More than just a decorative groove, the Bo-Hi contributes to the Katana’s balance and performance. The Bo-Hi makes the blade lighter, improves handling and produces a distinctive sound.
What is the Bo-Hi?
The Bo-Hi is a long groove cut along one side of the Katana blade. Often mistaken for an ornament, the primary purpose of this groove is to reduce the weapon’s weight while maintaining its strength. By removing a portion of the steel, the blacksmith slightly shifts the katana’s balance point towards the Tsuba (guard), allowing for better handling. The addition of a Bo-Hi thus creates a lighter blade, for fast cuts and fluid sequences.
Contrary to a persistent myth, the Bo-Hi was never designed to facilitate the flow of blood during combat.
Etymology
The term Bo-Hi has its roots in the Japanese language. It is composed of two kanji characters:
- Bo (棒): meaning “stick” or “bar”, in reference to the elongated, straight shape of the groove.
- Hi (樋): meaning “gutter” or “channel”, evoking the hollowed function of the groove on the blade.
The whole term Bo-Hi can therefore be interpreted as “stick-shaped groove” or “linear channel”, emphasizing the idea of a gutter running along the length of the blade.
Advantages of Bo-Hi
Weight reduction
Bo-Hi makes the blade lighter while maintaining its structural integrity. This gain in lightness is essential for practitioners of martial arts such as Iaido, where speed and precision of movement are paramount. By reducing the blade’s mass, Bo-Hi enables faster acceleration and more fluid cuts. This translates into more agile strikes with no loss of power.
Modification of the Balance Point
Another notable effect of Bo-Hi is the shift in the Katana’s balance point. By reducing the weight of the blade, this point is brought closer to the guard. Handling becomes easier, and the katana reacts more sharply to the swordsman’s movements.
Bo-Hi variations
There are several types of Bo-Hi: the single Bo-Hi (a single groove) or the double Bo-Hi (two parallel grooves). The choice between these types often depends on the desired aesthetic, but also on the weight distribution along the blade. A double Bo-Hi offers even greater weight reduction, while retaining a solid structure, but is rarer and requires greater expertise on the part of the blacksmith.
What’s more, some grooves run the length of the entire blade up to the Habaki and guard, while others cover only part of the blade.
- Full Bo-Hi: runs the full length of the blade.
- Partial Bo-Hi: limited to a specific portion, usually before the middle of the blade.
The sound of Bo-Hi: Tachi-Kaze
The Bo-Hi also plays an acoustic role, producing a distinctive sound called Tachi-Kaze when the blade splits the air. In effect, the Bo-Hi modifies the sound emitted by the blade during strikes. This sound is produced by the flow of air through the groove as the katana moves. A clean, clear Tachi-Kaze is often seen as an indicator of correct, precise technique. For practitioners of martial arts such as Iaido or Kendo, this sound is crucial, as it measures the quality of their cut and the precision of their gesture.
The shape and depth of the Bo-Hi can also influence the sound produced when cutting. A deeper groove will generate a deeper Tachi-Kaze, while a shallower Bo-Hi will produce a lighter, faster sound.
Bo-Hi is also aesthetically pleasing
In addition to its functional properties, the Bo-Hi is often seen as an aesthetic addition that embellishes the blade. The presence of a well-executed Bo-Hi is a sign of skilled swordsmithing, capable of shaping a blade that is both beautiful and formidably effective. It is often seen as a mark of mastery and technical perfection, as the creation of a Bo-Hi requires great precision to avoid weakening the blade’s structure.
On some top-of-the-range katanas, the Bo-Hi is in perfect harmony with the Kissaki, the tip of the blade. Blacksmiths who master this art manage to create a visual balance, where the curve of the Bo-Hi matches the shape of the Kissaki. Such attention to detail is rare today, and reflects the traditional craftsmanship of ancient Japanese swords.
One of the most widely held beliefs about the Bo-Hi is that it was designed to make blood flow more easily down the blade. This myth, often propagated by popular culture, is in fact incorrect. The Bo-Hi has always had a practical purpose: to reduce the weight and improve the balance of the blade. What’s more, a correctly executed Bo-Hi does not compromise the katana’s solidity; on the contrary, it sometimes reinforces the blade’s resilience by slightly increasing its flexibility.
Buying a Katana with or without Bo-Hi
The presence or absence of the Bo-Hi depends on the practitioner’s preferences and needs. A katana with Bo-Hi will be lighter, faster and better suited to fluid sequences and katas. This type of blade is ideal for martial arts requiring rapid movements, such as Kenjutsu or Iaido. On the other hand, a katana without Bo-Hi will be slightly heavier and offer a more solid feel for powerful cuts, ideal for the Tameshigiri (cutting test).