If you've spent much time nymphing deep runs, you've probably heard someone rave about the 20 incher fly pattern. It's one of those flies that just looks like "fish food" the moment you see it in a fly box. There's no fancy foam or high-tech synthetic flash—just a whole lot of peacock herl and pheasant tail that seems to drive trout absolutely crazy. I remember the first time I tied one on; I was a bit skeptical because it looked a bit like a messy version of a Prince Nymph, but three casts into a fast riffle, a solid brown trout proved me wrong.
The beauty of this fly is that it's not trying to be a specific bug, yet it manages to look like everything at once. Is it a large stonefly? Is it a beefy Mayfly nymph? Honestly, the fish don't seem to care about the taxonomy as much as we do. They just see a heavy, buggy profile tumbling along the bottom and decide it's worth a bite.
The story behind the fly
We can't really talk about the 20 incher fly pattern without giving a nod to its creator, Charlie Craven. If you know anything about fly tying, you know Charlie is a master of creating patterns that are both incredibly effective and relatively easy to mass-produce at the vise. He originally designed this fly to be a "confidence fly" for big water, particularly in the West where stoneflies are a staple of a trout's diet.
The name itself is a bit of a challenge. It's called the 20 incher because, well, it's supposed to help you land fish that hit that magical twenty-inch mark. It's a bold claim, but the fly usually backs it up. It has that perfect combination of weight, silhouette, and movement that triggers an aggressive strike from larger, more territorial fish that aren't interested in tiny midges.
What makes it so effective?
You might wonder why a bunch of feathers wrapped around a hook works better than some of the more modern, "realistic" patterns. The secret sauce of the 20 incher fly pattern is definitely the peacock herl. If there's one material that has caught more fish than any other in the history of fly fishing, it's peacock herl. It has this natural iridescence that reflects light in a way that looks like the trapped air bubbles or the metallic sheen of a real insect's exoskeleton.
Then you have the pheasant tail fibers. These are used for the tail, the wing case, and the legs. Pheasant tail is naturally mottled, giving the fly a lifelike, segmented look. When you combine the shimmering green of the herl with the earthy browns of the pheasant tail, you get a fly that stands out in murky water but still looks natural in clear mountain streams. It's the ultimate crossover nymph.
Tying the 20 incher fly pattern
If you're sitting down at your vise to tie some of these up, you'll find it's a pretty satisfying process. You'll want a 3XL long nymph hook because this is a big bug. Most people start with a gold bead head—tungsten is better if you really want to get deep—and then add some lead or lead-free wire wraps behind the bead. You want this fly to sink like a stone.
For the tail, you're using pheasant tail fibers. Then comes the ribbing; I usually go with fine gold wire to give it some extra flash and to protect the delicate peacock herl from the teeth of hungry trout. After you've secured the wire, you wrap a thick bunch of peacock herl forward to create a chunky, segmented body.
The "wing case" is just more pheasant tail pulled over the top of the thorax. The trick to making the 20 incher fly pattern look really "buggy" is in the legs. You take those same pheasant tail fibers and fold them back along the sides. It gives the fly a wide profile that creates a lot of vibration in the water. I like to finish mine with a little bit of dubbing right behind the bead to hide the thread wraps, usually in a dark brown or black.
How to fish it properly
This isn't a fly you want to fish on a light 2-weight rod with a delicate presentation. The 20 incher fly pattern is a heavy hitter. Most of the time, I'm fishing it as the lead fly in a two-nymph rig. Because it's so heavy, it acts as an anchor, pulling your smaller dropper fly down into the strike zone much faster.
You want to fish this deep. If you aren't occasionally ticking the bottom and losing a fly to a submerged log, you're probably not fishing it deep enough. I usually use a strike indicator and adjust it so the fly is drifting just inches above the riverbed. In faster water, don't be afraid to add a bit of split shot a foot or so above the fly to make sure it doesn't just sweep over the heads of the fish.
Another great way to fish it is with a "dead drift and swing" technique. Let the fly drift naturally through the head of a pool, and as it reaches the end of the drift, let the line tighten and allow the fly to swing toward the bank. Sometimes, that sudden change in direction and the way the pheasant tail legs flutter in the current is exactly what it takes to trigger a strike from a big trout that's been watching it the whole time.
When should you reach for it?
While you can fish the 20 incher fly pattern year-round, it really shines during the spring and early summer when stoneflies are active. When the water is a little high and off-color from snowmelt or rain, the dark, chunky silhouette of this fly is easy for fish to spot.
It's also a fantastic "searcher" pattern. If you're on a new piece of water and there's no obvious hatch happening, throwing a 20 incher into the deepest, darkest holes is a great way to see who's home. It's large enough to be a meal, but not so intimidating that a smaller trout won't take a crack at it too. I've caught everything from six-inch brookies to twenty-plus-inch rainbows on this thing, often in the same stretch of water.
Variations to keep in mind
Like any classic, people have tweaked the 20 incher fly pattern over the years. Some people like to use a silver bead instead of gold, or even a colored bead like orange or red to act as a "hot spot." I've seen some versions tied with a bit of flashabou over the wing case for extra visibility in muddy water.
If you find that the standard size is a bit too big for your local creek, you can always scale it down to a size 14 or 16. In those smaller sizes, it starts to look more like a dark Mayfly nymph or a caddis larva. But honestly, there's something special about the original size 8 or 10. It's a fly with presence. It says, "I'm a big bug, come and get me."
Final thoughts on this classic nymph
At the end of the day, fly fishing is often about confidence. If you believe a fly is going to catch fish, you'll probably fish it better—you'll be more focused on your drift, more attentive to your indicator, and quicker on the hook set. For me, the 20 incher fly pattern is one of those flies I can always rely on. It's been a staple in my box for years, and it's earned its spot through countless successful days on the water.
If you haven't tried tying or fishing one yet, give it a shot. It might look a little old-school compared to some of the flashy new patterns hitting the fly shop bins lately, but there's a reason it hasn't gone out of style. It just works. Whether you're targeting that trophy trout of a lifetime or just want to have a productive afternoon on your favorite tailwater, you really can't go wrong with this buggy masterpiece. Just make sure you bring a few extras—once the fish start hitting them, you won't want to run out.