Performance optimization of a trawl for Antarctic krill

Abstract

An optimization analysis test was carried out,based on a trawl currently in use for Antarctic krill(Euphausia superba).The buoyancy/sinking force ratio,and types of bridles and straps were chosen as factors in an orthogonal array test L9(34).From 135 towing trials using 3 expansion ratios(L/S) and 5 trawl speeds,the fol-lowing conclusions were drawn.(1) At higher trawl speeds and higher values of L/S,the drag resistance of the trawl was higher and the width of the net mouth was greater.(2) At a towing speed of 3 knots and L/S = 0.45,these factors influenced resistance(Fs) and power consumption(Ps) in the following order: gravity(A) cross rope(C) bridle(B).The order of their influence on the net mouth height(Hs) was ABC,and on energy dissipation(Ce) was CAB.The optimal combination for hydrodynamic performance was A3B3C1.At L/S = 0.5,the order of influence on Fs,Hs and Ps was CAB,the order of their influence on Ce was BCA,and the optimal combina-tion for hydrodynamic performance was A2B3C3.At L/S = 0.55,the order of influence on Fs and Ps was CAB,their influence on Hs was BAC,their influence on Ce was BCA,and the optimal combination for hydrody-namic performance was A2B3C3.(3) When L/S was extremely low(0.2),the horizontal expansion of the net mouth was approximately 23 m.Although the width of the net mouth was greatly increased,the trawl was ineffi-cient overall and there was a significant increase in the coefficient of energy dissipation.(4) The main method of adjustment to improve the overall performance of the trawl is to increase the effect of gravity by appropriately lengthening the bridle and the cross rope.Based on this study,to achieve a high expansion of the trawl at low towing speeds,we recommend the use of highly expandable otter doors.