5/7 Blood Knot
The Blood Knot is an excellent way to connect two pieces of monofilament. It’s thin in diameter and very clean (no protruding bits to catch weeds or other obstructions). Back in the days of gut leaders it was the knot of choice. When nylon appeared, the Blood Knot hung on, but with the same restriction that it had in the days of gut: it is not good for connecting pieces of mono that are more than .002″ different in diameter. This meant that an angler building a leader down from .020″ to 3X would have to connect .020″ to .018″ to .016″ to .014″ to .012″ to .010″ to .008. Whew, that a lot of knots. I can personally testify to the difficulty of following a leader formula that had 7 pieces, some of which were only 6″ long. Ugh! As I experimented with nylon, I realized that its properties were as different from gut as day is from night. They are simply not the same in any way except that one can get them in a variety of diameters.
In the olde dayes, if one reduced the diameter of the gut sections by more than .002″, hinging would occur. That is, the thinner section would collapse back onto the thicker section. Not so with nylon. One can reduce the diameter of each subsequent section of nylon in a leader by 35% and not get hinging. That means you can connect .020″ mono directly to .013″ mono and not have a hinging problem. Now a leader can go from .020″ to .013″ to .010″ to .008. Fast and efficient. Oops, the standard Blood Knot can’t make the jumps.
Here’s the problem. When the material only varies by .002″ the five coils of mono on each side of the knot pull up smoothly and evenly. When one exceeds this .002″ rule, the thin material pulls up faster than the heavier material, and the knot can’t come completely tight on the heavier material side. As I experimented with the knot, I discovered that as the diameter difference increased beyond the allowable .002″ more turns were required with the thinner material in order to make a strong knot. In fact, for each .002″ beyond the allowable, initial .002″, one more turn was required by the thinner material. Thus, when connecting .020″ to .014″ (a difference of .006″) the knot is perfectly balanced using 7 turns with the lighter material and 5 with the heavier material; thus, the 5/7 Blood Knot.
The reason, it turns out, is that the 7 turns of the lighter material takes the same amount of time to turn over and pull up as does 5 turns of the heavier material. Furthermore, once the 5/7 knot is tight, the distance from the center of the knot to each end of the knot is the same–the knot is balanced in the way it tightens and in its size from the center to each end.