1 14 



THE TECHNIQUE OF TRAWLING 



Km - — — 



The resistance of the water is at 

 right angles to the wire. 



7 6 s 4 3 2 I 



Position of the wire with various thicknesses. 



direction at its upper end, the diameter, the resistance of the trawl to the 

 water, and the depth. The three wires shown in the drawing vary greatly 

 in length: with a diameter of nine millimetres, a depth of 5,000 metres, 

 and a speed of two knots the length required is 9,600 metres; with a 

 diameter of 12 millimetres, 7,900 metres; and with a diameter of 16 

 millimetres, 6,700 metres. Thus it pays to work with wires which are not 

 too thin. 



It is also important to know the speed at which the trawl travels along 

 the bottom. In the oceans there is usually a comparatively shallow surface 

 current, and the deep waters underlying this mo\e s^ery slowly. Owing to 

 its shallowness the current will have no effect on the position of the wire 

 even though it is fast-flowing, particularly since the wire has its highest 

 tension in the surface and is therefore very reluctant to curve. The ulti- 

 mate direction of the wire is thus determined by its speed in relation 

 to the deep water layers, and this is obtained by the mathematical calcula- 

 tion referred to above. 



Deep-sea trawling is rarely carried out at speeds greater than two, or 

 two and a half, knots. Generally speaking, therefore, it is best to go 

 against the surface current when trawling; otherwise the ship may fail 

 to make adequate speed and will be difficult to navigate. The regular 

 collection of water samples at great depths provides a means of obtaining 

 information about the direction of the surface current. In this kind of 

 work the thin instrument wire will only remain vertical if the ship is 

 made to go against the surface current at the same speed as the current. 



If a very long wire is to have adequate strength it is essential that its 

 diameter should increase from the lower end to the upper one, otherwise 

 it will be incapable of bearing its own weight which is very considerable. 



