Katana with Bo-hi

[steve oakley]

I have a question about bo-hi on katanas. From reading, the bo-hi is done to hide faults in the blade Shinogi-ji or to supposedly lighten the blade.

 

My guess (speculation on my behalf) is that a a bo-hi extending into the nakago would have been originally done when the blade was made and that a katana with a bo-hi stopping at the habaki was possibly added later to hide faults. Happy to hear different views on this.

 

It seems that a bo-hi is not seen in as many older blades. Was this a preferential thing or because of some other reason. I have attached an image of a katana with a bo-hi that extends into the nakago. Is there a way to determine age according to how a bo-hi is done. I gone through previous posts but not much is covered on the bo-hi origins

 

regards

Steve

[sanjuro]

As you say, this is speculation on your part.

Bo hi were cut to lighten the blade and also reputedly to allow the blade to cut more effectively. The reasoning behind this last function was that the groove allowed air into the wound and prevented the severed flesh closing over the mune of the blade as it passed through a target. I'm not sure if this is correct but certainly a blade with bo hi makes a wound gape open more readily. Be that as it may, a bo hi that passes through the nakago is usually found on older blades. Sometimes this is because they were deliberately cut that way or if the blade is O suriage, because the original nakago has been removed thus moving the hi further down the blade. There are also hi that taper out in the nakago, which have been done deliberately. In short it was easier in the old days to cut through the nakago than to round out a hi on the blade. Bohi that stop prior to the habaki were cut on later blades but not simply to hide faults. Dont get your thinking fixed in concrete over this. I have blades with bohi (Koto and Shinto) which are virtually faultless and I have also seen blades where the bohi was cut later to hide faults. The problem with cutting a bo hi, at least from the smiths perspective is that it may bring out a fault that would otherwise have been hidden, and with a blade that exhibits a small fault, the cutting of a hi may in fact make a fault more obvious and more pronounced. Like all things nihonto, there are a multitude of variables and no single simple answer. :D

[Jacques D.]

Hi,

 

Bo hi were cut to lighten the blade and also reputedly to allow the blade to cut more effectively. The reasoning behind this last function was that the groove allowed air into the wound and prevented the severed flesh closing over the mune of the blade as it passed through a target.

 

Legends are long-living, the purpose of bo-hi is only to make the blade less heavy and more beautiful.

[sanjuro]

Jacques.

 

You will note I said 'reputedly'. I may or may not subscribe to this theory, but if you are determined to quote me and dispute me, then include the entire passage not just the bit you want to pick at....... The rest of the quote is below:

 

 

I'm not sure if this is correct

 

I included a mention of it for interests sake, nothing more. By all means disagree if you feel it to be necessary. However, since neither your view or the legend/theory that I mentioned can be proven correct or otherwise, It is an academic point.

 

You say....

less heavy and more beautiful.

I could easily dispute the latter part of this because beauty was not a primary concern in Koto blades. You may think of them as art sabers, but they were first and foremost a weapon in those bygone days. As I say, I could dispute it................ but I won't. :D

[Jimmy R]

Anyone interested in the technological advances made in edged weapons cannot ignore the fact that runnels exist in all and I mean all cultures that created them. They are a natural conclusion brought about by observing the behavior of edged weapons in a sucking wound.

Just a few things to mull over,

Jim

[CurtisR]

Although I have admittedly limited knowledge (always learning though) I read an article that described bo hi as being similar to an "I" beam ...it is strong, yet lightens the blade, even when encountering heavy resistance such as bone, etc.. My $.02 cents worth, but it made good sense to me.

 

Curt R.

[leo]

The bohi not only added beauty to a blade and reduced weight. If done properly, it even increased the structural stability of the blade at the same time. Physically this effect is achieved by enlarging the surface of the material.

It is the same principle as in a steel double-T-girder which provides much more structural stability for a building than a solid beam would. If you see the section of a ji with double bohi you will see a pretty massive double-t-girder.

 

As Jim stated, this was well known in most cultures who made edged weapons, even though I do not support the bloodgroove theory:

I´ve been hunting deer and wild boar all my life and stuck my knives(no grooves) into a lot of living and dead flesh, so personally I cannot confirm this "sucking" theory.

Apart from that, a human torso, which should be the primary target for a lethal thrust, consists of more cavities, soft organs and liquid carrying vessels than solid muscle!

(The bodybuilders among you are exempted

[CurtisR]

Thank you Martin - your post leads me to believe what I read was correct, although the DOUBLE bo-hi hadn't occurred to me. Makes perfect sense! Also, I can only attest to 18 stitches with a no-hi kissaki in the forearm during a poorly executed Noto. At the time, it didn't seem to 'suck' anything...just cut .

It was certainly a learning experience.

 

Best,

 

Curt R.

[runagmc]

I've heard the I-beam thing too, but it seems wrong to me. Think how strong an I-beam would be if it was solid. I think I-beams are made the way they are to make them light and to save material while still maintaining strength. It's all about the strength to weight ratio... solid would be stronger but also alot heavier. So, I would agree with Jacques that bo-hi are about weight and decoration... just my opinion..

[nagamaki - Franco]

Adding strength by removing material, hmmmm, . I would stick with the lightening and leave it at that unless it can be demonstrated in a cutting tool in which the groove is not formed into the blade at the time of forging and quenching, but cut in afterwards as is the case with nihonto, that strength is somehow added.

As far as cutting better, that one too, , first, the cutting edge is no sharper, second, in a striking event its mass X velocity = final momentum, which means if you remove material lightening the blade, the velocity has to then be increased to achieve same results, with all else being equal my money is on the nihonto without bo hi, thank you.

And, as for bo hi being original or added, suggest learning kantei with additional study, study, study, of blades with known and identified history of their bo hi.

[steve oakley]

The point about the bo-hi some refer to it as the blood groove, or how assists with extracting the blade from the body has possibly continued due to urban legend or whatever (maybe one for myth busters ). An article on the internet about some of the points just covered is here http://bshistorian.wordpress.com/2008/0 ... od-groove/

 

In regards to learning Kantei about the known or identified history of their bo-hi, as i said i have found little on this subject. Any references to this subject would be great.

 

As Mr.Sanjuro states that if the blade is O suriage, because the original nakago has been removed thus moving the hi further down the blade. From this I assume that generally it would have been an older blade shortened to reflect the times.

 

Yes i have seen bohi on koto blades but no many though. I would think that bo-hi was more common during Edo times when katana were more artistic? The reason i say this is that during the Muromachi period when blades were mass produced to keep up with the wars, it would have been time consuming to add a bo-hi.

 

Which leads me to this question, when was the bo-hi first seen on the Japanese blades?

 

regards

Steve

[Ken-Hawaii]

Adam & Franco, take it from a professional engineer that an i-beam is about 35% stronger than an equal-sized solid beam. You forgot to consider the actual weight of that extra solid material in the non-i-beam. This was a basic problem all us engineers get to solve in our statics & dynamics class in university.

 

In other words, the ancients knew that secret.

 

Ken

[Derek]

Very interesting info, ty.

 

-Derek

[drdata]

Very interesting. I like the stiffens blade, for a given weight theory.

 

Over at SFI someone recently posted a cross section of niku with bo-hi. Does look i-beamish:

 

http://www.swordforum.com/forums/showth ... o-and-niku

 

[Jean]

My guess (speculation on my behalf) is that a a bo-hi extending into the nakago would have been originally done when the blade was made and that a katana with a bo-hi stopping at the habaki was possibly added later to hide faults. Happy to hear different views on this.

 

It seems that a bo-hi is not seen in as many older blades

 

 

Steve,

 

You need to read, Nihon To Koza, Conoisseur's book of the Japanese sword, Bo hi with a kaku dome was a standard on Uchigatana in sue Koto Bizen. Read carefully the Connoisseur's Boook, p. 75

[sanjuro]

Franco.

 

in a striking event its mass X velocity = final momentum, which means if you remove material lightening the blade, the velocity has to then be increased to achieve same results, with all else being equal my money is on the nihonto without bo hi, thank you.

 

Hmmmm. You havent used a nihonto to cut anything have you? The act of cutting with a nihonto is not an impact as in mass X velocity type of action. What you describe is purely a blow and not a cut. A cut is a combination of acceleration through the arc of incidence with the target and equally the movement of the blade along its longditudinal plane (the action of cutting).

With a good sword a slightly built woman can deliver as telling a cut as a man can. If the sword is merely accelerated through an arc of incidence, then blade weight and blade speed become combined to inflict a heavier blow but an ineffective cut. A heavier blade is also more difficult to accelerate through an arc of incidence and therefore may not reach the same speed as a lighter blade travelling the same arc with an equal amount of energy applied to its acceleration.

 

Having introduced that into your thinking, I await rebuff etc etc.......

[Ken-Hawaii]

Good reply, Keith. I'd like to factor in hasuji, but frankly can't come up with an engineering equation to model it.

 

But I have two nicely-balanced shinken, one with & one without bo-hi. The heavier non-bo-hi blade cuts just fine in tameshigiri, but I find that I can easily cut three bamboo targets with the bo-hi blade, while struggling to cut two with the other one. Not very scientific, I'll admit, but after using them both for a decade, I can certainly tell there's a difference in cutting ability.

 

Ken

[sanjuro]

Ken.

 

Personal theory here:

 

Given that the technique of cutting is constant, (god knows it should be after a decade of tameshigiri), Could it be that the flex/reflex characteristics of your two swords is different? The non bohi blade having a greater flex through the target and reflexing after leaving the target, which would lessen its cutting ability as it reflexed progressing through a second consecutive target providing a braking action; whereas the less flexible bo hi blade has a more rigid nature that does not flex at impact to such a marked degree thus maintaining a more focused cut? If so it makes a good case for the added strength and cutting ability of a blade with bohi.

[Ken-Hawaii]

You know, Keith, I've never thought about it, but I definitely feel a much-lessened degree of what I'll call "vibration" with the bo-hi shinken. That could certainly be a function of flexion of the blades. I did think that the bo-hi blade felt tighter & more controllable, too. The heavy blade feels a lot more solid in my hand compared to the comparatively "whippy" bo-hi blade. The non-bo-hi blade weighs about 300 grams more, but since I'm around 190 pounds myself, I don't think that less than a pound would make any difference cutting-wise. It's possible that my hasuji is different as I cut through the bamboo when the blade exits the targets. I sometimes have a slight "scoop" at the end of my cut with the heavier blade, but almost never with the bo-hi.

 

Ken

[sanjuro]

Ken.

 

Not wishing to hijack this thread, May I ask if there is also a difference in the sori of the two swords? And if so, is the bohi blade the one with greater sori?

I have always cut better with a sword that has a more pronounced sori, and according to my old sensei this was because when the hands lead the monouchi through the cut, there is greater control. One reason I dont favour shin shinto katana is the comparative lack of sori, so my own shinken is a koto tachi style blade (with bohi), rather than a later style katana.

 

My apologies to everyone for this slight divergence from the main thread.

[nagamaki - Franco]

Ken, Keith,

 

I stand by my previous reply. Food for thought, you would think that if what you're saying is absolute that we would find all nihonto with bo hi, ah, but we don't, hmmmmm. Mind you, I'm taking cutting motion into account and not arguing the strength of I beams.

[Ken-Hawaii]

Keith, the sori of the bo-hi blade is 0.5 inch, & the sori of the other blade is 0.55 inch, so no real difference for practical purpose (& measuring sori isn't exactly the most precise measurement I can make, either). The heavier blade is about 1.5 inches longer, & the motohaba on each is virtually identical. Sakihaba is slightly larger (0.15 inch) on the longer, non-bo-hi, blade. The weight distribution, however, is significantly different on the blades, with the longer blade's CG about 1.8 inches closer to the kissaki, but that's offset by a longer tsuka.

 

So if you can make heads or tails of why they swing differently, I'd sure like to know! Too many variables for this brain! Given a choice, though, I use the bo-hi shinken for tameshigiri except for tatami omote cutting. BTW, if you know of a good source of tatami omote since tameshigiri.com stopped shipping it, please let me know! I'm running very low.

 

Ken

[sanjuro]

Franco.

 

you would think that if what you're saying is absolute that we would find all nihonto with bo hi, ah, but we don't, hmmmmm.

 

So true.... I dont think Ken and I were trying to make a case for absolute superiority of bohi blades, rather it was an exploration of the practicality of a bohi blade. No real mystery here, there are really no absolutes. The greater prevalence of bohi in later blades may in fact be partially attributable to greater strength or better cutting power or at least the belief that bohi contributed to these qualities, or it may just be the product of blade evolution and the tastes of the times or preference of the individual. Its an interesting topic to ponder however. What we were doing was discussing the nihonto in its practical sense rather than its aesthetic sense. You'll forgive our small digression (and concomitant fall from grace) as we moved away from the purely academic and nebulous 'art sword' view. :D

[jeremy]

Found a good commentary by Keith Larman on bo-hi which I read a while back, thought Id post it here.

 

"Then there's bo-hi. I'm not a fan from a performance standpoint . Think about what it's doing. If the blade has a low shinogi and a bo-hi, you're reducing weight creating an edge on the top of the bo-hi to grab target as the blade passes through. Then if the bo-hi is grooved deeply rather than smoothly and shallow , you've removed significant material from the cross section so a poorly angled cut should snap the blade. It reduces weight and reduces strength. On the other hand, tapering the shinogi ji to create a high shinogi can reduce weight similarly but without such a serious compromise of the lateral strength of the blade. And the high shinogi also reduces surface area at the same time. It seems to be a vastly better solution.

 

And remember that bo-hi were usually done historically for a variety of reasons. The first was to have a way of cutting out weld flaws or other "boo-boos" that appeared on the surface. This could save a blade from the trash heap. The second reason for cutting in grooves was to "correct" an old poorly balanced sword. Both of these methods work given the motivation, but ultimately both reasons are fixing something after the fact. The final reason was purely for aesthetics. Some just like 'em. This is a personal taste issue -- but it can become a safety issue if the blade is being made for tameshigiri."

 

Kind regards,

 

Jeremy Hagop

[IanB]

All when it comes to flexing during a cut, I suggest you watch this link. When this film was made it left most people staring in disbelief.

 

Ian Bottomley

[runagmc]

Adam & Franco, take it from a professional engineer that an i-beam is about 35% stronger than an equal-sized solid beam. You forgot to consider the actual weight of that extra solid material in the non-i-beam. This was a basic problem all us engineers get to solve in our statics & dynamics class in university.

 

In other words, the ancients knew that secret.

 

Ken

Ken, if you read what I said, I did consider the weight. I said a solid beam would take more force to bend but would weigh so much more that it would be less efficient and that's why they make I-beams... because I-beams have a better strength to weight ratio.

[runagmc]

The thing about the I-beam design is that it has much less resistance to torsion stresses or twisting so I would think a solid sword would take more stresses than one with a bo-hi.

[sanjuro]

True if we are talking of a straight 'I' section beam. A bohi blade however is firstly not a true 'I' beam but a 'I' section above and firmly affixed to a narrow 'V' section below. It is also curved against the upper and lower sections of the 'I'. The 'I' beam analogy is thus taken too far and does not directly apply when considering torsional characteristics.

[runagmc]

Your right that there are more contributing factors with a blade, but I think the rule is still the same as it would be with a straight beam. All other things being equal, removing material will never make a sword or a beam stronger. It will only make it have a better strength to weight ratio. That's my best analysis...

[runagmc]

And you're right Keith, the I-beam analogy is not perfect but it is the best one we have since there is not tons of data on katana strength with or without bo-hi.