a lining. A steel ring (or rings) is fastened in 

 the cod end to maintain the tubular shape. 

 A contamer, consisting of a specially constructed 

 perforated can, is fastened by draw strings on 

 the cod end of the net to retain the sample in a 

 relatively undamaged condition. 



8-20 The Depressor Assembly. — In front of 

 the net, the spread- V-shaped depressor vane 

 is rigged in such a manner as to deflect the 

 incoming water up and into the net during 

 towing operations, thereby depressing the vane 

 and attached net. A spreader bar attaches to 

 the leading edge of the top panel of the net. 



8-21 The Towing Cable and Bridle.— The 

 towing cable should be wire rope of sufficient 

 strength to withstand the towing strain, such as 

 K-inch cable. The net may be streamed off 

 the stern of the towing vessel, or from a boat 

 crane off the side. If streamed off the stern, 

 a boom or large A-frame will be necessary to 

 facilitate handling. The breaking point of 

 the towing cable should be determined, and a 

 dynamometer installed in the towing system to 

 prevent exceeding this point. 



The bridle consists of 3 lines, 1 of which is 

 divided in order to be attached to the ends of 

 the spreader bar; the remaining 2 are shackled 

 to the side arms of the depressor. 



8-22 Materials and Parts.— The following 

 table lists some of the specifications for the 10- 

 and 15-foot trawls. 



Bridle: 

 Material- 



Spread (feet)-. 

 Vane: 



Area (square 



feet). 

 Weight (lbs.).. 

 Material 



Net: 



Length (feet),. 



Inlet area 



(square feet) 



Material 



Liner: Material.. 



Cod end can: 



Material 



Length (inches) 

 Diameter (in- 

 ches) . 

 Number baf- 

 fles. 



10-foot trawl 



0.250-inch wire 



rope. 

 10 



21 



150 



0.125-inch steel. 



31 



80 



2.5-inch stretch, 

 No. 24 medi- 

 um lay seine. 



0.5-inch stretch 

 bait netting. 



Steel 



13.5 



9.5 



None 



15- foot trawl 



0.380-inch wire 



400. 



0.75-inch marine 

 plywood. 



72. 

 160. 



2.5-inch stretch 

 No. 36 medi- 

 um lay seine. 



0.5-inch stretch 

 bait netting. 



Aluminum. 



24. 



16. 



8-23 Assembling the Midwater Trawl. — 



In assembling the trawl, the towing bridle is 

 attached to the main hauling line with a ring 

 and swivel, and the two lower lines of the bridle 

 are shackled to the hinged side arms of the 

 depressor. The net is then fastened at the three 

 lower points to the trailing edge of the depres- 

 sor. The two upper hauling points of the net 

 are attached to the ends of the spreader bar, 

 which in turn is shackled to the third (divided) 

 line of the bridle. Figure 8-3 illustrates 

 the component parts of the trawl in assembled 

 condition. Modification and strengthening of 

 certain parts, such as the compression strut, the 

 tension member, and the hinge pins of the side 

 arms may be advisable, although the depressor 

 is designed to fold before the breaking strength 

 of the towing cable or netting is reached. 



8-24 Streaming the Trawl. — Placing the 

 trawl in the water is dependent upon the char- 

 acteristics of the towing ship and upon the 

 number of men and equipment available for 

 handling. Generally speaking, however, the 

 cod end is put over the side with bare way 

 on. As soon as the cod end is streamed and the 

 net is flowing freely, the depressor should be 

 lowered just below the surface. If the trawl is 

 lowered over the side rather than the stern, 

 fouling in the ship's screws can be avoided by 

 making a gradual inside turn until the trawl is 

 streaming well aft. 



If properly streaming, the V-shaped depressor 

 will not only cause the net to dive, but will 

 funnel additional water into the mouth of the 

 net, keeping the net billowed out. As soon as 

 this occurs, and the net is well clear of the ship, 

 the ship's speed should be increased to that 

 desired for trawling, plus the speed of the winch 

 as it pays out the towing cable. A continuous 

 watch on the dynamometer should be main- 

 tained, especially during lowering and retriev- 

 ing, or during changes in weather conditions, 

 to avoid straining or parting the towing cable 

 or trawl. 



An alternate and perhaps better way of 

 streaming the net is to pay out cable with the 

 ship having just enough way on to prevent the 

 trawl from fouling itself. This method allows 

 the trawl to sink more rapidly to the desired 

 trawling depth. When it is estimated that 

 this depth has been reached, the ship's speed 

 should be increased to the desired trawhng 

 speed. The trawl will then stabilize itself at 

 a depth dependent upon the trawling speed, 

 cable diameter, etc., and an adjustment will 

 have to be made in the amount of cable payed 

 out, unless this has been calculated beforehand 

 by means of a graph. 



H. O. 607 



83 



