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additional 50 feet above the deck. The wave flowed over the deck and rose up to the level of the bridge, before finally washing back into the sea. The tanker shuddered, rose, and continued on its way without further incident. Fortunately, we had no major damage. However, if you’d told me there were 69-foot-high waves out there, I wouldn’t have believed you—until I experienced one myself.”
Jannsen stated that he’d seen “sky-high waves, total green water,” but none that he felt was truly a rogue wave. The big waves that he’d seen in his experience came from storms. Once, when leaving Ireland in a 197-foot-long anchor-handling tugboat, he encountered gale-force winds. He described driving into “a hollow sea, green water over the bridge, the fish looking in at me….”
Jannsen is also familiar with South Africa and the Agulhas Current. He’d never experienced a rogue wave personally, but told me about seeing masts down, rails twisted and torn away, lifeboats ripped from their supports and washed overboard—all the result of large waves.
Although the focus of this book is on oceans, I would be remiss not to say something about large waves in our inland freshwater seas—the Great Lakes. Commencing in November, the Great Lakes are susceptible to terrible storms. Hundreds of ships have been lost on the lakes since they became a favored path for the transport of freight and passengers. The largest lake, Lake Superior, is about 350 miles long and 160 miles wide, with an average depth of 483 feet and a maximum depth of 1,332 feet. Lake Michigan, the second largest, is about 300 miles long, 118 miles wide, and about half as deep as Lake Superior.
Given the size of the lakes, the waves that can be created by winter storms are fetch limited. Table 2 shows that a steady wind of 30 knots blowing for 23 hours over a minimum fetch of 243 miles (about the length of Lake Michigan) will produce waves that are 19 to 34 feet high. These waves would theoretically have a period of 12.5 seconds, a speed of 38.2 knots, and a wavelength of 800 feet. Since the waves are fetch limited, you would not expect to see 100-foot waves in the Great Lakes. However, there are two complications: First, storms can come up quickly; second, in the deepest parts of the lakes, the depth is deep relative to the wavelength. As the waves move into shallower water,