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Trouts Journal

Understanding Fly Lines

Ivan Orsic / Sep 25, 2020

By Josh Jenkins from Scientific Anglers

In the age of fast action fly rods and brightly anodized fly reels, fly lines have fallen from the limelight. By definition, what separates fly casting from any other kind of fishing is the fact that the weight of the fly line carries the fly to its destination. In every other form of fishing, the weight of the lure or bait is used to load the rod and make the cast. In these cases, the line is simply a tether, but in fly fishing, the line becomes something dynamic as well.

Bear with me here: you could slap a fly line on a spincast combo and cast a fly. A fly line could load the rod and make a cast and the reel could retrieve line. Now imagine using a fly rod and reel with nothing more than thin monofilament for line. Would you be able to cast an Adams with any accuracy at distance? No, it would be impossible. You might be able to lob a heavy streamer, but then you’re relying on the weight of the fly to load the rod (not fly casting). If you consider all of that, an argument could be made that without fly lines, fly fishing wouldn’t exist.

Fly lines are basically a lengthy mass of line that loads the rod and carries a fly that has relatively little mass by comparison. So there’s two things to consider concerning the mass of the line: what is the total mass and how is that mass distributed along the length of the line. The first question is pretty easy to answer. A deer hair bass bug needs a heavier line than a standard dry fly. That same bass bug line is also probably too heavy for a delicate dry fly presentation. The total mass of the fly line is its “weight,” which is typically used to match to a specific rod. A number of years ago, line manufacturers began creating oversized lines for certain large fly applications. This basically means that the line is heavier than standard so that large flies can be cast on smaller fly rods. Overweighting fly lines works because most rods (especially fast action rods) can handle a range of line weights.

Tapering allows line designers to mold the fly line mass into different areas. Most weight forward lines have two tapers, a front taper and a rear taper. Imagine a corn dog; the stick is representative of the running line and the dog is like the head (combination of rear taper, belly, and front taper) of a fly line. The front taper transitions from the belly diameter down to the tip where the leader is attached. Front tapers control “turnover” or delivery of the fly. Short front tapers turn over aggressively and are great for large, heavy, and wind resistant flies. Long front tapers are better for small flies and delicate presentations. The rear taper is the transition from the running line up to the belly diameter. In general, short rear tapers are better for shooting line, where long rear tapers are ideal for false casting longer lengths and mending at distance.

It was not always easy to control fly line tapers however. The very first fly lines were made of horse hair. Soon after that, braided lines were created from silk. The braided construction of these silk fly lines allowed makers to selectively cut strands to taper the line. Silk line had to be dressed regularly to keep it floating and also had to be dried to prevent mold and mildew. The next big evolution in fly lines came with plastic coatings. Some still used a tapered, braided core with a thin sheath of plastic, but creating an accurate, tapered core proved to be difficult and costly. Modern fly lines use a level core and taper the coating, which is more accurate and repeatable in production. The coating and core not only determine the fly line mass, but also determine how the line acts in different environments. Supple cores and coatings are ideal for cold water trout fishing. Harder cores and coatings are reserved for tropical environments. If you take a trout line on a tarpon trip, you’ll notice that the coating feels sticky and doesn’t shoot well. If you were to take a tarpon line on a trout trip, you’ll notice that the stiff core and coating have so much coil memory that the line is unusable.

If we combine all of the points above, it’s fairly easy to lay out the differences between a tarpon line and a trout line. First off, the tarpon line will have a larger “weight” to cast large flies. Typical tarpon lines range from 10 weight to 12 weight for example. Assuming the trout line is mainly used for dry flies, a 4 or 5 weight would be sufficient. The tarpon line should feature shorter front and rear tapers than the trout line because a short front taper will aggressively cut through wind and turn over large flies with ease. A shortened rear taper allows the angler to quickly pick up and shoot line at moving fish.

By comparison, the trout line would benefit from longer front and rear tapers. Here, the long front taper delicately delivers the dry fly without spooking rising fish. The longer rear taper enables the angler to carry long distances of fly line while false casting for pinpoint accuracy. Once the line and fly have landed, the long rear taper also allows for mending to adjust the drift. Finally, the tarpon line should be built with a hard coating and core to combat the tropical heat, whereas the trout line should have a supple coating and core to reduce memory in cold environments.

Understanding these basic principles of fly line design can explain almost every aspect of a fly line. Total mass and mass tapering determine ideal fly size and how that fly is delivered while the fly line material determines what environment the line is best suited for. Roll all of these variables together, and you can decipher the complex world of fly line craftsmanship.


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