Trawl drag area and netting geometry
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Original versionThis report is not to be quoted without prior consultation with the General Secretary.
The bag of a trawlnet is considered as an elliptical cone, its wings as merely forward extensions of this cone. The dimensions of the cone are derived from the measured wingend spread and the headline height (gape) of the net and from its specification drawings. From these operational and constructional dimensions the mean angle of attack of the netting panels and the mean setting angle of the meshes are derived. All operational dimensions change with towing speed. Formulae are then given which with further inputs of twine diameter bar length and developed area of each netting panel allow an estimate of the drag area of the netting cone. The codend in the form of a tube, the net appendages and the ground friction are eachconsidered separately. Thus a total drag area is derived; multiplication by the hydrodynamic pressure and addition of the friction give the total geometrically derived drag of the trawlnet. This is compared with the measured drag over a range of speeds for 3 very different bottom drawls and two substantially different midwater trawls. Examples of the comparisons are presented. The method provides a means of predicting change of trawlnet drag with change of shape.