the weight will vary, but the method of jetti- 

 soning is either by mechanically turning a 

 thru-hull releasing shaft or by electrically or 

 hydraulically actuating a release mecha- 

 nism. In the latter procedure the electrical 

 power is derived from emergency batteries 

 within the pressure hull. In some cases an 

 electrically-actuated (explosive) device may 

 serve as the release mechanism by cutting a 

 restraining cable or the like. 



Equipment Jettison: 



To attain the same results as a weight 

 drop, equipment such as batteries, mechani- 

 cal arms, motors and the like can be jetti- 

 soned to lighten the vehicle. This procedure 

 is generally secondary to a weight drop be- 

 cause of the high cost of such equipment. 



Trim Fluid Drop: 



Those submersibles which use mercury to 

 attain bow trim (pitch) angles or roll (list) 

 angles also incorporate a method of dumping 

 the mercury in extremis. In the event that 

 the submersible rests at such an angle that a 

 gravity dump is not effective, it may — as 

 does DEEPSTAR 4000 — carry a reservoir of 

 compressed nitrogen which can force the 

 mercury out of the reservoir. In considera- 

 tion of the pollution aspects of mercury, the 

 U.S. Navy is presently working on an alter- 

 native to mercury as a trim fluid and has, for 

 the interim, made mercury dumping from its 

 vehicles impossible. 



High Pressure Ballast Tank Blow: 



Submersibles' main ballast tanks are used 

 to achieve surface freeboard and are emptied 

 of water by compressed air. Consequently, 

 low pressure air is all that is required for 

 blowing ballast on the surface. As an emer- 

 gency feature, in addition to normal variable 

 ballast tank control, a number of vehicles 

 carry high pressure air to blow the main 

 ballast tanks empty and lighten the vehicle 

 for surfacing from operating depth. This is a 

 common emergency feature on the shallow- 

 diving submersibles and is found on some of 

 the deeper vehicles (e.g.,ALUMINAUT). 



Manual Deballasting: 



As a backup to the above deballasting 

 procedure, a few vehicles include a hand 

 pump in the pressure hull which can be used 



to evacuate water from the ballast tanks if 

 the air blow system malfunctions. 



Personnel Egress: 



More than a third of past and present sub- 

 mersibles incorporate procedures to allow 

 the occupants to exit the vehicle when all 

 else fails to bring it to the surface. In theory 

 the procedures are simple and fall into two 

 categories: 1) Pressurizing the hull with com- 

 pressed air from the ballast blow tanks until 

 internal air pressure equals ambient water 

 pressure, which allows the occupants to open 

 the hatch, flood the hull, leave and ascend to 

 the surface; and 2) opening a thru-hull valve 

 to allow seawater into the hull until it com- 

 presses the air in the hull to a point where it 

 (the air) is equal to ambient pressure and 

 allows the occupants to open the hatch and 

 swim to the surface (in some vehicles both 

 systems may be used concurrently). In lock- 

 out submersibles the egress hatch is in the 

 bottom of the vehicle and it is not necessary 

 to flood the hull for exiting when internal 

 pressure is equal to ambient (Fig. 14.5). 



Fig 14 5 Divers approaching the lock-out hatch of BEAVER Air pressure in the aft 



sphere IS equal to water pressure and prohibits entrance of seawater (North 



American Rockwell) 



660 



