ENVIRONMENTAL 

 CONSTRAINTS 



Pressure 



A fundamental consideration in the design 

 of any vehicle transporting man or equipment 

 underwater is pressure. Pressure may be re- 

 sisted, as it is by the submersible's pressure 

 hull, or it may be compensated, as is the case 

 with many battery packs, propulsion motors, 

 etc. Once the submersible's operating depth 

 has been established, the pressure at that 

 depth will determine the dimensional and 

 compositional characteristics of the vehicle 

 and its components. 



Pressure in the ocean is a function of depth, 

 and for routine oceanographic calculations the 

 3.3-foot depth is equal to about 1 atmosphere 

 (14.7 psi). To moderate depths, say to several 

 thousand feet, seawater may be considered 

 incompressible and the following expression is 

 used: 



P = Pa + wh 

 where p is pressure in pounds per square inch 

 (psi), Pa is atmospheric pressure (psi), w is a 

 1.0-ft head of standard salt water equal to a 

 pressure of 0.4447 psi and h is depth in feet; 

 then 



p = 14.7 + 0.444h psi 



At greater depths, the compressibility of 

 water must be considered and, to obtain a 

 more accurate value, the density of seawater 

 may be taken as varying linearly from 64 pcf 

 at the surface to 66.6 pcf at 30,000 feet 

 (Fig. 2.1). Neglecting atmospheric pressure, 

 the pressure at depth h then is approximately 



p= 0.444h-l- 0.3 



\1,000/ 



psi (ref. 1) 



Hence, at 6,000 feet, the pressure on the 

 surface of a body is 2,674.8 psi acting normal 

 to every exposed surface. 



Seawater Conductivity 



Various devices in submersibles, e.g., mo- 

 tors, batteries, pumps, are immersed in a pro- 

 tective liquid which serves as an ambient 

 pressure compensator and an insulator 

 against loss of power to seawater. The intrin- 

 sic dielectric conductivity of seawater is ap- 

 proximately 4 mhos/m (milliohms/meter) or 4,- 

 000 times greater than that of fresh water; 



and, it increases with temperature, salinity, 

 frequency of the propagating wave and pres- 

 sure (1). A common cause of failure in electri- 

 cal systems is contamination of the compen- 

 sating/insulating fluid by seawater, where as 

 little as 0.1 percent contamination reduces the 

 resistivity of some fluids below recommended 

 limits (3). Various forms of corrosion (pit, crev- 

 ice, stress, layer, etc.) attack metals in seawa- 

 ter. Protective coatings and/or sacrificial 

 anodes should be considered in the initial de- 

 sign stage. 



Temperature 



The temperature of seawater (Fig. 2.1) has, 

 among others, two important effects on sub- 

 mersible diving: 1) The occupants must deal 

 with extremes of temperature caused mainly 

 by loss or gain of heat through the pressure 

 hull; and 2) the pressure hull material must be 

 capable of retaining its desirable characteris- 

 tics (crack arrest) under cold temperatures 

 encountered above and below the surface. 



Light 



Sunlight has been observed to penetrate the 

 ocean to depths as great as 2,300 feet (4), but 

 usable sunlight for detailed external viewing 

 generally terminates at 1,000 feet even under 

 the very best of conditions. Consequently, the 

 submersible user must rely on artificial light 

 sources for external illumination. Because of 

 the lateral and vertical variability of light 

 transmission properties and the frequent 

 blinding effects of backscatter throughout the 

 oceans, lighting for each diving mission is 

 approached on a case-by-case basis. 



WEIGHT DENSITY, LB/F=T- 



SALINITY, PARTS PER THOUSAND 



.11 -U IS Ifi 17 



Suflac 



TEMPERATURE, ^F 



TEMPERATURE. C 



Fig. 2,1 Seawater density, salinity, and temperature as function of ocean deptti 

 (From Ret (2)] 



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