The phenomenon of "dead water": where did Cleopatra's fleet disappear?

The phenomenon of "dead water": where did Cleopatra's fleet disappear?
The phenomenon of "dead water": where did Cleopatra's fleet disappear?
Anonim

The previously inexplicable braking and shaking of fully operational ships in the so-called "dead water" finally received a scientific explanation.

When the ship enters dead waters, the journey is paused. In the best case, a vessel with fully operational engines will slow down, in the worst case it will stop. A tailwind can help sailors, but even with full sails, the ship will move slower than it should.

The phenomenon of dead water was first noticed by the Norwegian researcher Fridtjof Nansen in 1883. Going to the north of Siberia, the traveler found himself in a zone where his ship slowed down so much that it became difficult for him to control. Nansen did not quickly pick up the necessary speed, and did not understand what had happened.

In 1904, the Swedish physicist and oceanographer Wagn Walfried Ekman described a similar phenomenon. In his laboratory, the scientist set up an experiment with water of various salinity, as in that part of the Arctic Ocean, where Nansen had "stalled" earlier. Ekman discovered that mechanical waves are formed at the interface between the layers. When the bottom of the ship interacts with these waves, they create additional resistance.

After Ekman's discovery, scientists realized that the phenomenon of dead water is caused by different densities of liquid layers. Differences in density can occur due to different salinity or water temperatures. But in any case, the captain of the ship has only two options. He can watch with vexation how the ship is dragging at a constant abnormally low speed, which was once felt by Nansen; or to stand on the bridge and sway after the ship, experiencing the abrupt excitement discovered in the laboratory by Ekman.

Understanding the cause and types of the phenomenon of dead water, scientists did not know the mechanism of capturing ships in wave captivity. It was only recently that physicists, fluid mechanics and mathematicians from the CNRS Institute of Natural Sciences and the Laboratory of Mathematics and Applied Sciences of the University of Poitiers first described this mysterious phenomenon. A press release for the study is available on the CNRS website.

The team of scientists classified the waves that occur when layers of liquid of different density come into contact with each other, and then simulated the movement of the ship along the waves mathematically described earlier. Simulations have shown that the dead water effect occurs when waves form something like a conveyor belt. Along this "tape", the ship is barely noticeable moving forward or backward, which looks like a slowdown from the side.

The experiment also showed that there are no fundamental differences between the phenomena observed by Nansen in 1883 and Ekman in 1904. Ekman's oscillations gradually damp, and the ship begins to move slowly and at a constant speed.

The work of scientists immediately gave rise to a new hypothesis on one of the oldest mysteries of mankind. It is still unknown why during the Battle of Actium (31 BC) Cleopatra's powerful ships were killed when they collided with Octavian's weak fleet. If we assume that the bay of Aktia, where the battle took place, was filled with dead waters, it becomes clear why the power of Cleopatra's ships did not help the ruler. Friction is inversely proportional to speed: the more you drag on a resisting surface, the more it resists. This means that the weak ships of Octavian in the dead waters could be more maneuverable and faster than the powerful fleet of the Queen of Egypt.

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