# The Truth of the Blood Groove

You might want to check the publication date…

You might have heard the term “fuller” used in HEMA circles as a euphemism for the blood groove found on swords. This is understandable, as we in the HEMA community want to distance ourselves from the violent roots of sword fighting, and rebrand as a respectable modern pastime. And you’ve also probably heard some un-truths related to the blood groove, so I’m here to straighten them out.

## What Is A Blood Groove?

The blood groove is a channel down the length of the sword, which can be seen on both European and Asian swords.

It is a common misconception that this is designed to allow blood to flow out of a wound, preventing a sword from getting stuck. But, given that I just said it is a misconception, why is this blood flow path not realistic? To understand why, let’s look at a little bit of fluid mechanics.

First of all is the issue of pressure. If you’ve ever had a blood pressure test you probably heard the result of something like “120 over 80”. This is a measure of your systolic and diastolic blood pressure, which (oversimplified) is the high and low of the pressure over the heartbeat cycle. These are measured in millimeters of mercury.

“But wait, isn’t measuring pressure in units of length weird and arcane?” you may ask. And the answer is yes. Before the invention of modern pressure measuring instruments the easiest way to measure the pressure of something was to hook it up to a stack of fluid, and see how high it could “push” the fluid up. If you have a fluid of a known density (like mercury) you know how much pressure it takes to push it up every millimeter, and the height that the mercury stack raises can be used as a measure of pressure you are applying.

What does this mean in terms of pressure, in a sensible unit of measure? A blood pressure of 120/80 mmHg translates into 16 kPa/10.7 kPa. Can we please get some context for this?

Why is there a star on the vacuum cleaner? Because it highlights an important point: that pressure is all relative. The values for air pressure are all set compared to a “zero” point. A vacuum cleaner, on the other hand, operates more along the lines of “20 kPa lower than the atmosphere in which it is operated”. Because if you had a vacuum cleaner operate at a fixed 80 kPa it would suck with a force of 20 kPa at sea level, and blow with a force of over 50 kPa at the top of Mt Everest!

This is the same principle at play when measuring your blood pressure. The pressure is measured relative to the air, which means that when you are at sea level your blood pressure is actually more like 100 kPa + 16 kPa. (And you thought it was too high already!)

The bottom line is this: your body is pressurized. You don’t need to drain the fluid to avoid suction; in a living body the circulatory system is pressurized and actively pushes blood outwards.

## Friction

The other thing that can impede removal of a sword (or any tool) is friction. We have two general types of friction, coulomb and viscous. Coulomb friction is the type you learned about in high school, because it is the type you can solve with high school math.

`Force of Friction = Normal Force * Coefficient of Friction`

“Normal Force” is not a reference to a half-assed estimate of what the force usually is, it is the term for the force of two surfaces pressing against each other. Let’s imagine you want to imitate your favorite video game block pushing puzzle, and move a 100 kg crate across the floor.

However there is a problem, the force of friction is too high and you can’t move the box!

• One solution is to take half of its contents out, lowering the weight of the box, and hence the normal force between it and the floor. And thus you cut the friction in half.
• The other option is to put some oil on the ground below the box, which lowers the coefficient of friction. The total force of friction would then be low enough that you can overcome it and move the full weight of the box. (Careful not to step in the oil.)

Now back to the sword. If the body exerts any sort of “clamping” force on the sword, it will be as a pressure exerted on the surface of the blade.

If you make the material thinner in the middle so it isn’t subject to this clamping force, would we get less friction on the blade?

Unfortunately for our would-be blood groove designer, hollowing out the center to relieve the contact friction is kind of a spin on the “what weighs more, 1kg of lead or 1kg of feathers?” question. The formula for friction is FORCE times the coefficient of friction. If you have the same force, but distribute it across the blade’s surface differently, you still have the same friction.

Could creating a blood groove reduce the total friction experienced by the blade? Only in the same way that any other change in the blade profile does. And it would not help relieve any forces which are external to pressure on the blade due to its width (any compression of the insertion point, possibly by a twist of the body it’s lodged into). Remember, the purported function of the blood groove was to allow fluid to flow and break the suction, so it’s a moot point to argue about friction anyways.

Bottom line is that there isn’t even a real mechanism for the blood groove to do any work.

End Note: Researching this section was terrible, as it looks like there is no research into this area, presumably because it’s a non-topic. Lots of studies on how the stab happens, but not what goes on after. Desperate keyword searches involving the suction of foreign bodies inserted into a person produced results ranging from unhelpful to naughty. The keywords were also really likely to give me hits on surgical suction knives, which are apparently a thing.

## Swords And Golf

The surprising history of the blood groove actually comes from golf. You may have heard that a smooth golf ball will not fly as far as the normal golf ball with dimples. Something that sounds crazy, but is actually true. How does it work?

First we need to understand a little bit about fluid flow. When we think about fluid we usually think about things like water (and blooooood!), it actually means either liquid or gas. So the study of the air passing over an airline? Fluid mechanics.

When a fluid travels over a surface you get a gradient near the surface, gradient being the fancy math word for “not all the same”. (Note that fluid traveling over a surface, or a surface traveling through a fluid, are the same thing.) The fluid far away from the surface is undisturbed, and moves at its normal rate. The fluid in contact with the surface will be stopped, and not moving. And all the fluid in between the stopped surface layer and the fast undisturbed flow will be at some speed in the middle. And you call it a gradient to sound smart.

## Golf and Water

When a ball flies through the air it must separate the air so it can pass through. This air flows around the ball, and creates a lot of disturbance when it gets to the end. The airflow diagram below is an example of the turbulence generated.

So how do the dimples on a golf ball help? The dimples act as kind of a “trap”, in that they cause this surface layer to be even more “sticky” than normal. Which probably sounds like a bad idea, since the golf ball is supposed to be flying through the air, rather than getting caught in it. But trapping this surface air pays off in the end. Because the air is clinging tightly to the golf ball it has a tendency to “wrap around” a little bit when it passes the widest part of the ball. And, because the golf ball is narrow enough, if we can get the air to wrap around enough it can meet up with itself on the other side!

This means the flow is no longer turbulent (as in the top picture) but instead known as lamellar flow. And one big benefit of this lamellar flow is that we have much less drag on the backside of the ball, which more than makes up for a little energy lost making the surface layer cling to the ball a little bit harder.

## We Came For Blood!

But enough about golf. That sport sucks3. We are here to talk about swords.

The blood groove on a sword functions in a similar way to dimples in a golf ball, allowing the blood to gain extra traction as part of a surface layer1

If you have a blood groove on your sword it means that the flow can smoothly converge after the sword has passed through the target2. Which has two important benefits:

1. Lower drag on the cut through the material. This is very important if you want to be able to cut through multiple people at once4.
2. Reduced spray from the cut4. While it looks impressive, a large blood spray can compromise the footing and leave you vulnerable to attack. Blood grooves keep the battlefield contamination to a minimum.

1 I might have made this up completely.

2 Franklin, S. (2022, April 1). The Truth of the Blood Groove, Citation 1. Sword STEM. Retrieved April 1, 2022

3 Duh

4 I think it’s theoretically possible? ¯\_(ツ)_/¯

(Good Lord, I hope I haven’t created something terrible when people take the final conclusion of this fluid flow section seriously.)