In 1893, Norwegian explorer Fridtjof Nansen described what must have been a frightening phenomenon under the circumstances. His ship became trapped in the Nordenskiöld Archipelago off Siberia, barely able to move, yet its engines appeared to be working normally and he wasn't confronted by any known force. It's taken 127 years, but physicists think they finally have an answer to how this phenomenon operates. It's also possible a rare appearance changed the course of history.
Nansen was no doubt not the first to be trapped in the manner he described, but he did provide a detailed description that was recorded and believed. The phenomenon where a boat can make no headway without apparent reason, now known as dead-water, has been experienced many times since.
From the start, Nansen recognized such entrapment occurred when a layer of fresh water sat above more salty water, something common in fjords, particularly when cold meltwater from glaciers drain into the sea. However, he did not understand why water layers created more drag than a well-mixed fluid.
Progress was made towards an explanation in 1904, when Swedish physicist Vagn Walfrid Ekman reproduced it in the laboratory, showing subsurface waves could occur at the point where the salty and fresh water met. If these waves counteract the force provided by the ship's propeller, they can bring it to a near complete halt.
In Proceedings of the National Academy of Sciences, a team at the Universite de Poitiers argue there are actually two phenomena that can cause dead-water: kinematic drag and dynamic resistance. They name the former after Nansen (Nansen-wave making drag) and the latter after Ekman (Ekman wave-making drag).
The authors used a rope to tow a model ship in a stratified canal and filmed the wake it produced with high-resolution cameras. They report the Ekman wave-making drag is amplified in enclosed spaces, leading past researchers (who also used narrow canals) to focus on it. However, in more open waters the Nansen-wave making drag quickly becomes dominant. While acknowledging more work is required to fully understand the nature of the Nansen drag, the paper claims “that we have solved a more than 1-century mystery with respect to the true nature of the dead-water effect.”
Since its discovery, scientists and mariners have observed dead-water can occur wherever two layers of water with different densities fail to mix. One suitable location is the Bay of Actium, Greece, where Mark Antony and Cleopatra's combined forces fought Octavian for control of the Roman world. Shakespeare, never the most reliable historian, attributed the outcome to a loss of nerve on Cleopatra's part, but the researchers think it is possible her ships got caught in dead-water, rendering her unable to come to Antony's aid.