Fly rods are tools, and like any tool, the basic design can be tweaked for a desired outcome. What does a fly rod do? It stores energy, translating potential energy into kinetic energy with each stroke, then recapturing that energy before sending it rebounding again. It’s the same basic function that tennis strings, a bow string, or a rubber band provide. And this energy the fly rod captures and releases is translated into the fly line, which shoots in the desired direction while also unrolling (kinetic energy).
So if that’s the function of the tool, how do rod designers tweak it? One thing they can do is change the rate at which the rod loads and unloads. This can be modified by the choice and arrangement of materials – if the fly rod features materials of a type that can more rapidly load and spring back, and the fibers in the fly rod are arranged in a way that maximizes this spring-back, you get a faster “recovery”. This means the total time to load and unload the rod decreases, making the stroke faster.
Another thing rod designers do is change the amount of, and location of, fibers. This changes the region of the rod that flexes, and will impact the “tip speed” of the rod. The faster the tip speed, the tighter loops the rod can throw, and the longer distances it can be thrown at. This is a faster action. To flip that around, the slower the tip speed, the more open the loop is that the rod throws, and the shorter the distance that the rod can throw. This is a slower action.
Note that this is an extremely general statement – the difference in today’s modern rods in terms of casting distance is not huge (say 10′-20′ for a decent caster). Modern fly rods, featuring carbon graphite, glass, boron, and other materials in fiber form are leaps ahead in terms of “recovery” and “tip speed”. Taking the cheapest Cabela’s Three Forks 8 wt model, and casting it side by side with a Helios 2, an experienced caster will notice the difference. That said, both will cast effectively, and both will catch fish.
When do you want a fast action rod versus a slower action rod? For me, I love fast action rods in the salt – they help me cut through wind (less time unrolling meaning less time for the wind to change the line trajectory) and I can boom them a little farther. When blind casting, my fly stays in the strike zone longer. And for sight fishing, they let you reach out and accurately harass fish at a slightly greater distance. That said, they have weaknesses. At closer distances for sight casting (20′-40′), which is where most drum, bonefish, etc are seen and cast to, they may not adequately load. These rods are happiest throwing a lot of line – too little, and they lose accuracy. If you use them in tighter quarters, you have to “over-line” them (e.g. use a 9 wt line on your 8 wt fast action rod).
Slower action rods excel at trout fishing. They help you slow down your cast, synchronizing your movements with the slower loading and unloading of the rod, and they minimize your jerky, fish spooking movements. The best part: As they unroll, they help your tiny size 22 Griffith’s Gnat land gently in the film, rather than with a splat. They also have a place with close quarters saltwater fly fishing and freshwater bass fishing where you are routinely fishing in the 20′-40′ range.
In terms of fish fighting ability, slower action rods are more gentle on an angler. They hinge father down, closer to the handle, and hence this fish does not have the same leverage as on a faster action rod. Fighting multiple bad-ass fish like false albacore will wear you out more quickly on a fast action rod. There is a reason why most top tarpon anglers like slower action rods – accuracy at close range, and improved fish fighting abilities.
One final comment: Slower rods are easier to learn how to cast. You feel them load with a minimum of movement. The stroke is less exaggerated. With fast action rods, the more energy you impart to them with your body, they more they give you back. I also liken casting a nice, stiff rod to throwing a boxer’s cross – the same muscles are engaged, and it becomes a much more athletically engaging effort (which is a reason why I love them).
So my general suggestion: If you are new to fly fishing, get a slower action rod. If you enjoy being a little more dynamic when fishing, fish primarily in the salt, or are a more experienced caster, a faster action may be for you.