of 0.635-centimeter (0.25 inch) aluminum plate, and welded to two 7.62- 

 by 3.81-centimeter (3 by 1.5 inches) cross channels (Fig. 7). 



An additional cross channel is attached across the edge of the runners. 

 The 9. 14-meter-high mast consists of three sections. The lower section 

 is a 6.03-centimeter (2.375 inches) outside diameter 2. 77-meter- long 

 (9.08 feet) aluminum pipe; the middle section is a 4.82-centimeter 

 (1.9 inches) outside diameter, 3. 03-meter- long (9.96 feet) pipe; and the 

 upper section is 3.34-centimeter (1.315 inches) outside diameter, 3. 04- 

 meter- long (9.98 feet) aluminum pipe. These sections are connected by 

 coupling joints and the bottom of the mast is attached to a flange on 

 the center platform. The mast is guyed at 12 points, in groups of four 

 guys each at elevations of 3.048, 6.096, and 9.14 meters (10, 20, and 

 30 feet) , and supports three or more adjustable spars to which the current 

 meters are attached. 



All members were constructed from extruded, aluminum alloy 6061-T6. 

 Aluminum is preferable because of weight and corrosion resistance; the 

 6061-T6 alloy also has higher tensile strength than other alloys. The 

 corrosion resistance is not affected by welding although there is some 

 loss of strength in the weld area or heat-affected zone. Alloy 6061-T6 

 is the least expensive of all the heat-treatable alloys produced. 



Where magnetic fields are present, stainless steel is not preferred 

 to aluminum (see discussion of current meters) ; also, aluminum better 

 absorbs impact loads from waves. 



Erection of the structure takes about 2 hours and requires no special- 

 ized tools for assembling. The main structure is similar to equipment 

 used in nearshore surveying; the difference being the spars, the skid 

 design, and the instrument powerpacks. The sled's weight is increased by 

 bolting steel bars to the runners, preferably to the bow part. 



Damaged sections of the sled can be repaired at the field site with 

 an arc welder. 



c. Operation Modes and Required Support . Two standard operations 

 have been practiced with the sled, the choice depending on the type of 

 flow-measurement experiment. One operation is locating the sled at a 

 desired point offshore and measuring currents in situ; the other requires 

 towing of the device by a LARC V or LARC XV (lighter amphibious resupply 

 cargo) vehicle along a given profile, usually alined normal to shore. 

 The latter operation is an incremental movement of the platform, usually 

 commencing in 9.14 meters of water to avoid submerging the antenna, and 

 currents are measured for a given length of time at stations in sequence 

 along the profile. During operation in this mode, the sled is decoupled 

 from the LARC during recording time to prevent any movement of the sled 

 due to drifting of the LARC. This procedure is repeated for each succes- 

 sive position of the sled along the profile (positions are usually pre- 

 calculated) . A pair of shore targets normal to the profile gives the 

 LARC operator his control when moving in on the profile. Actual locations 



19 



