Hydrodynamic performance of semi-pelagic self-adjusting otter boards in demersal trawl fisheries

Abstract

In this study, we show how a novel self-adjusting, semi-pelagic otter board have high hydrodynamic efficiency and reduce seabed contact. The otter boards have two adjustable flaps, which are controlled by on-board altimeters and actuators, modifying their lift and drag and altering their position in the water column. The actuators are governed by a Proportional Integral Derivative feedback system, which uses the altimeter data to maintain the otter boards at a preset target height above the seabed.Full-scale experimental trials were conducted to measure the hydrodynamic performance of this new design system and to compare it with a conventional seabed-contacting design. We demonstrate that the new system is highly efficient and has a lift/drag ratio of 4.2, which is 3.5 times that of the conventional otter boards. When the target height was set at 1 m, the SAO contacted the seabed at most 16% of the time; this decreased to 8% when the target height was raised to 2 m above the seabed; and there was no seabed contact when the target height was set at 5 and 10 m.