THE TECHNIQUE OF TRAWLING 



By B. KULLENBERG 



In deep-sea trawling we encounter difficulties not met with in inshore 

 commercial fishing. One of these is our imperfect knowledge of the topo- 

 graphy of the deep-sea floor, though to a large extent this has now been 

 overcome by means of the self-registering echo-sounder, which can re- 

 cord a profile of the bottom even at the greatest of depths. Thus, the first 

 task of a deep-sea research vessel is to find an expanse of fairly level bot- 

 tom in the area to be trawled. But since it would take a great deal of time 

 to put out marker-buoys, there can be no guarantee that exactly the same 

 area will be trawled as was reconnoitred. The result may be unpleasant 

 surprises in the shape of rough obstructions. If the situation threatens to 

 be risky, there is no alternative but to discontinue the trawl. 



Another difficulty is that of judging the length of wire which will be 

 necessary in order to trawl the bottom and avoid waste of effort by 

 dragging the free water masses. The time at the disposal of an oceanic 

 research ship is so precious that failures must as far as possible be avoided. 



In order to judge the minimum amount of wire that will be needed we 

 must examine its position in the water, and the easiest way of doing this 

 is by a theoretical analysis. The principle involved is the law of resistance 

 to a short piece of wire, or cylinder. If a cylinder moves through water at 

 right angles to its axis, then the resistance per unit length will be propor- 

 tional to the diameter of the cylinder and the square of the speed. The 

 proportionality factor, it is true, is slightly variable, but with the dimen- 

 sions and speeds with which we are concerned it may be regarded as con- 

 stant. If the cylinder moves longitudinally, the resistance will still be 

 virtually proportional to the diameter and the square of the speed, but 

 the proportionality factor will be only about seven per cent, of that govern- 

 ing the rectangular resistance. 



However, we are mainly concerned with the resistance to a cylinder 

 of which the axis forms an acute angle with the direction of motion, 

 because nearly every part of the wire moves in this way. It is reasonable 

 to divide the speed into two components, one at right angles to the axis 

 of the cylinder and the other parallel with it. The resistance has a trans- 

 versal and a longitudinal component, each determined by the correspond- 

 ing component of the speed. The longitudinal component of the resistance 

 is usually so small compared with the transversal one that the resultant 



