Student and Jack are both correct. Here’s some more crap to consider.
Fishing rods, no matter what action, are load-bearing tools. A fly rod doesn’t know or even care if it is throwing 3 weight or 6 weight line, as all it responds to is the load/weight it is carrying. If the load is more than it is capable of handling, it won’t perform. If the load is less than it is capable of handling, it won’t perform.
All rods, no matter what action, have a certain loading point on the performance/design curve that is the optimum point of performance for that rod. All rods have this optimal design point for carrying weight irrespective of action and that weight is determined by (a) length of line times (b) line weight (density). Now, the key to making a good rod of any action is making the performance curve as flat as possible for as long as possible on both sides of the optimal design point
Now, the industry standard for the weight standard for determining a rod’s line rating is 30 feet of line so the only variable is then line size. If a particular rod is under loaded at 30 feet of 4 weight line, you shouldn’t increase the feet of 4 weight line to 50 feet to increase overall weight you should increase the line weight to 5 holding 30 feet constant. Although never exact, the line weight used at 30 feet of line that comes closest to performance at the rod’s optimum design point is supposed to be the rod’s rated line capacity irrespective of action.
So, in theory, if you needed to throw 70 feet of 6 weight line, in the truest sense you should get an accurately labeled 8 weight rod of any action and underline it with 6 weight line.
But that is not what is happening today.
Traditionally speaking, fast action, just like medium and slow action, is a type of rod taper/performance and fast action really shouldn’t have anything to do with the ability to throw more line than any other taper. However, somewhere over the past 30 years fast action has become associated with bombing 90 foot casts.
Complicating this is rod companies are now labeling rods by who knows what standard. A 9 foot, fast action rod labeled “4 weight” by pure definition should not be able to carry 80 feet of 4 weight line in the air. This is not a true 4 weight rod – it is a 6 or maybe even 7 weight rod labeled 4 weight and people think it is a great rod because it throws 80 feet of 4 weight line. The fact is all the rod cares about is load and 80 feet of 4 weight line is probably pretty close in overall weight to 30 feet of 6/7 weight line. It would be like taking a Corvette chassis and engine and welding on a VW Bug body and then saying, “My VW Bug can go from 0-60 in 4.2 seconds and has a top speed of 190 mph.” It’s not a VW Bug it’s a Corvette labeled VW Bug.
That being said this is now where the performance curve of a rod comes into play.
When fishing you are not throwing the design weight every cast but rather using the rod over a range of performance points (i.e., line distances change continually, which changes the carrying weight). So really you want to get a rod that performs best over a range of casting distances that you will be fishing. This is where “fast action” starts to get associated with distance casting and slow/medium action gets associated with up close casting.
While I may be wrong my understanding of performance of a slow/medium action rod is that these rods have a flatter performance curve skewed below the optimum design point while the fast action rods have a flatter performance curve skewed above the optimum design point. In practical terms this is why slow/medium action rods are better able to load/cast at shorter distances than fast action rods while fast action rods are better able to load/cast at longer distances than slow/medium action rods. This is where the whole thing of line distance starts to come into play and fast action begins to get associated with longer distances.
Further compounding the problem is as I noted before, most fast action rods, except the high end ones, have too stiff tips. For close distance casting with a fast action rod you are casting off the tip with more of a flick of the wrist as opposed to a casting stroke. Poorly made fast action rods can’t do this and this is why many people end up over lining their fast action rods for up close work.
The fact is rod selection all comes down to how you are going to fish it. If you are going to fish mostly light tippets in the 15 – 30 foot range, more than likely a 3 weight fast action rod isn’t the rod you want. You probably would be best suited with a 4/5 medium action rod unless you got yourself a high end fast action rod that does have a soft tip. If you are fishing waters like the Delaware where you regularly need to make 50-60 foot casts and you’re not using tippets much less than 5x, a medium action 5 weight rod probably isn’t the rod you want.
As for salt water fishing, salt water fishing is not about finesse and tippet protection, it is about throwing a large wind resistant fly in windy conditions a long distance. This requires line speed and tight loops and while Lefy Kreh and Joan Wolff could certainly put us all to shame with a slow action rod, 99% of us benefit from a heavy duty fast action rod with a lot of backbone to accomplish this. I complete agree with Student that fast action is the way to go in salt water however, going back to pure technicalities, most 9 weight fast action salt water rods aren’t 9 weight rods based on the 30 foot rating criteria, they are 11 or 12 weight rods. But the fact is circumstances in salt water (long powerful casts) are best suited to fast action rods for the vast majority of us.
