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The two halves of the ROK Ship Cheonan are raised from the sea near the ROK-owned island of Baegryeong-do, which in turn is very close to the DPRK’s mainland. The Cheonan was sunk on 27 March 2010. (Note, I have “flipped” the image on the right to better match up with the front half.)

One of the “mysteries” surrounding the sinking of the ROK’s warship, Cheonan, is that the explosion split the ship in half, a result our popular culture has trained us to forget. After all, World War II movies always show a torpedo strike in the same way: one or two white streaks quickly approaching the ship followed by a localized jet of water where the torpedo struck the hull. Sailors stream out of their bunks to jump over the side as the ship keels over, taking in water. Below the water line, jagged holes, punched by the explosive force of the warhead, let in sea water. Compartments quickly (or if dramatic effect is needed, slowly) fill with water, drowning all the bit players, uh, sailors trapped below.

These movies have influenced our expectations for the damage caused by modern torpedoes even though there are much more efficient ways for a torpedo to destroy a surface ship. Consider the damage done by 200 to 400 kg of high explosive to the hull of the USS Cole. This high explosive was put essentially right beside the Cole’s hull the same way a “classical” torpedo would place it. The damage is considerable, but it is “classical,” in the sense that it is a hole punched in the side. (The damage is considerably greater below the water line, as can be seen by the increased “dent,” because the water increases the coupling between the explosively generated shockwave and the hull.


The hole punched in the side of the USS Cole by between 200 to 400 kg of high explosives in close contact with the hull. Note how the damage is considerably more wide spread below the water line. The terrorist attack on 12 October 2000 caused the tragic loss of 17 crew members and the injury of 39 others but did not sink the Cole.

Significantly more damage can be cause by the same, or even smaller, explosive detonated significantly below the keel of a warship. The destruction of the warship pictured below was caused by the explosion of 44 295 kg of high explosive well below its keel, between a quarter and an eighth of comparable to the explosive that holed the Cole but causing much more damage. Near the start of this sequence, which begins in the upper left corner and then moving right and then down, you see a “spray dome” forming on either side of the ship. This is caused by the initial shockwave of the explosion breaking the water’s surface. Caused by the interference between the upward-moving initial shockwave and its reflection from the surface, it is a region where the water density has been considerably decreased. Considerable damage is caused by this shockwave hitting the ship, as indicated by the plume of black smoke the boiler emitted after being violently shaken when the shockwave was transmitted through the hull. The hull could have been significantly damaged by that same shockwave.


This series of images show the destruction of an Australian warship by a torpedo with 44 295 kg of high explosive detonated well below its keel. The plume rising above the ship’s superstructure is caused by the collapse of a large gas bubble sucking sea water upward in a powerful jet.

The second major effect damaging the hull, and probably the one that caused the vessel to break in half, was a jet of water blasting its way through the ship. This jet was formed as the gas bubble created by the initial explosion collapsed upon reaching the ship’s hull.

This is the way modern torpedoes sink ships. Everything about the Cheonan’s sinking is consistent with either a torpedo or submerged mine blowing up beneath the ship’s keel.

<— A jet of water caused by a bubble bursting at the surface. It is formed as the pressure inside the bubble is suddenly released as it bursts, causing the water at the bottom of the bubble to rush in.

Update The day after I posted this, the South Korean government reached a similar conclusion.

Update (30 April 2010) (thanks to Josh Pollack for pointing this article out!) South Korean scientists at the Sound Engineering Research Lab of Soongsil University have performed an analysis of the acoustic signals (the media report mistakenly calls them seismic waves) associated with the sinking of the Cheonan. It would be very nice to see their actual analysis—and the data would be even cooler—but it appears on the face of it to be a very interesting result. Their main conclusion, again based solely on the media report of their findings, seems to be that the Cheonan was actually struck by a heavy torpedo. (The say the most likely candidate is the Chinese Yu-3 heavy torpedo with a 205 kg high explosive warhead.) According the article, the South Korean scientists believe the torpedo struck the hull of the Cheonan but since the explosive is significantly aft of the torpedo’s bow, the center of the explosion was a little over 2 meters away. They rule out the “kill mechanism” being a bubble jet.

Some of the other statements in the media report are open to more interpretation; again I wish I could see the original report. Apparently, they can hear secondary explosions through out the ship after the initial torpedo detonation. The media report seems to hint that these were caused by the fire associated with torpedo detonation. However, secondary explosions could just as easily—more easily in my opinion—be caused by the heaving of the ship in response to either the shockwave or the rush of water associated with the bubble formation and possible the jet that I think still might have formed depending on how strong the force of attraction due to the hull was to the bubble.

By the way, this standoff distance of a little over 2 meters probably represents the results of a careful analysis to determine the best distance at which to explode a large charge from a ship’s hull. It presumably balances all the effects we have discussed. I recommend people read John Field’s comments in the comments section.