66 



2.3.4 Towing Speed 



Towing speeds range from 1 to 8 knots (2 to llkm/hr) and average 3 knots 

 (6kni/hr) . The higher tow speed vehicles are generally employed in mid-water 

 operations while the slower vehicles are towed on- or very close (5 to 10m) - 

 to the bottom. Fast speed is not advantageous since it is not conducive to 

 photography of high resolution. 



2.3.5 Power 



Electrical power in all but three systems is supplied from the support ship 

 via an umbilical cable. Since propulsive power is supplied by the surface 

 ship also, the umbilical cable carries only power for instrumentation, and, in 

 the case of the RUFAS vehicles and S-', power to control the dive planes. 



Power requirements vary from fractional amperage at 110 VAC to 20 amps at 



440 VAC. Although no two vehicles call for identical power requirements, 



60 Hz, 115 VAC, 20 amperes is fairly representative. Significantly, the voltage 



requirements listed in Table 2.7 are those required at the fish. Voltage 



losses through 6,000m of cable are significant and the shipboard voltage is 



frequently quite higher than what is required. RAIE I, for example, is supplied 



400 VAC at the surface to provide 110 VAC at the fish. 



Vehicles which do not obtain their electrical power from the surface carry 

 batteries. Ordinarily the towing mission is limited to the battery endurance. 

 However, the NRL System's nickel cadmium batteries are maintained in a fully 

 charged condition throughout the towing operation by current from the support 

 ship via the umbilical cable. 



2.3.6 Vehicle Control 



Maneuvering a towed vehicle is simple in concept, but can be difficult in 

 practice. The most common approach involves lowering the vehicle to some 

 appropriate depth (or altitude off the bottom) and maintaining this depth by 

 winching in or winching out cable. Distance from the bottom is generally 

 monitored by a downward- looking echo sounder. CCTV can be used to maintain 

 an appropriate distance also. In addition to CCTV altitude control, DSS-125 

 employs a pair of collimated spot lights to assist in determining the size 

 of objects in the scene and the distance of the vehicle above the ocean bottom. 

 The difficulty is brought about by sea surface conditions which may impart 

 heave to the ship. According to Dr. F. N. Spiess, Marine Physics Laboratory, 

 San Diego, Ca. , ship's heave motion is reflected about 1:1 on the submerged 

 vehicle. At the very least it is Ir^j- Consequently, focusing the TV camera 

 or still cameras can be exceedingly difficult. For this reason, the DEEP TOW 

 system employs accumulators aboardship which act to reduce the vehicle's 

 vertical excursions. 



An alternative solution to accumulators is found in the DSS-125 system. Here 

 a 600kg (1300 lb) depressor is attached to the umbilical approximately 60m 

 (200 ft) forward of the vehicle (lead ballast weighing up to 1,100kg (2,400 lbs) 

 can be added to the depressor if necessary) . With this arrangement surface- 

 imparted heave is taken up by the depressor instead of the fish which trails 

 astern. There is, quite naturally, a penalty in this approach. Instead of an 



