Materials: Hard tank systems employ the 

 same material for the VBT as they do for the 

 hull, though in some cases a stainless steel is 

 used. Soft/hard tank systems vary in the 

 nature of the material used for the hard 

 component. ALVIN, SEA CLIFF and TUR- 

 TLE use titanium spheres, while PISCES IV 

 and V use HY-100 steel, the same material as 

 found in the pressure hull. The flexible bags 

 in the hard/soft tank system of TURTLE and 

 SEA CLIFF are composed of reinforced rub- 

 ber. 



Example: 



(a) The variable ballast system of 

 MAKAKAI provides not only positive and 

 negative buoyancy changes, but changes in 

 trim and roll (or heel) as well. From refer- 

 ence (4), ballast tanks are mounted on each 

 corner of the vehicle (Fig. 6.3), and each tank 

 has a capacity of 199 pounds of seawater. 

 Two ballast pumps (one supplying each side) 



pump water in or out of the tanks to provide 

 buoyancy changes. The non-water volume of 

 the ballast tanks is pressure-compensated to 

 10 to 20 psi above ambient by air stored in 

 four high pressure cylinders (thereby negat- 

 ing the need for pressure-resistant VBT's). 

 The high pressure air is reduced to ambient 

 pressure by a differential pressure-regulat- 

 ing system, and the ballast tanks are pro- 

 vided with relief valves to vent air on ascent. 

 To attain fore or aft trim, water may be 

 pumped fore or aft between tanks, and taken 

 onboard on one side of the vehicle and over- 

 boarded on the other side to attain roll an- 

 gles. 



(b) Although now converted to a hard 

 tank system, ALVIIS's original variable dis- 

 placement system was typical of other vehi- 

 cles and worked in the following manner (5). 

 Two large, flexible oil-tight bags were de- 

 signed to fit into floodable fiberglass com- 



TRIM TANK 



STARBOARD BALLAST 

 PUMPAND-VALVE UNIT 



PORT BALLAST 



PUMP-AND- 



VALVE UNIT 



BALLAST AIR 

 STORAGE 



BALLAST WATER LINES 

 COMPENSATING AIR LINES 



Fig. 6.3 MAKAKAI's ballast control schematic. (From Ref. (4)] 

 292 



