be seen with the dropping of small weights 

 which serve, primarily, to make the submers- 

 ible lighter, but also served, initially, to pro- 

 vide sufficient negative buoyancy to allow it 

 to descend; the same may be said of iron 

 shot. A further distinction, then, between 

 the two systems is that they are grouped 

 according to their primary ballasting func- 

 tion. 



Chapter 14 deals with other ballasting de- 

 vices in the form of an emergency weight 

 which is dropped to provide positive buoy- 

 ancy. Because these methods are not rou- 

 tinely employed, it will suffice to note that 

 they constitute another means of gaining 

 positive buoyancy. 



The following descriptions of various bal- 

 lasting/deballasting devices are brief by ne- 

 cessity, for only a handful of vehicles, e.g., 

 the PISCES series, use similar procedures 

 and components. Individual descriptions of 

 each vehicle's ballasting system is provided 

 in Chapter 4. Most of the systems perform 

 essentially the same function from vehicle to 

 vehicle. Consequently, a general description 

 of the system's function, location, configura- 

 tion, etc., where it is amenable to this for- 

 mat, is presented. The examples cited are 

 selected to include one system which is fairly 

 representative of all, or one that represents 

 an advancement over or significant depar- 

 ture from the general field. 



The capacity of main ballast tanks varies 

 from vehicle-to-vehicle and is not controlled 

 by any standards. The Marine Technology 

 Society recommends that main ballast tank 

 capacity should not be less than 10 percent of 

 the vehicle's displacement at normal diving 

 trim. The American Bureau of Shipping, on 

 the other hand, does not require a minimum 

 capacity, but must be satisfied that the vehi- 

 cle can stay on the surface without endan- 

 gering the safety of the vessel under normal 

 sea conditions (Sea State 3 or as defined by 

 the designer) and with adequate freeboard. 

 In most vehicles the 10-percent displacement 

 margin is generally attained or exceeded. 



Reversible Systems 



Main Ballast Tanks: 



Function: To provide large changes in posi- 

 tive and negative buoyancy and provide ade- 



quate freeboard for maneuvering and for the 

 ingress/egress of personnel to the pressure 

 hull. 



Operation: On the surface the MBT's are 

 empty. Vent valves are located at the top of 

 the tanks and flood valves at the bottom; the 

 latter may or may not be free flooding. To 

 dive the vent valves are opened by the opera- 

 tor and seawater flows in through the flood 

 openings forcing air out through the top. In 

 the majority of vehicles an indicator light or 

 dial warns when the MBT's ai-e fully flooded, 

 at which time the vent valves are closed. 

 When the MBT's are full, the submersible 

 may be at neutral buoyancy or slightly nega- 

 tive and begins to descend. If the former is 

 the case, smaller capacity variable ballast 

 tanks are flooded or releasable weights are 

 added to provide negative descent buoyancy. 

 In a few submersibles, the MBT's can be 

 fully blown at operating depth; in the major- 

 ity, the MBT's are not blown until the vehi- 

 cle reaches the surface where additional 

 freeboard is required. In all vehicles, com- 

 pressed air is used to blow the MBT's when 

 surfaced. 



Location: MBT's generally straddle the pres- 

 sure hull and are located as high on the 

 vehicle as practical to provide stability when 

 surfaced by raising the center of buoyancy in 

 respect to the center of gravity. 



Configuration: Virtually any configuration is 

 acceptable which is compatible with the pres- 

 sure hull shape and offers least hydrody- 

 namic drag. 



Material: Steel of various compositions, fi- 

 berglass, aluminum. Material whose 

 strength is sufficient to withstand wave slap 

 and the rigors of shock during transport and 

 at-sea handling. 



Exfimple: 



a) The submersible BEN FRANKLIN 



has four MBT's which straddle the pressure 

 hull fore and aft (Fig. 6.2) and provide ap- 

 proximately 18 inches of freeboard when dry. 

 The tanks are constructed of laminated- 

 polyester and fiberglass, V4 to ^/s inch thick, 

 and contain 11 fiberglass ribs (filled with 

 syntactic foam) spaced within each tank to 

 provide additional strength. Each tank has a 

 capacity of 162 ft^ and all four provide ap- 

 proximately 41,500 pounds of positive buoy- 



288 



