Advancing otterboard hydrodynamics in trawl fisheries. A novel in situ method for measuring angle of attack

The angle of attack (α) is a critical parameter influencing the hydrodynamic performance of otterboards in trawl fisheries, directly affecting hydrodynamic lift and drag forces, fuel efficiency, and seabed interaction. Traditional methods to estimate α are limited to controlled flume tank trials or indirect modelling approaches, which may fail to capture the full complexity of otterboard behaviour under real fishing conditions. This study presents a novel in situ technique for measuring the angle of attack using a compact, magneto-inertial data logger mounted on the otterboard. The sensor integrates a tri-axial magnetic compass and tilt sensors (pitch and heel) to estimate the otterboard's orientation relative to the vessel’s course over ground, corrected for local magnetic declination. Sea trials were conducted in the Adriatic Sea using four otterboard designs with distinct hydrodynamic profiles. Using non-metallic supports to separate sensors from steel otterboard components and optimizing sensor positioning significantly improves data quality and minimizes loss from magnetic interference. Despite significant challenges with magnetic interference, largely mitigated in a non-metallic otterboard design, this method yielded accurate measurements of α, ranging from 36.0° to 56.2° across the tested otterboards. The results highlight the significant impact of otterboard design on hydrodynamic efficiency and operational stability. This low-cost, scalable approach provides fisheries scientists and gear technologists with a practical tool to evaluate otterboard performance under commercial towing conditions, bridging the gap between laboratory studies and field applications.