Empirical dependences of the hydrodynamic drag coefficient of cod-ends made of T0 and T90 netting

Cod-ends are cylindrical netting structures, which are made of nettings of different material and mesh rotation, it can be a traditional T0 or rotated by 90° – T90. Cod-ends consist of a jacket, the mesh pitch of which is set by fisheries regulations or conventions of a particular fishing area, and the cod-end itself. The cod-end made of ropes is surrounded by a frame for strength. In some cases, in coastal fisheries and inland waters, only the bag without the jacket and frame is used, while the bag itself performs several functions: catching hydrobionts in accordance with fishing regulations and holding the catch. Such cod-ends are used for scientific purposes in the trawl fishery for ruffed grouse (Coregonus albula) in lake Vishtynetskoe, Kaliningrad region. Retention qualities of cod-ends are connected with their selectivity, it means retention of those hydrobionts, which can be removed from the water body without damaging aquatic organisms and without undermining biomass of coastal waters, lakes and reservoirs. As a rule, mesh T0 has less selective properties in comparison with T90, as in netting T90 there are no sharp angles of contact between the ropes of one mesh, which ensures that hydrobionts are not clamped in it, in addition, such netting is better from the point of view of catchability to hold hydrobionts with an oval body. Also cod-ends with netting T90 have less hydrodynamic resistance compared to netting T0, which positively affects the hydrodynamic resistance of the trawl system itself and cod-ends made of netting T90 contribute to the preservation of the cylindrical shape of the cod-end in the process of trawling and catch retention, this is due to the shape of the mesh and physical and mechanical properties of the ropes from which the netting of the cod-end is made. In the course of the research, empirical dependencies of the coefficient of hydrodynamic resistance of cod-ends made of netting T0 and T90, material polyamide were obtained. The least coresistance was shown by kapron cod-ends made of netting T90. The resistance of such bags is 11% less than traditional cod-ends.