14. DROGUES AND NEUTRAL-BUOYANT FLOATS 



J. A. Knauss 



Velocity measurements in the open ocean have usually been made by one of 

 two methods. Either the flow of water past a fixed point is measured by some 

 device such as a propeller-driven current meter, or else the distance a water 

 "parcel" moves in a given time is observed. The usual method of making this 

 latter observation is to place something (such as a drogue or float) in the water 

 and to follow it. For some applications this method of measuring velocity has 

 two very distinct advantages over the current-meter method. The first is that 

 the mean velocity can be measured very accurately. The error in the velocity 

 observation is determined by the uncertainties, A\ and A 2, of the positions of 

 the drogue at the times of launching and recovery, respectively ; 



v = (d ± Ji ± A 2 )/t, (1) 



where v, d, and t are velocity, distance and time respectively. Since navigation 

 is a very real problem in the open ocean, A\ and J 2 are usually not negligible ; 



Fig. 1. 



however, they are usually independent of both d and t and consequently the 

 error in mean velocity can be made very small by following the drogue for a 

 long enough period of time. The second advantage of this method is that the 

 drogue is a "self-integrating" device. By following a drogue over several tidal 

 cycles, it is possible to eliminate the effect of the tidal currents on the mean 

 velocity. Similarly, the effect of "small-scale turbulence" superimposed on the 

 mean flow is eliminated, by measuring velocity with a drogue. The main dis- 

 advantage of this method is that it takes a long time to make a single 

 observation. 1 



A well designed drogue system is one in which the drogue moves at nearly 

 the same velocity as the water at that depth. For a drogue designed to measure 

 surface currents this is usually not a serious problem. It is a very real problem 



1 There is no reason why a pressure-indicating device cannot be attached to the drogue 

 to measure its depth ; however, so far as I am aware, this has never been done, and the lack 

 of accurate information on the depth of the drogue must be considered a disadvantage. 



[MS received January, 1961] 303 



