A bo-hi, sometimes spelled bohi, hi, fuller, or blood groove, is carved into a Japanese sword’s blade to lighten it without significantly compromising its strength or durability.
It does this in the same way I-beams in the construction industry lighten loads without sacrificing strength.
Not only does adding a sword grip reduce the weight (by 2 to 3 ounces), but it also slightly moves the sword’s center of gravity closer to the handle than it would be without a grip (typically around .5-1″).
Ultimately, these alterations to the sword’s weight and balance point result in a blade that seems slightly more responsive to the user’s wrist and finger movements.
Lighter blades that are more balanced toward the hilt are faster, but their cutting power and durability suffer as a result (which is only noticeable under extreme cutting conditions).
Last but not least, a bo-groove can intensify the “Tachi-Kaze” (the Japanese term for the sound a sword makes when it cuts through the air) (sword wind sound). The blood groove is utilized as a form of auditory feedback to indicate to the sword student whether or not their slashes were effective, depending on the speed and precise technique of the practitioner.
Profiles vary widely in forged Japanese swords. Many variations of hi exist, but they all serve the same primary function: reducing the weight of a sword by eliminating steel from the blade.
A blade without a bo-hi is heavier and has a more frontal balancing point, increasing its strength at the expense of its grace. If appropriately crafted, bo-hi and no-hi can make remarkably similar cuts. Whether you choose a Bo-Hi or No-Hi blade is a personal preference that should be considered when deciding which type of training equipment to purchase.
You can accomplish the following with the use of the Bio-hi sword customization:
- Weight Reduction
In ancient times, the sword was a vital tool on the battlefield. It’s difficult to kill an opponent with agility if your blade is too cumbersome.
- Minimizing Material Waste
Steel production is exceedingly low, and the method used to process iron dates back centuries. The blade’s blood grooves (BO-HI) serve as a salvage mechanism, and large blood grooves can save as much as 20% of the steel used, allowing for more blades to be produced for the battlefield.
- Maintain Balance
Since every ancient sword is unique and crafted by hand, the blood grooves can fine-tune the weapon’s balance, CG, weight, and more.
- Increase Strength
The blood grooves along the cross-section of the sword create a “work” shape that saves weight and guarantees the blade’s strength.
The ‘Blood Groove’ Illusion
When thrusting towards the body, some individuals mistakenly believe that a bo-hi’s or any fuller’s primary function is to enable blood to drain down the groove and avoid suction from causing the blade to become lodged.
It sounds fantastic, maybe, but it is just that – a myth. The only purposes are to lighten and change the blade’s balance and provide auditory feedback.
Blood Groove Types
There are numerous less common variations of the usual full-length groove which ends just before or under the habaki (the blade collar).
The most prevalent blood groove is called b-ohi for the shinogi-zukuri blade shape, and it does not pass through the kissaki (tip).
Another great bo-hi type is the same as the one on the left, but this time going all the way through in the kissaki.
A bo-hi that does not go through habaki; instead, this form of bohi is reserved only for iaito; its function is to counteract the other points.
No geometry is required for the Unokubi-Zukuri (Cormorant’s Neck) Bohi. Such blades often have an added bo-hi, creating a very light blade.
A geometrical Bo-hi for the unokubi zukuri (Cormorant’s Neck).
Double-sided bo-hi with two blood grooves down the blade’s length. In place of one, we have two fullers. The effect on handling, however, is the same as that of a single, deeper fuller; the only difference is that it is maybe more aesthetically appealing.
In traditional blades, however, the kissaki tip is hand-shaped to match the exact curves of the area where the groove ends. However, few sword makers go to the extra time and money of hand carving the groove because of how rarely it is used (though, as is quite often the case with production swords, this shortcut is cosmetic only).