Untitled - CNR

Fishery and Sea Resourcesand at low towing speeds, the spread of thedoors could well be higher due to the extraspreading force produced by the groundshear. In this respect the current paper illustratesthe performance and impact on theseabed of an existing door and a new doordesign for demersal fisheries, with the mainpurpose of discussing the differences betweenengineering sea trials and the flumetanktests and the differences between bothtrawl doors.2 Materials and methodsIn order to specify the basic design of traditionaldoors, a review of common commercialdoor specifications was made beforethe scale door trials. A typical otterboardCambered vee type (termed ARdoor), commonly used in the commercialMediterranean demersal trawl fisheries,was selected as the reference door (Table1). A new experimental door (Clarck-Ydoor) has been developed by the door manufactureGrilli sas (Italy) in collaborationwith Prosilas sas (Italy) and CNR-ISMARof Ancona (Table 1). The experimentaldoor was designed to try to reduce hydrodynamicdrag coefficient and increasespread, and its design was based on themost advanced hydrodynamic concepts inimproving the water flux on the upper partof the trawl door to avoid vortices, whichare the cause of increased drag and cavitations.Since the trawl doors mostly operateunder approximately steady-state conditions,the steady-state hydrodynamic coefficientsare their most important hydrodynamicproperties. To find these coefficientsas accurate as possible, flume-tankexperiment was performed at the North SeaCenter flume-tank in Hirtshals (Denmark).This made it possible to find the hydrodynamicforces for various combinationsof orientation angles. The hydrodynamicforces are assumed to be pressure-inducedand, therefore for a given angle of attack,proportional to the square of the trawl doorvelocity relative to the water. It is, therefore,sufficient to do measurements for asingle velocity. The two large door models,AR and Clarck-Y, were designed, producedand tested in the flume tank. The scalingof the physical models was based on thenormal scaling rules. The linear scale factorused here is defined as the quantity inthe full-scale trawl divided by the correspondingquantity in the model. In generalterms, reductions to dimensions of a linearnature are made throughout the model bythe amount of the basic factor. The factorconcerning drag resistance, which is dependenton surface area for its value, variesproportionally with the square of the velocityof water flow. Weight and buoyancyforces that rely on volume for theirvalue are reduced by the cube of the basicscale. Both the AR and Clarck-Y modelswere tested in the flume-tank for a limitedrange of otterboard heels with the intentionto make some quantitative measurementsof the performance (otterboard spreadinganddrag-forces) and of the reaction forceon the seabed. The total downward force ofa door on the seabed (equal to the reactionforce) is the resultant of the door weight inwater, the downward hydrodynamic force,the upward force in the warp and the downwardforce in the bridle. Because the modelis held in a fixed position, two angles ofheel (0 and 30 degrees outward) have beentested. The spreading- and drag-force measurementshave been made using in-lineload cells, whereas to measure reactionloads new load cells have been developedto fit onto the door shoes. Two reactionload cells were required on each door in1876