Copyright 2019 - Woods Designs, Foss Quay, Millbrook, Torpoint, Cornwall, PL10 1EN, UK
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  • 36ft Mirage open deck catamaran

People often talk about sailing though a 2-4ft chop. But what does that really mean? How do people decide how high the waves are? Well, the Beaufort Scale is a useful indicator, but the Significant Wave Height is the "scientific" definition, and is "the mean wave height of the highest third of the waves".

I remember years ago sailing with a friend in some bumpy conditions. I thought the wave height was maybe three feet, the waves never came near the gunwale (we were motoring) and we only had light spray on deck. So I was very surprised to read in the log that my friend wrote down 4-6ft.

I also remember racing our Sagitta in a big Atlantic swell and have the sails go aback in every trough, as there was no wind there, only for them to be blown out with a crack at the crest.

Another time I left Plymouth and was surprised to see my fishfinder echosounder tracing a perfect sinewave with peaks of 13ft. I couldn't see the 40ft monohull sailing near me when I was in the troughs

It's actually easy to gauge wave heights. A typical 35ft boat might have 4ft freeboard. So a 4ft wave will be as high as the gunwale, that's pretty high. Most people talking about a 4ft chop actually mean 2ft. Even a 10ft wave would wash over a bungalow. A 20ft wave is very scary, just imagine it coming down the road towards you (tsunamis are different again of course)

Of course an ocean swell might be much higher than that, particularly in the Roaring Forties say, but their long wavelength means they aren't a real danger. It can even be fun, as a big swell allows you to surf for miles. A bit like driving through rolling countryside rather than over a ploughed field.

The steepest, biggest waves I've been in were on Eclipse, where they were definitely steep, breaking and over 20ft high.

Apart from the scariness of big waves and the potential damage they can cause there is another major factor why it is important to know the height of waves

That is because a boat (irrespective of the number of hulls or its ballast) is considered safe in waves whose height does not exceed the beam of a boat. In other words, if your catamaran is 20ft wide you are effectively safe from capsizing in waves until the wave height exceeds 20ft. I'm not saying that you will then capsize, just that you then become at risk of capsize.

So a wide beam power catamaran, for example, is much more seaworthy than most monohull powerboats