Dead-water

Internal waves: Dead-water

Dead-water is a nautical term referring to a drastic decrease of a ship’s speed compared to its normal speed that arises when traversing through water that is density-stratified. The decrease in speed was reported to be up to a factor 5, giving the ship the appearance of having been brought to a full stop. The fluid can be stratified vertically due to variations in temperature or salinity. The decrease in speed is due to the fact that the ship is generating internal gravity waves on the interface between two layers. This process of wave generation is, unwantingly, very efficient when the ship has a draught comparable to the depth of the upper layer and when energy intended for propulsion is lost to internal wave generation. This process has been carefully studied and explained by V.W. Ekman (1904), following earlier observations on the Barentsz Sea by F. Nansen. An example of the dead-water phenomenon is given in the accompanying movie. It shows six successive experiments, performed in identical circumstances in a two-meter long tank filled with a dyed fresh water layer, and a thicker, salt-stratified lower layer. A little boat is dragged across the channel by means of a small weight of a few grammes, attached to a tiny wire, that is guided with the aid of two paperclips. There is a remarkable variation in the time needed to cross the channel depending on the nature of the interfacial gravity waves that are generated. An AVI-movie (see below) showing a carriage paddling at same paddling-rate through homogeneous fluid [upper panel] and stratified fluid [lower panel] can be downloaded here (~ 40 MB)

Publications:
Maas, L.R.M. and Haren, J.J.M. van, (2006) Worden mooi-weer verdrinkingen door dood-water veroorzaakt? Manuscript ingediend ter publicatie bij Meteorologica.