A GENTLE nudge at the right time could stop a flooded ship from capsizing. So say researchers whose computer simulations show how the chaotic movements of a flooded ship can be controlled.
Once a ship takes on water it can capsize in seconds if the water is free to slosh from one side of the vessel to the other. The ship quickly becomes unstable, and the movement of the sea makes things worse. The waves outside the ship create a resonance effect with the water inside, which causes the ship to roll dramatically. This movement often becomes chaotic and that spells disaster for the vessel.
Kunihiko Mitsubori of the Japan Coast Guard in Hiroshima and Kazuyuki Aihara at the University of Tokyo created computer simulations of flooded vessels to see if they could control the chaotic motion, and hence prevent a ship from capsizing. Although purely theoretical at the moment, their idea may one day help prevent disasters like the sinking of the ferry Estonia. The Estonia sank in the Baltic Sea in 1994 after its bow door broke loose, allowing seawater to flood the car deck. The vessel sank within 30 minutes with a loss of more than 800 lives.
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After the Estonia disaster, stricter regulations were put in place. All roll-on roll-off ferries operated by north-west European countries at least must now be able to cope with up to 50 centimetres of water on the car deck. One way to achieve this is to build in partitions that stop the water sloshing about.
In principle, Mitsubori and Aihara鈥檚 idea could deal with a lot more water, and an alternative to partitions on the car deck would also please ferry operators. Dividing up the car deck dramatically cuts its capacity and the revenue it can earn.
The solution proposed by the Japanese modellers takes advantage of the fact that small nudges to a chaotic system can have huge effects. Although these effects are complex, they are not random. Chaotic motion is largely predictable over short periods of time, which is why meteorologists can forecast the chaotic effects of the weather tomorrow but not next month.
The team鈥檚 idea is that gently nudging a chaotically moving ship in just the right way should bring its future motion entirely under control. This might buy enough time to evacuate a ship before it sinks (Proceedings of the Royal Society A, DOI: 10.1098/rspa.2002.0982).
Exactly how to nudge the ship has yet to be decided. But the group has already tested a method in which a large weight is moved along rails from one side of the ship to the other in a way that reduces roll. Mitsubori says a similar device could be used to bring chaotic motion under control too. But they admit that getting the method to work in practice won鈥檛 be easy.
Steve Bishop, an expert in the nonlinear motion of ships at University College London, isn鈥檛 convinced the extra time bought by Mitsubori and Aihara鈥檚 system would help. Flooded vessels can heel over and capsize in seconds, even in relatively calm conditions, he says. 鈥淲ould this be enough time to bring the motion under control?鈥�
