I don’t think there’s another knot as closely linked to fly fishing as the blood knot. Although not especially fast or easy to tie, few knots rival it in terms of strength, smoothness of shape and versatility.
The blood knot is probably best known for joining monofilament segments of varying lengths, diameters and breaking strengths to produce tapered leaders that turn over easily, allow a fly to gently land on the water’s surface and help to produce a drag free drift. The blood knot’s smooth, elongated shape enables it to slip easily through rod guides and keeps it from picking up slime or debris in the water.
To tie a blood knot, overlap 4 to 6 inches of the two line segments being joined. I generally like to start with the lighter weight segment in my right hand and the heavier in my left. Place the right over top of the left. Begin wrapping so when the tag end of the lighter material is pointed up, it’s being wrapped toward you. Take 4 wraps in this manner. More about the number of wraps later.
After the fourth wrap bring the tag down and then come up between the two line segments where they intersect. Leave about two inches of tag extending beyond the intersection. With the thumb and index finger of your left hand, pinch that intersection point to hold it tight and keep it open. Now, take the tag end of the heavier material and start making wraps in the opposite direction. In other words, when the tag is pointed up, you're pushing it away from you to take wraps. Once again, make 4 complete wraps with the tag. Then, run the tag end through the open hole at the intersection in the opposite direction as the first tag. While making sure the tags don’t pull out, begin to gently draw the knot closed. When using mono, this is where you need to lubricate the knot, most people use saliva. Continue to draw the knot closed until it forms a nice, symmetrical barrel shape, like so.
When correctly tied, the wraps on both sides of the knot should be fairly even and running in the same diagonal direction. The tag ends should protrude in a gentle “S” shape. Up close, it really is a very pretty knot, if there is such a thing.
I’m going to once again tie a blood knot but this time using two different colored pieces of 12 pound mono. With 4-6 inch long tags, place the right over top of the left and take wraps so as the tag comes over the top, it’s coming toward you. I’ll take 5 wraps which is pretty standard for most blood knots. Pass the tag between the two lines that form the intersection and then pinch that intersection to keep it open. Get hold of the heavier tag and take 5 wraps in the opposite direction. So as it goes over top, it’s being wrapped away from you. After 5 wraps, feed the tag through the open intersection in the opposite direction as the first tag. Get the knot aligned, making sure the tags don’t pull out of the intersection and then gently draw the knot down a little bit, so it looks something like this. You can actually see the beginnings of the even-angled wraps and the gentle “S” of the tags before the knot has been closed. Lubricate the knot with saliva, lib balm also works well. Then rapidly pull in opposite directions on the standing line to firmly close and seat the knot. If done correctly, it should look something like this.
One of the wonderful things about the blood knot is the tags can be snipped off really close so there’s basically nothing protruding from the knot.
Here, I’m going to show how to tie a blood knot using more realistically sized materials. Just imagine you’re attaching a length of 6x tippet material to the end of a leader that tapers down to 5x. The knot tying procedure is exactly the same as before but managing the lighter weight, harder-to-see materials is definitely more challenging. Yes, you could use the simpler double or triple surgeons knot to do this, but the blood knot generally has a better shape and allows you to snip the tags off closer.
As far as number of wraps go, I’ve used as many as 7 on each side of 6 and 7x and gotten away with as few as 2 wraps going from 0x down to 1x. I think in most cases, 4-5 wraps on either side is plenty.
In addition to making tapered leaders and tying on tippet material, blood knots can be used in dropper rigs. Just leave one of the 2 tags long enough to tie on a second fly. Or you can add weight to the tag so if you get hung up on the bottom, the weight will simply be pulled off, or the tag will break while you attempt to free the rig.
It takes practice to tie blood knots quickly and correctly, but it’s well worth it. A blood knot is something that should be in every fly angler’s bag of tricks.