A submarine, by definition, needs to be able to supply its occupants with clean air, and its hull needs to be able to resist the intense pressure of the water around it. A bomb shelter needs to be able to supply its occupants with clean air, and ideally should be able to resist the intense pressure created by a nearby nuclear blast. And in the 60s the Navy still had tons of old submarines from the war that were too obsolete to send into combat - so why not turn them to a different use?
The plan was to use SS-212 Gato-class and SS-285 Balao-class submarines.[Sw] The Navy had built 205 submarines of these two classes between 1940 and 1946, although by 1961 less than a quarter were still available for conversion.[VHF] The study team inspected one such decommissioned submarine, the USS Sunfish (SS-281), at Mare Island Naval Shipyard.[Sw]
Figure 1: USS Sunfish in 1945[Wiki]
Their proposal was simple. First, workers would strip the sub of unneeded equipment and cut off the conning tower and other superstructure. Then, they would excavate a 360-foot-long, 22-foot-deep slip near a beach and allow it to fill with water. A tug would tow the submarine from its berth into the slip, where it would be covered with eight feet of sand.[Sw]
Figure 2: Cross-Section of Submarine Bomb Shelter[Sw]
The shelter occupants would enter via a horizontal zig-zag passage through the sand berm to a door cut into the bow near the forward torpedo room. If this passage collapsed, an emergency exit would be available through a vertical shaft to the surface from the aft torpedo room hatch. The shaft would ordinarily be kept filled with gravel and sand, to reduce radiation leakage; if the hatch was opened the gravel would spill into the ship, clearing the exit.[Sw]
Figure 3: Entry Into (left) and Exit From (right) Submarine Bomb Shelter[Sw]
Extensive work would still be needed to make the submarine livable. The electrical system would have to be completely redone - submarines ordinarily use DC current, whereas the power grid provides AC. A 30 kW diesel generator would also be installed in the former engine room. The ventilation system could be used largely as it was, but would be connected to filtered intakes from the surface fitted with blast closure valves. A separate shaft would vent the exhaust from the generator. A shallow well would provide brackish water for cooling, with the hot water piped to the ocean.[Sw]
Figure 4: Ventilation and Air Conditioning Diagram[Sw]
Gato-class submarines ordinarily carried a crew of 75 to 85, but the removal of the engines, torpedoes, and other equipment would free up a lot of room. With additional bunks installed, and occupants sleeping in shifts, the shelter could hold more than 276 people.
A Gato-class submarine, before it was transformed into a shelter, could reach depths of up to 300 feet. This is equivalent to a blast resistance of 130 psi overpressure - strong enough to survive a 1 megaton blast a kilometer away.[GD] And eight feet of sand would give a protection factor of about 180,000,000 - enough to block any plausible amount of radiation.
The study estimated the cost of conversion at $60,000 - about $454,000 today. On a per head basis, that was quite competitive with blast shelters offering equivalent protection. But the Navy had less than 50 submarines of this class available for conversion. Even if all were used, that was only enough space for 13,800 people. Naval Base San Diego alone has 26,000 workers today.[Mil] Even if all the subs were converted, it just wasn't enough space to bother with, so the idea was dropped.[VHF]
[GD]: Glasstone, Samuel, and Dolan, Philip J. The Effects of Nuclear Weapons. Third Edition. Department of Defense and Department of Energy, 1977. http://www.fourmilab.ch/etexts/www/effects/
[Mil]: Military.com. "Naval Base San Diego." Accessed 09/22/13. http://www.military.com/base-guide/naval-base-san-diego
[Sw]: Swalley, R. F. "Submarine Hulks as Protective Shelters." US Naval Civil Engineering Laboratory, 1961. http://www.dtic.mil/docs/citations/AD0251309
[VHF]: Van Horn, W. H., and Freund, D. Civil Defense Utilization of Ships and Boats. US Naval Radiological Defense Laboratory, 1963. http://www.dtic.mil/dtic/tr/fulltext/u2/421043.pdf
[Wiki]: "USS Sunfish." Accessed 9/22/13. http://en.wikipedia.org/wiki/File:USS_Sunfish;0828109.jpg