High Lift and Reduced Drag in a Fore-Wing-Dominant Biplane Trawl Door

Reducing trawl-door drag without compromising lift is important for reducing towing resistance and fuel consumption while maintaining the required horizontal opening of the trawl net. A fore-wing-dominant biplane trawl door was developed to reduce drag while retaining the high-lift capability of biplane configurations. Using a biplane-type hyper-lift trawl door without a spanwise slit as the reference, seven parameters describing the wing geometry and relative arrangement were optimized through an integrated framework combining orthogonal design, computational fluid dynamics, a backpropagation neural-network surrogate model, and a multi-objective genetic algorithm. Flume-tank experiments yielded a maximum lift coefficient of 2.08 at α = 27°, in close agreement with the CFD prediction. The present design maintained a maximum lift coefficient comparable to those of existing high-lift biplane configurations while exhibiting lower drag. At α = 20°, its lift-to-drag ratio was approximately 14.4–47.8% higher than those of four reference configurations. Flow-field and wing-resolved force analyses showed that the optimized arrangement reduced the suppressive influence of the rear wing on the fore-wing flow and shifted the larger share of lift generation to the fore wing. Although occupying only 35.5% of the total wing area, the rear wing contributed 40.2–45.0% of the overall lift but only 24.3–31.1% of the overall drag. These findings establish unequal but complementary hydrodynamic roles of the two wings as a design strategy for reducing trawl-door towing resistance and potentially lowering fuel consumption without compromising net-mouth spreading.