of the sled on the profile line are determined with transits while re- 

 cording is taking place. 



The sled is towed with a 0.794-centimeter-diameter (0.3125 inch) 

 galvanized steel cable coupled to harnesses on the sled and on the LARC. 

 Cable lengths vary depending on the mode of operations. Several 30.48- 

 and 91.44-meter (100 and 300 feet) sections of cable which can be attached 

 consecutively are carried onboard the LARC. About 152.4 meters (500 feet) 

 of cable are needed to turn the sled around in depths of 9.14 to 15.24 

 meters (30 to 50 feet) and to aline the sled on a profile headed toward 

 the shore. Care must be taken when using shorter lengths of cable since 

 the sled has a tendency to hydroplane and could possibly capsize if an 

 obstruction was hit. Extra cable is also added to avoid coupling and 

 decoupling when passing through the surf zone. 



While the sled is under tow, a minimum of two people are needed, a 

 LARC operator and a deckhand. Experimental designs requiring these oper- 

 ations are discussed in the next section. 



2. Instrumentation. 



The sea sled was principally designed for deploying current meters in 

 depths not over 9.14 meters. The sled's most important assets are the 

 versatility of measurements provided by the spars which can be raised, 

 lowered, or rotated, and the current-meter mounts which are also adjust- 

 able to alinements on any of three axes. Thus, sensors can be placed in 

 any configuration and at any elevation below 9.14 meters. In practice, 

 the sensors are seldom placed closer than 0.91 meter (3 feet) of the 

 bottom because streaming around the cylinders is expected to bias records 

 produced by the sensors. In addition to the current meters, a pressure 

 wave gage is commonly installed on the sled. However, the number and 

 kinds of instruments are not limited by the sled, as long as a method to 

 collect the data from the instruments is provided. 



Realining a sensor or moving it from one elevation to another can be 

 executed underwater by a diver. This operation requires the loosening of 

 setscrews of the cross fittings which support the spars. 



Upward or downward movement is limited to each 3.048-meter length of 

 each section of the mast. While the spars can be rotated in horizontal 

 plane or about their axes, the current meters can also be rotated 360° 

 about the spar axis; this is easily done even underwater by loosening a 

 pair of U-bolts holding the mount. If necessary, the sensors can also 

 be moved laterally. Rotation about the spar, if alined normal to shore, 

 will change a shore-parallel recording to a vertical or vice versa; to 

 aline in a shore-normal position will require complete removal of the 

 mount and the attached sensor (Fig. 8). 



a. Current Meters . Of the four types of current sensors commonly 

 in use (acoustic, inductance (EM), force, and mechanical), the mechanical 

 current meters are the most reliable in the surf zone. The principle of 



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