Study On Construction Optimization Of The Net Mouth Of Roped Stow Net With Anchor

Stow net originated in the 1980 s, in order to protect large yellow croaker, small yellow croaker, pomfret and other economic species, the stow net was developed from net of monkfish in Jiangsu province. After 1990 s, stow net constantly being optimized, Nets operating performance was improved, the fishing technology of stow net got better and better. The advantages of stow net continued to be highlighted. Stow net is widely welcomed by fishermen for its high scale, high catch capacity per single ship, low energy consumption, high profits, high catch efficiency and high yield.However, traditional stow net fishery is a common offshore fishery in China and rope stow net,which is always set by single anchor,is the most important construction type of stow net.This fishery has disadvantages such as the fishing ground is too large, the fishing effort is too high and the selectivity of net is poor. In addition, the rough construction of the net mouth,where all the meshes are used in AT cut,can easily cause folded or overlapped conditions of the panels near the middle of footrope and headline and then the uneven force of net panels may cause breakages of stow net and move of anchor.In order to solve these problems, a technical measure, including tapering the back and belly panels with cutting ratio in net mouth,making net mouth of stow net form a “winged ”structure. The working condition and hydrodynamic performance between designed net and control net were compared through the numerical simulation method, combining with observation in scaled model test in a towing tank and in the sea.In this paper, the traditional roped stow net with single anchor(230m×180m) was used as the contract net. The scaled model test was carried out in hydrostatic water tank of Scientific Research Institute of East China Sea Fisheries Research Institute of China On 28-29 March 2012. The model stow net is made following the Tauti’s modeling law, the λ and λ′ of control net is 25 and 3.95, the λ and λ′ of designed net I(the cutting ratio of net mouth is 1:5) is 25 and 2.73.In the model test,we measured the extension of net mouth(height of net mouth, width of net mouth),resistance of net,and observed the conditions of net.In order to establish the dynamical model of stow nets, numerical simulation based on the concentrated mass method was used and the designed nets I and II(with 1:5 and 1:7 cutting ratio on the net mouth web panel) and the control net were simplified through the mesh grouping method. Then MATLAB software was used to simulate the two nets. In the simulation, we take into account the resistance of net, extension of net mouth and tensile force of partly panels, rendered the Space in the shape of the stow net in 3D through the MATLAB graphics.In addition, through the two stages of the sea trials. In the sea trials, we observed the conditions of designed net and control net, calculated the net breakage rate.The designed net and controlled were compared through three ways, conclusions are including:(1) with the results of numerical simulation, resistance and extension of net are correlated with the velocity of flow. As the velocity increasing the resistance of net will increase while the area of net mouth extension decrease. The net mouth will expand outward, then shut inward, and remain stable until the last.(2) with the results of numerical simulation, if cutting ratio is 1:5, the resistance of designed net reduced by an average of 2.04%, comparing with control net, the area of net mouth extension reduced by 16.59%, tensile force of back panels reduced by 14.61%; if cutting ratio is 1:5, the resistance of designed net reduced by an average of 3.48%, comparing with control net, the area of net mouth extension reduced by 16.98%, tensile force of back panels reduced by 18.03%; if cutting ratio is 1:7, the resistance of designed net reduced by an average of 6.23%, comparing with control net, the area of net mouth extension reduced by 26.23%, tensile force of back panels reduced by 22.29%;(3) with the results of scaled model, if cutting ratio is 1:5, the resistance of designed net reduced by an average of 5.12%, compared with control net, the area of net mouth extension reduced by 16.80%(4) By comparing the results of scaled model test and numerical simulation, the difference of resistance between control net and designed net is less than 4%;the height and width of net is less than 11%, the results of numerical simulation is suitable for simulate the working condition and hydrodynamic performance of stow net;(5) with the results of sea trials, The net breakage rate was 3% for the design net and 12% for the control net;Comparing between the control stow net and designed net, the results of scaled model test and numerical simulation showed that the net panels of net mouth of designed stow net spread better than those of control net. The problems of being folded or overlapped and uneven force distribution of net panels were mitigated effectively, especially in high current velocity condition. According to the results of sea trials,it was found that the net breakage rate of designed net is less than that of control net. However, construction improvement in net mouth may lead to decrease of extension and catch. This study suggested that the cutting ratio of net mouth should be about 1:5.