I have a hard time understanding how a wave forms and why the scientists insist that the water in a wave doesn't actually move forward. I have a faint recollection of reading somewhere that a wave starts when a dimple forms on a flat sheet of water. The wind blows on the back of the dimple and pushes it forward turning it into a wave that grows bigger and longer as the wind blows longer and harder. But what starts the dimple?
The Oxford Companion to Ships and the Sea says quite categorically that "the water in a wave does not move forward in a horizontal direction but rises and falls below the surface, unless the force of the wind is enough to cause the crest of the wave to overbalance and break, when the water in the crest does move forward."
It also moves forward when a wave reaches a shallowing shoreline, of course, and the bottom of the wave is retarded by friction against the sea bed. I have no problem understanding that.
Interestingly, the relationship between wind speed, in miles per hour, and the height of the wave it generates, in feet, is approximately 2 to 1. This ratio from the U.S. Hydrographic Office suggests that a wind of 50 mph should raise a 25-foot sea.
In fact, however, there are many reports of waves 40 and 50 feet high in heavy gales in some oceans , which would require sustained winds of 80 to 100 mph, which seems unlikely outside of hurricane season. Perhaps these rogue waves are formed when one huge wave happens to ride on the back of another huge wave, thereby doubling its height.
The length of a wave, from crest to crest, is reckoned to be about 20 times the height, after the wind has been blowing steadily for some time and when there is no opposing current. Thus, a wave 25 feet high would be about 500 feet long and would race through the water at about 30 knots, because the speed of a wave (in knots) is the square root of the length from crest to crest (in feet) times 1.34.
For the sea to become fully developed, the wind must blow in the same direction for a certain minimum time, and the rule of thumb here is that the time in hours equals the wind speed in knots. In other words, a 20-knot wind will take about 20 hours to form the biggest waves it can.
Another little mystery for me is how a little oil can calm a nasty sea. I always imagine the tiny molecules of seawater swallowing the oil and experiencing the same relaxing effect that a glass of single-malt Scotch brings to a human being, but I'm not sure that's scientifically acceptable. So it will remain for me one of the many mysteries that makes sailing such a fascinating pastime.
The longest wave is quickly lost in the sea.
— Emerson, Representative Men: Plato.
Tailpiece“You’ve got to lose weight. “I’m putting you on soup and salad for a month.”
“OK, doc. Before or after meals?”
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