Probably the most sought-after surfing experience is the tube ride (a.k.a. "getting barreled"). A tube ride occurs when the top of the wave pitches over the surfer so that he or she is completely enclosed in an oval space behind the curtain of falling water. Inside the "green room," you are hurtling through a tunnel of water and the only way out (without wiping out) is straight through the opening in front of you. Hollow waves are foot-for-foot the most powerful variety of breaking wave, and good tube riding is really difficult. It requires timing, experience, and skill. The video shows us some world-class surfers making it look easy!
But why do some waves break as hollow grinding tubes while others crumble more gently and forgivingly? Let's examine a little wave dynamics in order to assess the situation. Ocean waves are created by wind blowing over the ocean surface, as the kinetic energy of the air is converted into potential and kinetic energy of the water. The biggest and most powerful waves are created in massive storms. As the swells generated by these storms travel over the open ocean, the originally chaotic "victory at sea"-type wave motion is gradually organized into cleaner lines. As with all waves, it is not the actual material (water in this case) that travels any distance through the medium -- it is the energy of the wave. As the wave energy passes through a point in the ocean, the water molecules rise and fall in a circular pattern but remain in the vicinity as the disturbance passes by.
While waves are in deep water, you can't surf them because they don't break. In order for a wave to break, there must be less than a 120-degree angle between the front and back face, and the wave doesn't get steep enough until it approaches shallower water. What happens in shallow water is this: the motion of the bottom of the wave is impeded by friction with the ocean floor while the top section of the wave continues to move, relatively unimpeded. It therefore "outruns" the lower part of the wave, forcing the front face to steepen until the top layers cascade over the edge. Waves generally break when the ocean depth falls below about 1.3 times the wave-face's height.
However, in order to get hollow tubing waves, you need energetic swells that travel abruptly from deep to shallow water. Tropical reefs (such as those fringing the north shore of Oahu) often produce some of the best tubing waves, not only because they're in the direct path of powerful swells originating in the North Pacific, but also because the ocean depth decreases from several dozen meters to a few feet over a very short distance as you move shoreward over the reefs. Offshore winds blowing into the wave faces also help steepen them and hollow them out.
Dropping in to a "macking tube" involves negotiating a steep takeoff and finding the right line inside the tube. Ride too high, and the water rushing up the face of the wave can throw you "over the falls," which is an extremely unpleasant experience for a surfer, especially if you hit the reef on the way down. Ride too low, and you lose momentum and get crunched when the tube closes, or knocked off your board by the descending lip of the wave. Find that perfect line, and -- if you're lucky -- the air ejected from the collapsing tube propels you out into the open, to applause and glory!
Adam Weiner is the author of Don't Try This at Home! The Physics of Hollywood Movies.