First your legs drop out from under you. Gravity yanks you down the face of a toppling blue wave, and it feels like coming over a hill in a sports car: weightless. Then your weight returns, pressing your toes into the nubbins of wax spread across the surfboard. You lean into your first turn. It’s important that the turn go well.

Three sharp, curved fins cut the water beneath the tail of your board, negotiating with gravity. Slender but stiff, they are the reason the wax is pressing into your toes. As the fins resist gravity’s planned route — straight down — the board slices off to one side, away from the plunging wave-crest and toward its rising shoulder. Your board’s fins have helped you solve a complicated physics problem: transferring the mundane tug of gravity into a smooth glide along a gleaming face.

Don’t underestimate your achievement. Physicists have long understood the principles involved, but the equations are so complex that computers are only now becoming powerful enough to model them. And don’t underestimate your equipment, either. Developing a fin that slingshots through a turn without sapping the board’s speed has consumed surfboard designers for more than 50 years.

Lacking hard numbers, surfboard shapers formed intuitive ideas of how water washes past fins. Through years of tinkering and test-driving, they developed their own vocabulary, some of it lifted from aerodynamics and some inspired by the feel of a board under their feet. They dreamed of fins that give surfers "drive," that "project" them through turns, and generate "lift" to combat the effects of "drag."

Now, companies like Future Fins of Huntington Beach, California, believe they can build on this vocabulary by delving into the field of computational fluid dynamics. Armed with computer models, data from test flumes, and ultraprecise tools, they are reassessing the ideas that shapers developed over decades.

"Trial and error is a powerful, potent development tool. But we’re extremifying the sport," says Michael Caldwell, CEO of Future Fins. Shambling and grizzled, Caldwell comes to work with his T-shirt untucked, flip-flops slapping. In his fifties, he is closely involved in fin design and leads prototype test-sessions down at the beach. "Everything we make is extreme and most people don’t even know it. They just like how it feels," he says. In Caldwell’s view, surfboard shapers operating on intuition and experience won’t be able to keep up.

Naturally, the traditionalists see things differently. "The ocean is our design studio," says Shawd Dewitt, a head designer for Rainbow Fins in La Selva Beach, Calif. "With the hydro testing, you can put a fin in a tank and run water past it, but surfing is more dynamic than that. It’s not happening in a straight line."

What makes this more than an academic debate is the $15 million surf fin industry. Surfers will pay for faster, nimbler fins, whether they come from an optimized, 3-D computer model or an outline traced onto fiberglass with an X-Acto knife. The trick, for Caldwell and Dewitt alike, is getting surfers to believe.

Ice axes and airplane wings

Surfers looking for the perfect fin have a formidable wish list in mind: Fins should hold a straight course, yield in turns, minimize drag, and transfer downward momentum into speed along the wave. Through the years, as shapers pursued such a design, the physics progressed from crude fins that worked like ice axes to sleek hydrofoils that worked like airplane wings.

Early fins merely stopped a board’s tail from slipping down the face of a wave as the rider angled across it. These fins were crude, long, and nearly rectangular. Like an ice ax, they applied drag to turn a cartwheeling fall into a straight descent. The first refinements to this fin shape came because the surfer, unlike the mountaineer, is not interested in slowing down.

In 1981, an Australian named Simon Anderson introduced a radical new design. He broke apart the single fin into three smaller ones, each shaped like a dolphin’s dorsal fin. He arranged the fins in a triangular pattern under the tail, a setup he called the "thruster": one out by each edge, and one about 8 inches farther back, on the center line. From above, the fins looked like a very skinny raindrop, with thick leading edges that tapered into narrow trailing edges. Shapers quickly realized they could make this aspect of the fin shape, called "foil," asymmetrical. Water moving across such a fin would push it sideways — the term, borrowed from aerodynamics, is "lift." The era of the airplane wing had begun.

Removable fins, adjustable surfboards

Until about 1993, fins were fixed in place on the surfboard with a sturdy coat of fiberglass. Once these "glass-on" fins were installed, shapers couldn’t tinker with them unless they sawed the fins off and built new ones from scratch. That all changed when a company called FCS invented a removable fin system.

At last, designers could think about, and test, fins independent of the surfboards. Adjusting a fin’s shape was simple, and as soon as the fiberglass resin cured someone could jump in the water and test it. FCS took the first crack by introducing three simple variants that were all the same shape but different sizes.

When a board tilts sideways into a turn, the flat sides of the fins press against the water. Larger fins allow the surfer to generate larger side forces, which resist slipping down the wave and instead accelerate the board along the direction of the center line. They work well when the forces involved are big: heavy surfers or tall waves. Smaller fins appeal to lighter surfers or surfers on smaller waves. The new FCS fins meant that for the first time surfers could think of their boards as adjustable.

But fin designers were quick to point out another benefit: economy. Price has always hemmed in the surfboard industry, whose typical customer is a 22-year-old in a hooded sweatshirt. "Most people don’t have enough cash to buy a huge quiver (of surfboards)," says Jimmy Robertson of LokBox, an affiliate of Rainbow Fins, "but you can buy a bunch of different fins and swap them out."

Imagine the prospect of breathing new life into old equipment, went the pitch. Did your last surfboard feel sluggish or unwieldy in the water? Try a smaller fin to loosen it up. Or perhaps you have a board that just doesn’t seem to go fast enough to keep up with the wave? Slap in a bigger fin for extra drive.

That was 10 years ago. Now, Future Fins’ catalog splashes 39 separate fin designs across the page under their tag line, "surf faster = surf better." To the uninitiated, the page could be a chart about dry beans: there are black ones, red ones, speckled ones, all pretty much the same shape and size. And yet Future’s array of fins is not out of the ordinary. Four other major fin designers — FCS, Rainbow, RedX, and Speeedfins — offer more than 20 fin types each.

It only takes a moment of watching at a popular surfing beach, like Santa Cruz’s own Steamer Lane, to understand what’s driving the variety. Surfers are everywhere, on all kinds of equipment, from 10-foot longboards to tiny chips of fiberglass shorter than the surfers riding them. Even among the best surfers, different styles abound: low-crouched figures zip across steep faces; a stylish silhouette cuts wide swoops on the shoulder. Even the waves vary, from the long walls at the rocky point to the slow, sloping faces just a hundred yards down the beach.

Still, with high-performance fins selling for $55 to $100 per set, it’s natural to wonder how much of a difference they will make. After all, a 22-year-old surf rat can think of a lot of things to spend $55 on.

Fiberglass and flex

Rainbow’s Dewitt doesn’t believe that continued research will ratchet up fin performance to another level. He focuses the Rainbow line on catering to fine variations in rider size, surfing style, and wave conditions. The company is family-run out of a low building set back 400 meters from the shore of Monterey Bay. A lifelong surfer, Dewitt tests new designs in the waves that are constantly audible from across the coastal scrub.

Rainbow makes fins essentially by hand from windshield-sized slabs of fiberglass. Dewitt traces fin outlines on the rectangular sheets, and workers shape the fins using bandsaws and disk sanders.

Dewitt uses custom-made router bits to put a precise curvature on the leading edge of each fin. Workers shape the rest by eye. For all the opportunity for human error, the finished fins are remarkably consistent. The glossy smooth, translucent blades — available in custom colors — have a seductive weight and shimmer in the hand.

It’s not all look, according to Dewitt. Fiberglass fins have just the right amount of flex, he says: enough to soften an abrupt change in direction, but not so much that it wilts under hard side pressure. "Just imagine if you made a fiberglass fishing pole," Dewitt says. "It loads up, bends, and then springs back, and is more responsive, whereas a plastic pole has limited flex to it and it doesn’t have a lot of response."

Dewitt is skeptical about Future Fins’ pursuit of subtle changes in fin designs. "It all comes down to wetted surface and drag," he says. The amount of surfboard bottom dragging through the water at any moment dwarfs any shape changes made to fins, he says. It’s especially true for a surfboard that is constantly changing direction. "It’s not like a sailboat where you can make a 1 percent difference and that’s huge in a 500-mile race," he says.

A microscopic attention to detail

The designers at Future Fins believe in the pursuit of small performance differences. A tall drafting bench along the main wall of their design office, wedged in the industrial district of Huntington Beach, supports three weathered PCs. Curtis Hesselgrave jiggles the mouse on the middle computer. He has been designing fins for 30 years, since the days when fins were cut out of wood and sanded into shape.

Now, he pulls up a computer-aided-design map of a fin’s surface, with the relief rendered in shades of blue. He is cagey about the details of their designs. "You have to understand," Hesselgrave says. "There’s a huge amount of proprietary information here that doesn’t make for good copy. It makes for good business."

Future Fins contracts with university engineers — he won’t say which ones — who use computer models to evaluate the flow characteristics of fin designs before they are ever built. Once the fin has the specifications the designers want, the computer sends up to 10,000 lines of machine code to the next room.

Next door is a soaring warehouse populated by four very expensive machines — three precision cutters and an injection molder — and two shiftworkers fine-filing new fins. Two of the machines look like ski-resort gondolas. The doors slide open to reveal a precise cutter hovering over a mound of aluminum shavings. This is where the computer code arrives.

When Hesselgrave presses "Go," the machine carves the fin shape into a block of aluminum, making a mold. A third cutting machine cuts fins directly into fiberglass sheets, in a nod to Dewitt’s belief in flex. The tolerance is one one-hundred-thousandth of an inch. "We are not screwing around here," Hesselgrave says. "This is not approximate."

The Future Fins operation is light on manpower, but heavy on capital. Each cutter costs as much as $100,000. Caldwell says the price tag is justified by the utter dependability of the finished fins, which look exactly the way they were designed on the computer. "If you’re making fins by hand you don’t learn anything, because you can’t ever make it again," he says.

It’s this consistency that has given Future Fins the ability to tinker with microscopic aspects of fin design. One of their new designs, the Vector Foil, tries to maximize the fin’s lift and direct it upward, instead of to the side.

To do this, Future’s designers took advantage of the principle that lift develops perpendicular to the fin’s flat surface. They shaped a zigzag into the fin so that part of the surface approaches horizontal. The new fin looks misshapen at first, like a cookie that has cooled half on, half off the cooling rack. The resulting lift "pulls the board out of the water," Caldwell says, "It reduces the wetted surface area, and that makes you more maneuverable and faster."

Instead of stealing excess speed from the drop down the wave — making the board hard to control — the goal is to keep the board from slowing down in the flats. Where previous shapers saw drive as the secret to speed, Vector’s designers are solving for glide.

It’s in these new designs that the physics starts to veer into the complex world of fluid dynamics and turbulence. When water flows over a fin at more than about 5 mph, turbulence develops in the sheet of water next to the surface. This sheet is called the boundary layer. At even higher speeds, the turbulence gets out of hand. Wild, careening eddies ricochet off each other and strike the fin and surfboard at odd angles, slowing it down. So when fin designers look for efficient fins, they chase after clean, smooth boundary layers.

Caldwell doesn’t claim to understand these complexities; he leaves the details to his unnamed engineering consultants. But for a story that investigates the claims of precisely designed fins, it seems only fair to consult an actual fluid mechanics engineer — preferably one who smells faintly of surf wax.

The view from the boundary layer

Jim Hench is a physical oceanographer and engineer at the Environmental Fluid Mechanics Laboratory of Stanford University. He models turbulence in coastal currents to estimate how sand moves around harbors, and he teaches fluid mechanics to undergraduates.

Hench has surfed South Pacific reef passes over shallow coral during research expeditions, and he rides the cold beachbreaks between Monterey Bay and San Francisco the rest of the time. Sporting the classic 1950s surfer crewcut, the 36-year-old postdoc is slight but energetic. He often punctuates his sentences, at least sentences about fluid mechanics, with an excited laugh.

The claim that Vector Foil fins can work up enough lift to cut drag on the surfboard’s bottom puzzles Hench. "You mean on the fin that’s out of the water?" he asks, pointing out that in hard turns, the board is on edge. "I’m going to have a tough time believing it’s adding anything to the story."

But even though the concept sounds faulty, Hench says, fins that generate high lift forces might still help a surfboard perform. He notes that lift from the fin on the inside of the turn would push downward. It would tend to snug the edge of the board into the wave, "which I’ve always thought was a good idea," he says.

And though Hench knows the concepts involved forward and backward, he points out that in practice the forces generated depend minutely on the dimensions involved — speed and fin size. Future Fins "could be right in listing all these different effects, but they might be very small compared to another effect."

In the end, he says, it’s probably just a matter of carving out the time and money to work through the math. "If we had a grant to study this for a few years," Hench says, "I’m sure we could weed out a bunch of red herrings, and get rid of some of the worst ideas we’re still batting around." Future Fins — and other companies who are investing in research and development — are venturing into the first stages of such a study. If they haven’t found the answer yet, Hench says, at least they’re looking for it.

Selling an ounce of speed

For all the high technology involved, there’s still plenty of room for argument about whether the new generation of fins has any advantages. Doug Haut, who after 40 years in Santa Cruz is one of Northern California’s best-known shapers, recently tried Vector fins. He found them faster than other fins, and started selling them in his shop.

"But some people are going to love them and some people will hate them," he says. "Everybody’s got such different styles. It’s not like one fin is going to be the answer."

Dewitt thinks most of the difference is in the mind. "Put it this way, if all those designs had so much benefit that they tout in all their ads, every single professional surfer would be on them."

Consumers have been slow to make up their own minds. Future Fins’ market share is on the order of 12 percent, according to an industry journal, with Rainbow somewhere in single digits. They’re both trying to catch FCS, which has commanded the market ever since removable fins came into being. And last year, FCS introduced its own computer-designed, lab-tested fin.

Caldwell thinks it’s ironic that surfers who check the waves on the Internet each morning have stayed satisfied with 1980s-era fins. Amateurs in other sports rave about equipment that offers only mild advantages — just try and talk a bicyclist out of wearing Spandex bike shorts.

When the waves go flat and surfers turn to another sport, they take computer-guided innovation for granted. "Nobody is hand-shaping golf clubs or tennis rackets," he says. "Everything (surfers) buy outside of their sport, they expect to have some kind of new technology in it." Golf is a good example: The painstakingly designed dimples on a golf ball are the reason even a middling golfer can drive 200 meters off the tee. Caldwell believes the same potential is hidden in the surfboard fin.

Once Caldwell and Future Fins have worked through the math, they still have to persuade surfers it’s worth paying for. "If you talk to surfers, they don’t understand these principles," says Caldwell. "But ah — so what? They don’t have to. That’s our job."

An example is professional surfer Rob Machado, whose fluid style is wildly popular. When asked what he liked about a new set of Rainbow fins, he simply smiled. "I don’t know," he said. "I just like the way they work."

Machado’s attitude is typical of many surfers, who think more about their surfing than their equipment. You would too. Deep in the middle of a perfect first turn, your eyes are already fixed on the top of the wave, watching it feather into droplets as it falls. As you carve toward the lip, water streams across your fins in exactly the pattern that designers like Caldwell and Dewitt spent months envisioning. Even now, they’re probably picturing changes they’d like to make. But you’re just thinking about your next turn.

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ABOUT THE WRITER

Hugh Powell
B.A. (biology), Huntingdon College
M.S. (ecology & organismal biology), University of Montana

ABOUT THE ILLUSTRATOR

Matt Farrar grew up on an island in the Puget Sound of Washington State. In 1996, he received his bachelor's degree in oil painting and intaglio printmaking from the small private college Cornish College of the Arts, in Seattle. Each year following, he has worked as a studio artist with modest success and many of his fine paintings are peppered about the United States in private collections. Upon finishing his Science Illustration graduate degree, Matt plans on continuing his career as a fine artist as well as working as a freelance illustrator and graphic designer.