One of the classic film images of sea battles is that of the torpedo slamming into the side of side of a ship, setting off a huge fiery explosion.
One of the classic film images of sea battles is that of the torpedo slamming into the side of side of a ship, setting off a huge fiery explosion. That might have been true 60 years ago, but modern torpedos do not kill their targets by "hitting" them.
The American Mk48 is a heavyweight among torpedos. It first saw service in 1972, and latest hot model is called the Mk48 ADCAP (Advanced Capability). It travels at highway speeds, and has a range of around 40 km when doing 100 km/h, or 50 km at 75 km/h. It weighs around 1600 kg (depending on the version) of which 267 kg are high-tech explosive, and which have the punch of half-a-tonne of TNT.
The Mk48 can be guided by a wire that reels out behind it, or it can find the target by itself. Once it gets close enough to the target, it first uses sonar to aim for the centre of the ship. When it's really close, it uses the magnetic signature of the target as a trigger to explode, when it's about 15 metres directly under its hull. The depth and location are quite critical. The 267 kg of high explosive almost instantaneously all turn into a huge volume of gas.
First, the actual explosion generates a very high pressure shock wave. This rams into the middle of the underside of hull of the ship at about 1.5 kilometres per second.
Second, the shock wave crushes the underside of the hull, and also lifts it up. It bends the ship bend upward in the middle, like a banana. The upper decks of the ship crack apart. After a few hundredths of a second, the shock wave has come and gone. But within a few more fractions of a second, the expanding bubble of gas from the explosion then hits the underside of the hull. The bubble reaches a maximum size of about 18 metres across, and it maintains the massive upward force on the bottom of the hull, once the shock wave has passed. So the ship is bent upwards in the middle in two stages - from the shock wave and then the expanding gas.
Third, after about half-a-second, the bubble (thanks to some fancy physics) begins to shrink. The ship then "sags" in the middle, and begins to "banana" in the other direction. This breaks the hull of the ship even more. Navy people call this sagging the "whipping" phase. It's actually very "useful" in breaking the back of a ship - after all, if you want to break a stick, it's much more effective to bend it back-and-forth, rather than bend it in only one direction.
Fourth, after about one second, the shrinking bubble has reached its minimum size, and begins to expand again. The water pressure around it is greatest directly underneath (being further from the surface) and least at the top (being closest to the surface). So it tends to expand upwards more than downwards - in fact, it pushes a lot of water upwards as a high-speed wall of water. This "bananas" the ship back in the first direction again.
Finally, the wall of water and the enormous bubble ram right through the hull. Combined, they can be powerful enough to completely rip the superstructure (that is, everything above the hull) clean off the ship, giving the appearance of a second explosion. The hull is often snapped into two separate halves.
On June 14, 1999, the submarine HMAS Farncomb fired its wire-guided Mk 48 torpedo at the decommissioned warship, the Torrens. The two "explosions" happened 1.3 seconds apart - two hits for the price of one. The plume of water and ship fragments shot 150 metres into the air. The Torrens split into two halves, and sank.
But it would all be very different if the torpedo slammed into the hull of the ship, and then exploded.
A ship in battle mode makes all its separate compartments watertight by shutting all the doors. The damage would be immense in a few compartments, which would get flooded. But the ship would probably keep on floating, with a severe list to one side. The remaining crew could be evacuated, and the ship could even possibly be repaired.
So while a missile kills a ship by going "Bang" on the body of the ship, a modern torpedo kills a ship by going "Bang" underneath it.
Please note that there is no red flame in any of this in real life. But there will be in a Hollywood movie, because they can't help themselves, and will add it in for extra.
Published 01 September 2005
© 2016 Karl S. Kruszelnicki Pty Ltd