底曳網の模型實驗

Abstract

The efficiency in keeping the mouth of net open was experimented with Otter boards of suitable shape and width attached to a kind of drag nets used in Japan. The models of 3/40 and 1/75 of the original drag nets, A and B, of which the net pieces are arranged as shown in Fig. 1 A and B, were made according to TAUTI'S principle1) of model net construction. The Otter boards, as shown in Fig. 2 A and B with the lengths converted into original, were attached to these nets, A and B, respectively. The tensions on the ropes of nets under various pulling speeds were estimated. Between the tension on the net R and pulling speed υ, for two original nets, we get the relations R= 0.036υ2+72 (net A) and R=0.097υ2+200 (net B), while for the model nets, R=0.026υ2+72 (net A) and R=0.050υ2+200 (net B), respectively. Hence in Fig. 4 A and B the tension from the model experiments are converted by the relations R=1.40×(0.026υ2)+72 (net A) and R=1.94×(0.050υ2)+200 (net B), into originals (crosses) for the same reason as the writer has mentioned in the case of the model experiments on sailing trawl net used in Mikawa Bay2), and compared with that of originals (circles). A fair coincidence is seen between them. The forms of model nets were photographed both from above and lateral sides under the pulling speeds, which are, if converted into the original, 1/2, 1 and 11/2 mile/hour (Fig. 3 A and B). Several dimensions of the mouth of net vary with the speed as shown in Fig 5 A and B.