ソコビキアミ ノ ケンキュウ II : キリカエアミ ノ コウカ ニツイテ

Four types of the model drag net whose baiting-part was supplied with, at the cod-head, four kinds of changing web net were used, experiments being performed at a tank of circulating water system in the laboratory. The models were made in a reduction to one fourteenth of the original scale after Tauchi's comparative method. The current velocities inside and outside of the net, were measured with small type current-meter, and as was performed in the Ist Report, the relationship between the current velocity and the height of net-mouth, and that between the current velocity and the resistance of net were analyzed. Results are as follows; (1) Current velocity inside the net was faster about 5~20% than that at the net-mouth ; the higher was the current velocity at the net-mouth, the lower was that beside the net-body, owing to the current beside the boby, showing eddy-state. Both at the side of the triangled webbing and at the rear of the cod-end there was noted a current of lower velocity, the value ranging about 45~80% of that of the net-mouth. (2) The height of net-mouth of the changing web net with large mesh (B_1 net) was ascertained to be larger than that of the other three nets ; the difference in the height being no less than provided that the interval between the wing ends was 42m. and the towing speed was 3 knots. In B_1 net mentioned above, current velocity inside the net was noted to be faster than those in other three nets. While relationship of the net-mouth height (h) and the towing speed (v) is to be sought by the following formula. In case of h=h'e^{-av} B_1 net ; Value of a is 0.27, h' is 12.9m.背網の1部分に構造の異なる切りかえ網を夫々に付加した4種の底曳網を田内法則に従って1/14に縮尺し,本学の大型回流水槽で実験した。実験は既報と同様に網の内外の流速を小型流速計でもって測定し,流速分布を求めると共に,網口中央高さ,網成り,網の受ける流水抵抗との関係について検討した。その結果 (1) 網中の流速は網口の流速より5~20%速く,網の側面の流速は網口の流速が高速になる程速くなる傾向があり,渦流状態になることが認められた。また低速部分が三角網と網の後方部分に形成され,いずれも網口の流速の45~80%の値を示している。(2) 大目合の切りかえ網を使用した網(B_1 net)は網口高さが顕著に高くなり3ノットで他の網より1mも高くなった。この網の場合の網中流速は他の網より速いことが認められた。網口高さ(hm)と曳網速度(Vノット)との関係はh=h'e^{-av}の一般式であらわされ,B_1 netの場合,aは0.27,h'は12.9mの値となる