What causes the ships to slow down or completely stop despite the engine being properly operational? This phenomenon was reported for the first time in 1893 and the experimental description was presented in 1904, without completely exploring the mystery behind “dead water”.
In 1893, the Norwegian explorer, Fridtjof Nansen, on his sail to north Siberia experienced that the waves were exerting a mysterious kind of force against his ship, considerably slowing down its speed. Swedish oceanographer, Vagn Walfrid Ekman, described experimentally in the laboratory that the interaction between the ship’s bottom and the waves produced at the interface of saline and fresh water layers, forming the upper part of the Arctic Ocean, create a drag force which causes the ship to slow down. This is commonly referred to as “dead-water” and is observed whenever water of varying density levels (due to temperature and salt content) mix.
This was attributed to two phenomena, the Nansen wave-making drag which results in continuous reduced speed and the Ekman wave-making drag which characterizes the back and forth motion of the ship. However, the mystery behind these two drags was not yet solved.
In a recent study published in PNAS on July 6, 2020, researchers from the CNRS and University of Poitiers unveiled the enigma behind such events. The team of researchers which included physicists, fluid mechanics experts, and mathematicians classified diverse internal waves mathematically and thoroughly analyzed the images obtained by experiments at sub-pixel level.
Their work explained that the speed of ships sailing in dead-waste changes because of the waves which act as a surging conveyor belt on which the ships experience a back and forth motion. Reconciling observations by Nansen and Ekman, they revealed that Ekman wave-making drag is a short-lived phenomenon as the ship eventually escapes it and reaches the constant Nansen speed.
This research was conducted as a part of the large project based on exploring the mysteries behind the loss of Cleopatra’s fleet of ships on facing the Octavian’s weaker vessels, during the Battle of Actium (31 BC). The divulged mystery of dead water has presented another hypothesis for explaining this emphatic defeat which was previously attributed to suckerfish that might have gotten attached with the body of the ships.