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Two Foot or Two Metres?

By Dr Rob Brander on June 8, 2011 in Other

Aloha. I just got back from Hawaii, birthplace of surfing and what appear to be the world’s largest pick-up trucks. Now we all know that the late great Duke Kahanamoku introduced surfing to Australia and was a generally cool guy who now has an excellent bar at the Outrigger Waikiki named after him, but the Duke and the Hawaiians have also left us with another ongoing legacy: a complete disdain for the metric system.

Example 1: Eavesdrop in on some conversations amongst surfers sussing out the waves when the surf is big. Waves will be towering over the heads of surfers standing virtually upright on their boards, but all the talk will be about the waves being ‘4 foot’. Unless the boardriders are midgets, this seems to make no sense at all.

Example 2: When I give talks to primary school kids, I show them a movie of a monster wave and ask them how high they think the wave is. I always get answers of ’60 foot’. How do these 9 year-olds even know what a ‘foot’ is? Didn’t we convert to metric in the 1970s? Where does all this jargon come from? Blame the Hawaiians!

Surfing really did start in Hawaii and with those big swells rolling in, the original surfers used to sit offshore on their boards estimating the height of the back of the wave as it passed them by. However, by doing this, they only really saw half the true wave height, something we call the amplitude of the wave. Now if you were sitting in front of that big swell wave as it reached shallow water, curled over and started to break, in the few seconds before you got pummelled, you’d be looking at the front face of the wave from the top (called the crest) to the bottom (called the trough). The vertical distance from the crest to the trough of a wave is the wave height. So the front face of the wave is the true wave height and is equal to twice the amplitude of the wave. In other words, double whatever the Hawaiians (and a lot of local surfers) are calling the wave height.

As more overseas people started surfing in Hawaii and Hawaiian surfers started travelling, their method of estimating wave height started to take hold and that’s the reason why you so often hear surf reports in ‘feet’ rather than ‘metres’. Even some of the popular surf report and prediction websites will list wave height in feet. But how accurate are these reports?

Well, these days, most websites report the true wave height based on measurements from offshore wave rider buoys, which are essentially giant beach balls floating offshore that record wave height, period and direction. Combined with physical theory of how waves change their shape as they enter shallow water and good bathymetric measurements, the estimates are pretty accurate… and correct. And most surfers are pretty savvy at nailing the real wave height just by looking at it. However, there’s definitely some machismo going on out there so don’t judge the waves until you see them for yourself!

1 COMMENT. SHARE YOUR THOUGHTS

  1. If amplitude is from the back, then wave heights are not necessarily ‘twice amplitude’ – there are many waves like Chopes that have next to no back on them, but can be 10ft at the front. Just saying…

    Posted by: tz | September 25, 2011, 4:13 PM |

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