Yunosuke Iitaka

The virtual mass of plane nets

A series of experiments was performed with the purpose of getting the fundamental aspects of the virtual mass effects acting on the net gear motion. Three types of webbings; monofilament with double sheet bend, lmotless multifilaments and multifilaments with sheet bend, were used in the experiments. The motion offramed plane nets were recorded as the change in the force through the strain gauge on a recorder. The virtual mass of webbing consisting of multifilament with sheet bend became larger than the other two types. When inclined angle with respect to the moving direction was 0°, the virtual mass represented the maximum value in all cases. The virtual mass effects acting on the mesh size appeared clearly when mesh size became relatively small. The results suggest that the virtual mass of webbing is variable and depends upon the mesh size and the surface shape of webbing in addition to the state of motion.

Model Experiments on the Sardine Purse Seine Operating in Hyuganada-V

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).