The Bird in the Waterfall by Jerry Dennis and Glenn Wolff



W A V E S
Waves Underwater
Jerry Dennis & Glenn Wolff

Dolphins riding effortlessly beneath the bow of a moving ship are applying the same principles used by humans who surf the breakers off Honolulu, with an important difference: The wave they are catching is an underwater one. The pressure of water being pushed ahead of a ship creates a constant underwater wave shaped much like a surface wave. Dolphins learn to tilt themselves at a slope that matches the slope of those submarine waves, so that, in effect, they're on a perpetual downhill ride. Slight adjustments of body angle allow them to surface periodically for air without losing velocity, then to return to their position on the front slope of the wave.

So-called "internal" or "submarine" waves also occur deep underwater when masses of water of different temperature or density strike forcefully against one another. They can be generated by earthquakes and underwater volcanic eruptions, or can result from tides or other deep currents. They've been measured over 300 feet high, with wavelengths of more than a thousand feet, and with periods lasting several minutes. You can generate them yourself with oil and vinegar combined in a bottle. Tip the bottle and small internal waves will travel lazily back and forth across the boundary where the oil and vinegar meet.

When a slow-moving ship enters the mouth of a large river, estuary, or fjord, it sometimes encounters mysterious resistance that can stop it dead in the water. Sailing vessels sometimes become unmanageable, refusing to respond to the tiller, and motorized vessels sometimes lose speed so abruptly they stop making progress and are occasionally stranded.

Mariners of old commonly believed that the phenomenon of "dead water" 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 with a thick layer of barnacles. Ancient accounts blamed remora, those sucker-lipped fish so often seen attached to the sides of sharks and other large fish. For centuries, attempts to break free of dead water included having the entire crew run repeatedly forward and aft along the deck, firing guns into the water, scooping quantities of seawater over the deck, pouring oil on the water ahead of the vessel, dragging a hawser from bow to stern beneath the ship's hull, working the rudder rapidly back and forth, and slashing and beating the water beside the ship with oars and other tools.

When tugboats with a vessel in tow encountered dead water they discovered that several tactics worked. The simplest was to stop for a few minutes, allowing the stern waves to pass, then proceed at full speed. Another was to shorten the towrope as much as possible to allow the tug's screw to mix the water around the towed vessel. The phenomenon of sticky or dead water can be explained by underwater waves. Dead water occurs only where there are layers of water of very different density, as when fresh or brackish water rests on top of salt water, or where warm water is on top of cold water. When portions of a vessel's hull are traveling in water of different densities, the disturbance creates submarine waves that bound off the interface of the two densities of water and create a zone of turbulence that increases resistance. In some places, such as the Norwegian fjords, where freshwater on the surface rests above very dense salt water a few feet below, the dead-water phenomenon is particularly noticeable. If the layers of water are mixed by wind, however, the effect is lessened and will not reoccur until the water has been calm for several days, allowing fresh and salt waters to stratify. Sailors have learned that accelerating their vessels before they reach dead water until they exceed about four miles per hour, the speed of submarine waves, allows them to break free of even the stickiest water.

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