is changed Forward of fixed outer surface and movable skin surface Connecting surface 25R: Downward connecting surface of the starboard side fixed outer surface and the movable outer plate surface when the valve size is changed 25L: Lower connecting surface of the portside fixed outer surface and the movable outer plate surface when the valve size is changed 26R: Upper side connecting surface of starboard side fixed outer shape surface and movable outer plate surface at the time of valve size change 31: Wave forming resistance curve based on fluid number at normal valve size 32: Fluid number base at time of valve size change Wave-making resistance curve 33: Cp curve 34 for normal valve size 34: Cp curve 35 for changing valve size 35: Wave-making resistance curve 36 for changing draft for normal valve size: Drafting for changing valve size Wave-forming resistance curve 41 at the time: Water line 42 on the rear fixed outer plate surface 42: Water line 43 on the front fixed outer plate surface 43: Water line 44 on the movable outer plate surface at the normal valve size Water line 45 on the rear connection outer plate surface at the normal valve size: Water line 46 on the front connection outer plate surface at the normal valve size 46: Water line 47 on the movable outer plate surface at the time of valve size change: When the valve size is changed Water line 48 on the rear connecting outer plate surface of the water: Water line 51 on the front connecting outer plate surface when changing the valve size 51: Fixed outer surface 52: Link mechanism portion 52K of the movable outer plate surface 52S on the starboard side 52: 52 Driving part 52P: 52 angle adjusting pulley and driving part 52W: 52 angle adjusting wire and link mechanism part 53: A connecting side on the rear side of the fixed outer surface on the starboard side and the movable outer plate surface in the normal valve size. 53S: 53 link mechanism portion 53K: 53 drive portion 54: forward connecting surface of starboard side fixed outer shape surface and movable outer plate surface at normal valve size 54S: 54 link mechanism portion 54K: 54 drive portion 55: rear linkage surface 55S: 55 link mechanism portion 55K: 55 drive portion of the starboard side fixed outer shape surface and movable outer plate surface when the valve size is changed 56: Front side connecting surface 56S of the starboard side fixed outer shape surface and movable outer plate surface when the valve size is changed 56S: Link mechanism portion 56K of 56: Drive portion Xa of 56: Rear side of the fixed outer plate surface and the connecting surface Connection position X coordinate Xf: Front side connection position X of fixed outer plate surface and connecting surface X coordinate X1: Rear side connection position X of movable outer plate surface and connecting surface at normal valve size X2: At normal valve size Forward side connection position X coordinate X3 between the movable outer plate surface and the connecting surface: Rear side connection position X coordinate X4 between the movable outer plate surface and the connecting surface when the valve size is changed X4: Connection with the movable outer plate surface when the valve size is changed Front side connection position with surface X coordinate a: Rear side connection position Y coordinate between the fixed outer plate surface and the connecting surface Yf: Front side connection position Y coordinate between the fixed outer plate surface and the connecting surface Y1: Between the movable outer plate surface and the connecting surface at the normal valve size Rear side connection position Y-coordinate Y2: Front side connection position Y-coordinate between movable outer plate surface and connecting surface when normal valve size Y3: Rear side connection position Y-coordinate between movable outer plate surface and connecting surface when changing valve size Y4: Front side connection position of movable outer plate surface and connecting surface when changing valve size Y coordinate Zt: Connecting surface uppermost Z coordinate Zc: Water line Z coordinate with the maximum valve width Zb: Connecting surface uppermost Z coordinate

Claims

A hull equipped with a bow valve, which can form two types of outer surface shapes corresponding to two types of Cp curves with wave forming characteristics that reverse the magnitude of wave forming resistance at a certain fluid number. The shape is composed of a fixed outer plate surface, a movable outer plate surface, and a connecting surface between the fixed outer plate surface and the movable outer plate surface, and the second outer surface shape is the same fixed outer plate surface as the first outer surface shape, It is composed of a movable outer plate surface used in common with the first outer surface shape, and a connecting surface between the fixed outer plate surface and the movable outer plate surface different from the first outer surface shape. A movable outer plate surface that can be repositioned to change to the outer surface shape and the reverse direction, and a drive unit and a link mechanism to move the connecting surface between the fixed outer plate surface and the movable outer plate surface, and one outer surface Outer surface shape formed with a connecting surface between the fixed outer plate surface and the movable outer plate surface that is not required when forming the shape Longitudinal ends of the connecting surface between the two fixed outer plate surfaces and the movable outer plate surface corresponding to the curvatures of the water lines at both ends in the longitudinal direction of the movable outer plate surface that are accommodated inside and change in two outer surface shapes. The hull is characterized by forming a smooth curved surface that does not cause a step when the outer surface shape is changed by making it the same as the curvature of the water line of the part.