1. What Is the Short Piece of Line Tied to the End of Braided Line For?
Follow a braided line from the reel to its tip and you may find a second line, often a different color, tied on. It is called a leader (in Japan, a "shock leader"). A Japanese outdoor magazine says it is usually about 1 m long and made of nylon or fluorocarbon [2]. The idea is not only Japanese. The Wikipedia article on fishing line describes the leader as the part nearest the hook, and notes that a leader rated weaker than the main line can act as a sacrificial "fuse" that protects more costly tackle [3].
The Japanese magazine gives two reasons for using a leader with braid. First, braid is weak against abrasion: when it rubs on rocks, its fine fibers can break almost at once, while nylon can wear for a while before breaking. Second, braid is weak at knots, and the leader supplies the knot strength braid lacks [2]. A Japanese tackle maker's beginner guide agrees that braid is weak against sudden shock and against rubbing on rocks and other underwater objects [4].
That is the answer in outline. The rest of the article asks why braid has these weak spots when it is so strong, using the numbers one step at a time.
2. How Can a Fishing Line Be Stronger Than Piano Wire?
Braided line is made by twisting or weaving many fine fibers of polyethylene together. The magazine describes four- and eight-strand lines as common, and says some products use twelve. It also says more strands can make a line rounder and stronger when pulled straight [2]. The fiber in the line described here is an ultra-high-molecular-weight polyethylene. Its Japanese maker, Toyobo, says it has some of the highest strength and stiffness of any organic fiber, "about eight times as strong as piano wire when compared at the same weight" [1].
The words "at the same weight" matter. Steel is heavy, so a given weight of steel makes a thin strand, while the same weight of this plastic makes a much thicker one. The "eight times" is therefore not a comparison of two threads of the same thickness, and it does not mean that one fishing line is eight times stronger than a piano wire. It says the plastic gives more strength for each gram.
It is also light. Compared with water (density 1), a maker's comparison table lists braid's fiber at about 0.97, nylon at 1.14, and fluorocarbon at 1.78 [5]. A Japanese tackle maker's technical page gives the same figures for nylon and fluorocarbon [6]. Among these values, only the braid fiber is lighter than water. They are typical values, and they vary a little between products.
3. How Much Strength Does Braided Line Lose When You Tie a Knot?
The same comparison table reports lower strength at a knot than when pulled straight. It gives the strength left at a knot, as a share of the line's strength when pulled straight: nylon keeps 70 to 80 percent, fluorocarbon 60 to 70 percent, and braid only about 40 percent. It also says braid is about 2.5 times as strong as nylon when pulled straight [5].
Here is a simple calculation to see what that means. Call the straight strength of a nylon line 1. After a knot it is about 0.7 to 0.8. A braid of the same thickness starts at 2.5 and keeps 40 percent, so 2.5 × 0.4 = 1.0. Its lead over nylon shrinks from 2.5 times to about 1.25 to 1.4 times. (This is my own arithmetic from one company's table.)
Sources disagree on the size of the effect. The Japanese magazine puts braid's knot strength at about half of nylon's, and says tying braid straight to a lure can cut its strength by more than half [2]. If braid were only twice as strong as nylon, the same calculation would give 2 × 0.4 = 0.8, no better than a knotted nylon line. The sources report different ratios, and the exact result may depend on the test and the knot. What the sources agree on is the direction: braid gives up a bigger share of its strength at a knot than nylon or fluorocarbon does.
4. Why Does It Matter That Braided Line Hardly Stretches?
The magazine gives a stretch of about 4 percent for braid and about 30 percent for nylon [2]. A little stretch has two effects. A line that stretches soaks up the jerk when a fish pulls suddenly, so the fish is less likely to get away, but it is slightly less sensitive. A line that barely stretches passes small tugs to the angler's hand, which makes bites clear, but it handles sudden shocks poorly. A Japanese tackle maker lists nylon as shock-absorbing, fluorocarbon as stretching less than nylon, and braid as having very high sensitivity but weakness against sudden shocks [4].
A testing-machine company, Shimadzu, pulled fishing lines apart in the lab. Each line was 0.175 (a thickness code, no unit given) and was gripped 200 mm apart. Four kinds of nylon stretched 36 to 41 mm before breaking, and one fluorocarbon line stretched 43 mm. Two metal lines stretched about one-tenth as much. Braid was not among the lines tested [7].
Be careful with those numbers. 36 to 43 mm out of 200 mm is roughly 18 to 21 percent, but that is how far the grips moved, not a measurement of the line alone, so it should not be compared directly with the magazine's 30 percent. The safe reading is that nylon and fluorocarbon stretch a lot, and a stiff line stretches little. A nylon or fluorocarbon leader adds a section that can stretch, and the sources describe stretch as helping to absorb sudden pulls.
5. How Do Nylon and Fluorocarbon Behave Differently in Water?
Anglers choose between the two leader materials for their behavior in water. Kureha, the company that makes the Seaguar brand, says its fluorocarbon line absorbs no water, sinks quickly because it is dense, is hard to see in water because light bends in it almost as it does in water, and stretches less than nylon, which makes bites easier to feel [8]. The Japanese tackle maker's technical page agrees that fluorocarbon absorbs less water than nylon and is harder for fish to see, and lists nylon as one that absorbs water and ultraviolet light and wears out faster [6]. The Wikipedia article also says fluorocarbon is denser and less buoyant than nylon and has a refractive index close to water's [3].
All of these are descriptions from makers, who sell the products, and the sources give no numbers for how much harder to see or how much faster to wear out. They are best read as a general direction. Fluorocarbon is the one that sinks and hides; nylon is the one that stretches. Neither wins everywhere, so anglers pick by the job.
6. How Did Silk, Nylon, Fluorocarbon, and Braid Come to Be Fishing Lines?
Traditional fishing lines were made of silk [3]. Nylon came out of the research of the chemist Wallace Carothers at DuPont in the 1930s and went into production in 1939, first aimed at the silk-stocking market [9]. The sources I read do not say when nylon first became a fishing line. Kureha says that in 1971 it developed and put on sale the world's first fishing line made of fluorocarbon, which it describes as polyvinylidene fluoride [8]. That "world first" is the company's own claim. For braid I could confirm only the material: I did not find a reliable date for the first braided fishing line.
The new materials did not replace the old ones. All three are sold side by side today, and tackle makers describe different strengths for each [4]. Using a braid with a nylon or fluorocarbon leader is a way of getting some of the good points of both.
7. How Can You Check These Numbers Yourself Without Touching a Hook?
Try changing 2.5 to 2 in the calculation in section 3. What number do you get for 2 × 0.4? Then look at what each number compares. "Eight times" compares weight, "2.5 times" compares straight pulling, and "40 percent" compares a knot with the same line before it was knotted. If you have a chance, ask an adult who fishes to show you the label on a spool of line for its material and thickness. Hooks and cut line can hurt, so look only at the label, and ask an adult before touching any tackle.
Two questions this article cannot answer: how different knots change a line's strength, and when braided fishing line first went on sale. They would make good next searches.