底曳網の模型實驗

抄録

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.