Skimming Physics

The Delaware coast has great skimboarding beaches. They're so good, in fact, that the 2007 East Coast skimboarding championships were held at Dewey Beach, Delaware a few weeks ago. My son and I stopped by after the waves died down and were too small to surf.

In case you're not familiar with skimboarding, it's one of the coolest and least recognized extreme sports. Check out this video from the 2004 championship, which was also held at Dewey Beach.



Skimboarding, surfing, wakeboarding, and kiteboarding are among the popular skimming sports. In all four sports, athletes glide across the surface of the water on one type of board or another. (Skimming is also known as planing, but who would want to practice a sport called "planeboarding?")

Most boats plow through the water, pushing the water they pass through to either side. It's tough work, which means that they move very slowly compared to the sorts of speeds we're used to on dry land. Twenty knots (about 23 miles per hour) is a pretty good clip for a boat. To go faster, or reduce the amount of energy it takes to move, watercraft have to skim.

In unpowered skimming sports, particularly surfing and skimboarding, speed is less an issue than conserving energy. The more the board pushes water aside, the more quickly the surfer or skimmer will lose energy and slow down.

So, how does skimming work? In water more than a few inches deep, a surf board or skim board glides along the surface of the water with a small angle of attack -- that means that the board is not perfectly level relative to the water surface, but instead raised up a little more at the front, and lowered down a bit at the back.

You can get the idea if you stick your hand out of the window of a moving car. Tilt the leading edge of your hand up, and you will feel lift pulling upward, tilt it down and the force pushes your hand down. (It's still called 'lift' when it pushes down, except that now it's a "negative lift" or "downward lift." I know, it's not a good term, but it's what we're stuck with because most engineers and physicists who first started looking into this stuff were interested in upward forces, not downward. In race car engineering, they tend to call negative lift "down force.")

When a surfer is moving fast on a wave you can see the trail they leave behind that results from the board pushing water straight down below the board. Unlike a boat, surfers and other skimmers don't create a v-shaped wake (at least not much of one, provided their speed is high).

A boat's wake comes from pushing water sideways, which is a very inefficient way to get around in the water. That's because, when it comes down to it, a boat's primary function is to stay afloat. Moving from place to place is secondary.

Most surfboards, skimboards, and related skimming equipment barely float at all. When surfers sit on their boards waiting for waves, their boards tend to be mostly under water.

Once they get moving, a surfboard or skimboard doesn't float so much as fly. That is, the board's buoyancy doesn't matter at all once the board is moving fast enough. What's important is the size of the board, or more specifically the surface area of the board. The more surface area available, the more lift the board will have. The more lift the board has, the slower you have to go to start skimming over the water instead of plowing through it.

The cool thing about skimboards, in contrast to all the other skimming sports, is that they also use another phenomenon to glide along. They benefit from something called the ground effect.

When skimming on a very thin layer of water, about a centimeter or so, the board pushes the water down but the ground gets in the way. Instead, it has to flow out toward the sides of the board. But water, like all fluids, resists flowing through small spaces. The term for this is viscosity. Molasses is very viscous and flows slowly, air is not very viscous and flows quickly, water is somewhere between the two.

Because of the ground effect, skimboarders can glide for a very long time over shallow water. As soon as they enter water more than a centimeter deep, the board acts like a surfboard instead of a skimboard.

The same thing happens to airplanes. When they are high up, they have lots of air below them, and they fly by pushing air down with their wings. When they come in for a landing, the air can't escape from under the wings and the plane begins to act like a skimboard on shallow water. Pilots are familiar with the ground effect. In fact, it helps when landing because it feels as though they are coming down on a cushion. On the other hand, if a novice pilot isn't prepared for the extra cushion of the ground effect, they can end up skimming over the runway and missing their landing altogether.

The ground effect makes skimboarding unique among extreme sports (or any other type of sport, as far as I know). No other sport takes advantage of this particular bit of physics.

So go skimming! It feels very cool to glide along on one of those things, and it gives you something fun to do at the beach when the waves are too small to surf.

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