Prepared by the U. S. Navy
August 28, 2017
The Navy’s involvement with HUNLEY’s recovery began with the discovery of the wreck and continues through project oversight and involvement of staff at the Naval Heritage and History Command (NHHC). In 2013, the Hull Response and Protection Branch at the Naval Surface Warfare Center Carderock Division (NSWCCD) began an analysis of the underwater explosion of HUNLEY’s torpedo on the loss of the vessel. This work, motivated by the discovery of a fixed spar torpedo, has been performed in close collaboration with the conservation team at Clemson University’s Warren Lasch Conservation Center (WLCC) as well as NHHC.
The effort has been funded by the Office of Naval Research (ONR) and internal NSWCCD research funds with technical collaboration of the University of Michigan, Naval Surface Warfare Center Indian Head EOD Technical Division (NSWCIHEODTD) and Aberdeen Test Center (ATC). The findings of this research have provided substantial insight into the sequence of events during the attack as well as HUNLEY’s operations and all aspects of its naval architecture. A key accomplishment has been success in performing advanced numerical simulations of the attack utilizing a validated model of the black powder explosion. This model has been validated by a carefully conducted testing program performing full scale testing of a modern replica of HUNLEY’s torpedo.
The study’s efforts with regards to the torpedo explosion have been documented as follows:
Peer-reviewed conference paper presented at the Society of Naval Architects and 2015 Marine Engineers (SNAME) World Maritime Conference entitled “Investigating the loss of HL HUNLEY.” Article is awaiting publication.
Science Meets History: Incident Analysis of H. L. Hunley
Conference presentations at the Shock and Vibration Symposium, the premier conference for shock and weapons effects on ships, submarines, and other structures.
Early efforts were featured in ONR Future Force Magazine: "How Did Hunley's Crew Die?"
An internal Navy report providing full details of the Navy’s black powder testing report was published in 2016: Harris et al., “Characterization of the underwater explosion output of black powder,” Naval Surface Warfare Center Indian Head EOD Technical Division, Report IHTR-3505 (June, 2016)
Below are a number of technical issues that have been examined or considered in the Navy’s thorough and scientific black powder testing performed to determine if HUNLEY’s crew was killed by their own weapon.
1. Blunt Trauma and Accelerative Injuries Are the Documented Causes of Injury or Death Resulting from Underwater Shock Events. The most common crew injuries resulting from dynamic underwater explosion shock events have been on "deck-slap" and blunt trauma (see Hirsch, "Man's response to shock motions", Report 1797, David Taylor Model Basin, 1964 for the statistics of injury during 16 separate contact charge attacks). Such injuries have also been the focus of ONR funding through the Human Injury and Treatment (HIT) program. As a result of this prior experience, the Navy’s research into HUNLEY’s crew incapacitation has been focused on using crash dummies to evaluate the influence of accelerative loading. It has been concluded that, in the case of HUNLEY, crew death from the explosion is unlikely due to the relatively low motions that are imparted to the crew as a result of the pressure pulse characteristics of black powder as compared to more modern military-grade explosives. This conclusion is consistent with the events surrounding the semisubmersible torpedo boat CSS DAVID. CSS DAVID, which utilized a similar explosive charge to HUNLEY, attacked the USS NEW IRONSIDES with no crew injuries resulting from the explosion.
In contrast, crew injury or death from elevated internal pressures resulting from underwater explosion attacks where the structure remains intact is not known among the Navy weapons effects community. The lack of such injuries or deaths is emphasized by the lack of systematic hearing loss among crews experiencing such attacks as hearing loss occurs at far lower levels of severity than internal organ injury. Given the multiple occurrences of significantly more severe explosive attacks on submarines during World War II, a systematic loss of hearing among submariners would be telling. In contrast, hearing damage is endemic to gun crews as a result of repeated exposure to overpressures.
2. The Additional Effects of Internal Pressures Necessary to Cause Death Do Not Appear in HUNLEY. Research into real-world attack scenarios has revealed that explosions capable of generating sufficient levels of internal pressure to cause death would also be severe enough to result in hull damage or rupture. The work performed by Johnson (Johnson DL, Yelverton JT, Hicks W, Doyal R. Blast overpressure studies with animals and man: Biological response to complex blast waves. Albuquerque, NM: EG and G Inc; 1993), indicates that sheep exposed to internal blast pressure pulses experienced “Moderate to Lethal” injuries only at a slow-rising long-duration blast pressures above 32 psi or over 2 atmospheres. A similar pressure level would have undoubtedly caused tremendous damage to HUNLEY’s internal structure and bulkheads as well as failure of riveted seams and various penetrations. However, no such damage has been recorded by the conservators at WLCC. Furthermore, failure of penetrations would have resulted in HUNLEY rapidly taking on the 50-75 gallons of water needed to overcome neutral buoyancy and sinking the boat in close proximity to USS HOUSATONIC instead of 1000 feet away where the wreck was found.
3. HUNLEY Reacts Similarly to Modern Submarines. Navy research into the possibility that structural differences between HUNLEY and a more modern ship or submarine would result in fundamentally different outcomes with regards to internal air shocks from an external underwater explosion have been discounted. There is no evidence to suggest that more modern plating and stiffener arrangements would generate substantially lower levels of internal air pressure than HUNLEY’s ring stiffened cylindrical hull. The generation of an internal air shock can occur as the result of motion of the outer hull plating and the severity of this shock depends on the acceleration which is a function of the density and thickness of the plating as well as the presence of structural supports.
4. Pressure and Impulse Characteristics of Black Powder Underwater Explosions. Important factors to consider in any discussion about blast effects are peak pressure, shape of the pressure-time curve, and area under this curve or impulse. Black powder, unlike TNT and other similar high explosives, does not generate an instantaneous pressure rise or shock wave. The Navy’s recent testing of the underwater explosion output of black powder, performed as part of the investigation into HUNLEY, revealed pressure traces seen below:
It is clear that as compared to TNT, a common reference explosive, the pressure levels, rise time, and decay characteristics are entirely different. When impinging on a structure, the long duration low amplitude pressure wave applies a low level of loading for a long duration. Unlike a shock loading of equal impulse, the black powder pressure pulse causes lower accelerations and thus provides substantial time for the structure to deform. This precludes propagating a severe pressure wave internally. Furthermore, consideration of the fraction of impulse delivered during the time over which the structure responds and reaches its maximum amplitude is required when evaluating structural response. Biggs (Introduction to Structural Dynamics, 1964) gives a convenient rule of thumb that total delivered impulse is only an adequate indicator of peak response if the total duration of impulse delivery is less than about 1/10th of the response period of the structure. For longer duration loadings the peak response must consider the full details of the pulse shape.
5. TNT Equivalency of Black Powder. In comparing explosives, TNT equivalence value is often used to establish relative behavior. However, the application of this concept to black powder can be problematic. If black powder produced a similar waveform to TNT, the comparison between quantity of TNT and black powder would be reasonable. However, black powder provides an entirely different waveform as seen above as a result of its deflagrating rather than detonating reaction. Statements that black powder detonates are incorrect except when additional stimulus is provided. Thus, equivalence for TNT required careful interpretation when referring to black powder and extreme caution must be used in interpreting results. This is observed in the wide range of values provided by Napadensky (Napadensky HS, Swatosh JJ Jr. TNT equivalency of black powder, Volume 1: Management summary and technical discussion. Chicago, IL: IIT Research Institute, 1972): Equivalencies for Black Powder ranged from zero to 43 percent for impulse and zero to 24 percent for pressure. It is important to note that these equivalency results are primarily for black powder charges that are tested in air, heavily confined, and that most tests were boostered with additional high explosives. The Navy’s underwater testing of black powder explosions concluded that the peak pressures of HUNLEY’s torpedo explosion were only 2-3 percent of the peak pressure from a similar quantity of TNT. Furthermore, it was concluded in IHTR-3505 that as a result of the very low peak pressure for the black powder, comparisons (to TNT) in the context of peak pressure and energy are of limited value. In contrast to findings regarding the pressure wave, black powder was found to be effective at generating an oscillating gas bubble that eventually would have collapsed with a powerful force onto USS HOUSATONIC’s hull.
6. Pressure Outside and Inside the Boat. Another important factor for consideration is the rate of rise of elevated pressure inside the hull vs the rate of rise of elevated pressure outside the hull. The elevated pressure inside the hull occurs due to structural response and is of entirely different character from the external pressure environment. Specifically, the rise time of this internal hull pressure is slower and the peak pressure of substantially lower magnitude than the incident wave external to the hull. The Navy’s testing of a full-scale reproduction of HUNLEY’s torpedo shows this rise occurred over approximately 2 milliseconds. Any consideration of the pressure environment on crew injury requires careful examination of the loading timeline and its interaction with the hull’s natural response frequencies as the susceptibility to injury depend on these loading details.
7. Medical Issues. The DoD medical community has known for decades that the blast response of an animal or human organ can be analyzed in a similar manner to any other mechanical system. Therefore, the response to an elevated pressure from a blast is strongly influenced by the character of the pressure wave that is experienced, namely the rise time, peak value, and decay time of the pressure wave. For example, Chiffelle ("Pathology of blast injury", Defense Atomic Support Agency, 1966) notes regarding this topic: The degree of blast injury could be directly correlated with the rate, magnitude, and character of the pressure rise and fall; the duration of the pulse; and the size and species of animal involved. Similarly, Richmond and White ("A tentative estimate of man's tolerance to overpressures from air blast", The Symposium on Effectiveness Analysis Techniques for Non-Nuclear Warheads Against Surface Targets, U. S. Naval Weapons Laboratory Dahlgren, Virginia, October 30, 1962) notes: Also to be emphasized is the statement that man's tolerance to air-blast overpressures having other than classical wave forms can be expected to be quite different; i.e. an animal's tolerance to smooth-rising overpressures or those rising in steps sufficiently separated in time is higher than is the case for ideal or near-ideal pressure pulses. Bowen et al. (“Biophysical mechanisms and scaling procedures applicable in assessing responses of the thorax energized by air-blast overpressures or by non-penetrating”, Annals of the National Academy of Sciences, V. 152, Article 1, 1968) concludes that the response time of a 70kg mammal is 2.1 milliseconds and that only the impulse delivered during this period of response should be considered with regards to injury assessment. For a typical shock wave, this time period accounts for much of if not all the impulse. However, for black powder the majority of impulse is delivered after this time period. Thus, it is critical to distinguish between the effects of black powder used by HUNLEY and those of high explosive— the use of injury criteria based on sharp rising pressure waves is not correct when compared to the slower rising pressure wave generated by a black powder explosion.
8. The Influence of Charge Casing and Confinement. HUNLEY’s torpedo was lightly cased according to historical records and submerged in 8 feet of water. Water provides a fixed level of confinement independent of the case material as a result of both hydrostatic pressure and inertial restraint during the dynamic case expansion process resulting from the explosion. The Navy’s black powder testing concluded that using either thicker PVC or thin sheet aluminum casings did not affect the explosive output. Regarding the influence of black powder burn rates, a direct examination of underwater explosion data on large hexagonal grains that exhibit a slow burn rate (Hilliar, H W, “Experiments carried out on the pressure wave thrown out by submarine explosions”, Admiralty Experimental Station, Essex, UK, 1919) demonstrates the strong effect of powder details on the emanating pressure wave. In fact, the Navy’s earliest analysis efforts utilized a numerical model for which pressure generation was explicitly a function of black powder burn rate (see Sasse, R, “A comprehensive review of black powder”, BRL Technical Report BRL-TR-2630, Aberdeen Proving Ground, MD, 1985 for details on this subject). Further complicating research into the matter is that it’s difficult to extrapolate small-scale explosive data to a full scale event because it is well known that such phenomena do not scale well for the propellant-type explosives such as black powder.
9. Scaling. The scaling of explosive phenomena can be successfully accomplished. However, great care must be taken in scaling loading and structural scales. Furthermore, deflagrating reactions do not scale as the reaction rate has a non-linear dependence on pressure. As such, larger charges exhibit faster reaction rates. Thus, the Navy chose to do full scale testing of HUNLEY’s torpedo. Numerical modeling was accomplished using a highly accurate numerical model of the submarine based on direct measurements and geospatial laser and structured light scans. This is a necessary step in capturing the correct structural response. Unfortunately, it is extremely difficult to construct an accurate scale replica since the materials become unworkably thin— a 1/8th inch plate at 1/6th scale is only 0.02 inches thick. Neglecting ring frames and other internal structure will result in entirely different frequency response that alters the interaction between the incident underwater explosion and the structure.
10. Reasonable Assumptions Regarding Actions of Union Forces. When HUNLEY finally came to rest it did so about 1,000 feet from USS HOUSATONIC, a distance which requires that HUNLEY remained floating for some time after the attack. The crew of USS CANANDAIGUA, which arrived shortly after the attack, as well as Sailors aboard HOUSATONIC would surely have been on the lookout for the cause of the explosion. The Navy’s study has concluded that HUNLEY propelled itself away from USS HOUSATONIC either on the surface or while submerged and that HUNLEY’s demise likely occurred in the minutes or hours after the attack, not as a direct result of the torpedo explosion.