Predict the sea state

Consider wind and waves when planning and, as Norman Kean explains, you'll be able to predict and avoid rough waters

Checking swell forecasts and interpreting them will help you avoid rough water, says Norman Kean

T he basic driver of sea state is of course the wind. The stronger the wind, the greater the distance over which it blows unimpeded (termed the ‘fetch’), and the longer it blows for, the bigger the waves – up to a limit, for the wind strength.

Waves – as our stomachs know – are not generally regular. A typical winddriven wave pattern is a combination of many wave trains, each with different wave height (trough to crest) and period (the time interval between crests). When these combine, the result appears as groups of waves. Waves passing one spot will build to one or two big ones, and then diminish again before the cycle repeats, while a short distance away the same thing is happening, but not in step, so to speak, and the sea surface is a continuous grid of these fan-like wave groups.

The pattern is often best appreciated from the air, in breezy conditions. In a small vessel it’s often possible to steer between the groups in such a way as to dodge the biggest waves. If you’re watching waves break on a beach, it’s remarkable how often two or three big ones arrive in succession. In a random wave pattern, consisting of combinations of many wave trains of different heights and periods, about one wave in 25 will be twice the average height, and given several thousand waves – say 12 hours at sea – there is an excellent chance of meeting one three or even four times the average. Casually labelling these as ‘rogues’ or ‘freaks’, as the media often do, is thus not entirely appropriate, but it meets the need for sensational headlines.

Significant wave height is the average of the highest one-third of waves. This is regarded as the figure of greatest interest to sailors, and it’s the one that’s quoted in buoy reports and wave height forecasts. Long waves with long periods move faster, survive longer and travel further, and the most extreme example of this is a tsunami.

In this picture of Bernard Stamm’s Cheminées Poujoulat battling a Force 10 in Biscay at the start of the VELUX 5 Oceans solo round the world race, the wave patches can be clearly seen

Wind-driven waves interact with storm-driven swell from a different direction to create patches of waves Wave direction

Wave patterns are not regular because multiple wave trains with different periods interact resulting in a pattern with variable amplitude Wave envelope