. The Composition of sea-water : comparative and descriptive oceanography. Seawater -- Composition. 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
. The Composition of sea-water : comparative and descriptive oceanography. Seawater -- Composition. 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 A2, of the positions of the drogue at the times of launching and recovery, respectively; v = (d ± Ji ± A2)/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 J2 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 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 pro
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