水中を動く網地が受ける水の抵抗-V : 網地の吹かれ

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Fishing nets, when set under a current of waters, are liable to deformation. Since the synthetic fiber came in use for fishing nets, the more interests have been paid for decreasing the deformation of net due to currents so as to enhance the fishing efficiency. In continuation to the previous paper1), experiments have been carried out to clarify the different inclinations of nets constructed of various kinds of fibers, the materials of which are listed in Table 1. Using a net stretched to a frame with an area of one square meter to make the mesh square, one side of it was kept on the water level, and the vertical distance h was measured at a current speed v in a way shown in Fig. 1. Next, the inclination of the frame without the net was also determined in the same way. Then, the inclination of the net alone was calculated by deduction this inclinate effect of the frame itself from the former one. Thus, the relations between the inclination h and the current speed v were obtained as shown in Fig. 2 according to different materials of the nets. From the calculation, the weight of the net per unit in water w, under water speed v and angle cf inclination θ, is expressed on the bases of diagrams in Fig. 3. by a formula: w=a•d/l•tanθ•v2 where, d is the diameter of thread, l the size of a mesh and d the coefficient of resistance. It is noticed that the nets which are made of endless synthetic fibers such as Amilan (Polyamid), Saran (Vinylidene chloride), Kurehalon (Vinylidene chloride), Tevilon (Vinyle chloride) and Silklon (Polyvinyle alcohol) are larger in the value of a than the nets of natural cut-fibers. In other words, a is the coefficient chiefly depending upon roughness of the surface of the netting cord. In order to decrease the inclination of nets, it is desirable to make the value of w as large as possible. a•(d/l)

水中を動く網地が受ける水の抵抗-V : 網地の吹かれ

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