The Representation of Oceanic Movements and Kinematics 345 



"Swinging round" is the oscillation of the ship with the cable about a certain point 

 which in the extreme case will coincide with the fixed end position of the cable at the 

 sea bottom. Between one position of the vessel at A to another at B there will be a 

 change in angle y corresponding to a change in the course of the vessel from ^ to a 

 or vice versa. If the twisting forces of the wind and the current acting on the ship are 

 in equilibrium the position of the vessel will be stationary for a constant heading. 

 If, however, there is a change in these forces the position of the ship will alter and it 

 will tend towards a new equilibrium position. Thus, for example, if the wind conditions 

 are constant a periodic tidal current will move the vessel from a position A io B and 

 back again in about 6 moon hours. If the combined length of the cable and the length 

 of the vessel until the suspension point of the cable and the length of the vessel to the 

 suspension of the current meter is projected on the sea surface, then the length of this 

 projection is denoted by r. Since 



AB = ry 



180^ 



for r 500, 1000, 2000 m and y = 20° the velocity v of the vessel will be v = 0-8, 1-6, 

 3-2 cm/sec. These speeds are thus rather small provided the swinging round period is 

 sufficiently long and will scarcely cause errors of any importance in the current 

 measurement. The current meter is displaced from Aio B during such a movement and 

 will thus simulate a current from B io A which will be superimposed on the actual 

 current in current measurements, "Swinging" (Schwingen) can be regarded as an 

 extreme case of swinging round. The centre of the swing is shifted to the point where 

 the anchor cable is attached to the bow of the vessel. These oscillations will be recog- 

 nizable from the occurrence of a cable azimuth. In swinging movements with a period 

 of about an hour, the simulated current velocity will remain small also for large 

 values of y due to the small distance r (y = 60°, r = 60 m, r = 1-7 cm/sec), but when 

 the period becomes short errors will increase so strongly that the current measure- 

 ments will be unusable (y = 60°, r = 60 m, period = lOmin, v = 10-5 cm/sec). 

 However, an instrument suspended at a certain depth (such as a very long and strongly 

 damped pendulum) will react to the movement of the vessel and it is improbable that 

 it will behave very differently from the vessel. If the period of the current meter plus the 

 suspension wire and the swinging period of the ship are very dilTerent then the current 

 recorder at a deeper level will be unable to follow the movements of the vessel and 

 the measurements will give good results. If, however, the period of the entire system is 

 of the same order of magnitude as the swinging period there may be rather large dis- 

 placements of the current meter and the measurements will be erroneous. (Examples 

 are given by Defant, 1932; Defant and Schubert, 1934.) 



The third type of ship movement is yawing (Gieren). The pull of the cable and the 

 forces acting on the vessel (wind and current) keep the ship in an equilibrium position. 

 If there is a change in the wind or the current the vessel will be displaced into a new 

 equilibrium position whereupon the cable will either tighten or slacken. Thereby, 

 the heading of the vessel will not change, but the angle between the cable at the bow of 

 the vessel and the vertical will be altered. However, at a deep anchorage the change in 

 this angle will be small, since the upper part of the cable will almost always be approxi- 

 mately vertical. The displacements of the vessel due to yawing may be considerable 



