Wind and wave


It could be argued that the material in this chapter does not fall within the brief of a manual of seamanship. The seaman’s art, however, is all about helping the ship to live with the conditions she finds, so it sits comfortably as a final section. Assessing the likely effect of waves is a vital part of every skipper’s portfolio of skills, and they are better placed to make judgments if in possession of the facts about what the waves are likely to do. Although a full exposition of weather at sea rightly belongs in a volume of its own, certain areas of practical interest to the seaman on passage or in harbour are sometimes not covered in general texts. Such questions as how quickly the sea may build in rising winds, what sort of fetch will produce seas big enough to trouble the boat, and whether an approaching squall cloud is likely to generate crushing wind forces are important ones. They and certain others are therefore dealt with below.


Waves


Anatomy of a wave Length and height Waves are defined in terms of height and length. The length is measured from crest to crest, while the height is noted from the bottom of the trough to the top of the crest – not from some mean sea level. Fetch Waves that are of interest to seamen are largely generated by wind. If this is blowing off a weather shore, the water immediately under the land will have no significant waves. As the distance to leeward increases, wave height grows until it reaches the maximum that can be generated by that wind speed. The distance the wind is blowing uninterrupted across water is called the ‘fetch’.


Wave height increases with fetch Wave length


For strong winds, a surprisingly large fetch is required before full wave height is achieved. A 20-knot wind, for example, requires 150 miles of fetch before the last of the ‘lee’ offered by land finally disappears. The waves created by a force 8 gale are to some extent impeded as far to seaward as 450 miles, but in such winds, or stronger, it takes a good deal less fetch than this to render the seas dangerous to small craft, especially when affected by currents or shoaling water. Time factor Waves do not appear by magic as soon as the wind kicks up. They develop with time. Quantifying this is not easy because of numerous extraneous factors, but it can vary significantly. In the storm which wrought havoc on the Fastnet Race in 1979, the waves rose from 2m to 8m in 9 hours. In a more typical late Autumn storm in the Western Approaches to the English Channel, heights might take a day to grow from 3m to 11m.


Wave height


148 | MANUAL OF SEAMANSHIP


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