The effect of mesh orientation on netting drag and its application to innovative prawn trawl design
Balash, C and Sterling, D and Binns, J and Thomas, G and Bose, N, The effect of mesh orientation on netting drag and its application to innovative prawn trawl design, Fisheries Research: An International Journal on Fishing Technology, Fisheries Science and Fisheries Management, 164, (April) pp. 206-213. ISSN 0165-7836 (2015) [Refereed Article]
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.