Set Speed: Mind-blowing wave science

The chances are you already think the ocean is a place of mystery that you’re yet to fully master.

But, you have no idea just how interesting it is!

I’ve been doing a lot of reading about surf science lately, delving into books about waves that include equations that would make Einstein’s eyes water (I quickly skip over these).

Every now and again I read something that just blows my mind and makes me realise I actually know very little about the ocean (although I can summarise the important parts for ocean swimmers really well!).

The latest OMG moment came when I read something completely new about how groups of waves (sets) propagate (travel) across the ocean’s surface.

If you assumed, like me, that the individual waves in a group remain in the same position in the group throughout the journey, then this next part is going to come as a huge surprise.

It turns out that that is not what happens at all.

Let’s start with a reminder that waves are energy moving through the ocean, they are not a body of water moving from one place to another.

A single wave is unable to propagate very far on its own, but a group of waves can.

A group of waves moves across the ocean at ‘group speed’, yet individual waves within the group travel at twice the speed of the group (phase speed).

As the group travels along the surface of the ocean, all the waves in it are constantly moving from the back of the group to the front and then disappearing.

Because new waves constantly form at the rear of the group, the number of waves remains the same.

Confused? Need a visual?

It’s like if you took a moving escalator, lay it flat on the ground and then pulled the whole thing along. Each stair would appear at the back, travel to the front and disappear.

As I say to my clinic participants when I talk about this completely mind-bending piece of wave science; make sure you have full comprehension of just what you’re trying to say before you use it as your next dinner conversation starter!

For the most part, this phenomenon will have no bearing on how you experience a set in the surf. But it could explain that wave that pops up ‘out of nowhere’ when you think you’re through the set and out the back.

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