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



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-10 10 



U (crn/sec) 



Figure 3 



Current velocity vectors during each of the 

 13 experimental tows are shown with arrows. 

 Vessel direction during these tows is shown 

 with the dark lines. The axes, labeled U and 

 V, indicate the latitudinal and meriodonial 

 components of the velocity vectors in cm/s. 



2500 



2000 - 



1500 - 



1000 



Warp offset (m) 



Figure 4 



Mean tension lin kilograms) measured on both port and starboard 

 door legs is shown plotted as a function of warp offset in meters. 

 Note that on the side with the shorter warp (negative offset), 

 tension is higher than when the warps are equal and that on the 

 side with the longer warp, the tension is lower. 



sured distance between wing tips becomes increasingly 

 greater than the effective net width (i.e., the distance 

 from wing tip to wing tip projected on a plane perpen- 

 dicular to the direction of travel). The method used to 

 estimate effective net width was based on headrope 

 geometry and is described in Appendix A. 



Footrope shape viewed from the net mouth 



Because of footrope geometry, the importance of footrope 

 bottom contact to overall net efficiency varies along 

 the length of the footrope. This feature is true not only 

 because escapement probability likely changes with the 

 angle of the footrope in relation to the direction of travel 

 but also because the proportion of the net width spanned 

 by a unit length of footrope varies. To help visualize the 

 latter effect better, we projected the off-bottom distances 

 from their positions along the footrope onto a plane that 

 was perpendicular to the direction of travel and spanned 

 by the effective net width, using the footrope shape 

 model determined for each offset increment described 

 in Appendix A. 



Results 



Twelve successful tows consisting of 24 sets of treat- 

 ments were completed during the experiment. Bottom 

 current velocities ranged from about 5 cm/s to about 



30 cm/s and current direction was approximately paral- 

 lel to the trawl towing direction (Fig. 3); consequently 

 the mean current velocity perpendicular to the towing 

 direction was quite small (1.8 cm/s). 



Bridle tension and geometry 



For the single haul in which both tension meters worked 

 successfully, the bridle tension, combined over both 

 sides, did not change significantly when regressed on the 

 warp offset (df=ll, P=0.69). This result indicates that 

 any distortion of the trawl due to the offset treatments 

 was not sufficient to appreciably affect the combined 

 tension and, therefore, the hydrodynamic and frictional 

 drag of the net and the bridles. However, changing the 

 relative length of the warps resulted in a progressive 

 transfer of the tension to the shorter warp (Fig. 4). For 

 example, based on the average combined bridle tension 

 (3248 kg), when the difference in warp lengths was 

 11 m, the shorter warp carried 72% of the total net and 

 bridle drag. 



Headrope speed through the water 



The velocity component perpendicular to the headrope 

 (U) decreased with increasing warp offset, whereas the 

 absolute value of the velocity tangential (velocities from 

 the port side of the sensor are opposite in sign to veloci- 

 ties from the starboard side) to the headrope (V) at its 



