Smith 



each whole-degree Celsius isotherm and prints the results every 12 

 seconds on a continuous 19-inch-wide tape. At the normal towing 

 speed of 6 knots, the printout is equivalent to a temperature- depth 

 cross section every 120 feet. The thermistor outputs are also 

 recorded in digital form every 12 seconds on magnetic and paper tape. 



Marine Advisors Model B-7C Ducted Current Meters (fig. 1) 

 attached to the thermistor chain were used to determine current 

 speed and direction (Christensen, 1966). The B-7C meter operates 

 on the pulse rate generated by the alternate opening and closing of a 

 reed relay by a magnetic slug in the tip of each impeller blade. 

 Meters Ci, C2j C3, and C4 (fig. 1) provide continuous relative 

 water-motion observations at 43, 259, 505, and 750 feet (usual maxi- 

 mum depth of the chain), respectively. * The output of each meter is 

 recorded continuously on a chart recorder. Constant instrumient 

 errors in the data are eliminated by towing the current meters in a 

 square pattern and averaging the readings from oppositely directed 

 tow legs (fig. 1). This procedure allows for the resolution of the two 

 orthogonal components of the relative current vector at the level of 

 each meter and, subsequently, for determination of the relative cur- 

 rent vector itself. The square pattern usually involves 5 minutes on 

 each side after the chain becomes mechanically stable following each 

 90- degree course change. Several time intervals have been used but 



5 minutes has proved to be adequate. Stability is recognized from a 

 steady output from the pressure sensor at the bottom of the chain and 

 steady pulse rates from the current meters. 



In order for current vectors measured this way to have any val- 

 idity, a reasonable estimate of ship's speed over the ground must be 

 made. When considering currents in deep oceans it is usually assumed 

 that horizontal pressure gradients vanish at some intermediate depth. 

 Geostrophic flow calculations have been made using this assumption in 

 areas of eddies and, in general, agree well with direct current obser- 

 vations (Reid, 1963). However, the level at which motion vanishes 

 remains controversial. Wind- driven currents in the ocean decrease 

 in strength very rapidly with depth (Ekman, 1905). At the normal 

 depth of current meter C4, 750 feet, the wind effect is negligible 

 under usual oceanic conditions, and currents referenced to this level 

 will be nearly equal to their true values. Errors in this method of 

 reference-level selection could arise near the eddy centers where 

 "domes" and "dishes" in the structures are most pronounced and 

 differences may exist between the true current and the current rela- 

 tive to 750 feet; the peripheral current measurements are more relia- 

 ble, however. 



Another apparent problem is determining the direction of flow 

 through the ducted meters. Ocean currents are rarely as great as 



6 knots, the towing speed of the chain. The chord-to-thickness ratio 

 of the current meter fairing is 15 and that of the other fairings is 9. 

 The fairings will align themselves with the ship; therefore, the flow 

 *In the 1964 study only meters Ci and C4 were used; in 1966 all 

 four meters were used. 



397 



