The hydrodynamic drag and shape characteristics of two panel demersal trawl nets
The aim of the work described in this thesis was to investigate the hydrodynamic characteristics of two panel demersal trawls and to develop methods for predicting net drag and shape. The investigation started with a comprehensive analysis of existing data on net performance. Tentative prediction formulae for net performance were developed. The effects of groundgear, sweep length, buoyancy and net taper rate on the drag and shape were examined. Four experiments were then carried out to improve knowledge of the hydrodynamic characteristics of trawls. 1) Groundgear friction drag was measured for the three main types (bobbin, disc and rockhopper), with these parts rigged to have the same shape as in fishing conditions on both mud and sand; 2) The effect of buoyancy and wing spread on net drag and shape were investigated on five models nets having distinct differences in design in a flume tank; 3) Netting panel drag at low angles of attack (0o to 15o) was investigated. The effects of netting parameters (ie setting angle, Reynolds number, angle of attack, d/a ratio, solidity and so on), on the coefficient of drag based on twine area, were examined. The measurements significantly increased and improved knowledge of the hydrodynamic features of netting at low angles of attack; 4) Observations were made on flow through netting in a small flume tank, by measurement of velocity profiles and by visualisation. The velocity profiles at different positions showed the effects of setting angle, mesh type and angle of attack on water flow through the netting. Visualisation of flow through netting revealed details of the process of water flow. This study has provided a group of empirical equations to predict drag and shape and a sufficient data on groundgear friction and panel drag to enable analytical calculation of net drag with satisfactory accuracy.