The Perfect Storm

very highest waves measured anywhere in the world, ever.

Scientists understand how waves work, but not exactly how huge ones work. There are rogue waves out there, in other words, that seem to exceed the forces generating them. For all practical purposes, though, heights of waves are a function of how hard the wind blows, how long it blows for, and how much sea room there is—“speed, duration, and fetch,” as it’s known. Force 12 winds over Lake Michigan would generate wave heights of thirty-five feet after ten hours or so, but the waves couldn’t get any bigger than that because the fetch—the amount of open water—isn’t great enough. The waves have reached what is called a “fully developed sea state.” Every wind speed has a minimum fetch and duration to reach a fully developed sea state; waves driven by a Force 12 wind reach their full potential in three or four days. A gale blowing across a thousand miles of ocean for sixty hours would generate significant wave heights of ninety-seven feet; peak wave heights would be more than twice that. Waves that size have never been recorded, but they must be out there. It’s possible that they would destroy anything in a position to measure them.

All waves, no matter how huge, start as rough spots—cats’ paws—on the surface of the water. The cats’ paws are filled with diamond-shaped ripples, called capillary waves, that are weaker than the surface tension of water and die out as soon as the wind stops. They give the wind some purchase on an otherwise glassy sea, and at winds over six knots, actual waves start to build. The harder the wind blows, the bigger the waves get and the more wind they are able to “catch.” It’s a feedback loop that has wave height rising exponentially with wind speed.

Such waves are augmented by the wind but not dependent on it; were the wind to stop, the waves would continue to propagate by endlessly falling into the trough that precedes them. Such waves are called gravity waves, or swells; in cross-section they are symmetrical sine curves that undulate along the surface with almost no energy loss. A cork floating on the surface moves up and down but not laterally when a swell passes beneath it. The higher the swells, the farther apart the crests and the faster they move. Antarctic storms have generated swells that are half a mile or more between crests and travel thirty or forty miles an hour; they hit the Hawaiian islands as breakers forty feet high.

Unfortunately for mariners, the total amount of wave energy in a storm doesn’t rise linearly with wind speed, but to its fourth power. The seas generated by a forty-knot wind aren’t twice as violent as those from a twenty-knot wind, they’re seventeen times as violent. A ship’s crew watching the anemometer climb even ten knots could well be watching their death sentence. Moreover, high winds tend to shorten the distance between wave crests and steepen their faces. The waves are no longer symmetrical sine curves, they’re sharp peaks that rise farther above sea level than the troughs fall below it. If the height of the wave is more than one-seventh the distance between the crests—the “wavelength”—the waves become too steep to support themselves and start to break. In shallow water, waves break because the underwater turbulence drags on the bottom and slows the waves down, shortening the wavelength and changing the ratio of height to length. In open ocean the opposite happens: wind builds the waves up so fast that the distance between crests can’t keep up, and they collapse under their own mass. Now, instead of propagating with near-zero energy loss, the breaking wave is suddenly transporting a huge amount of water. It’s cashing in its chips, as it were, and converting all its potential and kinetic energy into water displacement.

A general rule of fluid dynamics holds that an object in the water tends to do whatever the water it replaces would have done. In the case of a boat in a breaking wave, the boat will effectively become part of the curl. It will either be flipped end over end or shoved backward and broken on. Instantaneous pressures of up to six tons per square foot have been measured in breaking waves. Breaking waves have lifted a 2,700-ton breakwater, en masse, and deposited it inside the harbor at Wick, Scotland. They have blasted open a steel door 195 feet above sea level at Unst Light in the Shetland Islands. They have heaved a half-ton boulder ninety-one feet into the air at Tillamook Rock, Oregon.

There is some evidence that average wave heights are slowly rising, and that freak waves of eighty or ninety feet are becoming more common. Wave heights off the coast of England have risen an average of 25 percent over the past couple of decades, which converts to a twenty-foot increase in the highest waves over the next half- century. One cause may be the tightening of environmental laws, which has reduced the amount of oil flushed into the oceans by oil tankers. Oil spreads across water in a film several molecules thick and inhibits the generation of capillary waves, which in turn prevent the wind from getting a “grip” on the sea. Plankton releases a chemical that has the same effect, and plankton levels in the North Atlantic have dropped dramatically. Another explanation is that the recent warming trend—some call it the greenhouse effect—has made storms more frequent and severe. Waves have destroyed docks and buildings in Newfoundland, for example, that haven’t been damaged for decades.

As a result, stresses on ships have been rising. The standard practice is to build ships to withstand what is called a twenty-five-year stress—the most violent condition the ship is likely to experience in twenty-five years. The wave that flooded the wheelhouse of the Queen Mary, ninety feet up, must have nearly exceeded her twenty-five-year stress. North Sea oil platforms are built to accommodate a 111-foot wave beneath their decks, which is calculated to be a one-hundred-year stress. Unfortunately, the twenty-five-year stress is just a statistical concept that offers no guarantee about what will happen next year, or next week. A ship could encounter several twenty-five-year waves in a month or never encounter any at all. Naval architects simply decide what level of stress she’s likely to encounter in her lifetime and then hope for the best. It’s economically and structurally impractical to construct every boat to hundred-year specifications.

Inevitably, then, ships encounter waves that exceed their stress rating. In the dry terminology of naval architecture, these are called “nonnegotiable waves.” Mariners call them “rogue waves” or “freak seas.” Typically they are very steep and have an equally steep trough in front of them—a “hole in the ocean” as some witnesses have described it. Ships cannot get their bows up fast enough, and the ensuing wave breaks their back. Maritime history is full of encounters with such waves. When Sir Ernest Shackleton was forced to cross the South Polar Sea in a twenty-two-foot open life boat, he saw a wave so big that he mistook its foaming crest for a moonlit cloud. He only had time to yell, “Hang on, boys, it’s got us!” before the wave broke over his boat. Miraculously, they didn’t sink. In February 1883, the 320-foot steamship Glamorgan was swept bow-to-stern by an enormous wave that ripped the wheelhouse right off the deck, taking all the ship’s officers with it. She later sank. In 1966, the 44,000-ton Michelangelo, an Italian steamship carrying 775 passengers, encountered a single massive wave in an otherwise unremarkable sea. Her bow fell into a trough and the wave stove in her bow, flooded her wheelhouse, and killed a crewman and two passengers. In 1976, the oil tanker Cretan Star radioed, “…vessel was struck by a huge wave that went over the deck…” and was never heard from again. The only sign of her fate was a four-mile oil slick off Bombay.

South Africa’s “wild coast,” between Durban and East London, is home to a disproportionate number of these monsters. The four-knot Agulhas Current runs along the continental shelf a few miles offshore and plays havoc with swells arriving from Antarctic gales. The current shortens their wavelengths, making the swells steeper and more dangerous, and bends them into the fastwater the way swells are bent along a beach. Wave energy gets concentrated in the center of the current and overwhelms ships that are there to catch a free ride. In 1973 the 12,000-ton cargo ship Bencruachan was cracked by an enormous wave off Durban and had to be towed into port, barely afloat. Several weeks later the 12,000-ton Neptune Sapphire broke in half on her maiden voyage after encountering a freak sea in the same area. The crew were hoisted off the stern section by helicopter. In 1974, the 132,000-ton Norwegian tanker Wilstar fell into a huge trough (“There was no sea in front of the ship, only a hole,” said one crew member) and then took an equally huge wave over her bow. The impact crumpled inch-thick steel plate like sheetmetal and twisted railroad-gauge I-beams into knots. The entire bow bulb was torn off.

The biggest rogue on record was during a Pacific gale in 1933, when the 478-foot Navy tanker Ramapo was on her way from Manila to San Diego. She encountered a massive low- pressure system that blew up to sixty-eight knots for a week straight and resulted in a fully developed sea that the Ramapo had no choice but to take on her stern. (Unlike today’s tankers, the Ramapo’s wheelhouse was slightly forward of amidships.) Early on the morning of February seventh, the watch officer glanced to stern and saw a freak wave rising up behind him that lined up perfectly with a crow’s nest above and behind the bridge. Simple geometry later showed the wave to be 112 feet high. Rogue waves such as that are thought to be several ordinary waves that happen to get “in step,” forming highly unstable piles of water. Others are waves that overlay long-distance swells from earlier storms. Such