Bottom Margin Research Articles

巾着網の網成りと環網に働く張力について

In order to find out the most fruitful method for studying the state of the purse seine in operation, the test were made in the water with under-water current on 1/150 and 1/50 scale models by Tauti's principle and the real seine. The results obtained from those experiments are as follows: 1) As shown in Fig. 1 the force acting on the corkline was concentrated chiefly on wings and back (Max; 10kg/m). 2) From the model experiments, the tension of the purse line in operation depends on under-water current. In actual observations the tension was 150kg in setting, 325kg in hauling and 1750kg in surfacing. Those figures confirm the results obtained from the model experiments. 3) The sinking velocity of the bottom margin was about 7.0m/min and the sinking depth of the net foot was almost 102m. 4) As shown in Fig. 5 the normal transformations by operation of the net were influenced by the current, and the conditions most favourable for operation were Type A (no current), and B (mashio).

日向灘イワシ巾着網模型実験-VI

It is a question whether Tauti's law on model experiment is tenable in such a small model as 1/250 or not. In order to check the validity of this law, a relatively large scale model experiment had to be made. An experiment was made with a 1/10 model in the sea to compare the results with the results obtained with the 1/250 model. The result is shown in Fig. 2, which shows a quite satisfactory agreement. Moreover, in the present experiment, the be haviour of the bottom margin of the net during pursing was observed using a newly deviced depth-meter and chemical tubes (Fig. 3).

日向灘イワシ巾着網模型実験-IV 一定の速度で環締した場合の網裾形状の時間的変化にいついて

As the fourth step in the experimental studies of the purse seine, a series of model experiments was made with the purpose of investigating the timely change of the depth of bottom margin of the net in the course of operation. The model gear and the experimental apparatus were the same as were used in the previous experiments1, 2). For two cases, when it took, (I), 1/2 second (about 2.1 minutes for full scale) and, (II), 1 second (about 4.2 minutes for full scale) after the net had been set into the standing water, the pursing operation was begun mechanically with a speed of 16.5cm./sec.. Then, the transformation of the net in whole course of operation was filmed with a speed of 16 pieces per second (Fig. 1). As the results of these experiments, the relation between the depth of bottom margin of the net and the time required to attain that stage was obtained as shown in Fig. 2, from which the following facts for full scale of net were presumed; a. For about 4 minutes (that is the second 1/4 duration of whole time (T) required to the setting and pursing) after the pursing was begun, the bottom margin of fish bag might still continue such a movement of sinking that it had done up to that time. b. The bottom margin of fish bag may sink to the deepest depth, that is, about 60 meters (40% of the stretched depth (L) of webbing of fish bag) at about 8 minutes (1/2 stage of the whole time) after the setting was begun. c. If the pursing is begun at 2 minutes (1/8T) after the setting was finished*, the maximum depth to which the bottom margin reaches may increase about 10 meters (7% of L).

日向灘イワシ巾着網模型実験-II

As the second step in the approach to the final aim that was related in the previous report1), a series of model experiments of the purse seine was made, with the purpose of knowing the sinking movement of the bottom margin of the net. The model gear which was reduced to 1/250 to the full scale was made based on TAUTI's principle2) (Table 1, 2 and 3). This model net was laid out in the standing water through a circular frame (Figure 1), and the sinking appearance of its lead line was observed and photograghed by the use of a stroboscopic illumination (Plate 1). As the results of this experiments, it was supposed that the sinking of the bottom margin is in a uniform motion at the eary stage and that in the case of this type of gear, the pursing operation is begun when the bottom margin has sunk at the depth of about 15 fathoms (Figure 4). A few practical examples of records of echo sounder, endorsing the above results, were added (Plate 2 and 3).

日向灘イワシ巾着網の模型実験-III 呈味アミノ酸中特にアラニン,グリシン含量に就て

As the third step in the experimental studies of the purse seine in the stratified layer current*, a series of model experiments was made with the purpose of investigating the timely change of the tension of the purse line and the shape of the net in the coarse of pursing opera ?? ion. The model gear is the same as was used in the previous experiments 2). The pursing apparatus and the tension selfrecorder of the purse line were specially designed (Fig. 1). In the two cases; A, when the bottom margin of the net has sunk completely and B, when it is in course of sinking, the pursing was begun and then the tension was selfrecorded (Plate 1), at the same time, the transformation of the net was photograghed (Plate 2). As the results of this experiment, it was observed that in the case A, the tension of the purse line begins to act as soon as the pursing begins (Fig. 2), but in the case of B, it does not act untill the first 1/6 stage of pursing procedure is finished, and then it increases sharply (Fig. 3). After the first 1/2 stage, the changes of tension in both cases take the same pattern (Fig. 4). The case B was considered to be more dangerous than the case A for the occurrence of kankui**.

On a Simple Instrument, Siomi-ito, for Detecting Underwater Current

Siomi-ito, which is a simple device for detecting underwater currents, is locally in use among Japanese fishermen in coastal fishing grounds, especially in the case when they are going to set their encircling net. When such a net is laid out in a water where there exists a strong underwater current relative to the surface one, it happens not seldom to occur that the net is entangled itself by the disturbed, current, thus a part of the bottom margin of the net is caught in the purse line and pulled into a purse ring with the purse line*. When it goes wrong, there is no help to mend matters. Fishermen, therefore, are in a great care in finding out the underwater strong current before they shoot their net. In such a case, a Siomi-ito is tried to use by the chief of the crew on board the skiff. This consists of dropping several boulder weights attached to strings of different lengths. Each weight is subjected to the current of the corresponding depth according to the length of string. The inclination of the string at the surface of the sea will be determined by both the profile of the current velocity and the length of the string. A skillful fisherman can toll the particularly unfavourable violent underwater current by a glance of the inclination angles of the strings. The judgement is said to be hardly possible only by experienced fishermen. It is said that they judge erroneously at times and the crew suffers from getting out of the trouble. Although the detection of underwater current is an urgent problem in fishing operations, the suitable device for fishery purpose has not yet been investigated scientifically. The difficulty seems to arise from the fact that the current velocity at each depth has to be measured successively as a vector quantity, even if it is a rough estimation. Judging from the fisheries point of view, on account of its simplicity, the Siomi-ito may be a hopeful gear for haul seine fishery of a small scale, operating in the water where there is some possibility of occurence of predominant underwater current. Siomi-ito's which are now in common use in Japan are made at fisherman's own way; each skipper has his own idea and one should not expect to find two Siomi-ito's just alike. The authors were intended to make clear the theory of Siomi-ito and to study the proper way of its make up and manipulation for fishery use.

日向灘イワシ巾着網模型實験-I

When a purse seine is laied out in the waters where there exists a strong under-water current, it happens not seldom to occur that the net is entangled by the current and a part of the bottom margin of the net is pulled into a purse ring with the purse line. For the purpose of getting out of this trouble, a series of model experiments based on TAUTI'S principle was made. The shape of the whole net and that of the purse line were photographed at various stages of pursing for various directions of current, when the cork line was held in a fixed circle. The results are summarized in Fig. 4 and 5, from which it is supposed that the entangling of the net may occure at a deeper part of the purse line at a comparatively eary stage of the pursing.