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In 1893, the browser, scientist and diplomat Norwegian Fridtjof Nansen , the first man to cross Greenland, he experienced something very strange when I was traveling around the archipelago Nordenskiöld, north of Siberia, towards the North Pole: his ship, the “Fram”, was slowed down by a mysterious invisible force. I could barely maneuver and much less even reach the normal speed. This was the first time that someone I watched this enigmatic phenomenon, that the Nansen named “dead water” , so powerful that it is even able to stop motor boats that work properly.
Fridtjof Nansen – Wikipedia
So enigmatic was what happened that no one found explanation until a decade later, when in 1904 the physicist and oceanographer, Swedish Vagn Walfrid Ekman showed in his laboratory how in that area of the Arctic ocean formed a wave of drag between the salt water and dense of the sea and the thin layer of fresh water melted from glaciers, generating resistance. However, this was not sufficient to unravel all the secrets of the phenomenon.
Now, an interdisciplinary team of the Centre national de la recherche scientifique (CNRS), the French equivalent of the CSIC, and the University of Poitiers has managed to explain it for the first time: the changes of speed in the ships caught in the dead water due to the waves acting as a conveyor belt rolling in the ships are moved forward and backward. And not only that. The researchers believe that this phenomenon could well be the culprit of the powerful ships of Cleopatra and mark Antony lose against the weakest of Octavio in the Battle of Accio (31. C.).
From the fjords to the Orinoco
The dead water occurs in all the seas and oceans where they mix waters of different densities (due to salinity or temperature). “Occurs in the fjords of scandinavia, of course, but also in the mouth of rivers like the Orinoco in South America, for example, due to stratification of the density (flow in rivers with sediment on the salt water of the sea)”, explains in an email to ABC Germain Rousseaux, a researcher at Poitiers and co-author of the study. “It is also found in cold lakes and mountain in summer because there is stratification of the temperature, and therefore, there is the risk that swimmers will drown “, he continues.
“oh, Captain, we’re stuck!”, bullet provided by the researchers – Morgane Parisi – www.StudioBrou.com
The dead water denotes two phenomena of drag observed by the scientists. The first, the drag of Nansen, causes a constant rate, abnormally low. The second, cited by Ekman, is characterized by oscillations of speed in the boat trapped.
But the cause of this occurrence was unknown. The new work, published in the journal PNAS, has used a classification mathematical of different internal waves and the analysis of experimental images at the scale of a subpixel. Has shown that these variations in speed are due to the generation of waves specific to act as a conveyor belt rolling over which the boat moves forward and backward. Scientists have also reconciled the observations of Nansen and Ekman. Have shown that the regime of oscillating Ekman is only temporary: the boat ends up escaping and reaches a constant speed of Nansen.
This “conveyor belt” can cripple a motor boat, but what size and for how long? “It’s a tricky question because it depends on several parameters: the density and depth of the upper layer, the lower layer, the depth of the boat, their way…,” says Rousseaux. “Nansen reported of several hours of delay , because each time the ship returned to sea, the dead water occurred as soon as the stratification has the good properties to trigger the generation of internal waves,” he adds. Yes, the researcher does not believe that the dead water is sufficient to cause a shipwreck.
The defeat of Cleopatra
Interestingly, this work is part of a larger project that investigates why, during the Battle of Accio the large ships of mark Antony and his ally Cleopatra had nothing to do against the weakest of Octavio. How could the gulf of Arta (or Accio), which has all the characteristics of a fjord, having caught the fleet of the Queen of Egypt in the waters dead? The scientists believe that now we have another hypothesis to explain this resounding defeat, which in ancient times was attributed to the ability of the legendary remora to stop the boats whose hulls are adhered to.
“This is a work in progress, but the gulf of Arta, where took place the battle takes a truly exceptional property, that is to say, it is the only fjord in the Mediterranean sea. The interior of the gulf is connected to the rivers, therefore, there is an important supply of fresh water,” says Rousseaux.
The scientists will address this issue in a later work. Up to now, ” we show that the ships of Antonio were not able to ram of Octavio because they felt a wall of resistance whose magnitude depends on the relationship between the draft and the depth of the water,” says the researcher.