Seeking maximum effectiveness and efficiency for large multi-sail penaeid otter boards
School of Engineering
The greatly-stretched lateral opening of penaeid trawls is typically achieved by using relatively large otter boards with an aspect ratio of ~0.5 at high angle of attack (AOA; ~40°) and low efficiency (lift-to-drag ratio; L/D ~1). Alternatively, the ‘batwing’ otter board utilises a unique rigging system to allow a flexible sail with an aspect ratio of ~1 to operate at a lower AOA (~20°). As a result, high effectiveness (lift) and efficiency (L/D ~3) are achieved for small- and medium-size gear. However, for large trawlers it is impractical to simply scale up the current batwing design as this will excessively increase the trawl's height and generate unnecessary drag. Hence, here we experimentally searched for greater hydrodynamic forces from a multi-sail configuration (comprising a jib and main sail) by varying the twist and camber of the two sails. The results suggest that the jib needs to be set at a low AOA compared to the main sail for balanced pressure loading of the two sails, and the main sail requires medium twist to control detrimental centre of pressure characteristics. Ultimately the desired objective for large-sized batwing boards might best be achieved using a two-mast configuration that incorporates three sails.