MEASUREMENTS OF THE WATER-FOLLOWING CAPABILITY OF HOLEY-SOCK AND TRISTAR DRIFTERS

Since 1985, a number of measurements have been made in deep water to d etermine the water-following characteristics of mixed layer drifters w ith both holey-sock and TRISTAR drogues at 15 m depth, The measurement s were done by attaching two neutrally buoyant vector measuring curren t meters (VMCMs) to the top and the bottom of the drogues and deployin g the drifters in different wind and upper ocean shear conditions for periods of 2-4 h. The average velocity of the VMCM records was taken t o be a quantitative measure of the slip of the drogue through the wate r, observed to be 0.5-3.5 cm s(-1). The most important hydrodynamic de sign parameter which influenced the slip of the drogue was the ratio o f the drag area of the drogue to the sum of the drag areas of the teth er and surface floats: the drag area ratio R, The most important envir onmental parameters which affected the slip were the wind and the meas ured velocity difference across the vertical extent of the drogue. A m odel of the vector slip as a function of R, vector wind and velocity d ifference across the drogue was developed and a least squares fit acco unts for 85% of the variance of the slip measurements. These measureme nts indicated that to reduce the wind produced slip below 1 cm s(-1) i n 10 m s(-1) wind speed, R > 40. Conversely, if the daily average wind is known to 5 m s(-1) accuracy, the displacement of the R = 40 drifte r can be corrected to an accuracy of 0.5 km day(-1).