水中を動く網地が受ける水の抵抗-IV

Abstract

Since synthetic fiber was adopted for fishing nets, many problems have given rise due to its different properties from natural fiber. Among them there is configuration of the nets which is affected by currents more than that of natural fiber nets because threads of synthetic fiber such as Amilan or Vinylon are smaller in specific gravity. For a preparatory study of the configuration of nets, tests have been carried out to clarify different inclinations of various threads in a current. In addition, experiments have been conducted on dyeing treatments of synthetic fiber threads for the purpose of decreasing the inclination, using dyeing materials such as coal-tar or its mixtures prepared with a commercial stone-powder, 15-, 30-, 45-percent in weight of the coal-tar base. Specification of the threads used for experi-ment is shown in Table 1. Keeping one end of the threads, each 130 centimeters long, on the water level, the vertical distance h from level to the other end was measured under current speed v. From Fig. 1 showing the relationship between v and h, it is noticed that the thread inclination denoted by h becomes larger as the diameter of thread d increased or coal-tar dyeing is used. Fig. 2 shows the comparison of h of various kinds of threads within almost the same value of d, and Fig. 3 the relationship between d and h by kind of threads, when current flows at 15 centimeter per second. From these figures, the thread inclination are obtained, in increasing order as Saran, cotton, manila- twine, Amiran, Vinylon; and in case of no dyeing, h of Vinylon is 40 to 60 percent, and Amilan 60 to 75 percent, of the inclination of cotton hc. However, in coal-tar dyeing, the difference decrease nearly to 10 percent. Different inclinations of threads due to dyeing treatments are given in Figs. 4 and 5. By coaltar dyeing, Vinylon is most effectively decreased in the thread inclination. -From Fig. 6 indicating the forces acting on thread in a current, next formula may be led Pθ=ω•cos θ. Taking the resistance coefficient Cp, the resistance Pθ is expressed by Pθ=σ/2g• d•Cp•v2 Where σ is the specific gravity of water, and g the acceleration of gravity. Hence, Cp=cosθ/v2 bull;2g/σ•ω/d. In order to decrease the inclination of thread, i.e., to increase the value of θ, it is desirable to make values of Cp as small and ω/d as large as possible. On the base of the values of Cp for tested threads (Figs. 8, 9), it is obvious that Vinylon is the largest in the value, and Amilan the least, in case of no dyeing. Whereas, in coal-tar dyed threads, Cp of Vinylon thread decreases almost to the same as of Amilan. On the other hand, in regared with the increasing ratio of ω/d by dyeing, Amilan is the largest followed by Vinylon (Table 2). Thus, the fact that Vinylon is remarkable for decreasing Cp, and Amilan for increasing ω/d, in coal tar dyeing, has led to a conclusion that the dyeing applied to these two kinds of threads is efficient for decreasing the thread inclination by current.