Damn the Torpedoes?

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Back when I was a kid, it was typical to see movies about World War II on TV. Lauding the brave fighting kids that America sent to stop the Nazis and the Japanese. A few of those movies included submarines, depth charges, and torpedoes.

Later on, during the height of the cold war, there were a slew of movies about submarines.

I guess I should tell you that I have never had the desire to enter a submarine. Being in a confined space under the water just sends chills up my spine. (I was in a submarine once- for an hour that felt like a year.) But, watching the movies was fine.

Except when there were torpedoes and depth charges. And the problem is that the movies really don’t tell the true story.

Rachel Lance (this was her PhD research at Duke), and Drs. B. Capehart, O. Kadro, and C.R. Bass of Duke University discerned what happens underwater when a nearby explosion occurs.  We know when an explosion occurs above ground, all sorts of shock waves emanate from the explosion, and they have enough force to toss desks, people, and such around like they were weightless.

Somehow, I had assumed that an explosion in water would be more attenuated. After all, water is far more dense than air and far more viscous, so there should be significantly greater resistance to the blast force. Except that assumption is dead wrong!

Oh, sure, injury from flying objects is not really an issue underwater at moderate distances from the blast, but the blast force propagates long and strong underwater. And, while the US Navy had long held that injury was possible when the pressure wave reached 500 psi, the first thing this research group determined  was that a force half that (1800 kPa or about 265 psi) provides a 50% risk of fatal injuries. Even a 20 kg blast provides a 20% risk of pulmonary injury, when the explosion is a kilometer away from a person.

H L Hunley recovered from Charleston Harbor

Given this research, it wouldn’t surprise you at all that the team was called in to determine why the H.L. Hunley- the confederate submarine that vanished after attacking (and destroying) it’s Union prey (the Housatonic) with a torpedo (filled with black powder) on 17 February 1864 in Charleston harbor– succumbed. After all, when it was finally found in 2000 and brought to the surface,  every crew member was seemingly fine, situated at their battle stations- but dead. The 40 foot ship was totally intact. Air hatches were  closed. Bilge pumps were set in the off position.  Why did no one even attempt to escape the ship as it sank to the bottom of the harbor?

Crew positions in H L Hunley submarine

As the team (now comprised of Drs. Rachel Lance , Lucas Stalcup, Brad Wojtylak, and Cameron R. Bass) reported in PLOSOne,  it was the submarines own weapon that did the ship and its crew in.

You see, when a torpedo attacks something underwater and explodes, a series of pressure waves travel in the water – and through the body of anyone who happens to be nearby. And, those waves could have enough pressure to cause blood vessels in the brain to explode (hemorrhage) and the lungs to deflate. (This is one of the premises behind the development of a neutron bomb.)

Moreover, this was 1864.  it wasn’t the torpedo we see in movies that shoots out of a submarine.  No, it was attached to the submarine with a long pole!  So, the submarine was pretty close to the explosion.

H L Hunley and Torpedo
H L Hunle with its torpedo

Lance et. al. found that when a bomb exploded and hit the underside of the hull- the waves were deflected. But– a secondary pressure wave formed within the interior of the ship and bounced around. So, the researchers then examined what happened when black powder (the explosive used in the torpedo that sank the Housatonic) was used at the same presumed distance. That caused the keel of the Hunley (a replica, of course) to be subject to 1100 psi of pressure (about what one would experience when the water depth is 2400 feet). And, the pressure within the submarine rose to 28 psi or 2 atmospheres- in a split second.

That rapid spike in air pressure within the submarine meant that there was an 85% chance the crew would succumb to pulmonary problems. But, it’s just as likely that the pressure spike didn’t kill them outright; instead, it could have knocked them out. And, with no conscious body on board to steer or maneuver the submarine, it simply sank.

Now, THAT’S the rest of the story.Roy A. Ackerman, Ph.D., E.A.

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