Sec. 76.30 



DESIGN OF SPECIAL-PURPOSE CRAFT 



I Wood-Slat Decklor Perforoted- Metal Deck for Ventino of Superstructure 



811 



Hatch 

 Freebocird 



Fig. 76.V Schematic Section op Double-Hulled Submersible Showing Main-Ballast Flooding and 

 Venting Arrangements and Buoyant Volumes 



equal volume of air, while at the same time 

 exercising control of the flooding and venting 

 among a number of separate tanks, involves a 

 rather elaborate system of large flood valves in 

 the lower part of the outer hull and correspond- 

 ingly large vent valves along the tops of the 

 main-ballast tanks, indicated schematically in 

 Fig. 76.V. 



Somewhat similar water-handhng systems are- 

 required for taking aboard, pumping out, and 

 shifting what is known as variable-ballast water 

 within the pressure-proof boundaries of the sub- 

 marine. The admission, discharge, and transfer 

 of water serve as compensation for the expenditure 

 of useful weights such as food, stores, and lubri- 

 cating oU; for the taking aboard of additional 

 useful weights; and for the moving of these 

 weights fore and aft and transversely within the 

 pressure-proof boundaries to suit the mission of 

 the vessel. 



Because a submarine is not burdened with 

 vulnerable deck hatches and rarely requires 

 personnel to work on deck at sea, except possibly 

 for a limited region amidships, its freeboard to 

 the superstructure deck can be much lower than 

 that of a surface vessel. With this low freeboard 

 it is expected that soUd water will be taken 

 aboard. Several expedients are adopted for un- 

 loading this water expeditiously. The topside is 



given a large tumble home and the deck edges 

 are often well rounded into the side, so that the 

 beam of the flat or cambered deck is rarely more 

 than 0.7 of the maximum beam at the surface 

 waterplane. The deck forward, excluding the 

 rounded turtleback on each side, may be not 

 more than 0.5 times the beam at the corresponding 

 section. The perforations and slots in the deck, 

 placed there to provide rapid venting of the free- 

 flooding superstructure volume, afford a means 

 of sluicing away the deck load of water before it 

 sUdes aft during an up pitch and dashes against 

 the deck erections amidships. 



The freeboard at the extreme stern may be 

 diminished to zero, so far as wavegoing charac- 

 teristics are concerned. If, however, the surface 

 speed is high enough to produce a crest of appre- 

 ciable height on the stern wave, the outer hull 

 should be carried up at least to the top of this 

 crest, with the vessel in running trim. 



The flooding ports or openings along the lower 

 edges of the superstructure volume, just above 

 the surface waterline, represent an unsolved 

 problem as far as combining rapid flooding of the 

 superstructure with minimum drag along the 

 surface waterline is concerned. A continuous 

 flooding slot along the lower edge of the super- 

 structure, used at various times, is not an accept- 

 able solution because of the relatively large 



