the MBT. Passing through a hull valve each 

 line runs through a check valve, a pressure- 

 restricting diaphragm and into a blow valve 

 aft and just below the vent valves. 



b) The All Ocean Industries submers- 

 ible employs compressed air to blow the 

 MBT's, but in a manner distinctly different 

 from BEN FRANKLIlS's and apparently from 

 all other vehicles. It operates in the follow- 

 ing manner: Two MBT's vent directly into 

 the pressure hull; to dive, the operator opens 

 a vent valve which allows water into the 

 tanks and forces entrapped air from the 

 tanks into the pressure hull. Because pres- 

 sure will build up in the hull, a second valve, 

 called a snorkel valve, is opened to allow the 

 escape of air outside the hull. When a little 

 water appears at the vent valve, it is se- 

 cured, as are three flood valves leading to 

 the MBT's. The snorkel valve is also closed 

 after the MBT and flood valves are secured. 

 With the MBT's full, the vehicle is still 

 slightly positive and VBT's must be flooded 

 to attain negative buoyancy. To blow the 

 MBT's on the surface, the two MBT flood 

 valves are opened and air from one of two 

 scuba tanks outside the hull is introduced 

 into the MBT. When bubbles can be observed 

 coming out of the MBT's, the tanks are as- 

 sumed empty, and the hatch (dome) may be 

 opened. 



Variable Ballast Tanks: 



Function: To provide small scale buoyancy 

 adjustments. 



Operation: Two approaches are used to at- 

 tain variable ballasting systems. In the most 

 generally used approach there is a hard tank 

 into which seawater is introduced by means 

 of the ambient pressure differential to attain 

 negative buoyancy and then expelled by 

 either compressed air or a pump to attain 

 positive buoyancy. In the second case, gener- 

 ally on the deep vehicles, a system is used 

 which employs a pressure-resistant tank con- 

 nected to collapsible (flexible) oil-filled bags. 

 When surfaced, the spheres are partially 

 filled with oil and air at atmospheric pres- 

 sure; the bags are fully collapsed. To gain 

 positive buoyancy when submerged, the oil is 

 pumped into the bags which expand and 

 displace seawater, thus, providing positive 

 buoyancy. To gain negative buoyancy, the oil 



is permitted to flow back into the rigid tank. 

 More accurately, this hard/soft tank system 

 is termed a variable displacement system 

 since the vehicle weight remains constant. 

 Evident from Table 6.6 is the absence of air- 

 blown VBT's below 2,500 feet for reasons 

 given earlier. When hard tanks and water 

 are employed as the variable ballast system 

 on present submersibles, high pressure 

 pumps are used to expel water from the 

 tanks below 5,000 feet. 



Location on Vehicle: As is evident from Table 

 6.6, there is no standard location for variable 

 ballast systems. In some vehicles the sys- 

 tems are in the pressure hull (this arrange- 

 ment saves the expense accompanying a sys- 

 tem exposed to seawater and ambient pres- 

 sure), and in others the system is external to 

 the pressure hull, thereby saving limited in- 

 ternal space in the pressure hull. Those vehi- 

 cles using variable displacement (hard and 

 soft tanks) systems must locate at least the 

 soft part of the system external to the hull. 

 As a general rule, most VBT's are situated 

 below the vehicle's center of gravity to keep 

 the center of gravity low. This is the case 

 with AUGVSTE PICCARD and BEIS FRANK- 

 LIN, where the VBT's are in the vessels' 

 keels. When two or more tanks are used, they 

 are balanced port and starboard or fore and 

 aft; in the latter situation the system may 

 also serve as a trim system by differentially 

 filling the tanks. 



Configuration: Systems external to the pres- 

 sure hull employing only air and hard tanks 

 have no particular geometrical configuration 

 on the shallow submersibles, but are gener- 

 ally spheres or cylinders on the deep vehi- 

 cles. Where the tank is external to the pres- 

 sure hull it must withstand the same forces 

 as the pressure hull. Because the tank may 

 be pressurized above ambient during ascent, 

 tensile stress is a factor. Where the VBT is 

 within the hull at atmospheric pressure, ten- 

 sile stresses are the overriding considera- 

 tion. 



Soft/hard tank systems are required to 

 withstand ambient pressures and for this 

 reason the hard tank component is always 

 spherical. Being either pressure-compen- 

 sated or collapsed, the soft tank component 

 can be any shape. 



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