Abstract
Dynamic responses of tuna purse seine to currents were numerically studied with regard to the vertical subsidence and global load distribution, and determinant attributes were identified. We rebuilt the submerged geometry of a purse seine net using the lumped mass method with hydrodynamic coefficients obtained from measurements of the prototype material, as well as a mesh grouping method, which corrects the twine diameter and netting material density for the equivalent net by introducing the compensation coefficient. Uneven tension distribution showed that it was vulnerable to high loads in the bunt area at shooting and along the lead line at pursing. High loads were present at the convex sections of net circles in the direction of the current. Higher shooting speeds resulted in well-balanced sinking, while a reduced hanging ratio of netting panels was beneficial to faster sinking. While large mesh sizes reduce water resistance and increase sinking velocity, extensive use should be cautioned in terms of the sensibility to stress. Comparing two options of different proportions of large-mesh panels, mesh sizes doubled for 15 strips versus 5 strips, suggesting that the 15 strip option would pose a higher risk of strand vulnerability, while the 5 strip option may be a more balanced alternative with a lower mesh density and a lower mesh stress.
Highlights
Tuna purse seine is one of the largest and technologically sophisticated fishing gears, which has been the predominant contributor to the total annual landings of various tuna species [1,2,3]
To estimate the empirical formulae of hydrodynamic coefficients, we performed tests on nylon netting in a circulating flume tank of Tokyo University of Marine Science and Technology (TUMST), with the working section of 9.0 m in length, 2.2 m in width, and 1.95 m in depth
Those sets shared a common feature that the middle part of the net was almost normal to the current, revealing that improvement of hydrodynamic coefficients was still needed for our model
Summary
Tuna purse seine is one of the largest and technologically sophisticated fishing gears, which has been the predominant contributor to the total annual landings of various tuna species [1,2,3]. Owing to effective monitoring and prediction of the net shape in the numerical approach, more details and arguments were put forward, for example, the transformation of the float line and purse line as the seiner drifted toward inside the net circle [19]; the substitution of a large-mesh panel with a prototype for superior performance of sinking and mechanics [20]; and the possibility of controlling the depth of the lower edge of the net by regulating the shooting speed to avoid grounding [21]. We identified design alternatives and how current interferences would account for the rationality of the structure and operation of the purse seine from mesh size, hanging ratio, shooting speed, and current pattern, which influence net deformation
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