The geometrical gear shape of a bottom trawl

An estimation of the gear shape during a bottom trawl can be achieved through geometrical modeling of the trawl system. This can be implemented by processing the field data obtained using the Scanmar system. The gear shape from the square through the bag net was assumed to be the upper and lower parts of different elliptic cones of which the cross section was an ellipse and the shape of the float rope to be of exponential function. A system of nonlinear equations was constructed to represent the gear shape of the bottom trawl net in water. When the equations were solved based on the data obtained from bottom trawl experiments with various warp lengths, the cross section of sweep and filtered volume, the eccentricity of the upper ellipse, the functional shape and inclination angle of the float rope, and the contribution of the upper side panel to the net height could be obtained in relation to towing speed and scope ratio. As the cross section of sweep at the mouth and the projected total cross section decreased a little bit with increased towing speed, the filtered volume tended to increase with increased towing speed. The gear shape at mouth of the bottom trawl was not so much changed compared to that of a mid-water trawl.