# Study on Reducing Towing Drag by Varying the Shape and Arrangement of Floats and Gears
## 4 Discussion
In addition, when combining ground gears, it is important to consider which ground gear has the lowest drag in the wing and entrance of the net, and how to reduce the drag by attaching the ground gears to the net. For this combination of ground gears, measurement of all shapes and materials of ground gears with various attack angles needs to be performed, and the redesign and rearrangement of ground gears must be considered through model and field experiments. Finally, a preliminary calculation to reduce the drag for the pair trawl fishery (Figure 10) was performed using the findings obtained in this study. Figure 10: A typical pair trawl fishery gear setup. Conventional design of pair trawl fishery gear had 744.64 kg of sinking force and 335.10 kg of buoyancy. In float parts, only sphere-shaped floats were arranged in a conventional design. Figure 11 shows the net used in the pair trawl fishery designed in 2016 (General Foundation Japan-Korea-Japan-China Agreement Measures Fishery Promotion Foundation, 2017). The total length of the net was approximately 53 m, and the lengths of the head rope and ground rope were 55.15 m and 64.55 m, respectively. Floats and ground gears are mainly attached to the head rope and ground rope, respectively, but some floats are attached to the lacing lines of the net (No. 6F in Figure 10). Total buoyancy of the floats used for the net was 3286.32 N and total sinking force was 7302.68 N (Figure 10). Sphere-shaped floats were used in the conventional design as shown in Figure 11. Three types of ellipsoid and sphere-shaped floats (F5, F9, F10) with similar buoyancies (3237.96 N) to the present design were chosen to replace the suitable parts of the net. The ellipsoid float (F5) was arranged to place parts where the attack angles of the mounting positions (front part of the wing and lacing lines) were close to 0\({}^{\circ}\). The drag of F5 at a 0\({}^{\circ}\) attack angle was used for three F5 floats at the edge of the head rope (No. 1F and 2F), and the drag at 30\({}^{\circ}\) was used for eight F5 floats on the two lacing lines (No. 6F). Spherical floats F9 and F10, were chosen for the other mounting positions. Their attack angles were set by considering the attack angles of mounting positions as follows: F9 on No. 3F of the head-rope with a 30\({}^{\circ}\) attack angle (front two floats) and 60\({}^{\circ}\) attack angle (one close to the entrance). Five F10 floats in No. 4F and four F10 floats had 60\({}^{\circ}\) attack angles, while the other F10 floats had 90\({}^{\circ}\) attack angles. For the 5F part, four F10 floats were placed at 60\({}^{\circ}\) attack angles. The attack angle of F9 was set to 60\({}^{\circ}\). Four F5 angles were set at 30\({}^{\circ}\), as explained. To estimate the drag reduction of the ground gear, three types of cylindrical and spherical ground gears with 7311.94 N of sinking force were considered. The detailed arrangements of ground gear in the ground rope design for the pair trawl fishery are  
Figure 11: The comparison between conventional design and suggested design of float parts in the pair trawl fishery gear for lower drag. Conventional design used all sphere shape float. In suggested design, ellipsoid-shaped floats (F5) were arranged in float parts with low attack angle in 0–30\({}^{\circ}\). And small numbers of lager floats were rearranged from many numbers of smaller floats. presented in Figures 12 and 13. To minimize the drag while maintaining the total sinking force, sixteen G8 and three G9 gears replaced the nineteen conventional rubber ground gears in the No. 5G part of the ground rope (Figure 13). G8 and G9 were set at an angle of 30\({}^{\circ}\). For Nos. 1G, 2G, 3G, and 4G, the ground rope parts were not replaced because they were already designed to reduce drag, and the redesigned ground ropes could have a larger drag than the conventional design. In addition, for No. 6G and No. 7G, the ground rope parts were not replaced because the ground gears used for No. 6G and No. 7G were larger than G10.