The Whip-Like Physics of Fly Fishing

It's like pitching a cotton ball at major-league speeds: up to 600 feet per second.

By Curtis Rist|Thursday, September 18, 2008
RELATED TAGS: PHYSICS
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Joan Wulff handles a fishing rod with grace and explosive power. In 1960 she set the women’s unofficial world record for distance casting—an astonishing 161 feet. Now the grande dame of fly-fishing at age 81, Wulff has not lost her touch. Standing on the banks of the Beaverkill River near her fabled fishing school in New York’s Catskill Mountains, she raises and points the tip of an 8½-foot rod just over her shoulder, gives a sudden backward snap of the wrist and sends 40 feet of line sailing straight back behind her. As the line hangs in the air, she drops her arm down toward her waist. Then she launches the line forward, depositing a feathery fly silently out on the water exactly where she wanted it. “If you do it right,” Wulff says, “the fly shoots by your head like a bullet.”

While other fishermen lazily dangle bait in the water, fly-fishers sweat over strategy and technique, struggling to imitate with rods, lines, and lures the delicate landing of a fly on the river’s surface. To catch a fish, a fly-fisher sends out a wispy lure attached to an almost invisible 7- to 15-foot-long leader, which is in turn attached to a thicker and much longer braided line. The trick is launching the virtually weightless fly to a distant spot with deadeye accuracy. The thicker line must hit the water first, allowing the leader and fly to then gently alight upon the water just the way an insect might settle on a ripple, persuading the prey (trout and salmon, often, or perhaps a largemouth bass) to bite. “The point is to make it all look as natural as possible to the fish,” Wulff says.

Natural, of course, does not mean easy. In bait fishing, a sinker adds heft—and a fish-scaring splash—to a cast. In contrast, propelling an ultralight fly the way Wulff does is akin to pitching a cotton ball at major-league speeds. The only weight resides in the fishing line itself, which uncoils from a reel during the cast. The relatively small backward and forward motion of the fly rod sends the line streaking through the air as far as 236 feet, the current record.

Propelling a line and attached lure such a great distance requires deft control of the body’s ability to impart momentum—the product of an object’s mass and velocity—to the rod and line. During a cast, the fly-fisher achieves peak energy in the flick-of-the-wrist stage, which “gives you all the momentum you’re going to get,” says Jeff Kommers, a technical staff member at MIT. While working on his Ph.D., Kommers entertained classmates with his restless efforts to parse the physics behind the process. Following the moment of peak energy, the arm’s snapping force passes through the rod to the flexible tip, which then wiggles back and forth. This motion translates the force from the arm to the speed of the line and fly. Because momentum—created by the arm, absorbed by the rod, and passed into the thin fishing line—must be conserved, the tiny mass of the line achieves tremendous velocity, “especially at the end,” Kommers says.

Bullwhips operate under the same principle as the fly line: Energy travels from the arm to the thick end of the whip all the way down to the tapered tip, which accelerates wildly as the mass decreases. The characteristic crack of the whip results not from the tip’s snapping to the ground but from its literally breaking the sound barrier, producing a small sonic boom. “Obviously, fly-fishermen aren’t interested in making mini sonic booms when they try to catch fish,” Kommers says. Fortunately, the design of the leader and the fuzzy fly at the end of the line make that very hard to do. “The highest velocities I calculated were about 600 feet per second, quite a bit less than the speed of sound,” Kommers reports. And air drag typically keeps velocities much lower than that.

The type of rod also affects the speed and distance a line travels. Salespeople call some rods fast and others slow in describing how far they might cast a fly. Graig Spolek, a mechanical engineering professor from Portland State University in Oregon, set up an experiment to quantify these vague terms. He hauled a bundle of rods into a darkened gymnasium, set up a strobe light, and took a sequence of photographs to examine the movement of the rod and the line during the forward cast.

Rods have to be stiff enough to handle the lunges of a caught fish but flexible enough to transfer energy effectively from the arm to the line during a cast. This allows the tip to wiggle quickly back and forth, which Spolek calls the rod’s frequency. In another experiment, he counted how many times the tip of each rod moved back and forth per second, and this number, he discovered, allowed him to predict which rods would cast the farthest. High frequency gives you a faster tip speed, creating swifter line speed. “And that allows you to cast a long way,” Spolek says.

Still, Spolek was not satisfied. “Who cares about finding a unified theory of the universe?” he asks. “What we need is a unified theory of fly-fishing.” In 2006 he took a yearlong sabbatical to make progress toward that theory. In his research he deconstructed the mechanical limitations of the rod that cause its tip to swirl during casting, preventing the line from aiming true. To make a rod, manufacturers tightly wrap graphite-fiber fabric around a thin steel cylinder. Spolek found that if the manufacturers changed the way they roll the fabric, they could minimize the swirling problem. He also concocted a new way to measure the flexibility of fishing line leaders, the part of the line nearest the lure, using a technique he had previously developed to gauge the bending stiffness of the tiny wires in pacemakers. In this way, he discovered that newer fishing lines made of fluorocarbon were actually less flexible than standard nylon lines.

Of course, a perfect cast still depends on the skill of the person heaving the rod—but science is helping there, too. Noel Perkins, a mechanical engineering professor at the University of Michigan, has built a fly-casting robot that can imitate the casts of both experts and novices. His technology could enhance the sport of fly-fishing by permitting researchers to analyze the differences between casts. The end result could be better advice for newcomers to casting. And because the robot can cast repeatedly at the same angle and force, it can test fishing lines and rods objectively. Perkins and his team have also created a fly-casting analyzer that attaches to the reel. Using motion-sensor technology, it gives a detailed printout about the rod’s speed and angle over the course of a cast—feedback critical for casters to develop control over distance and placement.

In the meantime, Joan Wulff offers her own, starkly simple explanation for how she handles a rod with such flawless grace: “It’s all in the arm,” she says.

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