Trawls energy efficiency is greatly affected by the drag, as well as by the swept area regarding pelagic trawls and by the swept width for bottom ones. The drag results in an increase of the energy consumption and the sweeping influences the catch. In order to reduce the drag per swept area (or width) a numerical tool dedicated to the automatic optimisation of the trawl design has been developed. Based on a finite element method model for flexible netting structures, the tool modifies step by step a reference design. For each step the best-modified design, in terms of drag per swept area (or width), is kept. Such a methodology was used in two cases: which show a 43% increase in energy efficiency regarding the pelagic trawl case and 27% for the bottom trawl one. (C) 2009 Elsevier B.V. All rights reserved.