Update: South Korean corvette was sunk by conventional torpedoes
caused the sinking. Mr. Yoon made the comment after inspecting the wreck of the ship. The rear half of the ship was salvaged on 15 April and the remaining half on Saturday.
Modern torpedo design
One of the questions surrounding the sinking of the Cheonan is why did explosion split the ship in half. These questions are the result of many Second World War movies depicting a torpedo attack, but in which the ship under attack does not split in half. Rather, Geoffrey Forden writes, most of these movies show a torpedo attack 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 fill with water, drowning to sailors who are trapped below.
The images these movie offer have little to do with the damage caused by modern torpedoes. The design of these torpedoes allow them to destroy a surfaced ship such as the Cheonan much more efficiently.
Before discussing modern torpedoes, we should consider the damage done by 200 to 400 kg of high explosive to the hull of the USS Cole. Suicide bombers put the high explosives essentially right beside the Cole’s hull — the same way a Second World War torpedo would do. The damage to the Cole was considerable, but it was “classical,” in the sense that it was a hole punched in the side of the ship.
Note, though, that the damage to the Cole was considerably greater below the water line, because the water increases the coupling between the explosively generated shockwave and the hull.
Forden offers a series of pictures showing the destruction of an Australian warship by a torpedo with 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 sequence shows that the explosion of comparable quantity of high explosives as was used against the Cole — but well below the ship’s keel — caused much more damage: the explosives which holed the Cole, split the Australian ship in half.
Near the start of this sequence we can 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. Forden writes that this region, caused by the interference between the upward-moving initial shockwave and its reflection from the surface, 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.
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,” Forden concludes. “Everything about the Cheonan’s sinking is consistent with either a torpedo or submerged mine blowing up beneath the ship’s keel.”
The Los Angeles Times’s John M. Glionna quotes Daniel Pinkston, a North Korean expert for the International Crisis Group think tank, to say that Washington officials who are following the Cheonan case told him they would be “absolutely astounded” if it turns out the ship was hit by a torpedo. “They’re giving more credibility to a mine,” Pinkston said. “There seems to be a hole in each of the scenarios being looked at. Some of it sounds like stuff from a James Bond film.”
