Dead water is a phenomenon where a ship experiences resistance that severely slows its forward progress, despite the surface water appearing calm and the engine running normally. This effect, which can reduce a vessel’s speed to a fraction of its normal rate, is a form of hydrodynamic drag. The sensation for mariners is often described as feeling as if the ship is being held back by a hidden force. Dead water is not caused by surface waves or currents but is a direct result of energy dissipation occurring beneath the waterline. It is a consequence of navigating in a layered water environment.
Creating Density Layers
Dead water occurs when a body of water is distinctly stratified, separated into layers of different densities that do not easily mix. This stratification is typically due to a difference in salinity, known as a halocline. The phenomenon is common in coastal areas, fjords, and near large river mouths where freshwater flows out over the denser, saltier ocean water. Freshwater is significantly less dense than saltwater, causing it to float on top. This creates a sharp boundary layer, or interface, between the two water masses, termed a pycnocline. The stability of these layers prevents mixing. The upper layer of brackish water can be quite thin, sometimes only a few meters deep, which is the necessary environmental condition for the drag effect to manifest.
How Internal Waves Cause Drag
The slowdown experienced in dead water is caused by the generation of massive internal waves that form beneath the surface, along the boundary between the two water layers. These waves are distinct from the waves visible on the water’s surface. When a vessel moves through the upper, less dense layer, its hull creates a pressure disturbance that presses down on the pycnocline. This pressure pushes the lighter water down into the heavier water, creating a trough in the interface, which then oscillates back and forth. The ship’s forward momentum is effectively spent creating and maintaining a train of these large, slow-moving internal waves. The energy that would normally propel the ship is transferred from the vessel’s hull and propeller wash into the internal wave field. This transfer of energy results in a significant increase in drag, often called wave-making drag. Because the density difference between the two water masses is small, the internal waves move very slowly but carry a large amount of energy. The ship becomes trapped by its self-generated wave system, which acts like a brake. In extreme cases, the total resistance on the ship can increase by three to four times compared to moving through uniform water.
The First Scientific Observations
The dead water effect was a well-known, though unexplained, obstacle for mariners for centuries, primarily in the narrow, stratified waters of Scandinavian fjords. The first detailed scientific account came from the Norwegian explorer Fridtjof Nansen. During his 1893 Arctic expedition aboard the ship Fram, Nansen documented how his vessel was mysteriously slowed to less than 1.5 knots, even with the engine at full power, while navigating near the Siberian coast. Nansen correctly observed that this extreme drag only occurred in areas where a layer of fresh water rested on top of salt water. He described the ship being held back as if by a “mysterious force.” Following Nansen’s return, he requested that the phenomenon be scientifically investigated, which led to the work of Swedish physicist and oceanographer Vagn Walfrid Ekman. In 1904, Ekman successfully replicated the dead water effect in laboratory experiments using a towing tank with two layers of water density. Ekman’s experimental work formally proved that the increased drag was caused by the generation of internal waves at the density interface. He established the physical mechanism that converts the ship’s kinetic energy into the potential energy of these submerged waves.
Navigating Through Dead Water
Mariners who encounter dead water can employ several strategies to mitigate the severe drag and regain normal speed. The effectiveness of these methods depends on the precise stratification conditions and the ship’s characteristics. One common solution involves changing the vessel’s speed to break the internal wave pattern that has trapped the hull. Navigators may try to accelerate significantly to a speed high enough to pass through the critical velocity of the internal waves. Successfully exceeding this critical speed can allow the ship to “break free” from the drag-inducing wave system. Conversely, slowing down drastically can also help, as it minimizes the energy transferred to the internal waves and allows the vessel to slip out of the self-generated wake. Another practical adjustment is to change the ship’s trim, which alters the depth of the hull’s pressure point in the water column. By shifting cargo or ballast, a vessel can be made to ride higher or lower, potentially moving the main source of pressure away from the pycnocline. Navigators also try to steer the ship into deeper channels or towards areas less affected by river runoff, where the stratification layer is either deeper or has begun to mix, thus weakening the dead water effect.