![]() Ifthe layers of water are mixed by wind, however, the effect is lessenedand will not reoccur until the water has been calm for several days,allowing fresh and salt waters to stratify. In some places, such as the Norwegian fjords,where freshwater on the surface rests above very dense salt water a fewfeet below, the dead-water phenomenon is particularly noticeable. Whenportions of a vessel's hull are traveling in water of different densities,the disturbance creates submarine waves that bound off the interface ofthe two densities of water and create a zone of turbulence thatincreases resistance. Dead water occurs only where there are layers ofwater of very different density, as when fresh or brackish water rests ontop of salt water, or where warm water is on top of cold water. The phenomenon of sticky or dead water can be explained byunderwater waves. Another was to shorten the towrope as much as possible toallow the tug's screw to mix the water around the towed vessel. The simplest was to stop for afew minutes, allowing the stern waves to pass, then proceed at fullspeed. When tugboats with a vessel in tow encountered dead water theydiscovered that several tactics worked. For centuries, attempts to break free of dead waterincluded having the entire crew run repeatedly forward and aft alongthe deck, firing guns into the water, scooping quantities of seawaterover the deck, pouring oil on the water ahead of the vessel, dragging ahawser from bow to stern beneath the ship's hull, working the rudderrapidly back and forth, and slashing and beating the water beside theship with oars and other tools. Ancient accounts blamed remora, thosesucker-lipped fish so often seen attached to the sides of sharks andother large fish. Mariners of old commonly believed that the phenomenon of "deadwater" was caused by a "crust" of freshwater sticking to the vessel,slowing it the way it would be slowed if the entire hull was coated witha thick layer of barnacles. Sailing vessels sometimes becomeunmanageable, refusing to respond to the tiller, and motorized vesselssometimes lose speed so abruptly they stop making progress and areoccasionally stranded. When a slow-moving ship enters the mouth of a large river,estuary, or fjord, it sometimes encounters mysterious resistance thatcan stop it dead in the water. Tip the bottle and small internal waveswill travel lazily back and forth across the boundary where the oil andvinegar meet. You can generate them yourself with oil andvinegar combined in a bottle. They've been measured over 300 feet high,with wavelengths of more than a thousand feet, and with periodslasting several minutes. They can be generated byearthquakes and underwater volcanic eruptions, or can result fromtides or other deep currents. So-called "internal" or "submarine" waves also occur deepunderwater when masses of water of different temperature or densitystrike forcefully against one another. Slight adjustments of body angle allow them to surfaceperiodically for air without losing velocity, then to return to theirposition on the front slope of the wave. Dolphins learn to tilt themselves at a slope that matches the slope ofthose submarine waves, so that, in effect, they're on a perpetualdownhill ride. The pressure of water being pushed ahead of a shipcreates a constant underwater wave shaped much like a surface wave. The Bird in the Waterfall: Waves UnderwaterÄ olphins riding effortlessly beneath the bow of a moving ship areapplying the same principles used by humans who surf the breakers offHonolulu, with an important difference: The wave they are catching isan underwater one. ![]()
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