If you had the right board, could you surf a wave all the way across the ocean? It’s tempting to think that you could, but unfortunately there is something about ocean waves that makes it impossible.
As far as I know, nobody has ridden an ocean wave for more than about 2 km. The most famous ‘longest waves in the world’ are Chicama in Peru or Pavones in Costa Rica – spots that receive swells from thousands of kilometres away, but only become surfable for a tiny fraction of that voyage, right at the end.
But why do we have to make do with surfing them at the very end of their journey? Why can’t we ride them all the way from the storm centre to the coast? Maybe all you need is the right board?
A friend of mine has a hydrofoil board. I’ve seen him gliding along above the water on unbroken swells, way outside the line-up. A hydrofoil takes energy from the water movements underneath the wave – inside the water rather than on the surface. The board itself doesn’t touch the surface so there is much less friction than with a normal surfboard.
He asked me why, in theory, he couldn’t just keep riding his hydrofoil board on an unbroken wave for thousands of kilometres across the ocean, literally chasing the swell from one side of the ocean to the other.
I thought about it. I realised that there is something about ocean waves in deep water that makes this impossible. If you tried to surf a deep-water wave in the open ocean, you would only get so far before the wave would die out underneath you. It’s all to do with the way waves interact with each other as they propagate across the ocean.
Wave groups, or sets, are formed when many ‘wave-trains’ of different wavelengths mix together. The result of this mixing at any particular point on the ocean surface depends on whether a trough coincides with a crest, or a crest with a crest, or anything in between. When you add these wave trains together you get bigger waves followed by smaller or no waves. In other words, groups.
In deep-water, longer waves travel faster than shorter ones (radial dispersion – see my article HERE). If we mix together several wave trains, but taking radial dispersion into account, we end up with a bizarre result: The individual waves in the group travel twice as fast as the group itself. As a result, individual waves appear from nowhere at the back of the group, move to the front of the group and then mysteriously disappear. Therefore, the groups are maintained intact but each individual wave inside the group only lives for a short time.
The distance travelled by an individual wave during its life, between being born at the back of the group and dying at the front, is exactly twice the length of the group itself
Not only does each individual wave not last very long, but it also doesn’t get very far. The distance travelled by an individual wave during its life, between being born at the back of the group and dying at the front, is exactly twice the length of the group itself.
So, if you tried to surf a wave across the deep ocean, you would only get as far as twice the length of the group. As a simple example, imagine a group of four waves travelling across the sea. The distance between each wave in the group (the wavelength) is 100 m, so the length of the whole group is 400 m. Therefore, if you caught a wave at the back of the group you’d be able to ride it for 800 m until it fizzled out at the front of the group. That’s far enough to make your legs feel like jelly, but nowhere near the thousands of kilometres travelled by the swell itself.
Want more articles looking in to the science of surfing? Go right HERE and have a browse.