A comparison of hydrodynamic forces in knotted and knotless netting, using both helix and conventional ropes for midwater trawls

Abstract

This study evaluated the hydrodynamic forces produced by whole panels of hand-made (geometrically different) rope meshes typically found in midwater trawls. Experimental treatments included both knotted and knotless meshes constructed of helix composite ropes, conventional braided nylon (PA) ropes, and twisted polyethylene (PE) ropes. Direct measurement of lift (L) and drag (D) forces were conducted using controlled flume tank experiments on panels oriented at 21° relative to the flow. For knotted netting, our results showed that the hydrodynamic forces (L and D) were statistically higher for helix ropes compared to conventional ropes of the same diameter. For knotless netting constructed of helix ropes, our primary finding is that the arrangement of helix ropes (S and Z lay) in individual meshes significantly affects the resulting D and Cd of whole netting panels. Arranging the mesh bars in the S-Z-S-Z lay pattern exhibited 29% and 32% lower drag (D) compared to S–S–S–S and S–S-Z-Z lay meshes, respectively. We also use computational fluid dynamics (CFD) to characterize the fine-scale flow-field around helix and conventional ropes. The resulting streamlines and pressure contours provide a functional explanation for the empirical measurements collected in the flume tank. These results can be useful in informing the design of midwater trawls.