4) Displacement of objects as they affect 

 buoyancy. 



5) Non-interference with hookup points if 

 the vehicle is to be launched/retrieved. 



6) Strength of the exostructure. Clearly, 

 the fully encumbered exostructure must 

 be able to withstand the anticipated 

 rigors of shock loading both at sea and 

 during transport. 



7) Accessibility of components which may 

 require routine removal and servicing 

 without completely disassembling the 

 framework. 



8) Shape of the exostructure — that it pro- 

 vides a framework compatible with the 

 final desired vehicle configuration. 



9) Method of attachment to the pressure 

 hull must be such that no concentrated 

 or restraining loads exist. 



When such questions are resolved, the se- 

 lection of a material remains. In this case it 

 is critical to ascertain the likelihood of corro- 

 sion because of contact of dissimilar metals 

 at the point of exostructure-to-pressure hull 

 attachment, and where bolts or nuts of dis- 

 similar metals may be used to join the exo- 

 structure together. Finally, the material se- 

 lected must be lightweight in order to main- 

 tain a favorable W/D ratio and must lend 

 itself to easy fabrication and assembly. Alu- 

 minum is a prime candidate because of its 

 low density. However, some aluminum alloys 

 are susceptible to crevice corrosion. The dan- 

 ger of galvanic corrosion requires that alu- 

 minum must be insulated from steel compo- 

 nents. Both steel and aluminum are used in 

 present submersibles because of their availa- 

 bility, ease of fabrication, maintenance, long 

 useful life and cost. 



While all submersible pressure hulls, as far 

 as is known, are securely and quasi-perma- 

 nently affixed to their exostructures, there 

 are exceptions. In the case of ALVIN, SEA 

 CLIFF and TURTLE, the pressure hulls are 

 attached to their exostructures by one steel 

 shaft which penetrates the bottom of the 

 pressure hull and affixes to the framework. 

 In the event of an emergency, such as foul- 

 ing or loss of positive buoyancy systems, the 

 thru-hull shaft may be manually rotated 

 from within the hull to activate a cam and 

 release the pressure sphere and sail from the 

 main body. Being positively buoyant, the 



sphere is capable of ascending to the surface. 

 Electrical connections are the quick discon- 

 nect variety which break away as the sphere 

 ascends. 



FAIRINGS 



The fairings of submersibles serve three 

 purposes: 1) Reduce hydrodynamic drag; 2) 

 minimize the potential for fouling with sub- 

 merged ropes, cables or other objects and 3) 

 allow the vehicle's hatch to be opened on the 

 surface without swamping. On the other 

 hand, three opposing arguments may be ad- 

 vanced against fairings: 1) Submersibles op- 

 erate at such low speeds that reduction of 

 hydrodynamic drag is unnecessary; 2) if the 

 submersible offers sufficient visibility, such 

 as an acrylic plastic hull, the operator can 

 see all potential hazards and avoid them; 

 and 3) opening the hatch before the vehicle is 

 safely aboard its support ship is inherently 

 dangerous and should be avoided. The count- 

 ering arguments are quite valid and would 

 suffice but for one obstacle: There is nothing 

 predictable about diving or the ocean. Chap- 

 ter 14 deals with the fatal and near-fatal 

 hazards experienced to date, and it would 

 suffice here to present two incidents to show 

 the danger of not having fairings: 1) On 17 

 June 1973, the acrylic plastic-hulled JOHN- 

 SOI\ SEA LINK became entangled in the 

 debris of a scuttled destroyer at 360 feet off 

 Key West, Florida. The unfaired SEA LINK 

 was held 31 hours until pulled free of its 

 restraint. Two men in the aft aluminum 

 pressure cylinder perished as a result. 2) 

 Diving in the Gulf Stream, the submersible 

 DEEPSTAR 4000 became separated from its 

 support ship SEARCH TIDE while sub- 

 merged. Upon surfacing, neither the ship's 

 radar nor radio direction finder could locate 

 the submersible. As a final resort, a trunk 

 surrounding the hatch was inflated and the 

 hatch opened, thereupon allowing the opera- 

 tor to fire off flares which were seen by the 

 surface ship and allowed recovery. No per- 

 sonnel perished. 



It is debatable that reduction of hydrody- 

 namic drag is necessary. If the vehicle is 

 towed, however, some protection against 

 drag and wave slap must be provided for its 

 cables and external instruments. There is no 

 question of the need to reduce fouling poten- 



265 



