What Doomed the Crew of the HL Hunley?

On February 17, 1864, the Confederate submarine H L Hunley became the first combat submarine to sink a warship when she snuck up on and rammed her spar torpedo into the hull of the screw sloop-of-war USS Housatonic outside Charleston Harbor.

Hunley promptly disappeared with all hands not to be rediscovered until 1995 and raised from the harbor bottom in 2000. (See ECW post “Making Naval History: The CSS Hunley.”)

The submarine’s remains are on display at the Warren Lasch Conservation Center in North Charleston, South Carolina, the subject of continuing research seeking to understand what happened to her. One of the mysteries—how the crew died—was addressed by a fascinating article in the March 2020 issue of Smithsonian magazine by Rachel Lance.[1] The article is from her new book on the subject published in April this year, which has received excellent reviews.

The author describes the dramatic events of Hunley’s attack. “As the black powder exploded, the copper casing ripped open, releasing the fearsome pressures of explosive black powder into the water and against the wooden hull of the Housatonic. A spray of shattered wood planks burst upward from the deck of the ship. The submarine had hit its target, punching a lethal blow through the boat’s underbelly. The force of the blast rippled through the entire ship, and even the sailors at the bow nearly 200 feet away instantly understood that their vessel would soon be on the ocean floor.” And so would Hunley.

In 2000, the recovered submarine rested in a huge, custom-built water tank as “archaeologists clad in protective coveralls and wearing respirators sorted patiently through the muck and silt that had slowly filled the hull of the submarine as it lay on the bottom of the ocean floor.”

They expected to find two officers and six crewmen horrifyingly bunched at the hatches attempting to escape or huddled together in the agony of drowning, suffocating, or asphyxiating—as seen in modern submarine sinkings. “The crew of the Hunley, however, looked quite different. Each man was still seated peacefully at his station.” This was against all human instinct.

Theories posited that Hunley sank with the crew inside either because the torpedo explosion breached the submarine’s hull, or a lucky shot from Housatonic struck, or she collided with another object. In all these circumstances, the men would not so calmly have accepted imminent death, but no one attempted to escape. “Something killed these men. Something that left no trace on the boat or their bones.”

As a PhD student in biomedical engineering at Duke University, Rachel Lance researched injury biomechanics—the various mechanisms by which human beings are injured and killed—with a focus on explosions. She also had been a civilian engineer for the U.S. Navy building underwater breathing systems. Most of their study cases were World War II sailors in the water gruesomely hit by subsurface explosions. Then she was asked to apply injury computer models to Hunley’s men.

A period schematic of the claustrophobic iron tube showed seven men sitting in a row hunched over the large crank that turned the propeller with the captain seated just forward, his head in the tiny conning tower. The explosive charge, 135 pounds of black powder in a copper container, was firmly affixed at the spar end 16 feet forward of the bow at a down angle of 45 degrees. “That is a lot of powder. It’s a lot of any kind of explosive.” The designers believed that the explosion recoil would push Hunley back out of danger.

Lance created a three-dimensional, volumetric computer model to calculate oxygen levels in the sealed submarine. It became apparent that the crew would have suffered increasingly difficult, panicky breathing and sharp headaches for 30 to 60 minutes before becoming incapacitated by lack of oxygen and increasing carbon dioxide.  “It was implausible that the crew would have stayed peaceful and quiet for this length of time during such symptoms.”  As causes of death, the theories of suffocation or asphyxiation were eliminated.

Similar symptoms had appeared in other circumstances. After the devastating firebombing of Dresden in World War II, victims in bomb shelters and basements were found to have died without moving, without signs of struggle, often still seated in their chairs. They were not outwardly wounded; they were not blown wildly across the room. The phenomenon is called “primary blast injury,” a “strange and horrifying fluke produced by the bizarre physics of an explosion. It is usually the result of a shock wave.”

“A shock wave is a particular kind of pressure wave, and it can have a terrible impact on certain human tissues.” Rapidly expanding explosive gasses ram forward a densely packed front of air molecules at speeds faster than sound. Although the wave pressure might not be high enough even to disturb the body’s position, the increase in pressure from zero to maximum is instantaneous.

Such waves move easily through dense water, and since the human body is mostly water, they do not affect most tissue. However, a labyrinth of air bubbles in the lungs slows the shock wave rapidly, redirects its kinetic energy, and shreds the lung tissue. Brain tissue can be similarly affected with no outward signs of trauma.

The author conducted a carefully calibrated experiment with a six-foot model dubbed the CSS Tiny in a pond with omnidirectional pressure gauges inside. The Bureau of Alcohol, Tobacco, and Firearms assisted in obtaining 283-gram black powder charges, also at 1/6 scale to the original, and conducting the tests. The first charge was attached to the bow of the model, the countdown commenced, and the button pushed.

“First, I saw the plume of the geyser of water. Then I felt the pier vibrate. Last of all, I heard the blast. [My assistant] yelled from shore that he could feel that charge through the ground…. There it was, the data from the pressure gauge tracking across the monitor of my computer. The squiggly neon green line—plotting pressure versus time—showed the jagged, erratic scream of bouncing waves trapped inside the hull of the boat. It had had sharp peaks, peaks with rapid rises—peaks that weren’t technically shock waves but still rose to maximum in under the two-millisecond rise speed that would hurt human beings.”

Blast after blast confirmed the findings. “And like the actual Hunley, the scale-model Tiny refused to show any damage itself, even after repeated blasts, even as it transmitted the pressures inside.” Although other explanations are possible, “analysis showed that the amount of pressure ricocheting around inside the metal tube, combined with the quick rise time of the wave, would have put each member of the Hunley’s crew at a 95 percent risk of immediate, severe pulmonary trauma. The kind that would leave them gasping for air, possibly coughing up blood.”

“Inside the submarine that night, they all had items in their pockets that spoke of their belief that they would go on living. The smokers brought their pipes. George Dixon, in his 20s with a head full of blond hair, brought his pocket watch. The watch broke at the time of the attack, locking the hands forever at 8:23 p.m. Dixon’s head dropped against the side of the hull. His ankles were lightly crossed, and one hand fell to his thigh, his body propped up by the hull wall and his small captain’s bench.”

Aboard Housatonic, the deck disintegrated; most crewmen ran for the bow amid a noxious cloud of smoke as the mighty ship heaved to port. Five sailors died. “The submarine drifted on the outgoing tide. With no one alive to operate the bilge pumps, eventually, it started to sink. Water rushed in, bringing the little boat to the sand but leaving an air space, inside of which, over the decades, stalactites would grow. The HL Hunley and its crew settled to a quiet grave 30 feet beneath the dark blue waves.”

From Rachel Lance, In the Waves: My Quest to Solve the Mystery of a Civil War Submarine (Dutton, 2020).

[1] Rachel Lance, “The New Explosive Theory About What Doomed the Crew Of The ‘Hunley:’ A blast-injury expert takes aim at the mystery of what sank the most famous—and lethal—submarine of the Civil War,” Smithsonian, March 2020. All quotes from this article.