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The Science of Surfing
page 3

Examine the bottom of a surfboard and you will note that it curves gently up at the tail and at the nose. This is called the bottom rocker of the board. The rocker at the rear helps you to turn the surfboard.

Lori cutting a turn
Look at the path recorded in the water behind Lori, and you can see she’s turning. Look at the board, and see that it’s tilted to the right and to the rear. Lori is pressing down with the toes of her rear foot to make the board turn.

Push on the Water and It Pushes Back
You’re sliding down the face of a wave and notice that the wave is breaking to your left and is peeling to the right. You want to turn right. You shift your weight onto your back foot, then push down with your heels or toes—whichever part of your foot is on the right side. You’ve moved the force you exert on the surfboard back and right from where it was in balance. The board responds by rolling a bit to
the right and pushing the right rail, or edge of the board, into the wave, while the rocker at the tail of the board pushes on the water.

A person watching from the shore can see the water spray to the left side of the board as you push the water out of the way. The forces of the water on the curved bottom of the rear of the surfboard cause the board to rotate to the right.

Turning Faster
If you are surfing with your legs bent into a crouch so deep that you can put a hand out and touch the wave beside you, then in the middle of a turn you can straighten your legs, accelerating your body upward. This will push the board down into the water with a force greater than gravity, and this extra force will make the board turn faster.

Compare how surfers navigate turns on a longboard and a shortboard. Click the pictures to see video of a person riding.

Turning on a short board
The shortboard surfer only needs to makes the smallest movements to turn.
The longboard surfer has to do much more of a dance to stay afloat.

Longboard Versus Shortboard Turns
Shortboards are short, with thin, pointed noses. Longboards are long, with wide, rounded noses. If you pick up a short piece of wood, a foot-long 2" x 4" for example, and try to twist it rapidly side to side, you’ll find that it’s easy to turn. But try to turn a three-foot-long piece, and your turning motions slow down. The same is true for surfboards. A longer, heavier, surfboard turns more slowly than a short, light one. Physicists call the resistance of the board to rotation its moment of inertia.

Longboard Versus Shortboard Speeds
A longboard might be slower to turn than a shortboard, but it goes faster when you are paddling it. A general rule for boats is that longer boats have a higher maximum speed limit known as hull speed; they go faster than shorter boats. The same rule applies to surfboards. This is because a boat or a board makes its own waves as it moves through the water. At the hull speed, the wave it makes is highest at the front of the board and lowest at the rear, so the board is always going uphill.

You can see the wave generated by the surfboard itself in the earlier photo of the blonde surfer paddling out to sea. As she paddles faster, the crest of the wave moves toward the back of the board. But a board that is sliding across the surface of the water is not limited by its hull speed. So when a shortboard gets up to speed and slides across a wave, its speed is no longer limited by its length.

Lori cutting a turn
All good rides must come to an end.

If a surfboard were to slide through the water sideways, the water would hit the fin or fins at the rear of the board with a strong sideways hydrodynamic force. This force would twist the surfboard around its center of mass until the board aligned with the direction of motion. The fins help a surfer turn by keeping the surfboard from sliding sideways across the water.

Eventually, the ride comes to an end. You step back on your board, digging in the rear and applying the brakes. The wave rolls under you and you end up on the back of the board, smiling and ready to paddle out to catch another wave.

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