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Hydrodynamics; how is it that a 200lb person can ride giant waves on small little chunks of foam (Surfboards) ?
Asked by jmyerz
Riding a wave is a sweet mix of buoyancy and planing.
Buoyancy is all about displacement and density. I think the best example of buoyancy is an ice cube floating in water. Ice and water are made of the exact same molecules, but pound for pound, ice takes up more space. When you place an ice cube in a glass of water, the ice cube will sink until the weight of water it displaces is equal to the weight of the ice cube, and since ice takes up more space, the “extra” volume of ice stays above the surface of the water. In other words, ice is less dense than water, so it floats.
The only requirement for floating in the ocean is that you have to be less dense than ocean water. Here’s how the 200 pound surfer with a little chunk of foam plays out:
- A cubic foot of sea water weighs about 64 pounds.
- A cubic foot of foam weighs about 2.5 pounds.
- The buoyancy of a cubic foot of foam in sea water is 61.5 pounds of lift (the difference between the water and the foam).
- To float a 200 pound surfer, we need about 3.25 cubic feet of foam.
So that’s all well and good if we’re just sitting in the water. But what about all those killer videos of surfers slicing down the face of a wave, barely touching it at all? How does that work?
That’s where planing comes in.
Planing is what happens when a surfboard is moving fast enough that the force of the water pushing against it exceeds the force of its buoyancy. The faster the board is moving, the stronger the effect. Think about water skiers. They barely float when they’re sitting in the water, but quickly rise to the surface when the tow boat takes off: the force of the water pushing against the skis is enough to lift the rider out of the water. Surfboards operate on the same principal, but instead of being towed by a boat, they’re accelerated by “falling” down the face of a wave.
You could, in theory, surf with a board that’s heavier than ocean water if you were towed into the wave at a high enough speed … But it wouldn’t be nearly as much fun!
(photo copyright Justin Myers)
