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 accumulations of energy can travel in threes—a phenomenon called "the three sisters"—and are so huge that they can be tracked by radar. There are cases of the three sisters crossing the Atlantic Ocean and starting to shoal along the loo-fathom curve off the coast of France. One hundred fathoms is six hundred feet, which means that freak waves are breaking over the continental shelf as if it were a shoreline sandbar. Most people don't survive encounters with such waves, and so firsthand accounts are hard to come by, but they do exist. An Englishwoman named Beryl Smeeton was rounding Cape Horn with her husband in the 1960s when she saw a shoaling wave behind her that stretched away in a straight line as far as she could see. "The whole horizon was blotted out by a huge grey wall," she writes in her journal. "It had no curling crest, just a thin white line along the whole length, and its face was unlike the sloping face of a normal wave. This was a wall of water with a completely vertical face, down which ran white ripples, like a waterfall."