More on big waves

A breaking wave (also known as a rogue, a giant, a freak, an extreme wave, or a superwave) is an enormous wall of water that collapses and falls with great force because the wave is too high for its forward speed. The wave accelerates and spills— with lots of air and foam—into a wide band of white water. Or the wave plunges, with solid water projected forward in an arc.

If the wave is small, the detached crest doesn't amount to much. If the wave is large—on the order of 30 to 35 feet or higher—the avalanching crest may contain tons of water that can violently engage a 1,000-foot ship or a 50-foot yacht. The associated troughs, or "holes in the sea," can be lethal to a vessel.

The appearance of these waves is almost the same as those colossal waves that break on Hawaiian beaches and attract big-time surfers and marine photographers. As I mentioned earlier, the difference is that the waves near the beach tumble into white water because the waves' energy is concentrated in a shallower water column. This increases the energy density, and meanwhile, by the dispersion relation, the waves' velocity decreases as the water shoals. This causes the waves to compress, peak up, and collapse.

If a breaking wave at sea gets mixed up with a strong opposing ocean current, the wave's behavior is exaggerated even more. Or said another way, current against wind means that higher waves appear more frequently. These effects are well known in the northeast-flowing Japanese Kuroshio Current, the North Atlantic Gulf Stream, and the Agulhas Current that runs south along the east coast of South Africa, plus a hundred other places. Even at the mouth of your local river, the waves can heap up more frequently (and reach higher than you'd expect from the shoaling bar) when there's a strong onshore wind meeting the river current and an ebb tide.

There are ways to work around these problems, and the point of this book is to discuss them in detail. In the upcoming chapters we'll see how the yacht Banjo—trying to lie a-hull in dangerous surface conditions—mixed it up with a breaking wave and a deep trough and came out second best. There's the Winston Churchill, a handsome 55-foot wooden cutter that entered the 1998 Sydney-Hobart race, hopefully to win. She was knocked apart when she fell off a huge breaking wave and into a cavernous trough off the southeast corner of Australia. The boat sank at once, and only six of her nine-man crew were rescued.6

I've seen a videotape taken from a U.S. Coast Guard helicopter hovering above a large yacht that's lying sideways to a breaking wave. As the film clip proceeds, it shows the boat falling into a trough and making a sickening crunch that was loud enough (even above the noise of the helicopter) for the entire aircrew to groan in disbelief and sympathy.7

It has long been known that if a line comes from a strong point (a sea anchor or a drogue) somewhere away from the boat, the line can be used to turn the vessel so that she shows her bow or stern to the storm. The real-world question is how to do this safely and maintain control of the boat.

Sailors have long talked about immense waves that are 50, 60, 70, or even 80 feet high. Just as regularly, scientists have dismissed these claims as delusional fantasy, like three-eyed fish or blue mermaids. Nevertheless, some of these sightings are hard to dismiss.

In 1933 in the Pacific, the U.S. Navy oiler Ramapo, on her way from Manila to San Diego, sailed into a huge storm that lasted seven days. The officers on the bridge did some clever triangulation using the ship's mast and measured a wave that was 112 feet from peak to trough—the same height as a ten-story building. The period of this wave was 14.2 seconds, and its length was 1,128 feet. The Ramapo survived the storm because her length—478 feet—fitted into these big waves and allowed the ship to head downwind without harm.8

In recent years at least five large cruise ships (the Italian Michelangelo in 1966, the British Queen Elizabeth II in 1995, the German ships Bremen and Caledonian Star in 2001, and the Norwegian Dawn in 2005) have suffered severe damage 75 feet or more above the water from waves estimated to have been 90 to 100 feet in height. Other well-found old and new ships are suffering losses at the alarming—and hard to believe—rate of one ship (over 2,500 gross tons) lost or destroyed every 51/2 days. And this rate is increasing.9

In February 2000, a British oceanographic research ship in a gale west of Scotland measured waves up to 95 feet. Seven research scientists on board wrote in the journal

Geophysical Research Letters about "the largest waves ever recorded by scientific instru-

Still, many oceanographers were skeptical and stuck to the numbers that came out of their beloved Linear Model, a bell-shaped graph that gives the probability of wave heights.

"It's like . . . children in a class," says Dr. Jim Gunson of the British Meteorological Office. "There is an average height of the children and most . . . are around that height. Some are quite a bit taller or shorter, but the chance that a child is three or four times the height of the average child is very, very small."11

As we saw in the last chapter, significant wave height—the average height of the highest one-third of the waves—is a better measure of sea state than average wave height when a mariner wants to evaluate the challenges he or she will face. The significant wave height will be half again the average wave height, and seas of that size, while not prevalent, will not be rare either. Out of every 30 waves, an average of five attain significant height.

If we isolate the highest 10% of all waves, their average height will be 25% higher than the significant wave height and twice the average wave height. Out of every 30 waves, one or two on average will be this high. There is nothing mysterious at work here—we are simply working our way toward the tail of a normal bell-curve distribution.

According to the Linear Model, therefore, if there's a big storm in which the significant wave height is 39 feet, one or two of every 30 waves could have a height of around 50 feet. The chance of a 100-foot wave, however, is virtually nil, and this was the accepted wisdom among scientists until recently. Twelve years ago, however, something happened that made scientists, ship designers, and marine insurers begin to lose faith in the famous Linear Model.

The big Draupner oil rig platform in the North Sea is 100 miles from land and has to take the weather as it comes. To check the wave heights, a scientist rigged a laser device, which in a heavy storm regularly recorded waves 40 feet high. On New Year's Day in 1995, however, the recording device suddenly picked up a steep wave that was 85 feet (26 meters) high from trough to crest. The amplitude (sea level to crest) of 18.5 meters was more than three times the average height of the wave train. This was proof positive that giant waves were real; maybe the sailors who had claimed to see the monsters out there somewhere were not so dumb.12

In 1991 and 1995 the European Space Agency put satellites with synthetic-aperture radar into orbit. In 2000, a team of scientists directed by Dr. Wolfgang Rosenthal set

Boating Secrets Uncovered

Boating Secrets Uncovered

If you're wanting to learn about boating. Then this may be the most important letter you'll ever read! You Are Going To Get An In-Depth Look At One Of The Most Remarkable Boating Guides There Is Available On The Market Today. It doesn't matter if you are just for the first time looking into going boating, this boating guide will get you on the right track to a fun filled experience.

Get My Free Ebook

Post a comment