Performance of new hyper-lift trawl door for both mid-water and bottom trawling

Abstract

A hyper-lift trawl door (HLTD) which is not heavily dependent on the bottom for its hydrodynamic performance was developed for both bottom and midwater trawling. Flume tank experiments were conducted on simplified HLTD models with an aspect ratio of 1.0 and camber ratios of 15%, 20%, and 25%. When the distance between the lower wing-end plate of the HLTD and the sea bottom approached 0.01c (where c is chord), the maximum lift coefficient decreased 32–39% compared with that in a free stream sufficiency off bottom. When a steel shoe was installed with a gap of 0.1c between the shoe and the lower wing-end plate, the maximum lift coefficient was greatly improved both in free stream and near the bottom. For HLTDs with camber ratios of 15%, 20%, and 25%, the lift coefficient was 2.14 at the attack angle (α) of 35°, 2.33 (α = 38°), and 2.45 (α = 40°) when in the free stream, and was 2.23 (α = 28°), 2.37 (α = 30°), and 2.46 (α = 32°) when near the bottom. Additionally, towing tank experiments with a 1/10 scale trawl model and four types of otter boards, including vertical V-type, biplane type, rectangular flat type, and the HLTD with a steel shoe, were carried out. It was found that the spread performance of the HLTD was best even though its wing area was only 70% of those of other doors. Furthermore, we compared the performance of the HLTD (with a steel shoe) with that of the vertical V-type door for bottom trawling at sea, and found that the HLTD had superior performance in spreading a trawl. Therefore, the HLTD with a steel shoe was verified as suitable for application in both mid-water and bottom trawling.