Measuring Wave Height | Boating Magazine

Measuring Wave Height

Here’s how to tell how big the waves you’re looking at really are.

Measuring Wave Height

In a boat it’s easy to overestimate wave heights. This is because of a phenomenon discovered by William Froude in 1861. He found that, no matter how your boat is situated on a large swell, what you feel to be “straight down” is actually at right angles to the wave. So when you think you’re looking out on a level line to judge a wave, you are actually looking on an angle, distorting your judgment. The only time to make an accurate appraisal of the waves is when you are at the bottom of a trough, midway between two waves, and you are sitting relatively level. In a small boat your eye might be about four feet above the waterline. If you can sight along several wave tops in a row when in a trough, then the waves are about three feet high, which is what most of us judge to be “six-footers.” Sight along the gunwale, about two feet off the water, to judge smaller waves. Stand up to raise your eyes to six feet or more to judge larger waves.

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