Hydrodynamic characteristics of knotted and knotless purse seine netting panels as determined in a flume tank

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).

Effect of Cutting Ratio and Catch on Drag Characteristics and Fluttering Motions of Midwater Trawl Codend

“…This ratio was 13.32% (±0.42%), 14.51% (±0.54%), 15.05% (±0.49%), 15.60% (±0.46%), 15.47% (±0.35%), and 15.82% (±0.40%) for the codend without cutting ratio and the codends with cutting ratios 6:1, 5:1, 4:1, 7:2, and 3:1, respectively. However, the results reported by Tang et al [25,26] on netting showed that the frame drag force accounted for a maximum of between 17% and 20% of the total (netting drag + rigid frame drag) using streamlined frames, close to those obtained in this experiment. Therefore, the application of a rigid frame reduced the impact of turbulent or vortex flow on the experimental hydrodynamic force and the geometrical shape of codends.…”

“…This study also demonstrated that the increase in cutting ratio led to the increase in the angle between the flow direction and codend surface (attack angle) during the operation. However, Tang et al [25] reported that the relationship between the nylon mesh drag coefficient and attack angle (0 • -20 • ) has a positive correlation, and the netting drag coefficient increased as the attack angle increased, decreasing as the Reynolds number increased before moving asymptotically toward a constant value. This trend was confirmed by the experiment carried out in this study, demonstrating that the drag coefficient for the empty codend increased as the Reynolds number decreased and attack angle and cutting ratio increased.…”

Variations in hydrodynamic characteristics of netting panels with various twine materials, knot types, and weave patterns at small attack angles

“…Tang et al . 11 investigated the differences in hydrodynamic coefficients between knotless and knotted PA netting panels by setting normal to the flow 1.23–1.35 times higher than that of knotless netting; a remarkable difference was found in the lift coefficient between different netting types at small attack angles.…”

“…Because the underwater shape is affected by current, fishing gear structures, and fishing operations, the movement of fishing gear is shown in an approximate angle state along the current direction. For a trawl, the angle of inclination between netting and flow is generally less than 15° during relatively stable towing 11 . In purse seine fishing operations, the small-attack-angle movement of netting along the current direction is a common phenomenon during shooting and sinking.…”

How to Deal With Seafloor Marine Litter: An Overview of the State-of-the-Art and Future Perspectives

“…Dedicated target identification algorithms need to be implemented to map and recognize as many categories of benthic marine litter as possible. -A state-of-the-art Lagrangian model, taking into account sinking speeds based on previous works (Takagi et al, 2007;Gabitto and Tsouris, 2008;Monroy et al, 2017;Tang et al, 2018) should be further developed. The model will have to use input velocities provided from simulations performed using a regional ocean model.…”