The effect of mesh orientation on netting drag and its application to innovative prawn trawl design

© 2014 . Prawn fisheries around the world comprise fuel intensive enterprises currently stressed financially by rising diesel costs. An avenue for relieving the situation is to improve the energy efficiency of trawling by raising the productivity of fishing per litre of fuel consumed. This paper presents work to develop a new prawn trawl design that leads to reduced trawl system drag. The trawl has a 'double-tongue' format, which refers to extensions forward of the upper and lower panels to form two additional towing points for the trawl. For this design concept, named 'W' trawl, drag generated in the trawl is largely directed to the centreline tongues and transferred forward to the trawler through a connected sled and towing wire. The associated reduction of drag-transfer to the wings makes the trawl substantially easier to spread and results in smaller otter boards being required and subsequently reduced overall drag of the trawl system. The study determined the effect on frame-line tensions of implementing T0 (diamond) and T45 (square) mesh in the main body and side sections of trawl models of conventional and 'W' configuration, with the aim to establish an optimal combination of mesh orientation for the principle parts of the 'W' trawl. The objective was to achieve minimum netting drag and beneficial strain transfer within the trawl such that maximum trawling performance (catch per unit of fuel) might be obtained in the field. T45 mesh in the side sections of the trawl was found to exhibit a progressively lower drag compared to T0 mesh as the flow speed increased, but the extent of drag reduction was not of practical significance. The 'W' trawl showed a capacity of redirecting 59% of the total netting drag to the centre line tongues when T45 netting was implemented in the body section, and only 40% when T0 orientation was used. However, the introduction of bracing ropes (at E=. 0.71) along the upper and lower centrelines of the T0 version of the "W" trawl improved the drag transfer to the tongues from 40% to 50% of the total drag. Overall, the most practical and economic configuration of the model 'W' designs tested produced an estimated drag reduction of 8.3%. ±. 0.6%, compared to the conventional trawl. It is expected that drag saving benefits in practice will be more substantial as the tested trawl models were not completely representative of practical commercial gear in that they had minimum twine area to make the experiment most sensitive to the drag-effect of mesh orientation

Prawn trawl shape due to flexural rigidity and hydrodynamic forces

Energy efficiency and ecological sustainability have become vital issues for the Australian and global prawn fisheries. The scientific community and innovative industry operators have introduced fishing gear modifications for drag and unwanted catch reduction. This project has investigated the potential for further drag reduction, focusing on the extent to which prawn net flexural rigidity affects the drag. A novel experimental technique was developed to quantify flexural rigidity for nets. The concept of the technique was to measure the mesh opening under various loads applied in longitudinal and transverse directions. A relative difference between values showed a resistance of the mesh to bend, and the results were fitted into an existing analytical solution. The geometric parameters of nets were measured applying a digital photogrammetric method. Four prawn trawls built from the netting being assessed for flexural rigidity were examined in a flume tank for drag and shape over a range of velocities. A stereo-vision system was developed to acquire the 3D shape image. The net flexural rigidity and drag showed a piece-wise linear relationship. Another main finding was that the drag coefficient was weakly dependent on the Reynolds number in the typical range for prawn trawl regimes of 100

Vortex-induced motion of a free-standing riser below the critical mass ratio

The presented work studied the vortex-induced motion (VIM) response of a free-standing riser (FSR) with varied riser length and buoyancy can (BC) mass with an ultimate aim to find a combination that would reduce the motion of the system. Specifically, four model configurations were experimentally tested in a flume tank over a range of flow velocities, with the BC motion recorded by a submersible camera positioned directly above the model; consequently, inline (IL) and crossflow (CF) amplitudes were estimated with a motion tracking software. In the pre-resonant flow regime, non-dimensionally, minimal differences were observed between the CF amplitudes, and the IL motion was reduced with a longer riser. Given the extreme length of full-scale FSRs and inherent low natural frequency, it is impractical to increase the riser tension to a point where VIM would not occur under normal environmental conditions. Alternatively, increasing the mass ratio of the BC so that it is above the critical mass ratio of 0.54 (the ratio of the mass of the body to the mass of the fluid) would limit the resonant flow velocities to a finite range, but a larger BC may not be an economically viable solution, and because of the increased diameter, it would experience a larger CF amplitude during resonance. Further study into the prevention of VIM of an FSR by varying the riser length and BC mass is unlikely to be beneficial

Prawn trawl drag due to material properties - an investigation of the potential for drag reduction

Rising fuel costs, impending oil deficit and global concern for greenhouse gas emission necessitate improvements of energy efficiency in commercial fisheries. Innovative high strength netting materials provide the potential for a positive outcome in relation to the energy efficiency of prawn trawling. Usage of high strength netting in Australian prawn trawling applications has returned mixed results over the last 10 years. The presented work investigated the drag of prawn trawl models constructed from innovative and traditional materials. The major finding was that drag reductions achieved with innovative materials were not directly related to the associated reductions in twine area: knot area and orientation produce amplified effects on drag. It was also shown that netting stiffness affects trawl shape and hence the corresponding drag