Choosing a fly rod can be a little confusing, but the same goes for fly line, too. The selection of rods and lines is straight-up immense, and the marketing behind them all can get rather repetitive and even sometimes ridiculous. While both rods and lines are designed for usages ranging from very specific to more general, it's important to try and pair both together as best you can. You can certainly pick out any fly rod and couple it with any matching floating line, but the performance won't always be optimized. Whenever I choose a fly line, I always consider the head/taper design and grain weight so that it will allow the rod to perform at its very best. The difference may not always be huge, but why not try to aim for peak performance?
Rod action basically refers to how a rod flexes. A fast-action fly rod will bend more in the upper part of the blank while casting, while something like a moderate-action rod will have a deeper flex. Since that fast-action rod bends more towards the tip, it will typically also have a faster recovery rate, meaning it will return to being straight quicker than a rod with a longer, deeper flex. Other factors can affect recovery rate though, too.
As seen above, AFFTA (American Fly Fishing Trade Association) sets forth standardized grain-weight ranges for fly lines labeled from 1–15 weight sizes. Each given grain weight is a measure of the first 30-feet of fly line, minus the short level tip section. For example, a WF5F fly line should have a grain weight between 134–146 with an ideal weight being 140 grains. I said should in that sentence because these days those numbers aren't always adhered to. This is why you may see a WF5F fly line with a grain weight that actually puts it squarely in the 6-weight zone according to the AFFTA chart. No, that's not a mistake on the manufacturer's part! You see, as fly rods have evolved, so have fly lines. The result is that manufacturers often build fly lines heavier than the guidelines indicate.
In addition to grain weight, another important fly line component is the design of the head/taper. The heavier, more aggressive fly lines I mentioned above often have a shorter overall head with more grain weighting concentrated towards the front of the head. I like to call this "usable weight." Aside from extra power, this also allows the angler to access more of the head's weight with less line out, thus loading (flexing) the rod a bit more. RIO Grand fly lines are a perfect example of both the weight and head/taper examples. These lines are built a full line size heavier than industry standards, and also concentrate extra weight towards the front of the line's head—perfect for use with fast rods. On the other side, RIO Gold fly lines sport longer heads that are less aggressive, along with 30-foot grain weights that fall within the proper AFFTA specs.
Image Courtesy of Redington
The basic rule of thumb I keep in mind is that for fast or extra-fast actions, I typically gravitate more towards the heavier, more-aggressive lines. With softer rod actions, I seek out less-aggressive lines that fall more within average specs.
Fast and extra-fast action fly rods are quite common these days, and lines that are built heavier than the standardized specs (like the RIO Grand) perform well on these rods. As mentioned, a rod in this category flexes more towards the tip and thus feels stiffer. While these rods boast extra power to throw a big fly or cast a long ways, a heavier-than-average fly line helps these rods load and cast more efficiently without feeling so numb, especially at close range when there's not much line out. If you have a very fast rod paired with a line that's not quite as heavy, the rod may lose feel on short presentations since there's not enough weight to load the rod well. All of this talk might make you say that not following the standardized specs really throws off the whole ratings system, and you'd definitely have a point there, but that's a whole other debate!
In contrast to an extra-fast or fast rod, a moderate-action rod readily flexes more deeply down the blank and therefore doesn't require such an aggressive line to help it load. With the longer, more proportional head and lighter grain-weight rating, a RIO Gold line would likely be the better choice here out of the two lines previously mentioned. While softer rods like these are easier for beginners to learn on and are good choices for gentler presentations, they don't have quite the punch or available casting power as a faster rod.
Someone might be casting one of the finest rods on the planet, but it may not feel that way if the line is too light or too heavy for the rod action.
In my experience this is nothing that has ruined a fishing day (even in the most drastic example I can think of), but when you get a good rod/line pairing you'll know it. Casting is more effortless, accuracy improves, and satisfaction peaks. If you haven't already gotten into the habit, start paying close attention to line descriptions and studying head/taper diagrams closely. Compare all the different lines that interest you....don't be so quick to just pick out any old fly line to pair up with that brand new rod!
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