Table O-l. Trawl specifications 



Bridle: 



Material - 



Spread (feet).. 

 Vane: 



Area (square 



feet). 

 Weight (lbs.). 

 Material 



Net: 



Length (feet).. 

 Inlet area 

 (square feet). 

 Material — 



10-foot trawl 



0.250-inch wire 



rope. 

 10 



150 



0.125-inch steel. 



Liner: Material. 



Cod end can: 



Material 



Length (inches) 

 Diameter (in- 

 ches) . 

 Number baf- 

 fles. 



2.5-inch stretch, 

 No. 24 medi- 

 um lay seine. 



0.5-inch stretch 

 bait netting. 



Steel. 

 13.5.. 

 9.5... 



15- foot trawl 



None. 



0.380-inch wire 



rope. 

 15. 



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. 



0-10 Assembling the Midwater Trawl.— As 



a guide in assembling the midwater trawl the 

 following procedures are listed : 



Step 1. Attach the net at three points to the 

 trailing edge of the depressor vane. 



Step 2. Attach the upper two hauling points 

 of the net to the ends of the spreader bar. 



Step 3. Attach two lines of the bridle to the 

 hinged side arms of the depressor; attach the 

 divided arms of the third bridle line to the ends 

 of the spreader bar. 



Step 4. Attach the bridle to a i^-inch towing 

 line with a ring and swivel. 



Step 5. Stream the net off the stern of the 

 towing vessel using a boom or from a boat crane 

 off the side. Install a dynamometer in the tow- 

 ing system according to instructions given in 

 chapter L, paragraph L-30. 



0-11 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 streamed, 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 tliis 

 occurSj and the net is well clear of the ship, 

 the ship's speed should be increased to that de- 

 sired 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 trawling speed. The 

 trawl will then stabilize itself at a depth de- 

 pendent upon the trawling speed, cable diam- 

 eter, etc. 



0-12 Towing the Trawl.— When it is esti- 

 mated that enough cable has been payed out to 

 place the trawl at the desired trawling depth, 

 the ship's speed should be slowed to the intended 

 trawling speed simultaneously as the winch is 

 stopped. Reasonable maneuvering can be accom- 

 plished by the ship during trawling. The length 

 of the trawling period should be at least several 

 hours. At an early point in operations, a trial 

 series of tows should be run so that a graph can 

 be drawn showing the necessary amount of 

 cable to be payed out for a certain depth when 

 hauled at a certain speed. 



0-13 Retrieving the Trawl.— After the trawl- 

 ing period is over, the ship should be slowed 

 to the desired trawling speed less the speed of 

 cable recovery by the winch. The slowing of the 

 ship as the winch begins to retrieve the wire 

 must be a smooth operation so that the actual 

 net speed always remains the same. Any time 

 the retrieving action is stopped, the ship's speed 

 should be increased again to the desired trawl- 

 ing speed. Caution should be taken at all times 

 to see that the actual trawling speed of the net 

 is kept constant to avoid excessive strain from an 

 increase in speed, and to avoid allowing en- 

 trapped animals to escape with a decrease in 

 speed. 



0-14 Additional Instructions. — Any increa,se 

 in trawling speed may cause the trawl to 

 dive more steeply. This additional deepening 

 must be taken into consideration if tows are 

 being made close to the bottom. 



Because of the additional strains due to the 

 surging of the towing ship during hea-t'y swells, 

 the trawl normally will be used in fair weather. 

 Special emphasis should be placed on trawling 

 when a pronounced deep scattering layer is in- 

 dicated on the echo sounder. 



0-5 



