A FLY LINE seems like a pretty simple product — little more than some plastic material gooped around a thin piece of string. The whole thing can be made to float or sink by adding microspheres (tiny air-filled glass bubbles) or heavy metal dust in the coating. However, this apparent simplicity can lull us into overlooking just how much we ask a fly line to do. It has to help us cast by remaining slick; it must withstand the rigors of constant casting, being dragged over rocks, and being stepped on; and it must help us keep our flies in the strike zone, whether it's on the surface or 10 feet down.

As line prices have increased and anglers are deluged with new marketing terminology, it's time to ask, What does it all mean? Is there a way to build a better fly line? And why does my damn tip keep sinking? The answers are not so straightforward as you may think, and there's a fair amount of subjectivity involved. But to get you started, here's your guide to fly-line technology.

Cores and Coatings

"Let's start at the beginning," says Nate Dablock of Cortland Line Company. "Today, most fly lines are made of PVC plastic — the same stuff in plumbing pipes — baked or 'cured' around a nylon core." According to Bruce Richards, the chief line designer for 3M/Scientific Anglers, "Although they are incredibly difficult to make, basically, we make a fly line a lot like you'd make a candle."

For fly lines, you take a core — usually woven nylon, but that depends on the application (see sidebar) — and run it through a machine that lathers it in liquid PVC plastic. (Airflo uses polyurethane instead of PVC.) As the line comes out the other end, it is cooked in a giant toaster to cure the PVC. Just like candle-making, the process must be vertical, so as the plastic runs off the line, it won't belly or sag. To make a tapered fly line, you run your "candle" through what Richards calls "a variable-orifice die," which can open or close to adjust the amount of PVC allowed to stay on the line. It opens wide to make a head, then closes tighter to make the running line. Nowadays, all this stuff is controlled by computers.

Making a fly line is kind of like cooking. "The plastic in a PVC line starts out at room temperature in a vat, where it looks like pancake batter," explains Tim Rajeff, Airflo's North American distributor. Scientific Anglers' Richards agrees with that characterization: "We make that 'batter' by mixing powdered PVC resin with a special mix of plasticizers and other chemicals, and mixing in Microballoons® for flotation [or metal dust for sinking lines], then coloring, and finally chemicals to make the plastic slick and flexible." That last ingredient — cagily called "chemicals" by line designers — is where expensive modern fly lines really differ from the bottom-of-the-barrel lines of yesteryear. It's also the area where fly-line designers face forces pulling in opposite directions.

PVC wants to be hard, like a plumbing pipe. "The only way you get it to stay soft and supple is by adding a class of chemicals called plasticizers," adds Neil Webb, a designer with Cortland. (Naturally, line companies are close-mouthed about the chemicals they use.) Softness alone isn't enough, though; fly lines must also be durable and slick. And, in most cases, the lines still have to float.

So, starting from a basic quandary — how to make hard plastic soft, slick, durable, and buoyant, all at once — different manufacturers have come to similar solutions from divergent paths.

Softness is a major engineering headache for all manufacturers. PVC fly line naturally loses its plasticizers over time — mostly due to exposure to ultraviolet light — which is why your lines dry out and crack. Manufacturers can't just add more plasticizers to keep the line from cracking because they have to worry about durability. Some manufacturers, such as Cortland, Orvis, and Rio, use "topcoats" to armor the PVC. A fly line is subjected to enormous stresses — Airflo's Rajeff says that a 6-weight rod will move the tip of the line faster than 200 mph as it comes whizzing by your head. All the while, the line is being scraped against the rod guides. If a line sports too many plasticizers, it becomes too soft and sticky, with a tendency to pick up dirt. And if there's not enough PVC to provide an underlying structure, the line will fail. Thus, softness is more or less inversely proportional to durability; designers must balance the competing forces to get a working fly line, and some compromises (such as eventual cracking) are unavoidable.

Slickness requires another balancing act. Scientific Anglers has touted its "Advanced Shooting Technology" (AST) for several years, but Cortland's Duraslik and Rio's Extreme Slickness Technology also include slickeners that go to the core. To make such a fly line, you must mix slickening chemicals into the "pancake batter" from the very beginning. As the outer surface gets worn down, new layers of embedded slickeners are constantly exposed.

But, because a fly line sees only a certain amount of wear during its useful life, other manufacturers — such as Orvis, with its Wonderline — take a different approach. "The Wonderline process is a very slick topcoat we apply at the factory in Manchester," explains Steve Hemkens, a product developer for Orvis. "Our 'Generation 2 and 3' Wonderlines have had progressively better adhesive to help hold the topcoat to the PVC line." As usual, there are drawbacks to both approaches: for instance, many to-the-core slickeners are liquid-based, so they can grab dirt. The slicker you make the line from the beginning, the more likely it is to quickly get dirty (ultimately affecting both slickness and floatability). Rio attempts to overcome this by using its Extreme Slickness Technology, which is a slickener based on solids. And gloss coatings can eventually wear off.

Cortland's Neil Webb believes that sinking lines wear more quickly than floaters. "The heavy dust in the line jacket is very abrasive, but we've got a product coming this fall that we believe will extend the life of sinking lines."

High and Dry

Most anglers know that microspheres have something to do with making a fly line float, but there's more to the story. Back before World War II, most fly lines were woven out of oiled silk, which floated only because the oil in them repelled the water; these lines were hydrophobic but not buoyant. After a half day or so, the oil in a silk line would lose its effectiveness and the line would sink, irritating the angler, who had to stop fishing to dry out his line or flip it around to fish the dry end. (This is why we have double-taper lines.) Engineers today rely on both hydrophobic chemicals and buoyancy to float a fly line.

A fly line's floatability is measured by its specific gravity — the ratio of the density of a given substance to the density of water. Water's specific gravity is a nice, round 1.0. If the specific gravity of a line is less than that of water, it will float. The smaller the number gets, the higher the line will float. Most fly lines have a specific gravity of about 0.8, although some have managed to get down to about 0.65. This has its own set of challenges, though. "Fly lines float best where they are thickest and can contain the most microspheres," explains Richards, "so that means the head is always going to float higher than the running line or the tip."

microspheres themselves are the source of some recent developments, which may begin to influence the entire community of line designers. Traditionally, microspheres have been tiny glass spheres, about the size of a grain of sugar, filled with air.

Because they are rigid, they don't cost designers too much in the way of structural strength. On the other hand, glass is heavy, which means that designers may not be able to use as many microspheres as they might like for a given line size. The new developments have come from using material other than glass for the spheres. Plastic — the logical alternative — costs manufacturers some strength, but it has unique properties (such as the ability to expand when the line is heated to cure), which can add new tools for designers seeking higher-floating lines.

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