Before leaving the issue of tapers, here's a brief aside about line weights. Line weights have been standardized as the weight of the first 30 feet of the fly line (absent any level tip section). Most anglers assume that rod makers design their wares to load with that amount of line. Instead, they normally design rods to load with the length of line "typical" of the fishing situation. For our typical 8-weight, that length of line might be 40 or 45 feet instead of the standard 30. Thus, a good caster who can aerialize 40 feet or more of line might find a line such as the SA Mastery Bonefish or RIO Bonefish balances perfectly. A less accomplished caster might become frustrated trying to carry this much line overhead, and may find that the rod doesn't load well with a shorter length extended. A line with a shorter head, such as the Ultra 3 Bass/Saltwater, may work better.
Line tapers are, of course, easily and typically "modified" every time we change the length and taper of the leader. It's not unusual to go out for an afternoon of bonefishing (substitute any favored species) and to take along, say, an 8-weight rod matched with a 9-foot leader. As the afternoon breeze dies, we might switch to a 12-foot or, in extremely skinny water, even a 15-foot leader. That change will provide a "softer" presentation, but it's not just because we have 3 to 6 feet more leader. For a given length of cast, we also have 3 to 6 feet less line extended.
Core and Coating
Fly-line coatings are commonly either polyvinyl chloride (PVC) or another polymer, such as polyurethane. Practically, one has no inherent advantage over the other as far as I can tell, in spite of manufacturers' admonitions to the contrary. Durability is a primary consideration, and lab tests tend to indicate differences. Oddly, those tests tend to favor the lines of the manufacturer conducting the tests. Regardless, each coating has been used to make great casting lines and, on occasion, some that don't quite measure up.
Manufacturers do put various goodies in the coating that are important. There may be "magic dust" to make the line float or sink, as discussed below. Most add plasticizers to keep the line supple and lubricants to make it shoot well.
One aspect of the coating-core configuration that gets compliments and, more often, complaints is the relative stiffness or limpness of a particular line. Such criticism could be directed at virtually any fly line, since most are designed to perform best under a relatively limited range of conditions. Stiffness or limpness is determined by varying both the chemistry and material in the core and the coating of the line. Cores, in particular, can affect stiffness.
Cores are typically either braided multifilament nylon, braided monofilament nylon or monofilament nylon, and the choices aren't related to strength. The multifilament cores are generally suppler and thus best suited to all-purpose and cool-weather lines. The braided monofilament cores are typically stiffer, and are used more in temperate and tropical lines. Monofilament core lines had their claim to fame in the early tarpon "slime" lines, but the popularity spread to other applications, including fresh water. Because the suppleness of monofilament can be controlled, today's monocore lines can be tailored for tropical or cool-water conditions.
Again, let's use two SA lines for comparison. The Mastery Bonefish line described above has a braided monofilament core. The tapers and coating of an SA Mastery Saltwater line are identical to those of the Bonefish line, but the Saltwater line has a braided multifilament core. (OK, the colors are different, too.) And because it is less stiff than the Bonefish line, it is the better choice in cooler weather, as it retains less memory.
Regardless of the approach, the objective is to produce a line that performs best for a given fishing situation. For an angler to expect that a single line will perform as well in tropical heat as in mid-November nor'easters is as unreasonable as expecting the same clothing to perform equally well in both situations.
Whether a fly line floats or sinks is determined largely by additives the manufacturer incorporates into its coating. Typically, hollow-glass microballoons are added to make the coating less dense than water and thus produce a floating line. Tungsten powder or other dense materials (but no longer lead) are added to coatings to make them sink. Controlling the proportion of tungsten in the coating makes it possible to control the sinking rate of the line.
Floating lines are far and away the most used type of fly line. But with more and more anglers fishing in off-flats locations, intermediate and sinking lines are becoming staple lines. The increasing demand has resulted in increased attention from manufacturers, and the benefit to us anglers is an abundance of better-casting, better-performing lines.
Not too long ago, sinking-head lines were either homemade or offered by manufacturers with a level section of sinking line integrated into the running line - no tapers. These lines, and the heavily weighted flies often matched to them, resulted in the "chuck-and-duck" style of casting. False casting was, at best, not a pretty sight. Today, several manufacturers offer integrated-head lines that include features such as density-compensated front tapers and a thicker transitional section of floating running line. Such lines can be false cast and are much better than the earlier lines.
Manufacturers provide approximate sinking rates for their sinking lines. Determining the density of line that is best suited to a particular set of conditions still involves some trial and error, but usually your local fly shop will have the skinny on which setup works best for a particular application. Compare that with the advice experienced fly-fishers offer (see "What the Pros Use"), and you'll likely be "in the meat."