Twist-and-camber effects on the performance of simple hydro-sails for efficiently spreading penaeid trawls

Penaeid-trawling fuel efficiency can be enhanced by using otter boards with high lift-to-drag ratios (L/D), which generally occurs at low angles of attack (AOA). The batwing otter board is such a low AOA solution; incorporating a unique towing arrangement and a high aspect ratio, flexible, hydro-sail to spread trawls with minimal seabed contact. This study investigated the effects of twist and camber on the performance of two simple batwing sails in a flume tank. Six degree-of-freedom loads were measured for AOAs between 0° and 40° for five twist and camber combinations. The major findings were that: (1) changing the mast rake angle from 30° to 45° reduced L/D at the design AOA of 20° by 12%, considering each sail's optimum setting; (2) stall angle increased for the higher rake angle, despite the associated sail having the greater aspect ratio; (3) irrespective of rake angle, camber was positively associated with lift whereas twist decreased drag and substantially increased L/D at the design AOA; and (4) a L/D ratio of ∼3 was achieved or exceeded at 20° AOA for both sails, which is ∼3 times greater than for contemporary otter boards, implying a 20% drag reduction for otherwise equivalent penaeid-trawling systems.