Article body copy
Norwegian mariners called it dødvann—dead water. They’d known for centuries that patches of seawater in narrow fjords could mysteriously sap a ship’s speed, drastically slowing it or stopping it altogether. In his 1897 book, Farthest North, explorer Fridtjof Nansen wrote of his encounter with dead water north of Siberia in 1893: “We could hardly get on at all for the dead-water, and we swept the whole sea along with us.” Dead water, Nansen noted, occurred “where a layer of fresh water rests upon the salt water of the sea,” as happens in northern fjords when snow and ice from mountains melt into the ocean.
Nansen’s report of dead water was investigated by scientists at the time, including the Swedish oceanographer Vagn Walfrid Ekman. In 1904, Ekman published research that showed dead water was caused by hidden waves in a dense subsurface layer of salt water that slowed the forward motion of a ship. Today’s speedy ships easily overcome these submerged waves, and for most mariners dead water is now largely forgotten.
But more than 100 years later, scientists are still exploring the phenomenon, and a new investigation has uncovered more details about its underlying mechanics.
In France, physicist Germain Rousseaux and his colleagues at the National Centre for Scientific Research re-created the laboratory experiments conducted by Ekman using modern techniques. They also investigated a different oceanographic phenomenon that Ekman first described, in which a ship repeatedly slows down before surging forward—an oscillation they’ve dubbed the “Ekman effect.”
In their experiments, the scientists dragged scale models of boats at different speeds through tanks containing fresh water lying on top of salt water, assessing how the boats’ passage affected the surface and underwater layers. They found that the two different phenomena—the drag of dead water described by Nansen, and the surges in speed described by Ekman—are both caused by waves in the hidden saltwater layer. In particular, the Ekman effect occurs when the subsurface waves bounce off the sides of a laboratory tank and create an oscillation in the surface speed of a vessel. Rousseaux says the Ekman effect could make it difficult for slower ships trying to navigate narrow passages, such as fjords and sea locks.
The new study also suggests other consequences of dead water. Leo Maas, a physical oceanographer at the Netherland’s Utrecht University, who was not involved in the research, says that although the effects of these subsurface waves are negligible for most motor-driven boats today, they could still affect swimmers, since their hands often interact with deeper layers of water. He points out that stratification in salt and fresh water can result from differences in water temperature, particularly in hot and calm weather. “This effect might play a role in tragic fair-weather drownings,” he says.
Though not a threat to ships today, Rousseaux and his colleagues think the effects of dead water and other related phenomena may have been much more significant in the past. In fact, they may have even changed history.
At the Battle of Actium in 31 BCE, warships commanded by Octavian, Julius Caesar’s adopted son, defeated a larger fleet led by Roman general Mark Antony and Egyptian queen Cleopatra. The battle led to Antony and Cleopatra’s defeat and subsequent deaths by suicide, and to Octavian becoming Augustus Caesar, the first emperor of Rome. Ancient legends blamed the loss on remoras—suckerfish that were said to have attached themselves to the hulls of the Egyptian ships, preventing them from reaching ramming speed.
But Rousseaux notes the battle took place in a shallow passage between the Ionian Sea and the Ambracian Gulf—an almost entirely enclosed sea, and one of the few places in the Mediterranean where the water can be stratified by salinity. Rousseaux thinks the physical hindrance of dead water, rather than the fish, might have slowed the Egyptian fleet.
The team now hopes to study whether the combination of shallow and potentially dead water impeded the large Egyptian warships, propelled the smaller Roman warships to victory, or both.