Preliminary study of influence of trawl codend length on its oscillation characteristics

As a key component of the trawl, codend directly influences the trawl's hydrodynamic performance and structural stability. In order to investigate the effects of codend length on the hydrodynamic characteristics of a midwater trawl codend, we designed three models (1.0 m, 1.5 m, and 2.0 m; denoted as codends A, B, and C), measured the drag and shape in a flume tank under varying flow velocities and catch conditions, as well as analyzed the oscillation characteristics using Morlet wavelet and Fast Fourier Transform (FFT) methods. The results indicate that: 1) Longer codends without catch exhibited more pronounced end bulging, and their horizontal extension increased with flow velocity. 2) Longer codends with catch showed greater amplitudes of longitudinal oscillation. 3) Codend drag increased with length and flow velocity without catch, but showed an initial decrease followed by an increase with catch. 4) The amplitude of drag oscillations increased with codend length, particularly for codends with catch. The dominant low-frequency ranges were 0–1 Hz for codends A and B, and 1–2 Hz for codend C. 5) Morlet wavelet analysis effectively captured dynamic fluctuations in drag and shape, whereas FFT provided a direct quantification of oscillation amplitude characteristics.