Polyurethane foam coated by 

 PVC sh»»t (colored orange) 



50-70cifl 



Figure 15. Schematic of drogue used by Japanese (from Sasaki, 1977), 



Further advantages and disadvantages of dye and float methods are 



discussed by Galvin and Savage (1966) 



b. Eulerian. Propeller-type current meters were first used in the 

 surf zone by Sonu (1969b). These bidirectional meters used a three-bladed 

 impeller mounted within a duct. Paired reed switches sensed both rotation 

 speed and flow direction through magnetic coupling in the blades. Thres- 

 hold velocity was 5 centimeters per second. The meter's response is non- 

 linear near the threshold speed due to impeller inertia and thus not 

 truly bidirectional. This type of meter is also phase-dependent in that 

 there is always a real timelag between forcing function and meter 

 response. In CERC's development of the Towed Oceanographic Data Acquisi- 

 tion System (TODAS) , the ducted impeller type (mechanical sensor) was 

 chosen over acoustic EM and force meters (Teleki, Musialowski, and Prins, 

 1976) . The reasons for this choice were the simple but rugged construc- 

 tion, zero drift, and antifouling characteristics (suspended sediments, 

 seaweed etc.) of the propeller type (Teleki, Musialowski, and Prins. 1975). 

 A 4-inch duct (8 inches long) directed the flow through a five-bladed 

 impeller. The measurement range was to 2.6 meters per second (0 to 5 

 knots) with a threshold speed of 2 centimeters per second. They found 



14 



GALVIN, C.J., and SAVAGE, R.P., "Longshore Currents at Nags Head, North 

 Carolina," Bulletin No. 2, U.S. Army, Corps of Engineers, Coastal Engineering 

 Research Center, Washington, D.C., 1966 (not in bibliography). 



55 



