Fig 5 1 1 Reinforcements and retainers for the pressure fiull of ALVIN showing window retainers (background), lift padeyes (center), penetrator shims (right and foreground) and 



release hooks (center foreground) (WHOI) 



with a taper of 4.1; viewport reinforcements 

 in deep submersibles are slightly less than 

 100 percent. 



Hatches: 



The majority of hatch or personnel access 

 openings are circular and designed to fit as a 

 cone in the pressure hull; the smallest of 

 these is in the DEEP ST4K-series vehicles 

 where 15.75-inch diameter prevails. Two ex- 

 ceptions to the above generalization exist: 1) 

 certain shallow-diving vehicles, e.g'.. PC-3A 1 

 & 2, NEKTON A^,C, where the hatch is a 

 circular dome disk and fits flush over a cylin- 

 drical conning tower (Fig. 5.12) and 2) certain 



lock-out vehicles where the hatch may be 

 oval shaped. The reason for an oval shape in 

 the lock-out vehicles, though not immedi- 

 ately apparent, is really quite simple and 

 compelling. Within the lock-out portion of a 

 submersible the internal pressure may not 

 only equal, but may exceed, ambient (exter- 

 nal) pressure during decompression on the 

 surface. For this reason a double-acting 

 hatch is required. This takes the form of an 

 internal hatch and an external hatch. In 

 order to initially install or remove the inter- 

 nal hatch from the pressure hull the access 

 opening must be other than circular, other- 

 wise it would be impossible to insert the 

 larger diameter circular hatch through a 

 smaller diameter penetration. 



256 



