Читать книгу Waves and Beaches - Kim McCoy - Страница 33

Eventually some of the great waves predicted by statistical methods encounter a ship. They may sink it, but if there are survivors they will describe with awe the collision with a rogue wave. A rogue wave is a great solitary wave whose crest towers above the rest and scares the living daylights out of the luckless mariners in its path. Sometimes these waves express themselves as extra-deep troughs—“holes in the sea” into which a ship can fall, to then be overwhelmed by the next crest. It is a matter of debate which is more dangerous, but doubtless these super-crests and super-troughs account for many of the ships that have simply disappeared at sea with all hands. In the past twenty years, hundreds of container ships and supertankers, some over 660 feet (200 m) have sunk; rogue waves and heavy weather have sunk many more ships in the distant past.

With decks awash, large waves torment a tanker in a storm. The contortions of the hull—creaks and shudders—are heard below the roaring of the winds by its trusting crew. On board the 1,140-foot (347-m) long Esso Languedoc off Durban, South Africa. The vertical masts are over 80 feet (25 m) tall. ESA/Philippe Lijour

A Cape Horn sailor, Captain William H. S. Jones, who made his observations from large sailing ships, once wrote: “It is strange but true that in high southern latitudes where seas 50 feet high and 2,000 feet long roll forward in endless procession, occasionally one sea of abnormal size will tower above the others, its approach visible for a considerable distance.” It is easy to be impressed by a wave that towers over “ordinary” 50-foot-high waves.

In late 1942 the 81,000-ton, 1,000-foot-long (300 m) passenger liner Queen Mary was serving as a World War II troopship. On one occasion, while loaded with 15,000 American soldiers bound for Glasgow, she encountered a winter gale 700 miles off the coast of Scotland. The seas seemed very large, even to observers on a ship whose deck is some 60 feet (20 m) above the still-water level. Suddenly “one freak mountainous wave” struck the ship broadside. An eyewitness reported that the Queen “listed until her upper decks were awash and those who had sailed in her since she first took to sea were convinced she would never right herself.” (Nearly all who have been to sea have been aboard a ship that rolled so far they were convinced she was going over.) But then, after hanging balanced on the brink of eternity for a few seconds that seemed very much longer, the Queen righted herself again.

On July 22, 1976, the tanker Cretan Star sailed from a Persian Gulf port loaded with 28,600 tons of light crude oil. On July 28, the master reported that the ship had encountered very heavy weather that had caused some damage and oil leakage. The last message received said, “vessel was struck by a huge wave that went over the deck and caused damage in the number 6 tanks—damage cannot be surveyed due to prevailing weather conditions.” The ship was not far from Bombay at the time, and on August 2, searching aircraft from that port reported a black oil slick 4 miles (6.5 km) long and 1.5 miles (2.5 km) wide. That was all. A subsequent inquiry noted that when the southwest monsoon blows it reaches its greatest strength in July off Bombay and periodically piles up “episodic waves of vast proportions.” The occurrence of rogue waves, sometimes called extreme waves (waves that are over twice the significant wave height) depends upon many factors including water depth and sea state. In Waves in Oceanic and Coastal Waters (2010) by Leo Holthuijsen, the statistics of deep-water and coastal waves are discussed in great detail. In a very general sense, one wave in roughly twenty-five is over twice the height of the average wave, one in 1,000 is over three times average height, and only one in 300,000 exceeds four times the average height. Although infrequent, if you encounter a rogue wave at sea, you will remember.

One part of the ocean is so well known for rogue waves that all mariners were warned in advance by the British Admiralty’s book of sailing directions known as the Africa Pilot, published in 1878, and still in print. This danger zone is off the “wild coast” of South Africa between Durban and East London. A few miles offshore the gentle slope of the continental shelf reaches 100 fathoms (180 m) and then drops precipitously away into deep water, thus creating a wall-like barrier against which the Agulhas Current presses hard. This massive flow of water moves southwest at high velocity—often at 4 knots (1.2 m/sec) and sometimes 6 knots. The strongest current is in the main shipping lane moving south—a lane chosen by the ship operators themselves.

When a gale blows from the southwest and raises waves that move directly against this current, the wave length is shortened and the wave steepness greatly increases. Even when there is no local storm, a large swell coming north from the Antarctic Ocean meets the south-flowing current head-on and creates impressive waves called “Cape Rollers” or “Agulhas Swell.” Under some circumstances the unusually swift current actually doubles the height of the waves.

The question is: why? There are several explanations. One reason is that approaching waves are refracted, or bent, toward the higher current velocity, especially at an abrupt change in velocity such as occurs along a steep continental boundary. This concentrates the wave energy over the strongest current and focuses certain waves there. In the special case of a ship moving with the current, the ship’s velocity causes an additional steepening of the wave (relative to the ship). Moreover, if the ship is moving at roughly 18 knots (9 m/sec) in one direction aided by a current of 8 knots (4 m/sec) and encounters an oncoming wave moving through the ocean at 19 knots (10 m/sec), the velocity of the collision is the sum of these, or 45 knots (23 m/sec). Because the force of impact is proportional to the square of the velocity, in this very special case the current nearly doubles that force. If the wave is twice as high as an ordinary storm wave, a ship is likely to be in trouble.

This phenomenon has long been the subject of scientific attention, and it was known for many years before Professor J. K. Mallory of the University of Cape Town called attention to the concentrated loss of ships. Early Portuguese caravels were lost in this area. The losses have continued; for example, the passenger liner SS Waratah with 211 persons aboard that vanished there in 1909. (Off the Cape Hatteras coast of the United States, jets of the Gulf Stream seem to have caused the loss of ships in a similar manner.)

Smaller vessels are too small to span between wave crests. The larger waves heave and rotate the entire vessel like a cork. Grand Banks of Newfoundland. Philip Stephen/NPL/Minden Pictures

In 1973, the Ben Cruachan, a new British cargo vessel of 12,000 tons, was moving southwest off the coast of Durban, South Africa, when suddenly the bow of the ship was seen to extend out over a hole in the sea. A large, closely following crest then smashed down on the bow with such force that it broke the ship’s back. Similar incidents with equally bad results have been reported by the ships Neptune Sapphire (12,000 tons), World Glory (45,000 tons), and Wafra (70,000 tons). The supertanker World Horizon (102,000 tons) broke up in heavy swell off of South Africa, and ships of more than 200,000 tons have also been sunk. Rogue wave sinkings are not mainstream news.

Ships traveling this part of the South African coast, offshore another 20 miles (32 km) or so, would be well outside the part of the current that caused the damaging waves. However, the temptation of a free ride on a current of 4 knots or more, with its substantial savings in time and fuel, is too great. Generally, the largest ships are the hardest hit. Because of its great mass, a supertanker responds to waves more like a seawall than a floating object and tends to resist rigidly instead of giving way as a smaller ship would do. Occasionally, though, a small vessel encountering a rogue wave makes it into the news. Sebastian Junger’s book The Perfect Storm immortalized the loss of the fishing boat Andrea Gail (only 92 tons) off the coast of Nova Scotia in October of 1991.