# Study on Reducing Towing Drag by Varying the Shape and Arrangement of Floats and Gears
## 4 Discussion  
This study confirmed that (1) the buoyancy/drag ratio of the largest volume sphere-shaped float was highest when the attack angle was 30\({}^{\circ}\), 60\({}^{\circ}\), or 90\({}^{\circ}\), and that the largest volume ellipsoid-shaped float had the highest buoyancy/drag ratio when the attack angle was 0\({}^{\circ}\); and (2) the drag of the ground gear consisted of hydraulic and frictional drag, and the sphere-shaped ground gear was advantageous for reducing drag when the attack angle was more than 30\({}^{\circ}\). Cylindrical ground gear is better at reducing drag at a 0\({}^{\circ}\) attack angle. The hydraulic drag of float and ground gears is mainly thought to result from surface area and resistance coefficients. The concept of reducing drag has not previously been considered for float and ground gear designs, probably because previous research mainly focused on providing stronger buoyancy for floats and maintaining tight contact with the seabed for ground gears [2; 18]. Only one study has investigated float drag [19; 20]. The resistance coefficients of sphere-shaped glass floats covered by various mesh size wetting was between 0.4 and 0.7, which is lower than the results from our study, which found that the \"C\({}_{\text{D}}\)\" of sphere-shaped floats ranged from 0.9-1.7. It is known that a sphere with rough or dimpled surfaces, such as a golf ball, has a lower resistance coefficient than a sphere with a smooth surface [21]. In addition, the drag force in some floats (F6 and F7) was not statistically significant at an attack angle between 60\({}^{\circ}\) and 90\({}^{\circ}\), as shown in Figure 3. It is thought that because F6 and  
Figure 9: The relationships between (a) attack angle and buoyancy/hydraulic drag ratio of floats and (b) sinking force/hydraulic and frictional drag ratio of ground gears. F7 had different sphere-shaped floats with ears for which the rope tied up to the head rope, these sphere-shaped floats with ears are different from other floats (F8, F9, F10). If netting covers the sphere-shaped float, it might create a rough surface structure and consequently result in a lower drag. A study on covered sphere-shaped floats [19; 20] suggests another idea to reduce the float drag other than what we addressed in this study: size, shape, attack angle, and finally, surface structure. Studies on ground gears of towed nets have focused on their impact on the seabed. The impact of mobile fishing gear on the seabed is considered a serious problem for marine ecosystem conservation [22; 23]. In fisheries using towed gears, reducing the contact area between the ground gear of the towed net and the seabed is one of the factors that can mitigate this problem [24; 25]. Reducing the contact area of the ground gear also generates a lower frictional drag, and consequently reduces the drag of the ground gear, as presented in this study. Therefore, the reduction in contact area of the ground gear with the seabed is effective for seabed impact mitigation and fuel reduction of the fishery. One recent innovative study involves the development of a semi-circular spreading gear to generate a horizontally spreading force on the ground rope [26]. The semi-circular spreading gear was mainly designed to have a small contact area on the ground, but it also effectively reduced drag. However, the self-spreading ground gear is very sensitive to small variations in geometry and may lose ground contact. Therefore, to overcome these disadvantages, a future study on the combination of various ground gears, such as tire ground gear, cylindrical rubber ground gear, iron ground gear, sphere-shaped rubber ground gear, and self-spreading ground gear, is important. In this study, a few ground gears with different shapes and volumes were measured in the flume tank experiment, and only four larger cylindrical ground gears and one sphere-shaped ground gear from the flume tank experiment were measured in the towing experiment.