The American Fishing Tackle Manufacturers Association (AFTMA) decided many years ago that there needed to be a standard length of fly line to determine the range of flies cast with a particular “weight” of rod. They devised a numbering system for fly line that was based on the weight in grains (1 grain = 0.0023 once) of the first 30 feet of line. For example a 5-weight fly line is required to weigh 140 grains for the first 30-feet (less the level tip of the line) plus or minus 6 grains (134-146 grains).
This system applied to all “types” of line, weight forward (WF), double taper (DT) etc.
It then became the responsibility of rod manufacturers to produce rods that conformed to that standard.
The original AFTMA system was, and still is great system, but one has to understand that it’s based on 30 feet of line weighing a certain amount and a rod optimally loading with that particular weight. Once that is understood, you can correctly match a rod with the range of flies and the distance or situation you plan to fish.
So what does that mean to the average fly fisher? Well in a perfect world it would mean that if you bought a rod (say a 5 weight) and a line (5 weight again) then that rod would optimally load with the first 30 feet of line beyond the rod tip. Why is this necessary; well when a fly rod is overloaded with a line heavier than the manufacturer calls for, it causes the rod to flex more deeply. This creates larger loops on longer casts and wastes casting energy by not directing it at the target. Casting with 40 feet of line rather than the recommended 30 feet is akin to loading your rod with the next size heavier line.
Unfortunately manufacturers (both rod and line) have not stuck to this simple plan and we are presented with a situation where one rod will load better with 25 feet and another with 35 feet using the same line! Some lines err on the heavier side of the AFTMA range and (e.g. 5 weight = 146 grains instead of 140 grains) to provide more power to the caster. In addition, line manufacturers have now introduced lines that are actually heavier than the AFTMA number printed on the box for the same reason!
So where does this leave us!
Every rod has its “sweet spot” when being loaded by the fly line installed on it and it would be nice if we could discover this spot.
So my recommendation is that when you purchase a rod or even if you already own a rod, you should determine the optimum length of line to effectively load the rod for the type of fishing you plan to do.
How can you do this; firstly measure 30 feet from the front of the forward taper and mark the line with a permanent marker. Install a 9 foot leader, extend the line out the rod until the mark is at the rod tip and make a few casts. Then reduce the head length by bringing the mark closer to the rod butt (reduce head by a rod length) and cast again. Repeat this but this time increase the head length by approximately a rod length. If you have a video camera then film yourself casting for later analysis. You should be able to feel and see which head length produces the most effective cast and this should be your datum for future casting with that rod and that line.
Remember that in future you should load the rod with that amount of line and use the line’s velocity to increase your distance by “shooting” line to the more distant target. Try to use the good practice of retrieving line to the optimum length for the next cast rather than lifting extended lengths of line off the water and thereby overloading the rod.
It is quite permissible to install one size smaller line weight on your rod or even one size larger to produce casts in different conditions. Should you be casting small streams with shorter casts then using the next heavier line will allow you to load the rod with a shorter head. A line size smaller will allow you to aerialize more line without overloading the rod and produce a longer total cast.