A drag-reducing twisted brace at the end of a ship's double tail fin and its design method

Abstract

The present invention relates to a twisted diagonal brace for reducing drag at the end of a double tail fin of a ship and a design method thereof, comprising: determining the thickness and area of ​​the cross section of the diagonal brace; determining the geometric shape of the cross section of the diagonal brace; determining the skew angle between the central axis of the diagonal brace and the centerline of the tail fin; determining the longitudinal inclination angle between the central axis of the diagonal brace and the centerline of the tail shaft; using the streamline angle of the incoming flow in front of the diagonal brace at different waterline heights as the centerline angle of the cross section of the diagonal brace at different waterline heights, and smoothly transitioning the outer contour of the diagonal brace; and using fluid mechanics software to screen the twisted diagonal brace scheme with the least resistance. The twisted diagonal brace for reducing drag at the end of a double tail fin of a ship designed by the present invention ensures the structural strength of the diagonal brace itself and the vibration reduction requirements of the tail fin. At the same time, the angle of attack between the centerline of the cross section of the diagonal brace at different waterline heights and the incoming flow at each waterline height is close to zero, which can minimize the interference of the diagonal brace on the surrounding flow field and has a better self-drag reduction effect.

Description

Technical Field

[0001] The present invention relates to the technical field of ship appendages, and in particular to a drag-reducing twisted brace at the end of a ship's double tail fin and a design method thereof. Background Art

[0002] Compared with conventional twin-propeller ships, twin-skeg hulls have the advantage of high hull efficiency and are widely used in amphibious ships and inland vessels. However, twin-skeg hulls have the disadvantages of large and poorly uniform tail wake, weak lateral and vertical stiffness of the skeg, and easy vibration of the skeg, which greatly affect the performance of the propeller and shaft system, as well as the structural life of the twin-skeg. In order to fully utilize the advantages of the high hull efficiency of the twin-skeg hull and effectively control the adverse effects of twin-skeg vibration, it is often adopted to add a diagonal brace structure to the end of the twin skeg, rigidly connecting the twin skeg end and the diagonal brace structure to the hull structure to increase the stiffness of the skeg structure and reduce its vibration. The current design of the diagonal brace at the end of the twin skeg in the shipbuilding industry generally borrows the design method of the tail shaft frame, which has two typical characteristics. First, the centerline angle of the diagonal brace cross section is designed according to the streamline angle at the bottom plate of the ship, and second, the centerline angle of the diagonal brace cross section is the same at different waterline heights. For non-double fin ship types with relatively flat tail surfaces, the difference in streamline angles at different waterline heights is small, and the tail shaft bracket designed using this method has little impact on the resistance and wake flow field.

[0003] However, for twin-tail fin hulls, the tail fin shapes at different waterline heights vary greatly, resulting in large differences in streamline angles. If the design method of the tail shaft frame is still used to design the twin-tail fin end brace, and the streamline angle at the bottom plate is used as the cross-sectional centerline angle of the brace at different waterline heights, and the value is maintained at the same value, it will easily lead to the cross-sectional angle of the brace at some waterline heights being too large with the incoming flow, causing a large pressure difference between the inside and outside of the brace, which is very likely to generate cavitation, leading to chaotic flow field around the brace, significantly increasing its own resistance, significantly reducing the uniformity of the trailing flow field, and even causing the propeller cavitation performance to deteriorate. Therefore, it is urgent to provide a drag-reducing twisted brace at the end of a twin-tail fin ship and a design method thereof, comprehensively considering the differences in streamline angles at different waterline heights of the twin-tail fin hull, designing the cross-sectional centerline angle of the brace according to the streamline angles at different waterline heights, and making the angle of attack with the incoming flow at different waterline heights as zero as possible, so as to control the brace's own resistance to an acceptable range.

[0004] The main technical problem in the design of the diagonal brace at the end of the ship's double tail fin is how to determine the centerline angle of the diagonal brace cross section so that the angle of attack with the incoming flow at different waterline heights is zero as much as possible, thereby significantly reducing the diagonal brace's own resistance and its impact on the trailing flow field. Summary of the Invention

[0005] In view of this, the present invention aims to provide a novel drag-reducing twisted brace at the end of a ship's twin tail fins and a design method thereof. The twisted brace designed according to the present invention has a cross-sectional centerline angle designed based on the streamline angle at different waterline heights, and the angle of attack of the incoming flow at different waterline heights is close to zero, significantly reducing the brace's own drag and its impact on the wake flow field.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] A drag-reducing twisted brace for the end of a double tail fin of a ship comprises double tail fins and a twisted brace. The twisted brace is respectively connected between the ends of the double tail fins and the longitudinal strong structure position above the bottom plate of the ship. There is a lateral inclination angle between the central axis of the twisted brace and the centerline of the tail fin, and there is a longitudinal inclination angle between the central axis of the twisted brace and the centerline of the tail shaft, forming a drag-reducing twisted brace for the end of the double tail fin. The angle of attack between the centerline of the cross section of the twisted brace at different waterline heights and the incoming flow at each waterline height is close to zero, which can minimize the interference effect of the brace on the surrounding flow field, thereby achieving a better self-drag reduction effect.

[0008] Furthermore, the cross-section of the twisted brace has a thickness of 0.35d and an area of ​​0.45d. 2 , where d is the tail axis diameter where the diagonal brace connects to the tail fin.

[0009] Furthermore, the minimum side inclination angle between the central axis of the twisted diagonal brace and the center line of the tail fin is not less than 50°.

[0010] Furthermore, the minimum longitudinal inclination angle between the central axis of the twisted diagonal brace and the center line of the tail shaft is not less than 60°.

[0011] Furthermore, the centerline angles of the cross-section of the twisted brace at different waterline heights are not the same, and the centerline angles of the cross-section of the brace at different waterline heights are basically consistent with the streamline angles of the incoming flow in front of the brace, and the angles of attack of the incoming flow at different waterline heights are close to zero, thereby reducing the disturbing effect of the twisted brace on the surrounding flow field, reducing the twisted brace's own resistance and its impact on the trailing flow field.

[0012] A design method for a twisted brace for drag reduction at the end of a ship's twin tail fins comprehensively considers the structural strength of the twisted brace, the vibration reduction of the tail fin, and the resistance of the twisted brace. The design method includes: determining the thickness and area of ​​the brace cross section; determining the geometric shape of the brace cross section; determining the skew angle between the brace centerline and the centerline of the tail fin; determining the longitudinal angle between the brace centerline and the centerline of the tail shaft; using the front flow streamline angle of the brace at different waterline heights as the centerline angle of the brace cross section at different waterline heights, and smoothing the outer contour of the brace; and using fluid dynamics software to screen the twisted brace solution with the lowest resistance.

[0013] Furthermore, the design method specifically includes the following steps:

[0014] Step S1: In a 3D geometry software, complete the geometric modeling of the double-skeg hull, excluding the skeg brace, as Scheme 0;

[0015] Step S2: Using fluid mechanics software, set the incoming flow velocity according to the design speed to obtain the ship resistance and the incoming flow streamlines and angles of the diagonal support at different waterline heights;

[0016] Step S3: Refer to the relevant design requirements for the tail shaft support arm in the structure section of the China Classification Society's "Rules for Classification of Sea-going Steel Ships", and consider that the stiffness of the tail fin itself is greater than the stiffness of the single-sided tail shaft support, and determine the thickness of the cross section of the diagonal brace to be 0.35d and the area to be 0.45d 2 , where d is the tail shaft diameter where the diagonal brace connects to the tail fin;

[0017] Step S4: selecting a symmetrical streamline shape as the cross-sectional geometry of the diagonal brace according to the thickness and area;

[0018] Step S5: Within the minimum skew angle range, determine the skew angle between the central axis of the diagonal brace and the centerline of the skew fin based on the longitudinal strong structure position above the bottom plating near the upper end of the diagonal brace, on the outer side of the skew fin. That is, the minimum skew angle between the central axis of the diagonal brace and the centerline of the skew fin is not less than 50°.

[0019] Step S6: Within the minimum trim angle range, the trim angle between the center axis of the diagonal brace and the center line of the tail shaft is determined based on the position of the transverse strong structure above the bottom plate near the upper end of the diagonal brace and outside the tail fin. That is, the minimum trim angle between the center axis of the diagonal brace and the center line of the tail shaft is not less than 60°.

[0020] Step S7: stretching the cross section of the diagonal brace from the outer side of the double tail fin end to the bottom plate along the side inclination angle and the longitudinal inclination angle to determine the geometric shape of the entire diagonal brace, thereby determining the length of the diagonal brace, wherein the stretching line is the center axis of the diagonal brace;

[0021] Step S8: rotating the cross sections of the diagonal brace at different waterline heights counterclockwise on the port side and clockwise on the starboard side around the central axis of the diagonal brace to the angles of the incoming flow of the diagonal brace at the different waterline heights, and making the outer contour of the diagonal brace smoothly transition to obtain a twisted diagonal brace;

[0022] Step S9: In a three-dimensional geometry software, complete the geometric modeling of the twisted brace; install the twisted brace at the ends of the double tail fins and the strong frame position of the bottom plate structure according to the side inclination angle and the longitudinal inclination angle, as Scheme 1;

[0023] Step S10: Using structural finite element software, check the structural strength of the twisted brace and the stiffness of the twin tail fins to ensure that the structural strength of the twisted brace and the stiffness and vibration of the twin tail fins are within an acceptable range;

[0024] Step S11: Using fluid mechanics software, set the incoming flow velocity according to the design speed to obtain the ship resistance and the incoming flow streamline in front of the diagonal brace; the resistance difference between Scheme 1 and Scheme 0 is the resistance of the twisted diagonal brace itself;

[0025] Step S12: extract the front flow streamline angles of the diagonal brace at multiple waterline heights in Scheme 1, respectively serving as the centerline angles of the diagonal brace cross section at different waterline heights, and smoothly transition the outer contour of the diagonal brace to obtain a new twisted diagonal brace scheme as Scheme 2; repeat step S10 to ensure the structural strength of the diagonal brace itself and the stiffness of the double tail fins in Scheme 2; repeat step S11 to obtain the diagonal brace resistance of Scheme 2;

[0026] Step S13: Compare the diagonal bracing resistances of Scheme 1 and Scheme 2, and select the scheme with the smaller diagonal bracing resistance as the new scheme;

[0027] Step S14: Repeat steps S12 and S13 until a twisted bracing solution with the minimum resistance is selected.

[0028] Furthermore, in step S3, from the perspective of ensuring the structural strength of the twisted brace itself, a design method for the cross-sectional thickness and area of ​​the brace is provided.

[0029] Furthermore, in step S5, a design method for the minimum side tilt angle between the central axis of the diagonal brace and the centerline of the tail fin is provided from the perspective of ensuring the stiffness of the double tail fins and reducing vibration.

[0030] Furthermore, in step S6, from the perspective of ensuring the structural strength of the twisted brace itself, a design method for the minimum longitudinal inclination angle between the center axis of the brace and the center line of the tail shaft is provided.

[0031] Furthermore, in steps S11 and S14, fluid mechanics software is used to select a twisted brace scheme with the minimum resistance from multiple twisted brace schemes with the goal of minimizing the resistance of the twisted brace itself.

[0032] The present invention adopts the above technical solution, and compared with the prior art, has the following beneficial effects:

[0033] The twisted diagonal brace for drag reduction at the end of the double tail fin of a ship and the design method thereof of the present invention ensure the structural strength of the diagonal brace itself and the vibration reduction requirements of the tail fin. At the same time, the angle of attack of the diagonal brace with the incoming flow at each waterline height is close to zero, which can minimize the interference of the diagonal brace on the surrounding flow field and effectively reduce the diagonal brace's own resistance and its impact on the trailing flow field. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1This is a linear tail view of the tail portion of the double-fin boat of the present invention;

[0035] Figure 2 It is a linear side view of the tail portion of the double-tail fin boat of the present invention;

[0036] Figure 3 This is a linear top view of the tail portion of the double-tail fin boat of the present invention;

[0037] Figure 4 yes Figure 1 A4-A4 cross section of the diagonal brace at the connection with the bottom plating;

[0038] Figure 5 yes Figure 1 A3-A3 cross section of the diagonal brace at 1.25R;

[0039] Figure 6 yes Figure 1 A2-A2 cross section of the diagonal brace at 0.7R;

[0040] Figure 7 yes Figure 1 Cross section of the A1-A1 brace at the junction of the middle and tail fin;

[0041] Figure 8 This is a schematic diagram of the incoming flow streamline before the diagonal brace of the present invention;

[0042] The figures are marked as follows: 1-double tail fin; 2-centerline of tail fin; 3-centerline of tail shaft; 4-diameter of tail shaft d; 5-diagonal brace at the end of double tail fin; 6-centerline of diagonal brace; 7-side inclination angle α; 8-longitudinal inclination angle β; 9-cross section of diagonal brace; 10-minimum thickness of diagonal brace cross section; 11-minimum area of ​​diagonal brace cross section; 12-centerline of diagonal brace cross section; 13; centerline angle θ of diagonal brace cross section; 14-streamline of incoming flow in front of diagonal brace; 15-angle of incoming flow in front of diagonal brace; 16-angle of attack γ; 17-radius of propeller R; 18-vertical plane passing through the centerline of tail shaft; A1-cross section of diagonal brace at the connection with tail fin; A2-cross section of diagonal brace at 0.7R; A3-cross section of diagonal brace at 1.25R; A4-cross section of diagonal brace at the connection with bottom plate. DETAILED DESCRIPTION

[0043] The present invention will be described in detail and specifically below through specific examples to provide a better understanding of the present invention, but the following examples do not limit the scope of the present invention.

[0044] like Figures 1 to 8As shown, the present embodiment provides a twisted diagonal brace for reducing drag at the end of a double tail fin of a ship, comprising a double tail fin 1 and a twisted diagonal brace. The twisted diagonal brace is respectively connected between the end of the double tail fin 1 and the longitudinal strong structure position above the bottom plate of the ship. There is a lateral inclination angle between the central axis of the twisted diagonal brace and the center line of the tail fin, and there is a longitudinal inclination angle between the central axis of the twisted diagonal brace and the center line of the tail shaft, forming a twisted diagonal brace for reducing drag at the end of the double tail fin, and the angle of attack between the center line of the cross section of the twisted diagonal brace at different waterline heights and the incoming flow at each waterline height is close to zero, which can minimize the interference effect of the diagonal brace on the surrounding flow field and has a better self-drag reduction effect.

[0045] Preferably, the cross-section thickness of the twisted brace is 0.35d and the area is 0.45d. 2 , where d is the tail axis diameter where the diagonal brace connects to the tail fin.

[0046] Preferably, the minimum side oblique angle between the central axis of the twisted diagonal brace and the center line of the tail fin is not less than 50°.

[0047] Preferably, the minimum pitch angle between the central axis of the twisted brace and the center line of the tail shaft is not less than 60°.

[0048] Preferably, the centerline angles of the cross-section of the twisted brace at different waterline heights are not the same value, and the centerline angles of the cross-section of the brace at different waterline heights are basically consistent with the streamline angles of the incoming flow in front of the brace, and the angles of attack of the incoming flow at different waterline heights are close to zero, thereby reducing the disturbing effect of the twisted brace on the surrounding flow field, reducing the twisted brace's own resistance and its impact on the trailing flow field.

[0049] This embodiment provides a ship double tail fin end drag reduction twisted diagonal brace and its design method, such as Figure 1-8 As shown, the specific steps include:

[0050] Step S1: Complete the geometric modeling of the double-tail fin ship in the 3D geometry software (such as Figure 1 ,2,3, excluding the tail fin brace), as scheme 0;

[0051] Step S2: Using fluid mechanics software, set the incoming flow velocity according to the design speed, and obtain the ship resistance and the incoming flow streamlines 14 and the incoming flow streamline angles 15 of the diagonal brace at different waterline heights (e.g. Figure 5 shown);

[0052] Step S3: Figures 4 to 7 As shown, referring to the relevant requirements for the tail shaft bracket in the structure section of the China Classification Society's "Rules for Classification of Sea-Going Steel Ships", the thickness 10 of the cross section of the diagonal brace is determined to be 0.35d and the area 11 is 0.45d 2 , where d is the tail axis diameter where the diagonal brace connects to the tail fin4;

[0053] Step S4: Figures 4 to 7 As shown, according to the thickness 10 and area 11, the symmetrical streamlined NACA0018 is selected as the geometric shape of the cross section 8 of the diagonal brace;

[0054] Step S5: Figure 1 As shown, the minimum side inclination angle α between the central axis of the diagonal brace 6 and the centerline of the tail fin 2 is 50°; within the range of the minimum side inclination angle α, based on the position of the longitudinal strong structure above the bottom plate outside the tail fin and near the upper end of the diagonal brace, the side inclination angle 7 between the central axis of the diagonal brace 6 and the centerline of the tail fin 2 is determined to be 60°;

[0055] Step S6: Figure 2 As shown, the minimum trim angle β between the central axis of the diagonal brace 6 and the centerline of the tail shaft 3 is not less than 60°; within the range of the minimum trim angle β, based on the position of the transverse strong structure above the bottom plate outside the tail fin and near the upper end of the diagonal brace, the trim angle 8 between the central axis of the diagonal brace 6 and the centerline of the tail shaft 3 is determined to be 70°;

[0056] Step S7: Figure 1 As shown in FIG2 , along the side inclination angle 7 and the longitudinal inclination angle 8, starting from the outer side of the double tail fin end, the cross section 9 of the diagonal brace is stretched to the bottom plate to determine the geometric shape of the entire diagonal brace, thereby determining the length of the diagonal brace, wherein the stretching line is the center axis 6 of the diagonal brace;

[0057] Step S8: rotating the cross sections 9 of the diagonal brace at different waterline heights counterclockwise on the port side and clockwise on the starboard side around the central axis 6 of the diagonal brace to the diagonal brace front flow streamline angles 15 at the different waterline heights, and smoothly transitioning the outer contour of the diagonal brace to obtain a twisted diagonal brace;

[0058] Step S9: In a three-dimensional geometry software, complete the geometric modeling of the twisted diagonal brace; install the diagonal brace at the end of the double tail fin and the strong frame position of the bottom plate structure according to the side inclination angle 7 and the longitudinal inclination angle 8, as Scheme 1;

[0059] Step S10: Using structural finite element software, check the structural strength of the diagonal brace itself and the stiffness of the double tail fins to ensure that the structural strength of the diagonal brace itself and the stiffness and vibration of the double tail fins are within an acceptable range;

[0060] Step S11: Using fluid mechanics software, set the incoming flow velocity according to the design speed to obtain the ship resistance and the incoming flow streamline 14 in front of the diagonal brace; the resistance difference between Scheme 1 and Scheme 0 is the resistance of the twisted diagonal brace itself;

[0061] Step S12: Considering the processing convenience of the twisted diagonal brace, the front flow streamline angles 15 of the diagonal brace at four waterline heights in Scheme 1 are extracted, including: the connection between the diagonal brace and the tail fin (A1), 0.7R (A2), 1.25R (A3), and the connection between the diagonal brace and the bottom plate (A4), where R is the propeller radius 17. These are used as the centerline angles 13 of the cross section of the diagonal brace at the four waterline heights, and the diagonal brace is rotated around the center axis of the diagonal brace, and the outer contour of the diagonal brace is smoothly transitioned to obtain a new twisted diagonal brace scheme as Scheme 2; repeat step S10 to ensure the structural strength of the diagonal brace itself and the stiffness of the double tail fins in Scheme 2; repeat step S11 to obtain the diagonal brace resistance of Scheme 2;

[0062] Step S13: Compare the diagonal bracing resistances of Scheme 1 and Scheme 2, and select the scheme with the smaller diagonal bracing resistance as the new scheme;

[0063] Step S14: Repeat steps S12 and S13 until a twisted bracing solution with the minimum resistance is selected.

Claims

1. A twisted brace for reducing drag at the end of a double tail fin of a ship, characterized by: The invention comprises a double tail fin and a twisted diagonal brace. The twisted diagonal brace is connected between the ends of the double tail fins and the longitudinal strong structure position above the bottom plate. There is a side tilt angle between the central axis of the twisted diagonal brace and the center line of the tail fin, and there is a longitudinal tilt angle between the central axis of the twisted diagonal brace and the center line of the tail shaft, forming a drag-reducing twisted diagonal brace at the end of the double tail fin. The angle of attack between the center line of the cross section of the twisted diagonal brace at different waterline heights and the incoming flow at each waterline height is close to zero, which can minimize the interference effect of the brace on the surrounding flow field, thereby having a better self-drag reduction effect. The thickness of the cross section of the twisted diagonal brace is 0.35d and the area is 0.45d. 2 , where d is the tail shaft diameter at the connection between the diagonal brace and the tail fin; the minimum lateral inclination angle between the central axis of the twisted diagonal brace and the centerline of the tail fin is not less than 50°; the minimum longitudinal inclination angle between the central axis of the twisted diagonal brace and the centerline of the tail shaft is not less than 60°; the centerline angles of the cross section of the twisted diagonal brace at different waterline heights are not the same, and the centerline angles of the cross section of the diagonal brace at different waterline heights are consistent with the streamline angles of the incoming flow in front of the diagonal brace, and the angles of attack of the incoming flow at different waterline heights are close to zero, thereby reducing the disturbing effect of the twisted diagonal brace on the surrounding flow field, reducing the resistance of the twisted diagonal brace itself and the impact on the trailing flow field.

2. A design method for a twisted brace for reducing drag at the end of a double tail fin of a ship according to claim 1, which comprehensively considers the structural strength of the twisted brace itself, the vibration reduction of the tail fin, and the resistance of the twisted brace itself, and is characterized in that: include: Determine the cross-sectional thickness and area of ​​the twisted brace; Determine the cross-sectional geometry of the twisted brace; Determine the lateral inclination angle between the central axis of the diagonal brace and the centerline of the tail fin; determine the longitudinal inclination angle between the central axis of the diagonal brace and the centerline of the tail shaft; use the streamline angle of the incoming flow of the diagonal brace at different waterline heights as the centerline angle of the diagonal brace cross section at different waterline heights, and make a smooth transition of the outer contour of the diagonal brace; use fluid mechanics software to screen the twisted diagonal brace solution with the least resistance.

3. The design method according to claim 2, characterized in that: The specific steps include: Step S1: In a 3D geometry software, complete the geometric modeling of the double-skeg hull, excluding the skeg brace, as Scheme 0; Step S2: Using fluid mechanics software, set the incoming flow velocity according to the design speed to obtain the ship resistance and the incoming flow streamlines and angles of the diagonal support at different waterline heights; Step S3: Refer to the relevant design requirements for the tail shaft support arm in the structure section of the China Classification Society's "Rules for Classification of Sea-going Steel Ships", and consider that the stiffness of the tail fin itself is greater than the stiffness of the single-sided tail shaft support, and determine the thickness of the cross section of the diagonal brace to be 0.35d and the area to be 0.45d 2 , where d is the tail shaft diameter where the diagonal brace connects to the tail fin; Step S4: selecting a symmetrical streamline shape as the cross-sectional geometry of the diagonal brace according to the thickness and area; Step S5: Within the minimum skew angle range, determine the skew angle between the central axis of the diagonal brace and the centerline of the skew fin based on the longitudinal strong structure position above the bottom plating near the upper end of the diagonal brace, on the outer side of the skew fin. That is, the minimum skew angle between the central axis of the diagonal brace and the centerline of the skew fin is not less than 50°. Step S6: Within the minimum trim angle range, the trim angle between the center axis of the diagonal brace and the center line of the tail shaft is determined based on the position of the transverse strong structure above the bottom plate near the upper end of the diagonal brace and outside the tail fin. That is, the minimum trim angle between the center axis of the diagonal brace and the center line of the tail shaft is not less than 60°. Step S7: stretching the cross section of the diagonal brace from the outer side of the double tail fin end to the bottom plate along the side inclination angle and the longitudinal inclination angle to determine the geometric shape of the entire diagonal brace, thereby determining the length of the diagonal brace, wherein the stretching line is the center axis of the diagonal brace; Step S8: rotating the cross sections of the diagonal brace at different waterline heights counterclockwise on the port side and clockwise on the starboard side around the central axis of the diagonal brace to the angles of the incoming flow of the diagonal brace at the different waterline heights, and making the outer contour of the diagonal brace smoothly transition to obtain a twisted diagonal brace; Step S9: completing geometric modeling of the twisted brace in 3D geometry software; The twisted brace geometry is installed at the ends of the double tail fins and the strong frame position of the bottom plate structure according to the side inclination angle and longitudinal inclination angle, as scheme 1; Step S10: Using structural finite element software, check the structural strength of the twisted brace and the stiffness of the twin tail fins to ensure that the structural strength of the twisted brace and the stiffness and vibration of the twin tail fins are within an acceptable range; Step S11: Using fluid mechanics software, set the incoming flow velocity according to the design speed to obtain the ship resistance and the incoming flow streamline in front of the diagonal brace; the resistance difference between Scheme 1 and Scheme 0 is the resistance of the twisted diagonal brace itself; Step S12: extract the front flow streamline angles of the diagonal brace at multiple waterline heights in Scheme 1, respectively serving as the centerline angles of the diagonal brace cross section at different waterline heights, and smoothly transition the outer contour of the diagonal brace to obtain a new twisted diagonal brace scheme as Scheme 2; repeat step S10 to ensure the structural strength of the diagonal brace itself and the stiffness of the double tail fins in Scheme 2; repeat step S11 to obtain the diagonal brace resistance of Scheme 2; Step S13: Compare the diagonal bracing resistances of Scheme 1 and Scheme 2, and select the scheme with the smaller diagonal bracing resistance as the new scheme; Step S14: Repeat steps S12 and S13 until a twisted bracing solution with the minimum resistance is selected.

4. The design method according to claim 3, wherein: In step S3, from the perspective of ensuring the structural strength of the twisted brace itself, a design method for the cross-sectional thickness and area of ​​the brace is provided.

5. The design method for the drag-reducing twisted brace at the end of a ship's twin tail fin according to claim 3 is characterized by: In step S5, from the perspective of ensuring the stiffness of the double tail fins and reducing vibration, a design method for the minimum side tilt angle between the central axis of the diagonal brace and the centerline of the tail fin is given.

6. The design method according to claim 3, wherein: In step S6, from the perspective of ensuring the structural strength of the twisted brace itself, a design method for the minimum longitudinal inclination angle between the center axis of the brace and the center line of the tail shaft is given.

7. The design method according to claim 3, characterized in that: In steps S11 and S14, fluid mechanics software is used to select a twisted brace scheme with the minimum resistance from multiple twisted brace schemes with the goal of minimizing the resistance of the twisted brace itself.

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