Optimization and experiment of the mid-water trawl for ocean medium single trawlers

Medium-sized single trawlers are still lacking to catch the pelagic fishes in the Eastern Central Atlantic Ocean in recent years. In this study, an optimal design and test were performed on the mid-water trawl for the medium stern trawler, according to the structural and hydrodynamic parameters of the current large mid-water and large-mesh offshore trawl. Three kinds of perimeters were set for the specification of net mouths in the model and offshore production, including 552, 456, and 360 m. Specifically, the mesh size of net mouth was 12 m, the length of body net/circumference net-mouth was 0.217 m, and the length of wing/circumference net-mouth was 0.076 m. The results show that the average resistance of type 552 (Circumference net-mouth was 552 m ) was 151.82 kN, and the height of net opening was 28.74 m; the average resistance of type 456 (Circumference net-mouth was 456m) was 135.6 kN and the height of net opening was 24.96 m; the average resistance of type 360 (Circumference net-mouth was 360m) was 70.76 kN and the height of net opening was 20.20 m, when the trawl speed was 2.31 m/s, while, three kinds of expansion ratio were 0.35, 0.40, and 0.45 (wingtip space between /ground rope length L/S), respectively. A model test was also carried out in the Eastern Central Atlantic Ocean. It was found that the average yield per hour of type 552 was 0.13 t and the volume of filtered water per second was about 2442 m3 when the trawl speed was 2.06 m/s. Nevertheless, only 5% of the pelagic fish was captured after the detection of pelagic fish flocks, indicating the very low net output. When the trawl speed was 2.26 m/s, the average yield per hour of type 456 was 0.77t and the volume of filtered water per second was about 1927 m3. When the trawl speed was 2.52 m/s, the average yield per hour of type 360 was 1.13t and the volume of filtered water per second was about 1321 m3. The maximum towing speed of type 360 reached 2.67 m/s, and the maximum output of single net trawl operation in 4 hours reached 20t. Since type 360 presented the smallest net mouth and the water filtration volume per unit time, the output per unit time was much higher than that of the rest. Particularly, the mid-water trawls were adopted in the medium trawlers, in order to catch the small pelagic fish with better swimming ability, where the higher towing speed was required. Furthermore, a better yield was achieved, as the increase of the trawl speed in the range of 2.31-2.52 m/s. Once the mid-water trawl run a higher towing speed, the main engine of the fishing boat was consumed more power to overcome the fluid resistance. Therefore, the power consumption of the test net part (excluding otter board and warp) was recommended as 11%-16% of the rated power of the main engine, when the decreasing power was allocated to the net. In addition, the ultra-high molecular weight polyethylene (UHMWPE) fiber can be widely used to reduce the diameter and area of the test net, further reducing the resistance for the better performance of the test net. The finding can provide an important reference for the structural design and optimization of the mid-water trawl in the medium stern trawler on ocean fishing.