The angle of attack of a cambered otter board in a bottom trawl was estimated using a three-dimensional semi-analytic treatment of a towing cable (warp) system that was applied to the field experiments of a bottom trawl obtained by the Scanmar system. The equilibrium condition of the horizontal component and vertical component of forces was used to the three forces acting on the otter board in the horizontal plane. Those forces were the force on the warp at the bracket, hydrodynamic lift and drag forces on the otter board and the force on the hand rope attached just behind the otter board. Also the equilibrium of moment about the front edge of the otter board was used to find out the angle of attack of the cambered otter board. When the warp length was 120m and 180m long and the towing speed was between 1.23 and 1.90 m/s, the estimated angle of attack of the cambered otter board was ranged between 26.1° and 29.6°, respectively, though the maximum lift force was at the angle of attack 22.6°. The angle of attack of the otter board was tended to increase weakly with the longer length of warp (180 m) at the same towing speed in the experiment.

An estimation of the headline height of a bottom trammel net set across under uniform current was achieved numerically from a differential equations describing the forces of the net and compared with the measured value in a flume tank experiment. The analysis on the shape of the bottom trammel net with the headline free was based on the equilibrium equation of the bottom gill net which was modified and slack of the trammel net was varied with net depth as shown in the tank experiment. The differential equations were solved by a forth-order Runge-Kutta method. The estimated headline heights with varied slack was found to be closer than that with constant slack when compared with the actual values.