How Did Hunley’s Crew Die?

Hunley 1

Raised from its resting place off Charleston, South Carolina, in 2000, the Confederate submarine H.L. Hunley is still revealing secrets about the fate of its crew on the night of its sinking in 1864. (Photo by John L. Williams)

By Dr. Ken Nahshon, Jamie Cruce, Michael Miraglia, and Dr. Paul Hess

On 17 February 1864, the Confederate submarine H.L. Hunley attacked USS Housatonic, a Federal sloop-of war participating in the blockade of Charleston, South Carolina. The explosion resulting from the Hunley’s
torpedo sank the 1,240-ton ship in a matter of minutes, securing Hunley’s place in history as the first submarine to sink an enemy combatant. Although the attack on Housatonic was successful, Hunley was lost at sea due to unknown circumstances with no survivors. Though various theories about the cause of Hunley’s loss have existed for some time, the sequence of events during and after the attack remains a mystery.

In 1995, marine archaeologists sponsored by author Clive Cussler located Hunley’s wreck off the coast
of Charleston approximately 1,000 feet from the wreck of Housatonic. Five years later, Hunley was raised
from the sea bottom and moved to a specially prepared tank facility at the Warren Lasch Conservation Center
(WLCC), located at the Charleston Navy Yard. Once there, a team of archaeologists and conservators from Clemson University began working on studying and preserving the submarine.

WHAT HAPPENED?

Motivated by recent archaeological findings made at the WLCC, engineers in the Naval Surface Warfare Center Carderock Division’s Survivability and Weapons Effects Department hope to shed light on what may have happened to Hunley and its crew using the Navy’s most advanced modeling and simulation software and computational capabilities.

Recently, archaeologists at the WLCC uncovered a long wooden pole of a spar torpedo weapon system. It had been previously reported that Hunley used a line-operated torpedo system—one that was operated from a distance using a line to set off its explosive charge. In contrast, Civil War-era spar torpedoes usually consisted of an explosive charge fastened to a fixed-length spar used either in contact or proximity to the target vessel. Thus, Hunley would have been separated from the explosive charge only by the spar’s length, generating a far more
severe loading environment than that from a line-operated system. The Confederacy’s largest spar torpedo,
Singer’s Torpedo, consisted of 135 pounds of black powder and a spar length of approximately 16 feet, along
with a contact fuse. In this current study, the use of Singer’s Torpedo is assumed; while it is possible a different design was utilized, it is likely the largest available spar torpedo would have been selected.

THE TEAM AND THE TOOLS

Realizing the significance of this finding, researchers at the WLCC, together with Dr. Robert Neyland, head of the Underwater Archaeology Branch at the Navy History and Heritage Command, contacted Carderock for assistance in interpreting the implications of this finding on Hunley. Fortunately, Carderock’s Survivability and Weapons Effects Division—which performs analyses, testing, and vulnerability assessments of underwater and
air-delivered threats on Navy ships, Marine Corps vehicles, and other structures—possesses the necessary
computational capabilities to evaluate Hunley’s attack on Housatonic using modeling and simulation.

With financial support from both the Office of Naval Research (ONR) and the Naval Surface Warfare Center,
Carderock engineers began applying a newly developed high-fidelity modeling and simulation tool, Navy
Enhanced Sierra Mechanics (NESM). This tool, developed jointly by Sandia National Labs and Carderock,
consists of a structural simulation finite element code, Sierra Mechanics, fully coupled to a computational
fluid dynamics shock-physics code for underwater explosions, DYSMAS/FD, developed by the Naval Surface
Warfare Center Indian Head Division. Using NESM, the fully coupled interactions between explosive
products, water, and the responding structure can be captured. These features are critical to obtaining
the correct response of a floating or submerged structure to an underwater explosion event.

To perform numerical analysis of a ship, submarine, or other platform in NESM, an appropriate numerical description, in the form of a finite element model (FEM), is  required. The FEM consists of a numerical description that includes both geometrical and material  properties. Fortunately, archeologists at the WLCC were able to provide the necessary details to  develop the FEM including photos, drawings, and geometrical point-cloud scans of Hunley generated using both  structured light and laser scan techniques. The scans provided the submarine’s exact shape and dimensions and were used to generate an FEM of Hunley.

In addition to the FEM, the project developed a numerical description of the loading generated by a
Singer’s Torpedo. In contrast to modern mines or torpedoes filled with high explosives, the Singer’s Torpedo was filled with black powder, a propellant. Unlike high explosives, propellants do not readily detonate, meaning the conversion of explosive to reaction products occurs on a relatively slow timescale. In addition, black powder is known to burn, or deflagrate, in a way highly dependent on pressure and the size of powder grains. To capture the  appropriate physical phenomena, Carderock engineers developed a suitable burn model using a  gas-injection feature originally developed to capture the behavior of underwater air guns.

With a model to capture the loading implemented and an FEM ready to be exercised, Carderock  engineers began their analysis of the response of Hunley and its crew to the torpedo explosion using NESM on a supercomputer,  Kilrain, located at the Navy’s Department of Defense Supercomputing  Resource Center at Stennis Space Center, Mississippi.

Hunley 2

 

This simulation of the explosion that rocked USS Housatonic shows the contours of pressure indicating the elevated pressure regions in white (left) and a view of the bubble created by the explosion, in dark blue, at its maximum size (right).

ANALYSIS

Initial analysis results indicate the presence of a long-duration, elevated pressure loading near the explosive charge. This is a direct result of black powders’ slow-burning nature. In contrast to a high explosive, however, the observed pressures were found to be modest and result in a steady heaving motion  of Hunley. Simulations indicated that the hull would not exhibit structural damage. This finding is consistent with what is being found during archaelogical excavations but not intuitive given Hunley’s proximity to the explosion.

In contrast, the bubble resulting from the explosion’s reaction products was found to be in direct and sustained contact  with Housatonic’s hull, providing a long-duration, high- pressure loading that would be more than capable of rupturing the ship’s hull. Interestingly, the standoff of the torpedo’s spar was just long enough to prevent direct bubble loading on Hunley.

Despite the apparent lack of hull damage to Hunley, these heaving motions may have injured or incapacitated the submarine’s crew, caused failure on seals and other openings resulting in rapid flooding, or resulted in an unrecoverable trim state. It is important  to note that no apparent evidence suggesting an escape attempt by the crew has been found—all crew member remains were found in their battle stations, all hatches were in a closed configuration, and all detachable ballast weights were found to be attached.

Current analysis efforts are focused on evaluating the potential for crew injury, particularly blunt trauma.
To capture the crew response to the explosion and resulting motions, an FEM of an automotive anthropomorphic test device, commonly known as a “crash- dummy,” is being used. The device is close in size to the average Hunley crew member as estimated by the discovered human remains.

In addition to Carderock’s effort, a separate ONR-funded  effort being performed  by Dr. Matthew Collette of the University of Michigan Department of Marine Engineering and Naval Architecture is examining the weights and stability of Hunley’s design, as well as paths in which the boat may have sank to its final resting place.
This effort already has found that even a small inflow of water or an unstable trim state resulting from the heaving motions during the attack could have resulted in Hunley’s sinking.

Once the current analysis efforts are completed, Carderock engineers should be able to help  uncover the mystery of why Hunley sank. In addition, the continued  development of modeling and  simulation capabilities to perform advanced analyses such as those described above will facilitate an ever-increasing ability to design against or evaluate future threats to the Navy.

About the Authors:

Dr. Nahshon, Jamie Cruce, and Michael Miraglia serve in the Hull Response and Protection Branch at the Naval Surface Warfare Center Carderock Division. Dr. Hess is the Ship Systems and Engineering program manager at
the Office of Naval Research.

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