DESIGN AND TEST OF AN ENERGY-EFFICIENT MIDWATER TRAWL BY SIMULATION AND FLUME TANK

Abstract

In this study, we designed a suitable energy-efficient midwater trawl to harvest small pelagic fishes in the Ghanaian waters. A mass spring-model was used to describe and calculate the shape and movement of a midwater trawl system. Two different gears were simulated by the dynamic simulation software, SimuTrawl, developed in Pukyong National University, Korea by Marine Production System Laboratory (MPSL). One gear was made up of polyetylene (PE) netting while the other gear was PE in the first six panels and the rest of the fourteen panels were replaced with “Dyneema”, half the diameter of the PE netting twine. To check on the accuracy of the mathematical calculation of the simulated gear, we constructed a small scale model midwater trawl net based on the normal scaling rules and tested it in a 3D water circulating flume tank (10.2 m long x 3.2 m high x 2.8 m wide). The height of the net mouth of the simulation of PE and “Dyneema” netting (PE+DY) had higher opening between 19% and 25% than the height of PE netting. The net drag of PE+DY was reduced by 16% to 31% compared to the drag of PE. The model test likewise recorded a reduction in the drag of the PE+DY between 20% and 23% compared to the drag of PE. These results confirm the hypothesis that replacing a polyethylene midwater trawl gear with “Dyneema” netting material, half the diameter of polyethylene netting twine, from the 7th panel to the 20th panel (codend) will yield a higher height of net mouth opening and lower drag. This type of gear should be highly useful for the Ghanaian small pelagic fisheries.