Getting Into The Groove

About 40% of suriage and ubu katana have bohi which is a simple groove which fills the shinogi area of the blade. Recently on a question came up about the strength effect on blades. 

I wrote about this a long, long time ago but it bears some updating here, about why a blade has grooves and what the effects are function wise. 

What it isn’t

I had a client once adamant that he would not buy a sword with bohi because it was a sign that the sword was flawed. He picked this up from a certain book self-claimed to be full of facts, because in some instance some horimono was put in place to remove something unsightly and/or to try to modify an attribution.

There are a lot of games played in this world of swords, but cutting bohi is I think not a usual one (though it does happen, I have seen a few and had one client write to me that he was having them recut in an antique sword to be more to his taste… my reaction to that of course lost me a correspondent). 

So for the most part, they are not there for cosmetic reasons or to hide flaws. 


The primary function of bohi is very simple: it lightens the blade. 

A lighter blade will draw faster and strike faster. It can also hit harder depending on the skiller of the user, as a faster strike has a lot more kinetic energy than a slower strike with a heavier blade. Like a baseball bat though there is a balance involved that depends on the user, their arm length, strength and skill, to find the optimum blade length and weight. Bohi basically allows you to get yourself a longer blade of the same strength and weight than you would otherwise have.

This brings us back to one simple question as well: why swords at all?

It’s all in the wrist

Basically a sword will defeat many other weapons because of its speed. If you consider a tennis serve by anyone skilled, it will always be a much faster ball than a thrown ball. This is because the tennis racquet head moves much faster than the hand, as it is an extension of the arm, and the rotation of the arm in the socket has an upper limit based on human musculature and the condition and strength of the athlete. As long as that arm is rotating, and the wrist uncocking at a fixed velocity, you can increase the speed of the racquet head by lengthening the shaft. This also increases inertia so it becomes that much more difficult to rotate your arm and uncock your wrist at their highest velocity. 

The same physics apply with spear throwers and in the game of jai-alai where extensions to the arm allow for increased velocity of the object being thrown. If you keep thinking about it, you can see golf, hockey and lacrosse all apply the same physics benefits of extending the length of an arm and thereby increasing velocity at the end of the tool being used to extend the arm. 

A spear or polearm being primarily a thrusting weapon does not benefit from this magnification of velocity. This is where trouble begins for a polearm user once a sword user penetrates the safe zone provided by the longer weapon.

Why not an axe?

An axe concentrates most of its mass at the head, and as a result, hits hard. This is nice if you have a stationary target like a tree because you can charge an axehead up and it hits very hard. But this weight at the end of the radius of your swinging circle means it is slow to increase its velocity. That means it’s not so good for hitting a moving object: imagine trying to hit a baseball pitch with a sledgehammer. It is not going to happen. Even though an axe or hammer will hit hard, it is not likely to hit someone who is not going to stand there and take it. For practical purposes then such a design has great limitations (i.e. we assume your opponent would like to not be hit).

Range and Velocity

So in all of this, we really care about range and velocity, because range increase means your defensive perimeter is larger and your killing area is greater. Velocity matters because it lets you hit a moving target better, as well as hit before someone else can hit you. The faster your weapon, the more likely you are to win.

Looking back to the bohi, it lightens the blade without removing any bending resistance in the direction of the cut. This allows you either to have a faster sword at the same length as someone who has no bohi, or it allows you to have a longer sword at the same speed as someone who has no bohi. Both of which mean that you can strike before you get struck. Depending on your own build and technique, one or the other may be preferable. But both are highly advantageous. 

A bohi doesn’t affect the strength of a sword in the cutting direction, because resistance to bending is primarily a function of the width of the blade. A wider blade is less prone to bending in the direction of the cut. The cross section is not as important as the width. The math can be found when we look at descriptions of i-beams.

There are downsides to an i-beam type of shape, which are that side-to-side it is weaker so the sword is more likely to bend if struck from the side. As well it won’t resist torsion so well but that is not really a problem with a sword strike. However, an improper strike could result in a hagire forming due to torsion.

If your skill is high though, neither of those will be a problem.

So one can think of pro level golf clubs, which may be harder for a beginner to use but magnify the abilities of a pro level user considerably.


So summarizing the tradeoffs, the bohi will let you make a longer blade of the same weight and strength in the cutting direction or a same length blade of greater speed and the same weight and strength in the cutting direction or a wider and less prone to bending in the cutting direction blade of equal weight and speed. 

As long as you are working with the concept of in the cutting direction then a bohi is to your advantage. And that implies benefits to skilled users. It will always be inferior once you get away from the cutting direction.

Additional Benefits

There are a few additional benefits to the bohi. The first of which is that a bohi causes turbulence when swung and this produces an audible singing tone. This tone during training gives the user feedback about how true the blade is to the angle of the swing and as such, lets you improve. 

This tone is also intimidating, maybe giving pause to someone else thinking of entering the fight, and certainly a reminder to your opponent that death is very close by. 

People also forget that one of the primary benefits of a sword vs. any other weapon is its practical day to day nature. You can wear it. You can’t walk around in full armor all day every day and you can’t carry a yari or naginata down the street so easily. 

I submitted a daisho this year to Tokubetsu Juyo which passed, and the surface of the daisho rather than being lacquered is beaten plates of gold. This is an incredible set, and was made most likely as a gift and to project power, but the day to day utility of this set is not so high. The reason is that the quantity of gold causes it to weigh about three times what a normal daisho would weigh. If you have to wear that around all day long, you are going to feel it.

The simple lightening of the sword via bohi makes it a more practical “daily driver” … at the end of the day this means you are less fatigued and so you’re a better fighter, or you just lead a better quality of life for not lugging around extra weight all day long.

That brings up a side thought, as the geek argument of who would win in a fight, a fully armored knight or a samurai was on my mind as I was walking around in Kyoto at 40 degress celcius (about 103F). I thought if I ever had to fight a European knight in full armor, my choice of weapon would probably be a rock and to be bicycle shorts and nothing else.

Because, my plan would be to just run circles around this guy throwing the rock at him. If he decided to sit and guard my rock after it hit him, I would sit outside of his weapon range. If he wanted to chase me I would run away and go pick up my rock again. 

Either way, within 20 minutes, even if I never got a good hit in with my rock, the sun would take care of all of my problems. When he collapses from heat stroke or the sweat has completely blinded him, I am going to go up to him and bash his head in with my rock.

We really discount real world practicality as well as fatigue when we think of these scenarios, but the weapon designers surely do not. Though a sword is nearly useless against a knight wearing plate armor, the impracticality of plate armor on a day by day, hour by hour basis means that it is not something you will usually encounter. That it is also a portable oven in the 40 degree summertime means it’s a losing bet. While a sword is something you can grab and strap on and walk around with, a highly practical and pragmatic weapon for day to day life though not your first choice against a fully armored opponent.

Coming back to bohi, less weight does matter in terms of your endurance during a fight and as I mentioned, just finishing your day with less pain and exertion. 

The last benefit the bohi provides is simply cosmetic. It looks good (though some may disagree and it’s purely subjective). This comes in dead last though because bohi is entirely a functional improvement to a sword, and with nihonto, form should follow function. 

None of this would matter if the bohi weakened the sword in the cutting direction. It may indeed be a factor of something like 1% but the benefits far outweigh the drawbacks. As long as you can use it right, a sword with bohi is far more to your advantage than one without.

PS. they are not blood grooves, in any design of any swords, anywhere. Grooves always fill the same function: greater strength-to-weight ratio.