High porosity net drag at a low angle of attack in application to a representative prawn trawl

The presented work investigated the extent by which Reynolds number determines the drag coefficient for high porosity nets at a low angle of attack. A simple prawn trawl model that incorporates the main design features of prawn trawls employed in Australia was developed. Four trawl models of various high porosities were tested in a flume tank with respect to drag and shape over a range of flow velocities. The physical trawl model was analysed as a system of independent plane net sheets, each with an orientation to the flow estimated from analysis of stereo-vision data. The main finding was that the drag coefficient was weakly dependent on the Reynolds number in the range typical for prawn trawl operations, 1000<Re<1700. These combined findings imply that trawler operators can approximately estimate prawn net drag from a function containing trawl twine area, towing speed and spread ratio.