experts from the University of Poitiers explained from a scientific point of view, the phenomenon of “dead water”. We are talking about the phenomenon known to seamen since ancient times: ships, going at full speed, suddenly slow down or even stop completely, in spite of the normal operation of the engines. The results are presented in the journal PNAS.
the braking Effect or “dead water” for centuries have been attributed to supernatural power. Braking occurs there and then, if the sea water is strongly stratified (i.e. different temperature and density of water at different depths). Because warm water has a lower density and lower viscosity, then the circulation takes place at the surface heated layer and thicker and more viscous in cold water they are not mixed. The movement of the ship causes the mixing of waters of different densities – the result is resistance. In addition, the waves at the interface of salt and fresh water forcing the ship to change speed. Thus, two phenomena affect the deceleration of the vessel, but in different ways: one ever slows down, the other creates a fluctuating resistance.
by the Way, stretches of water where the ship refused to go, Fridtjof Nansen, a scientist, polar Explorer, called “dead water” in 1893: in the Arctic ocean, his ship in some areas, as if gripped by an unseen force. Apparently, a victim of the same braking could become the fleet of Cleopatra in the battle near the Greek promontory of Actium in 31 BC: powerful ships of Cleopatra were defeated by light ships of the Roman enemy Octavian. On the fatal outcome of the battle, experts say, could affect “dead water”.
the secret of the “dead water” in 1904 was trying to solve the Swedish oceanographer vagn Ekman Walfrid. In laboratory tests he found that this effect always occurs when sea water is strongly stratified. A moving ship generates waves at the boundary layers, and they slow down his movement.
But it was only a partial explanation. Experiments showed why the braking effect is sometimes oscillatory, and what is happening in the boundary layer of water. The team of Johan Fordring investigated this puzzle by experiments and simulation.
experimentally, scientists have found that, if the ship is where the water is strongly stratified, it is influenced by two effects. The first is the kinematic wave-like effect of inhibition, which was observed Nansen: the resistance arises from the mixing of waters with different densities. “Under the ship wave is formed-brake if the ship is moving almost the same speed as the waves themselves. In this case, reduced inertia, and the vehicle is difficult to move, he seemed to be attached to the wave,” explains suth phenomenon of scientists.
the Second effect is dynamic, caused by nonlinear internal waves: the speed change is due to waves that act as a conveyor belt. “Another internal wave formed on the nose, says Furdrine. – When acceleration of the vehicle is formed in a wave – like front depressions and the crests of the waves reminiscent of a bumpy track, that the ship will move forward and backward. This wave is under the ship causes fluctuations in speed as the conveyor intermittently moving”.
However, the researchers clarify, the first effect is a wave – like braking – almost always occur in stratified water. The second, known as the wave effect Eckmann, especially noticeable in harbours, fjords and other limited areas of the sea: the wider becomes the surface of the water, the weaker the length of the reflected wave.
the speed of the ship also plays a role: if his speed much more than the underwater vibrations, the effect of Ekman’s practically invisible, the researchers say.
Thus, the mystery of the “dead water”, which for centuries led the sailors to the confusion, there was a scientific explanation.
