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)