Currents 



Currents in the ocean and contiguous 

 waters range in horizontal speeds from less 

 than 0.05 knot (Pacific deep water) to 15.5 

 knots (Skjerstadt Fjord, Norway), and they 

 fluctuate rapidly both spatially and tempo- 

 rally (5). Where currents are strong, the sub- 

 mersible must be able to maintain control and 

 headway to conduct its task and maneuver 

 safely. 



Density 



Since seawater density varies not only with 

 depth (Fig. 2.1), but with temperature and 

 salinity as well, vehicle buoyancy calculations 

 must be based on the specific diving location. 

 In some instances, underwater discharge of 

 fresh or brackish water near the bottom has 

 caused significant loss of positive buoyancy on 

 a submersible working close to the bottom (6). 



Acoustics 



Light and radio waves attenuate rapidly in 

 the ocean. Depending on the frequency of the 

 signal and oceanographic conditions, sound 

 waves may travel for thousands of miles. 

 Sound, therefore, is used for communications 

 between ship and submersible, for tracking of 

 the submersible from the surface and for a 

 variety of data collection instruments. The 

 velocity of sound in seawater varies from 

 about 4,775 to 5,150 feet/second and increases 

 with increasing temperature, salinity and 

 pressure (5). If sound is traveling vertically 

 downward, the effect of refraction (bending) is 

 relatively slight; as the beam direction ap- 

 proaches the horizontal, refraction may be- 

 come quite great. The usual situation (Fig. 2.2) 

 is for sound speed to decrease initially with 

 depth as the temperature decreases; hence, 

 the upper part of the sound beam travels 

 faster than the lower part and a shadow zone, 

 into which the sound beam does not pene- 

 trate, is left near the surface. Such refraction 

 may occur at any depth in the ocean; its 

 effects can control the ranges from which a 

 submersible can be tracked from its surface 

 support and still maintain voice contact. 



Sea State 



The operational limits of submersibles' 

 launch/retrieval devices are determined by 

 wave height (the vertical distance from wave 

 trough to crest) and period (the time interval 



between successive crests passing a station- 

 ary point); the condition is generally termed 

 Sea State, and its boundaries are presented in 

 Table 2.1. Sea state, as defined in the accom- 

 panying table, is misleading as a measure of 

 the ability of a launch/retrieval apparatus, for 

 it does not take into account wave period. For 

 example, launch/retrieval may be ruled out in 

 low sea states if the period is on the order of 8 

 to 10 seconds; but, if the period is doubled or 

 greater, the frequency of the wave crest's 

 passage is less and time may be sufficient to 

 complete the hook-up of lift lines between 

 successive crests. One must be aware that the 

 sea surface is rarely calm and is in a constant 

 state of change. If a submersible system is to 



SPEED OF SOUND, 

 FT/SEC 



5000 



6000 



8000 



Fig. 2.2 Typical variation of speed of sound with depth in the ocean. 



15 



