Study On The Application Of PA Monofilament Netting In The Design Of Bottom Trawl

Marine fisheries,as the basis for human survival and development for a long time,are vital to the food,nutrition and employment of hundreds of millions of people.Bottom trawl fishing has the advantages of good initiative and high economic benefits,and is a very important operation method in marine fisheries.However,with the decline of fishery resources and increasing awareness of marine protection,trawl fishery production has been decreasing year by year.At the same time,in recent years,international fuel prices have remained high,making the bottom trawl operating costs high.Facing the dilemma of gradual shrinkage of bottom trawl output and continuous increase in fishing costs,reducing the resistance of bottom trawl and reducing fuel consumption have become important topics for bottom trawl research.This paper takes the bottom trawl net system as the research object,and carries out a series of research work around the energy saving and drag reduction of the net gear system.Through multiple tests,the nettings,trawl nets,and otter boards were connected in series to form a simplified bottom trawl net system.From this,the design factors（net material,mesh size,twine diameter,otter board type,parameters of otter boards,etc.）,operating factors（towing speed,warp length）,and environmental factors（substrate）are systematically studied.This has important reference significance for the development of energy-saving trawl systems,and also provides a theoretical basis for improving the automation degree and control performance of bottom trawl net gear systems.The specific research contents are as follows:The first chapter mainly analyzes the background and significance of carrying out this research;elaborates the research status of related research on drag reduction of bottom trawl system;and introduces the research content of this topic.Based on the hydrodynamic characteristics of the nettings,verifies the feasibility of the application of the PA monofilament netting to the bottom trawl.In terms of hydrodynamic characteristics of the netting planes,when the plane mesh is perpendicular to the water flow,the drag coefficient C_D9090 of the PA monofilament netting is 9.8%smaller than that of the PA multifilament and 17.3%larger than that of UHMWPE.When the netting plane was parallel to the water flow,the drag coefficient C_D00 of PA monofilament netting was 16.6%higher than PA and 35.3%higher than UHMWPE.Based on the physical characteristics of the nettings,verifies the feasibility of the application of the PA monofilament netting to the bottom trawl.In terms of tensile mechanical properties of the netting,the breaking strength of PA monofilament netting is27.7%higher than that of PA multifilament,and its percentage of breaking elongation is better than UHMWPE.PA monofilament nettings has its unique advantage in bottom trawl application.Combined with the results of the previous netting experiments,model nets were made to explore the performance difference of different material nets through the flume tank experiment.According to the characteristics of PA monofilament and UHMWPE,we replaced the wing and the square part of the PA multifilament bottom trawl with the PA monofilament netting and UHMWPE netting with larger mesh size and thinner twine,in the flume tank experiment.The results showed that compared with PA multifilament bottom trawl,PA monofilament net can reduce the resistance by 32.9%,and UHMWPE net can reduce the resistance by 36.2%.PA monofilament net and UHMWPE net can also reduce the in-pull force of the net mouth,and can be paired with a smaller otter board in the towing,so as to further reduce the drag of the trawl system.This study in order to further explore the performance of low-resistance bottom trawl net,combined the PA multifilament net,PA monofilament net and UHMWPE net with HLTD,vertical v-type,double-wing type otter board to form the net system for the towing tank test.The results showed that,compared with PA multifilament,PA monofilament net or UHMWPE net can reduce the total drag of the trawl system by about 30%.Among the combinations of nets and otter boards,the combination of PA monofilament net and HLTD is the best.And the resistance of the net system is not the sum of the drag of the net and the otter board.Factors such as substrate and towing height will also have a significant impact on the resistance of the net system.The last chapter summarizes the research work of this dissertation,puts forward the main conclusions and innovations of this dissertation,and reflects inadequacy,and looks forward to the next research direction and content.