346 



The Representation of Oceanic Movements and Kinematics 



and may be unpleasantly noticeable in the current recordings, even when the yawing 

 movements are unnoticed in the open ocean. Only careful determination of the 

 position will allow a decision to be made regarding the extent to which the move- 

 ment of the vessel due to yawing has affected the recordings. 



An excellent example of yawing movements was obtained at the "Meteor" anchor station 228 in 

 the strong North Equatorial Current and the intensive north-east trade-wind. Figure 140 shows that 

 there was a "freedom in the yawing movement" of 2473 m and the mean position of the ship was at a 

 distance of 2760 m from the anchor point when projected on the sea surface. Since while the vessel 

 was anchored, the direction of the wind was hardly varied and since there was a strong basic 

 current (22 cm/sec), and a wind force of 5-6 Beaufort, the weak tidal currents could hardly move the 

 vessel from its main position and the movement of the vessel must have been due to yawing. Actually 

 as shown in Fig. 141 which gives the positions of the vessel determined astronomically while at 

 anchor, there were almost only yawing movements. It can be seen that all factors (heading of the 

 vessel, position, etc.) fit excellently to give a very plausible representation of the movements of the 

 vessel while anchored (see also Defant, 1940«). 



iooo4 



2000 



30004 



4000 -r 



4486^^^^ 





Fig. 140. Anchor station 288 of the "Meteor": water depth 4486 m, length of cable 6003 m, 

 freedom for yawing 2473 m, anchor position — mean position of the ship 2760 m. 



Since a really fixed point is scarcely obtainable in the open ocean at great depths, 

 methods have been devised for the elimination of errors that occur for this reason in 

 current measurements. Witting (1930); Thorade (I933fl) have reviewed the three 

 methods so far used to replace the absolute method using a fixed point. 



The correction method consists essentially of a careful observation of changes in the 

 position of the ship relative to that of a buoy anchored by the shortest possible cable 

 and in the correction of the current recordings by use of these observations. Witting 

 (1905) has given a procedure for calculation using numerical and graphical methods, 

 but due to the complexity of the observations and the difficulty of evaluation of the 

 measurements it has seldom been used. In the difference method the vessel is not an- 

 chored to the bottom but is kept stationary with a driving anchor; the current re- 

 corder then gives only the movements of the water relative to the ship. To find the 

 true current it is necessary to know the absolute current at one of the depths investi- 

 gated. Hansen (1915) and later Helland-Hansen (1926), for want of other possi- 

 bilities, used a second current recorder close to the sea bottom, or as deep as possible, 

 and assumed that the water here would be almost motionless. For current measure- 

 ments in the ice drift off the North Siberian Shelf Sverdrup (1929) on the "Maud" 



