| Mid-water trawl is an efficient fishing gear for exploiting Chilean jack mackerel(Trachurus murphyi) in southeast pacific using the single boat technique. Fisheryassessment of recent years have indicated that the resources have been declining, andfishing ground is hard to be found. In this case, it needs to use the operatingcharacteristics of mid-water trawls to improve fishing efficiency by target fishing andtracking fishing. The current net productions are mainly rely on import and subjectedto foreign countries, the efficiency of former nets is not high, meanwhile, theinternational trawl reforms have toward a friendly, economical development.Therefore, it is imperative to optimize the domestic jack mackerel trawl. This paperaims to focus on the model experiment of domestic jack mackerel trawl, analyze theperformance of each nets, determine the basic trawl net and propose a optimizingdesign scheme of trawl. Meanwhile, model experience the designed trawl and discussthe differences between the optimized trawl and the basic one. It is expected to play areferable and promoting role in the China-designed Chilean jack mackerel trawldesign procedure.This study can be summarized as follows:(1)We analyze the ten kinds of Chilean jack mackerel mid-water trawl and sumup the parameters of the domestic fishing boats. The rated power range of the fishingboats is5152~5920kW, and the length is91.1~104.41m, and the width is16~20m.These are all imported second-hand boats. The L/C range of the ten kinds oftrawl is0.127~0.304, and the Lb/C range is0.119~0.260, and the Lw/C range is0.184~0.378.(2)Setting the four-piece jack mackerel mid-water trawl manufactured bySHANGHAI KAI CHUANG DEEP SEA FISHERIES CO.,LTD, of which theparameters are1040m×253.02m(149.4m),1248m×285.00m(160.2m),1440m×361.00m (228.6m),1440m×352.30m(242.4m) and1632m×444.00m(277.4m) as the full scalevalues, and through the model experiment, we text the resistance of the trawls and theheight of net-mouth, then conclude the hydrodynamic performance of each trawl. Itcomes to a conclusion that:(a)The expression of the height of net-mouth isH=0.0912CV-0.366.(b)The expression of the resistance of the trawls isRa=SV2/(25.12+2.26V) and Rb=0.0218(d/a)LCV1.708.(c)In the aspect of powerconsumption,1440-1,1440-2and1632trawl consume the power by the close atdifferent speed, while do more than1040and1248trawl.(3)We set the1632trawl as the basic trawl, then analyze its principal dimensions.The total length of trawl is1632.00m, and the straight length is444.00m, and the L/Cis0.272,and the Lb/C is0.171, and the Lw/C is0.273, and the netting area coefficientis0.00172, the netting area is571.60m2,and the virtual netting area coefficient is332843.60m2. According to the practical experience and the current Chilean jackmackerel resources, suggestion is given that the perimeter of the sample should beincreased to enlarge the sweep area and the straight length of the trawls should bestrictly controlled without reducing the guiding effect and the stability of the trawls.The increase of the length will affect the timing of the fishing production andefficiently lower the resistance.(4)The design trawl is confirmed as1768trawl. The total length of trawl is1768.00m, and the straight length is473.88m, and the L/C is0.268, and the Lb/C is0.143, and the Lw/C is0.276, and the netting area coefficient is0.00171,and thenetting area is529.67m2, and the virtual netting area coefficient is309491.90m2. Wecan infer that compared with the sample, the perimeter of the target increase by8.33%,and the straight length increase by6.53%, and the netting area coefficient decreasemerely by0.58%, and the total virtual area decrease by7.02%, and the fittest size ofnet code is120mm. The reason why the netting area, the netting area coefficient andthe virtual area decrease is while diameter of the line is maintained, the net grid areais appropriately reduced. Meanwhile, the buoyant device of the1768trawl follow thesoft canvas of the1632trawl. The sedimentation device also follow the trawls1632trawl. The sedimentation force is19073.105N. Calculated through the resistanceformula Ra=SV2/(25.23+2.24V) derived from the model experiment, the result turn tobe that resistance reduce by7.33%and7.34%in the towing speed of4.0kn and6.0kn.Calculated through resistance formula Rb=0.0218(d/a)LCV1.708, resistanceincrease by14.96%and14.95%respectively in the towing speed of4.0kn and6.0kn. Calculated through the formula of height of net-mouth H=0.0912CV-0.366, height ofnet-mouth of1768trawl is8.34%and8.33%more than1632trawl in the towingspeed of4.0kn and6.0kn. It can be inferred that the1768trawl gain advantages inoperating performance to1632trawl while we need more model experiments tovalidate and perfect the parameters of1768trawl.(5)The model experiment of1768trawl shows that when the L/S is0.40,0.45and0.50, height of net-mouth of1768trawl increase as the L/S decrease, and isslightly higher than1632trawl. It also comes to the resistance of the trawl. Bycompared with1632trawl,1768trawl does better in energy consumption coefficientand hydrodynamic performance which indicates that1768trawl is better in economicperformance and catch performance. However, vertical expansion coefficient andpower consumption of1768trawl do worse than1632trawl. With the theory tocalculate the sweeping areas of1768trawl and1632trawl, it comes to a result of10469.12m2and9304.87m2which shows that1768trawl gains a12.51%advantageto1632trawl.(6)Adopting the basic form of current nets resistance R=SV2/(A+BV) andR=k(d/a)LCVn, we learn the resistance estimation formula of Ra=SV2/(25.12+2.26V)and Rb=0.0218(d/a)LCV1.708though the model experiment. We use paired T test toanalyze the difference between the results from two calculations and validate it with a1632m×444.00m（277.4m）resistance sea field experiment. Results show that:(a)Analysis the correlation of the two formulas by the t-test, the minimum and maximumvalues of each trawl resistance was no significant difference (P﹥0.05) on thecondition of4.0kn and6.0kn.(b) On the condition of4.5~6.0kn, the results ofRa=SV2/(25.12+2.26V) were most approximate to the resistance of1632trawl on fieldexperiment. Taking an overall look of the date of field experiment, the further fieldexperiment should be done in future, which can improve the accuracy of the data andverify the accuracy of the results. Through the resistance formulaR=SV2/(25.965V-53.437) derived from the measured date,when the towing speed is at4.0kn and6.0kn, the resistance of1632trawl is181.377kN and201.045kN, while theresistance of1768trawl is168.070kN and186.298kN. It comes to be7.90%and7.34%in1768trawl than1632trawl. |
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