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Hydrodynamic Evaluation of a Generic Sail Used in an Innovative Prawn-Trawl Otter Board

[+] Author Affiliations
Cheslav Balash

University of Tasmania, Launceston, Australia

David Sterling

Sterling Trawl Gear Services, Brisbane, Australia

Matt Broadhurst

NSW Department of Primary Industries, Fisheries Conservation Technology Unit, Coffs Harbour, Australia

Arno Dubois

Delft University of Technology Ship Hydrodynamics & Structures, Delft, Netherlands

Morgan Behrel

ENSTA Bretagne, Brest, France

Paper No. OMAE2015-41335, pp. V006T05A003; 6 pages
doi:10.1115/OMAE2015-41335
From:
  • ASME 2015 34th International Conference on Ocean, Offshore and Arctic Engineering
  • Volume 6: Ocean Space Utilization
  • St. John’s, Newfoundland, Canada, May 31–June 5, 2015
  • Conference Sponsors: Ocean, Offshore and Arctic Engineering Division
  • ISBN: 978-0-7918-5654-3
  • Copyright © 2015 by ASME

abstract

In prawn-trawling operations, otter boards provide the horizontal force required to maintain net openings, and are typically low aspect ratio (∼0.5) flat plates operating on the seabed at high angles of attack (AOA; 35–40°). Such characteristics cause otter boards to account for up to 30% of the total trawling resistance, including that from the vessel. A recent innovation is the batwing otter board, which is designed to spread trawls with substantially less towing resistance and benthic impacts. A key design feature is the use of a sail, instead of a flat plate, as the hydrodynamic foil. The superior drag and benthic performance of the batwing is achieved by (i) successful operation at an AOA of ∼20° and (ii) having the heavy sea floor contact shoe in line with the direction of tow. This study investigated the hydrodynamic characteristics of a generic sail by varying its twist and camber, to identify optimal settings for maximum spreading efficiency and stability. Loads in six degrees of freedom were measured at AOAs between 0 and 40° in a flume tank at a constant flow velocity, and with five combinations of twist and camber. The results showed that for the studied sail, the design AOA (20°) provides a suitable compromise between greater efficiency (occurring at lower AOAs) and greater effectiveness (occurring at higher AOAs). At optimum settings (20°, medium camber and twist), a lift-to-drag ratio >3 was achieved, which is ∼3 times more than that of contemporary prawn-trawling otter boards. Such a result implies relative drag reductions of 10–20% for trawling systems, depending on the rig configuration.

Copyright © 2015 by ASME

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