What is Bo-Hi on Japanese Sword?

Published: 09/04/2021 Comments: 1

What is Bo-Hi

A Bo-Hi (also known as a 'bohi', 'hi', 'fuller' or 'blood groove') is the groove on the blade located just under the spine. Bo-hi is sometimes called blood grooves or blood gutters, but the idea that channeling blood is their only purpose or even their main purpose is a fault.

Advantage of Using Blood Grooves

1. Reducing Weight

A Bo-Hi blade of Japanese katana can be 20% to 35% lighter than a No-Hi blade without any sacrifice of strength or integrity of the blade. The lighter the blade and the more balanced towards the hilt it is, will, in turn, create a faster blade.

In addition, swords were used on the battlefield in ancient. If the sword is too heavy, it is not conducive to kill the enemy flexibly. 

2. Saving Materials

In ancient iron-smelting technology is backward and the steel output is extremely low. The blood grooves (BO-HI) on the sword can save the material of forging swords. The large blood grooves can save about 20% of the steel material so that more swords can be made for the battle.

3. Maintaining balance

Ancient swords are all handmade, there may be deviations, causing the center of gravity to shift. A bo-hi blade of a Japanese sword makes the balance point closer to the wielder. The blood grooves can adjust the balance of the sword. The lighter the blade and the more balanced towards the hilt it is.

4. The final benefit of a bo-hi blade is that the groove tends to amplify the sound of the sword as it wooshes through the air, which in Japanese is called "Tachi-Kaze" (sword wind sound). How loud it is will depend upon the speed and correct technique of wielding sword, and so it is used as an auditory feedback device to let the sword students know which of their cuts were good and which ones need more improvement.

Disadvantage of Bo-Hi

The lighter the blade, the less powerful its cutting action is and it is ever so slightly less durable.
The bo-hi sword felt better but is at a disadvantage when it comes to lateral strength, it is not very suitable for hard targets like bamboo.

What's the difference between Bo-hi and No-hi?

-That a Bo-hi blade has fuller, lighter, and makes the "Tachi-Kaze" sound.
-The Bo-hi reduces the strength and cutting power of the sword. Strength wise is more having to do with developing a bend on the blade. Cutting power has to do with removing mass from the blade.
-Bo-hi is lighter, thus easier to balance.
-No-hi blade has no groove, is considered "Stronger", and heavier than a Bo-hi blade. Both bo-hi and no-hi can cut very similar targets if both are properly made. The no-hi of course will take on thicker targets with less strength needed from the user compared to the bo-hi.
-a fullered sword is lighter but not as rigid as a non-fullered sword of the same size, yet more rigid than an unfullered sword of the same mass. A blade with bohi is not very suitable for hard targets like bamboo (but also thin swords without bohi are not suitable for that).

Types of Bo-hi

In Japanese swordsmithing, swordsmiths created and used a variety of different fuller(Bo-hi) types. In this article, we will explore the different types of hi.

  • Bo-hi: A continuous straight groove of notable width and runs lengthwise of the blade, known as katana-bi on tantō. 

  • Bohi for shinogi zukuri, through to the kissaki.

  • Bohi for unokubi zukuri(Cormorant neck), without geometry. Such blades often added bohi to the shinogi-zukuri part, creating a very light blade. Unokubi zukuri provides excellent balance in a strong cutting blade. 

  • Bohi for unokubi zukuri(Cormorant neck) with geometry.

  • Bohi does not through habaki, this style bohi always for iaito, the purpose is to balance the point.

  • Double bohi, it features two blood grooves that run lengthwise of the blade.

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