It’s that time of year again, the threat of autumnal storms and the big waves that accompany them. Wind isn’t a direct threat to the small boat sailor; the problem comes when his assassin, the wave, gets in on the act. 

I was once in a severe storm off the east coast of the USA in which the waves built to a considerable height and there was a point at which I didn’t think the boat was going to make it up the face of a particularly steep one. It was an illusion, of course, but pretty scary nonetheless. 

The probable maximum height of wind waves is around 80% of the wind speed. So, a 50 knot wind blowing over an area of ocean with unlimited fetch would produce a maximum wave height of about 40 feet. This height is achieved after it has been blowing for a day, having doubled in height since the first four or five hours of the storm. Further maximum wave height increase is more subdued, it takes two days to get that wave up to 50 feet in height.