This question comes up every 8 months or so. The blood groove on a knife probably is derived from the channel present on swords, where it is called a "fuller". There are some persistent myths floating around about the function of blood grooves, from "releases the vacuum when the knife is thrust into a person" to "no functional use, purely decorative". Let's talk about these wrong answers first, before we talk about the right answers.
Wrong Answer #1: Releasing the Body Suction
Basically, this theory postulates that the blood groove is present to facilitate withdrawing the knife from a person/animal. In this scenario, it is said that the animal's muscles contract around the knife blade, and that this causes a vacuum, which makes the knife difficult to withdraw. But on a knife with a blood groove, blood runs through the blood groove and breaks the suction, so the knife can be withdrawn with less difficulty.
One problem is that there's no evidence that this suction ever really happens. Also, over and over again people report that there is no difference whatsoever in the difficulty of withdrawing a knife with a blood groove vs. one without. This is one theory that has been tested and found wanting.
Yes, I realize you may have heard this myth from your deadly knife instructor, or read it in a book somewhere. But the experts agree that it is false. If your knife can cut its way in, it can just as easily cut its way out, with or without a blood groove.
And with that, I am going to change terminology from "blood groove" to "fuller", since we all now know the so-called "blood groove" is not playing a blood-channeling function.
Wrong Answer #2: Purely Decorative
There is a grain of truth to this one. Although a fuller does play a functional role, on a short knife the effect might be so small as to be insignificant. Many believe the fuller plays a strictly decorative role on knives or swords under 2 feet long. As the knife or sword gets bigger, the fuller plays an increasingly important role. On smaller knives, it is indeed probably just decorative.
Okay, so what substantive role does the blood groove/fuller play? The bottom line is, it does two things:
1. It stiffens the blade
2. It lightens the blade
That first statement has been the subject of some controversy, with some people sending me equations purporting to show that the removal of material cannot make the blade stiffer. I will table for now the question of "does the blade get stiffer, in some absolute sense, due to the fuller?" Rather, I'll weaken the claim to say that the blade *feels* stiffer to the user who is waving it around -- because it's stiffer for its weight.
I'll reproduce a post by Jim Hrisoulas which lays things out clearly (reprinted with permission):
When you fuller a blade you do several things:
1: You lighten it by using less material, as the act of forging in the fuller actually widens the blade, so you use less material than you would if you forged an unfullered blade. (In stock removal the blade would also be lighter, as you would be removing the material instead of leaving it there).
2: You stiffen the blade. In an unfullered blade, you only have a "single" center spine. This is especially true in terms of the flattened diamond cross section common to most unfullered double- edged blades. This cross section would be rather "whippy" on a blade that is close to three feet long. Fullering produces two "spines" on the blade, one on each side of the fuller where the edge bevels come in contact with the fuller. This stiffens the blade, and the difference between a non-fullered blade and a fullered one is quite remarkable.
Fullers on knives do the same thing, although on a smaller blade the effects are not as easily seen or felt. Actually looking at fullers from an engineering point of view they really are a sophisticated forging technique, and it was the fullered swordblade that pointed the way to modern "I" beam construction.
When combined with proper distal tapers, proper heat treating and tempering, a fullered blade will, without a doubt, be anywhere from 20% to 35% lighter than a non-fullered blade without any sacrifice of strength or blade integrity.
Fullers were not "blood grooves" or there to "break the suction" or for some other grisly purpose. They served a very important structural function. That's all. I have spent the last 27 years studying this and I can prove it beyond any doubt...
All Comments are Welcome and Appreciated.