Weinberg and Somerton Variation in trawl geometry due to unequal warp length 



27 



the reaction height with the lines depicting the bridle 

 shape at each offset (Fig. 8). The change in these lengths 

 on the short and long sides of the trawl is asymmetric 

 with changes in warp offset (Fig 9). For the long side, 

 bridle contact length increases linearly with positive 

 offset. However, for the short side, bridle contact length 

 decreases nonlinearly with warp offset — the greatest 

 changes occurring with small offsets. This difference 

 likely leads to a change in the total width of the herding 

 area with changes in warp offset. If, for example, it is 

 assumed that the angle-of- attack (a) is the same for the 

 long and short sides of the trawl, then the width of the 

 herded area declines to a minimum at about 8 m offset, 

 at which the herded area is reduced by W.SVr compared 

 to that at zero offset. 



Headrope shape and effective net width 



With increasing difference in warp length, the model we 

 used to describe headrope shape predicts three distinct 

 changes in shape. First, the headrope is distorted so 

 that the wing tip on the short side of the trawl precedes 

 that on the long side in the direction of travel (Fig. 10). 

 The difference in the forward position of the wing tips, 

 however, is much less than the warp offset. For example, 

 an 11-m difference in warp length resulted in an offset 

 in the position of the wing tips of only 2-3 m. This dif- 

 ference occurs because the increased tension on the short 

 warp changes the catenary in both the bridles and the 

 warps (i.e., both become effectively longer as the sag is 

 reduced). Second, the headrope is distorted so that its 

 center is increasingly displaced away from the midpoint 

 between wings and toward the short side of the trawl. 

 When this displacement occurs, the perpendicular at 

 the center of the headrope is no longer aligned with the 

 direction of travel. Third, the headrope is distorted so 

 that the effective width of the net (i.e., the wing spread 

 projected to the line perpendicular to the towing direc- 

 tion) becomes increasingly shorter than the distance 

 measured by the acoustic net sensors. The difference 

 between the effective and the measured net width is 

 negligible for offsets up to 7 m but rapidly increases at 

 greater offsets (Fig. 11). 



Footrope shape viewed from In front of the net 



The distance of the footrope off-bottom, when viewed from 

 a position in front of the net, increases with increasing 

 offset; however, the location of the maximum off-bottom 

 distance in relation to the midpoint between wings, 

 shifts slightly with increasing offset (Fig. 12). With off- 

 sets of 9 m or less, the position of maximum off-bottom 

 distance is at the corner of the footrope on the long side 

 of the trawl. However, with increasing offset, the shift 

 in the position of the footrope corner changes because 

 of the rotation of the trawl in relation to the direction 

 of travel; and at a 20-m offset the footrope corner on 

 the long side of the trawl is positioned, when viewed on 

 a plane perpendicular to the direction of travel, almost 

 exactly midway between the wing tips. 



14 16 18 20 



10 12 Id 16 18 20 



i ^ + + ^ 



pqqrt 



10 12 14 16 18 



Offset (m) 



Figure 6 



Mean door spread, wing spread, and headrope height 

 are shown ( + ) plotted against offset increment. The 

 means for all values of offset increment were fitted with 

 a cubic spline function (solid curve). Bootstrapped 959^ 

 confidence bounds are shown with shading. Also shown 

 are the mean door spread, wing spread, and headrope 

 height for treatments with zero offset (solid horizontal 

 line) and the corresponding bootstrapped 95% confidence 

 bounds Idashed horizontal lines). 



