Thanks to Wired for this:

First of all, you can’t just open the hatch when you’re trapped at the bottom of the ocean. But there is a way out—it requires physics and some audacity.

This is an excerpt from:

In the Waves: My Quest to Solve the Mystery of a Civil War Submarine by Rachel Lance, to be published April 7 by Dutton, an imprint of the Penguin Publishing Group, a division of Penguin Random House, LLC. Copyright © 2020 by Rachel M. Lance

In the Waves book

EVER SINCE HUMAN beings created the first submarines, there have been other, more claustrophobic people who have stared at the devices and thought: “Nope.” For many, the thought of the pipe- and equipment-filled narrow metal confines is enough to trigger a fear of drowning—even when they’re standing on dry land. But everyone who has ever looked at a sub, even those of us so enamored with the underwater beasts that we sleep in custom-sewn adult-sized submarine pajamas, has at some point wondered:

If the boat goes down, is there any way out?

Yes! Escape plans and tools are almost as old as submarine technology itself. Although the odds may always be stacked in favor of the merciless, cold depths of the sea, a few dozen lucky people have taken that unintentional ride down to the ocean floor and lived to see daylight again. Their stories teach us how to get out.

In 1851, German submarine inventor Wilhelm Bauer looked at two of his panting countrymen, slumped inside the hull of his creation. The boxy, 26-foot-long, human-powered early sub model was supposed to help win the ongoing war with Denmark, Germany’s neighbor to the north, but the odds of its successful use were looking grim. The crew of three had been trapped inside the submarine for hours, sitting and waiting for rescue.

The test day in Germany’s Port of Kiel had started normally. The men had crawled, as usual, through the hatch in the angular conning tower above the bow and taken their places: Bauer at the controls, and Witt and Thomsen each standing at one of the two massive hamster wheels that powered the boat’s propeller. Bauer gave the command. Witt and Thomsen lifted their legs and began to step on the spokes of the wheels, spinning them slowly like a giant human-powered waterwheel. The submarine began to move forward.

Brandtaucher German submarine in 1851

Bauer expected a graceful and smooth disappearance beneath the surface of the water, like an elegant metal seal. Instead the Brandtaucher (“Fire Diver”) plummeted unexpectedly, caroming wildly in an awkward, unstoppable, and rapid descent into a depression in the harbor floor that was 16 meters deep. As she crashed into the seafloor and shuddered to a final stop, the three men were hurtled unceremoniously into the bow of the boat. They pieced themselves together, shaken but uninjured. However, Bauer, Witt, and Thomsen slowly came to the realization that they couldn’t get the boat out of the hole. They were stuck.

At first, they just waited. And waited. For at least five hours, according to them, they sat, wondering when rescue would come. Their dive had been witnessed by onlookers; they figured it was just a matter of time until the German Navy hauled them back up to safety and fresh air. Someone had in fact noticed, and eventually the clanking of chains and anchors on the hull indicated that boats and divers were poking around the wreck site. But Bauer was growing concerned about the air … and the anchors.

All the men were panting hard, pale and sweating. Bauer himself had a splitting headache and felt like he was about to be sick. Bauer knew these were the signs of carbon dioxide buildup, caused by the fact that they kept inhaling the oxygenated air they had brought down with them and exhaling noxious CO2. Their blood was becoming more acidic with every breath from the invisible but dangerous CO2, and he knew that they did not have much fresh air supply left. He was also concerned about the anchors and chains that were striking the submarine so loudly, because he thought her thin hull might rupture from their repeated hits. The submarine had an escape hatch, but the pressures of the ocean held it firmly shut. Bauer reached up a pallid, trembling hand and gripped a seacock valve tightly in his palm, twisting it open. Water poured in and started to flood the submarine.

Witt and Thomsen immediately pounced on Bauer, one slamming him down and sitting on his chest, the other scrambling to restrain his arms and close the valve. Wide-eyed, they yelled that he was trying to commit suicide and drown them too. But Bauer had opened the seacock because he was a man who wanted to live, and because he was also a man who understood physics.

The pressure inside the submarine was roughly 1 atmosphere because it had been closed and sealed on the surface at 1 atmosphere. The pressure in the seawater outside, at a depth of 16 meters, was equal to about 2.6 atmospheres. Therefore, the pressure difference across the hatch of the submarine was about 1.6 atmospheres total. Converting the units, if Bauer wanted to force open the hatch to escape he would need to be able to move it against the 166 kilopascals of pressure pushing the hatch door closed.

The hatch door had a total surface area of roughly 1.5 square meters. And 166 kilopascals of pressure from the water times the 1.5 square meters of the door equaled 249,000 newtons of aquatic force shoving against the door. Let’s put that into relatable units; I choose to describe the force in units of Rachel. I personally am 160 pounds’ worth of human-being mass, comprised mostly of cake, which in metric units is 72 kilograms. Therefore, according to Isaac Newton, to calculate the force exerted by me on the Earth, my 72‑kilogram mass gets multiplied by the rate at which Earth’s gravity wants to accelerate me downward, which is 9.8 meters per second squared. Seventy-two multiplied by 9.8 is a total downward force of 711 newtons.

Therefore, I exert 711 newtons of force on the ground just by standing there, doing nothing productive, converting oxygen to carbon dioxide. The force on the hatch from the water was 249,000 newtons. If Bauer wanted to leave the submarine, he would have needed to be strong enough to lift the 350 Rachel Lances standing on the hatch door.

Bauer opened the seacock because he knew that he needed to equalize the pressure differential. If he could partially flood the submarine and bring the pressure inside up to 2.6 atmospheres, the total pressure difference across the hatch door would drop to zero. The door would swing open with ease, and all three submariners could swim to safety. More likely the door would have blown open violently as the buoyant air tried to escape and shoot to the surface, but either way, exit pathway achieved.

Talked down by Bauer and his mastery of the laws of pressure, Witt and Thomsen released their captain and allowed him to flood the sub. The increase in the partial pressure of the carbon dioxide was temporarily difficult to tolerate, leading to gagging and choking, but the submarine flooded quickly and the pressure was equalized. The trio got blown out through the liberated hatch door and rocketed safely to the surface like they were the “corks of champagne bottles,” as Bauer later put it.

Bauer, Witt, and Thomsen were the first three submariners ever to successfully escape a submarine. They did it in the year 1851, and they did it through a mastery of the scientific principles of the underwater world. The Brandtaucher was plucked out of its mud hole in the ocean and conserved. It is presently on display in a museum in Dresden, Germany, and is the oldest submarine ever recovered.

Not all of the submariners from that early generation learned the counterintuitive undersea physics required to execute a daring escape, however.

Horace Hunely submarine from the Civil War in 1863

A few years later, in the fall of 1863 and during the heat of the American Civil War, Confederate privateer Horace Hunley found himself clawing at the conning tower hatch of a small hand-cranked submarine that would soon be renamed in his memory. Three Rachels of force were pushing the small oval door closed, and Hunley did not think to equalize the pressure like Wilhelm Bauer did. Hunley was unable to bash his way to freedom, and the hatch door remained firmly sealed. All eight of the crew asphyxiated inside.

Starting around the early 1900s, submariners looking for a way out became less reliant on the savvy wits of a lone scientific hero among the crew. Instead, the more modern boats came fully equipped to let everyone out in a semi-organized fashion, through double sets of doors known as locks. Locks allow submariners to escape by first climbing through an inner door or hatch and sealing it tightly behind themselves. They then partially flood the small volume before the outer hatch will swing open, but this two-door system means they do not need to flood the entire boat. Once they swim out safely, the outer hatch is resealed against the ocean, the flooded volume of the small escape trunk is drained and opened, and a new set of escapees can climb in.

However, even with submarines designed to provide an easier exit, submariners of the early 1900s still faced the fundamental problem of being humans and not fish, and militaries everywhere began to design escape “lungs” to solve that problem. The lungs were devices that recycled an escapee’s breath, using a chemical reaction to remove carbon dioxide and adding more gas as needed. The lungs began to become standard, and were routinely stashed onboard submarines like the HMS Thetis.

HMS Thetis British sub from 1939

In June 1939, a thick slathering of sticky grease covered the nearly naked bodies of British naval officers Captain Harry Oram and Lieutenant Frederick Woods, who stood in front of their crew wearing only trousers. The layer of grease was supposed to provide them with some insulation against the frigid waters of Liverpool Bay, where their submarine had sunk during a sea trial conducted amidst the first rumblings of the war to come with Hitler. They were ready to show their compatriots on the downed submarine HMS Thetis that the Davis escape lungs they were putting on, which were untested in a real sunken submarine scenario, did indeed work as promised. The two men applied their nose clips and began breathing out of their mouths and directly into the square airtight bags strapped against their chests. They climbed into the lock of the flooded and crippled submarine, past the inner door. By this time all of the crewmen were nearly debilitated from the excruciating effects of their own exhaled carbon dioxide, but Oram had a plan to save them, which he had written down and tied around his wrist in case he ended up floating dead on the salty waves above.

The inner door was sealed behind Oram and Woods, imposing a robust metal barrier between them and their crew, and confining them in the small cylindrical volume of the escape trunk. With the inner door sealed, the outer hatch, held shut by the ocean, was all that remained between them and freedom. Water began to flood the trunk. The pressure began to equalize. Their Davis lungs recycled their breathing gas as planned, giving them new oxygen and removing their carbon dioxide even when submerged underwater. Once the pressure equilibrated to zero across the outer hatch door, the men pushed it open and swam the remaining 20 vertical feet between them, sunlight, and safety.

Momsen lungs

4 lucky sailors made it out of the HMS Thetis. However, a failure of the outer hatch door meant that 99 more would die inside. A few years later, nine Americans would execute a similar getaway when they used Momsen lungs, the American parallel to the Davis lung, to escape the downed USS Tang in the Pacific Ocean off the coast of China.

The Momsen and Davis lungs provided the crucial gas supply that allowed escapees to become fish, to let them rocket for the surface and freedom without the need to grow their own gills, even for those who did not know how to swim. The design worked. But the grease was a rudimentary plan at best, and the hypothermia that later destroyed the escaped crew of the Soviet submarine K-278 Komsomolets in 1989 emphasized that the ocean still had ways to win. The Komsomolets crewmen were able to climb out of their vessel while it floated on the surface of the Barents Sea before sinking, but they died waiting for a rescue that did not find them quickly enough.

Modern-day submariners are equipped with full-body waterproof suits called SEIE suits, an acronym that stands for Submarine Escape Immersion Equipment and is pronounced “sigh.”

SEIE suit submarine

They are brightly colored fabric pods that look a bit like inflatable Minion costumes, except orange. To break free from a downed vessel, each sailor dons one and waits patiently inside the escape trunk with a partner, staring at the rising water and each other through the clear plastic panels in the front that provide viewports out of the poofy heads of the suits. The hatches of the locks are controlled by someone else now—too many times has the panicked, premature release of one hatch door rendered the entire lock useless—and when the outer hatch opens, the inexorable, extraordinary positive buoyancy of the fully inflated suits rockets the escapees forcefully toward the sky. The submariners pop up two at a time, and each suit unfurls its own personal flotation raft, also bright orange, until from above they look like a smattering of neon orange sprinkles bobbing placidly across the surface of the ocean. At least in theory, assuming the submariners weren’t blocked from getting to a hatch by the mangled wreckage of their sub’s new, more twisted form, and they weren’t incapacitated by rising levels of CO2.

Pearl Harbor, Hawaii (Oct. 12, 2004) – A Sailor assigned to the Los Angeles-class attack submarine USS Key West (SSN 722), receives training with the MK10 Submarine Escape Immersion Equipment (SEIE). Several submarines have already installed the new system including Key West, one of 17 attack submarines homeported in Pearl Harbor. This suit allows survivors to escape a disabled submarine at depths down to 600 feet, at a rate of eight or more men per hour. The SEIE is designed to enable a free ascent from a stricken submarine and to provide protection for the submariner on reaching the surface until rescued. The assembly is comprised of a submarine escape and immersion suit, an inner thermal liner and a gas inflated single seat life raft. U.S. Navy photo by Chief Journalist David Rush (RELEASED)

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