Make it easier to pull out.

The "blood groove"is actually called a fuller and its intent is to give the blade strength while reducing the weight somewhat. No one knows exactly when it was developed, but it was a long,long time ago, like well before the Vikings were ravaging England and Northern Ireland and is quite a technological achievement.Its advantage is evident in the use of swords. There is some debate on whether or not it has value on a hunting knife. I have read arguements on both sides whether or not it actually aids in knife removal.

Here is an expaination from A.G Russell's website:
What is a Blood Groove For?
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 But on a knife with a blood groove, blood runs through the blood groove and breaks the suction, so the knife can be withdrawn with less difficulty.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.

Right Answers:

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 statment 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 (re-printed 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...

Source: rec.knives Newsgroup May 1998

It depends on the lenght of the blade. If it s a short knife it is pure asthetics.
If it is a log blade the fuller can be applied to physics, strengh, and the release of suction when puncturing flesh. That being said there is no groove on the american bayonet...

buckhunter; I have never seen a katana with a fuller. Different manufacturing technics maybe?
Carl, thank you.

I am not posting this as an argument, just what I heard, that on fighting knives at least, the fuller would allow whatever stabbed to 'bleed out' quicker.

Although this doesn't seem likely for a combat knife, as you would have to stand there too long.

The only modern adaptation I saw was at a store in Georgia, which was an arrow with a 'blood channel' in it. The advert said it was to bleed the deer out faster, and make a better trail for tracking. I don't think it ever caught on generally.

This is an interesting topic. Possibly of myth-buster type.

I am going guess two thing:
The difficulty of pulling a knife or bayonet out which I have read about in several literary works as difficult at times is either an exaggeration or fictional

-or-

Am I mistaking the serrated edge as the culprit and not suction? I beleive the original Marine Corp knife had a serrated edge?

It may have some history, but the purpose was, I think, always questionable. Today, it's decorative.

NM was right on. The fuller is all about keeping the blade light and strong, and has nothing to do with blood letting. My katana has a fuller, and it is a real one not some piece of mall junk. I checked the Cold Steel catalog I had handy and all their katanas seem to have fullers too.

buckhunter: the katana is a slashing weapon

I never wanted to get close enough to see if it worked on a bayonet. Perfectly happy to use it opening ammo crates and C-Ration boxes.