Angling for momentum – fly casting research – Brief Article

Curtis Rist

Angling for Momentum

The quest for a unified theory of fly casting

JOAN WULFF HANDLES A fishing rod with grace and explosive power. In 1960, she set the women’s unofficial world record for distance casting–a blistering 161 feet. Now the grande dame of fly-fishing at age 72, 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 8-foot rod just over her shoulder, gives a sudden backward snap of the wrist, and sends 40 feet of the 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 front of hungry fish, 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 Dacron 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. “The point is to make it all look as natural as possible to the fish so they’ll bite,” says Wulff.

Natural, of course, doesn’t mean easy In bait fishing, a sinker adds heft–and a fish-scaring splash–to a cast. By 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 nylon 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 world record.

Scientists have yet to come up with a comprehensive explanation of how the simple motion of a cast propels a line and attached lure such a great distance. Part of the answer lies in momentum, the product of an object’s mass and velocity. Except for gravity and drag from air friction, momentum is conserved in any interaction of objects. 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 physicist, Kommers kept fellow Ph.D. candidates at Massachusetts Institute of Technology entertained with his failed attempts to come up with a mathematical equation for the process.

Following this moment of peak energy, the arm’s snapping force passes through the rod to the flexible tip, which then wriggles 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 of the string,” says Kommers.

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 snapping to the ground, but from the tip literally breaking the sound barrier and producing a concussion of sound waves. “Obviously” says Kommers, “fly-fishermen aren’t interested in making mini sonic booms when they try to catch fish,” and they can’t–no matter how forcefully they cast. The leader and the fuzzy fly at the end of the line create enough air drag to slow things down. “The highest velocities I calculated were about 600 feet per second,” says Kommers,. “And that’s quite a bit less than the speed of sound,” which is 1,070 feet per second.

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 Oregon’s Portland State University, set up an experiment to figure out just what these vague terms meant. 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 effectively transfer energy from the arm to the line during a cast. This requirement means that the tip wiggles quickly back and forth, which Spolek called 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.

In another project, Spolek looked at rods made of different materials–solid wood, bamboo, solid fiberglass, hollow fiberglass, and finally graphite–a progression reflecting the changes in rod technology since the middle of the nineteenth century Grouping rods by identical stiffness and testing them, Spolek found that every shift to a new material resulted in an increase in the rod’s frequency and its ability to cast greater distances. “It’s what some people call good action in a rod,” he says. “It’s what makes the fly fly”

That, and a lot of practice. As a result of his research, Spolek now has a good understanding of the separate parts of the cast–the arm movements, the tip wriggle, and the line flying. Like Kommers, however, he has yet to come up with a tidy equation that combines the multiple variables, including the drag of friction based on the size of the fly or the length of the leader and the possible line lengths released during the forward part of the cast. “We shouldn’t underestimate how urgent the need for this is,” says Spolek, who dreams of taking a year’s sabbatical for further study of the problem. “I mean, who cares about finding a unified theory of the universe? What we need is a unified theory of fly-fishing.”

In the meantime, Joan Wulff offers her own explanation of how the fly cast works: “It’s all in the arm.”

COPYRIGHT 1999 Discover

COPYRIGHT 2000 Gale Group

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