blood groove

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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.

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withdrawn with less difficulty.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.

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:

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):

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...