CHAPTER 18


Composite waves At sea, waves do not conform accurately to theoretical models. For all sorts of reasons, a particular wave may catch up the one in front of it. When it does, the two form a single heap of water that may be half as big again as the mean wave height. This is predictable and is tabulated in Beaufort scales as the ‘probable maximum wave height’ for a given wind speed. Other disturbances usually affect the wave pattern produced by the wind. Cross-seas left over from a previous wind, or swells running in from far away, can combine with the main wave pattern to kick up much larger seas from time to time. Such seas may be unusually steep and more likely to break dangerously than the undisturbed deep-water wave. Tides and races A tidal stream or any other current running hard to windward heaps the waves up, causing them to break more readily. This is exaggerated in charted tide rips off certain headlands and over shoals in otherwise open water. Such places are to be avoided assiduously in heavy weather, especially on a weather-going stream. Certain ocean currents are equally notorious, notably the Gulf Stream as it runs up the Eastern Seaboard of the United States.


Shoaling water In anything other than extreme conditions, wave height and length in water over 40m deep is unlikely to be affected by the bottom. As the water shoals, waves become less high as the length diminishes. Further shoaling makes the waves shorter and steeper until, at a depth of between one-and-a-half and twice their height, they break. Navigation is now impossible, and no boat should be anywhere near water so shallow with a heavy sea running. Note that offshore shoals have an identical effect to coastlines, making areas far from the nearest land like parts of the North Sea and the Coral Sea dangerous, where the chart indicates depths that would otherwise be safe for navigation.


Waves breaking on a bar or shoal


Shoal


Shoaling areas are extremely dangerous in heavy weather. This area off England’s East Coast has been a graveyard for vessels for centuries. © Crown Copyright. Not to be used for navigation. Courtesy of UK Hydrographic Office.


MANUAL OF SEAMANSHIP | 149


Wind and wave


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