Abstract
Nylon (PA) netting is widely used in purse seines and other fishing gears due to its high strength and good sinking performance. However, hydrodynamic properties of nylon netting of different characteristics are poorly understood. This study investigated hydrodynamic characteristics of nylon netting of different knot types and solidity ratios under different attack angles and flow velocities. It was found that the hydrodynamic coefficient of netting panels was related to Reynolds number, solidity ratio, attack angle, knot type and twine construction. The solidity ratio was found to positively correlate with drag coefficient when the netting was normal to the flow (CD90), but not the case when the netting was parallel to the flow (CD0). For netting panels inclined to the flow, the inclined drag coefficient had a negative relationship with the solidity ratio for attack angles between 0° and 50°, but a positive relationship for attack angles between 50° and 90°. The lift coefficient increased with the attack angle, reaching the culminating point at an attack angle of 50°, before subsequent decline. We found that the drag generated by knot accounted for 15–25% of total drag, and the knotted netting with higher solidity ratio exhibited a greater CD0, but it was not the case for the knotless netting. Compared to knotless polyethylene (PE) netting, the drag coefficients of knotless PA netting were dominant at higher Reynolds number (Re>2200).
Highlights
Meshes of traditional netting panels are composed of bars and knots
Experiments were carried out to investigate the effects of solidity ratio, flow velocity and knot type on the hydrodynamic characteristics of plane netting in uniform flow at different attack angles
The following conclusions can be drawn from the experiments and subsequent analysis: 1. Both Reynolds number and solidity ratio have a significant effect on hydrodynamic coefficients, and solidity ratio is negatively correlated with the parallel drag coefficient, but the opposite is true when the netting panel is set normal to the flow
Summary
Meshes of traditional netting panels are composed of bars and knots. Netting panels can be weaved without knots, and are called knotless netting. Single English knot and double English knot are commonly used knotted types for fishing nets, while in knotless netting, twisted Cross and Ultra Cross (named by Nichimo Co., Ltd, Shimonoseki, Japan) are the basic knotless type according to the way the mesh is wove. One parameter of a netting panel is the solidity ratio, which is defined as the ratio of projected area of bars and knots to the outline area of a netting panel.
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