Studies On The Design And Model Experiment Of Pelagic Trawl With Large-Size Meshes

With the decline of offshore fishery resources and the deterioration of the marine environment,marine fisheries have gradually moved from offshore to broad ocean,deep-sea,ocean-going and polar regions.Pelagic trawl is the most used and most productive fishing gear,and is also the main fishing gear used in Gulf of Oman fishery at present,but it has some shortcomings such as poor performance,weak adaptability and insufficient innovation.In recent years,the Iranian government's efforts to regulate fisheries have been increasing,and the audit of fishing vessels and incoming personnel has become more and more strict,so it is urgent to improve the performance of ocean-going trawl gear,and the large-size mesh trawl with low resistance and good vertical expansion performance is one of the main development directions.This paper comprehensively analyzes the structural characteristics of large-size mesh trawls in China,and combines the conditions of environment and fishery regulations in Gulf of Oman fishery,Iran,Optimizing design a type of large-size mesh bottom trawl based on improved mesh type.And designing flume test of model net hydrodynamic performance is carried out to analyze fishing gear performance.Firstly,according to the fishing period,fishing object characteristics and other conditions,design a large-size mesh trawl which size of 398m×149m,net mesh size of 6 m,rope trawl net length of 30m.In order to enhance the stability of the net,reduce the number of hauling the net,reduce the resistance of netting and improve the fishing efficiency,the design net has the characteristics of long relative length of net body,short rope trawl net length and large-size mesh.Secondly,according to similarity criterion,design large scale?=24,small scale?_d=6.7,using floating and sinking ratio 0.4,0.6,0.8,making and assemble trawl model,according to the expansion ratio L/S 0.45,0.55,0.65 three gear,model flume test at 0.4~0.8 m/s towing speed,test and study the resistance and net port vertical expansion.The results are as follows:(1)Resistance increases with the increase of towing speed,which is approximately exponential growth.at the same towing speed,the greater the horizontal expansion ratio L/S,the greater the resistance.when the horizontal expansion ratio L/S increases from 0.55to 0.65,the increase of the resistance value is particularly obvious.(2)The net port vertical expansion of model decreases with the increase of towing speed,Between the different horizontal expansion ratio L/S,net port vertical expansion is not obvious in the low towing speed.When the towing speed increases to 0.6 m/s,the larger the L/S,the smaller the net port vertical expansion at the same towing speed.And when the horizontal expansion ratio L/S decreases from 0.55 to 0.45,the height of the net port vertical expansion is obviously raised.(3)In the comparison tests of 0.4,0.6 and 0.8,the net port vertical expansion increases with the increase of the floating-sinking ratio.When the floating-sinking ratio increases from 0.6 to 0.8,the net port vertical expansion increases obviously.Increasing the floating-sinking ratio has a certain effect on increasing the net port vertical expansion,at the same time,it will not have too much effect on the resistance of net.(4)According to the test data,the resistance and the net port vertical expansion of net are calculated,and the parameters such as coefficient of headline height,energy consumption coefficient and power consumption are calculated.The design net lengthens the net body and doubles the back of the net body as a net bag,which is beneficial to increase the catch and improve the stability of the net gear.It is more suitable for fishing production in the fishing ground of Oman Bay.However,through model test research and analysis,this net body structure may also cause problems such as increased net resistance and poor water drainage.It is recommended to appropriately reduce the length of the net body and the net on the premise of ensuring stability and catch.The body's back end adopts a more reasonable connection method and is continuously optimized in production tests.