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Choosing Fly Lines for Maximum Performance

Choosing Fly Lines for Maximum Performance

Arian Stevens - photo

This story was originally titled “Direct Connect: Choosing Fly Lines for Maximum Performance.” It appeared in the 2021 Fly Fisherman Gear Guide issue.

Fly rods are remarkably lighter, stronger, and more responsive than they were even ten years ago. Reels are more thoughtfully designed to accommodate specific fishing situations. Our breathable waders even have stretch panels for comfort. But perhaps nothing has improved our fly fishing as much as recent advancements in fly line design and technology. These innovations have allowed us to modify and adapt our fly-fishing traditions so the lines can perform in hot, tropical weather, carry extremely large flies (see “Gearing Up for Muskies” on page 54), descend quickly, sink in a straight line for more sensitivity, stretch less for more efficient casting and hook-sets, shoot farther, float higher, and last longer.

Fly line tapers play a huge role in this, and you should choose a taper tailored for your specific situation. But you should also pay careful attention to the fly line’s core and coating. The core can be monofilament or multifilament. Monofilaments are used to produce stiffer lines for tropical saltwater fishing and for clear intermediate-sinking lines. Manufacturers have also become extremely proficient at using tight multifilament weaves (for stiffer line) or loose multifilament weaves (for limper line), and at modifying the coatings themselves to suit specific temperature ranges.

Braided nylon multifilament lines have from 20 to 30 percent stretch, which is helpful for stretching the line to remove the memory coils. Stretch in a line can also act as a shock absorber when you are playing a large fish. Less stretch gives you better sensitivity for feeling strikes, and more powerful hook-sets.


When Airflo lines first came out in the 1980s, they had cores of Kevlar with less than 1% stretch, and they suffered from excessive coiling that was difficult to remove. Airflo now uses a multifilament polyester core called Power Core that stretches about 7% to 12%. And RIO now uses a similar multifilament polyester core called ConnectCore Plus in their premium lines. (Dacron backing is a type of polyester multifilament.)


Fly line coatings are made from polyvinyl chloride (Scientific Anglers, RIO, Cortland) or thermoplastic polyurethane (Airflo). Thermoplastic polyurethanes (TPU) are highly elastic and are used to make roller coaster and skateboard wheels, synthetic fibers like Spandex, hoses, and coverings for cables and electrical wires. Pure polyvinyl chloride (PVC) is a white, brittle solid, and it’s used for household plumbing, doors, and windows. It can be made softer and more pliable with the addition of plasticizers, and in this flexible form it can be used to make imitation leather, inflatable products, and food packaging containers.

PVC can be chemically altered for specific uses, and all fly line manufacturers have their own proprietary “secret sauce” of plasticizers, lubricants, and additives that can work to make the line slicker so it slides through the guides more easily, and/or more durable so it stays slippery for longer. Examples of this include Scientific Anglers Amplitude lines with the AST PLUS slickness additive, and RIO’s new SlickCast coating on its Elite series fly lines.

Manufacturers have also adapted line weights to the changing rod market. As carbon fiber material and rod components become lighter and stronger, tackle makers are building rods that are less cumbersome, and often much stiffer and capable of casting greater distances than earlier rods. The problem for beginners is that these rods are harder to load when casting within their range. And the problem is the same for expert fly fishers who sneak close to a trout on a small creek, bass anglers who are casting up and around structure, and tarpon fishers who are searching under cloudy skies and don’t see the fish until it’s within 30 feet. In all these real-life situations you need a line that loads at short casting distances, so line manufacturers frequently overweight their products. According to American Fly Fishing Trade Association (AFFTA) standards, a 5-weight is supposed to weigh 140 grains in the first 30 feet. A 6-weight is supposed to be 160 grains. But in reality, the most popular lines are heavier than that. The Scientific Anglers Amplitude MPX is a half size heavy at 150 grains. A 5-weight Elite RIO Grand is 160 grains, and a Cortland Trout Boss is also 160 grains. Fly fishers used to discuss overlining or underlining their rods, and what they meant was putting a 6-weight line on a 5-weight rod. No one does that anymore, since you can buy a heavier line that still says “5-weight” on the box. It’s not uncommon for lines to be even two line weights heavy.

Heavier lines are not universally better than lighter lines, but they will help you cast larger flies and load the rod deeper in close fishing situations. A true-to-weight line will help you carry longer lengths of line in the air, and cast and fish at longer distances. It’s also better for presenting smaller flies accurately with stealth. Just be aware of what you’re buying, so you can get the line that matches your fishing situations.




*Ross Purnell is the editor and publisher of Fly Fisherman.

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