Between 1925 and 1927, the burgeoning US Naval submarine force suffered two high-profile submarine failures — the loss of the S-51 in 1925 as well as the S-4 in 1927 — together resulting in the deaths of 71 US servicemen. Seeing an opportunity to save lives, Charles B. "Swede" Momsen set about devising a means to pluck submariners from the briny deep. His solution remains the only submarine rescue system to ever actually work in practice.
Diving bells are surprisingly ancient inventions. Aristotle first described the concept of what's known as a Dry Bell in the 4th century BC: "...they enable the divers to respire equally well by letting down a cauldron, for this does not fill with water, but retains the air, for it is forced straight down into the water". They were also purportedly employed by Alexander the Great to dive the Mediterranean Sea. The first modern diving bell was constructed in 1535, by Guglielmo de Lorena.
Most modern dry diving bells work on the same basic principal: a fully enclosed, negatively buoyant habitat protected from the immense pressures of the surrounding water with fresh air pumped into the chamber via hoses from the mother ship. For rescue operations, such as evacuating surviving crew members from a flooded submarine, specialised diving bells are employed. These bells are designed to mate with the hatch of the sub, pump out any water caught between the rubber skirt on the bottom of the bell and the hull surrounding the hatch, creating a tight seal that allows the hatch to open and seamen to escape into the rescue bell where they are ferried back to the surface. The rescue bell can then be lowered back down to the sub to pick up another load of passengers.
Back to Momsen, in 1926 he was himself captain of the S-1 submarine when he began kicking around ideas for what he would later dub, a "rescue chamber". However his ideas initially fell on the deaf ears of naval brass, even after he was transferred to head the Bureau of Construction and Repair (the navy's construction, repair and fabrication arm until 1940). It wasn't until the S-4 was accidentally struck by a Coast Guard Destroyer off the coast of Cape Cod in 1927 — then sank in just 30m of water, killing all 40 crew on board — that public pressure forced the Navy to seriously consider Momsen's proposal. Eventually, Momsen's higher-ups relented and authorised the development of a submarine rescue device.
By the end of 1928, three prototype diving bells had been constructed by the BC&R and had begun testing off the coast of Florida. However it wasn't until two years and three design tweaks later that the submarine rescue chamber was officially unveiled. By that time, Momsen wasn't even on the project any more, having transferred onto a separate project that became known as the Momsen Lung. Instead, Lieutenant Commander Allan Rockwell McCann was placed in charge of the project that now bears his name.
The McCann Rescue Chamber is a 3m tall pear-shaped diving bell, 2m across at its widest point. The interior of the bell is split into upper and lower sections separated by a water-tight hatch. The upper section holds up to 10 men and supplies them with fresh air while pumping out waste gas. The lower section is ringed in ballast and holds the sealing skirt. When the bell comes in contact with the flat surface of the submarine, the skirt creates a tight seal so that the water in the lower section can be pumped out and people can exit the submarine. An eyelet on the top of the bell connects to 120m of half-inch steel cable which is used to haul the vessel back to the surface. To ensure the bell was properly positioned over the hatch, naval divers would first have to attach cables to the top of the sub's hatch that ran up through a pulley system on the bottom of the bell. As it was lowered, the haul down winch, as it was called, would automatically centre the bottom of the bell around the hatch.
It did suffer from some glaring technical shortcomings, such as the inability to compensate for currents, or access pressurised hulls, or reach escape hatches of subs lying at extreme angles. But the McCann remains the only Submarine Rescue Chamber in history to actually save lives. This opportunity came in 1939 when the USS Squalus sank off the coast of Portsmouth, New Hampshire during a test dive, due to a faulty induction valve that flooded the sub's aft section, killing 26 men immediately. The remaining crew moved to the forward compartments and signalled for help. Coincidentally, it was Lt Momsen that commanded the USS Falcon, which performed the rescue. The McCann chamber made a total of four trips to the Squalus' resting place, 73m down, to save the remaining 33 crew members. [Wikipedia - IBSSG - Navy - Bowfin - Global Security]