dead water

Dead water is the nautical term for a phenomenon which can occur when there is strong vertical density stratification due to salinity or temperature or both. It is common where a layer of fresh or brackish water rests on top of denser salt water, without the two layers mixing. The phenomenon is frequently, but not exclusively, observed in fjords where glacier runoff flows into salt water without much mixing. The phenomenon is a result of energy producing internal waves that have an effect on the vessel. The effect can also be found at density boundaries between sub surface layers. In the better known surface phenomenon a ship traveling in a fresh water layer with a depth approximately equal to the vessel's draft will expend energy creating and maintaining internal waves between the layers. The vessel may be hard to maneuver or can even slow down almost to a standstill and "stick". An increase in speed by a few knots can overcome the effect. Experiments have shown the effect can be even more pronounced in the case of submersibles encountering such stratification at depth. The phenomenon, long considered sailor's yarns, was first described for science by Fridtjof Nansen, the Norwegian Arctic explorer. Nansen wrote the following from his ship Fram in August 1893 in the Nordenskiöld Archipelago near the Taymyr Peninsula: "When caught in dead water Fram appeared to be held back, as if by some mysterious force, and she did not always answer the helm. In calm weather, with a light cargo, Fram was capable of 6 to 7 knots. When in dead water she was unable to make 1.5 knots. We made loops in our course, turned sometimes right around, tried all sorts of antics to get clear of it, but to very little purpose." Nansen's experience led him to request physicist and meteorologist Vilhelm Bjerknes to study it scientifically. Bjerknes had his student, Vagn Walfrid Ekman, investigate. Ekman, who later described the effect now bearing his name as the Ekman spiral, demonstrated the effect of internal waves as being the cause of dead water.