How much weight does a bohi save?

[piryohae3]

It doesn't look like enough metal is cut out to make much difference but looks can be deceiving. Wondering what the weight difference is between a sword that has one VS one without.

[ROKUJURO]

James,

weight might be a less important point compared to balance and speed.  If the blade of a fighter is 1/10 of a second faster than that of his opponent, it may just be enough of a difference.....

[ChrisW]

Samurai lived and died by the blade in battle, as Jean says, every millisecond counts.

[Ken-Hawaii]

Actually, the bo-hi is intended to strengthen & stiffen the blade. Being an engineer, I had to prove that to myself, so I created an AutoCAD 3D model of a blade, & ran what's called a Finite Element Analysis (FEA). I don't recall the exact increase in stiffening after a dozen years, but it was around 70%.

[piryohae3]

70% increase in stiffening? That's incredible!

[ChrisW]

Makes sense, a bohi is a form of an elongated arch which does resist force.

[Ken-Hawaii]

James, think about why I-beams are used in construction, instead of plain old steel bars.

[Rivkin]

I think the greatest question is what is being compared. Solid construction to a hallowed arc with no weight consideration - solid material will always be stiffer. With weight given a consideration it will be the other way around, but the question then becomes does it make sense to actually leave the weight savings as is.

A while ago I did a large table of weights including swords with and without bohi - and for the same period they were basically on top of each other. So instead of securing the weight savings, the sword would likely be proportionally scaled along some other dimensions - producing stronger weapon for the same given weight.

 

Overall the weight often tends to be rather narrowly fixed to a certain range for each weapon and each length. You don't often see a koto katana with 70cm or so nagasa that goes beyond 800 gram by itself, and they tend to be around 670-750 gram on average. Shinshinto will be somewhat heavier on average. 1.1kg seems to be the absolute maximum weight that a truly mass-adopted blade would take, and for some reason in Asia those were universally popular specifically in the 14th century.

 

Kirill R. 

[Nobody]

It doesn't look like enough metal is cut out to make much difference but looks can be deceiving. Wondering what the weight difference is between a sword that has one VS one without.

 

The following site says that the weight reduction is about 100 g.

Ref. http://nosyudo.jp/koshirae/

 

Its expected effects may be the change of its balance and/or the reduction of its moment of inertia as well as its weight reduction.

[Nobody]

Actually, the bo-hi is intended to strengthen & stiffen the blade. Being an engineer, I had to prove that to myself, so I created an AutoCAD 3D model of a blade, & ran what's called a Finite Element Analysis (FEA). I don't recall the exact increase in stiffening after a dozen years, but it was around 70%.

 

 

That theory is a misunderstanding I have sometimes seen.

The blade strength with bo-hi may decrease, but it never increases. That is unquestionable in strength of materials.

 

I just found an estimation example. (unfortunately, the paper is written in Japanese.)

Table 1 shows that Ra (cross section) and Ri (second moment of area) both decrease when bo-hi increases. Only the bending strength per UNIT AREA (Ri/Ra) is improved with bo-hi.

 

Ref. https://www.jstage.jst.go.jp/article/pscjspe/2015S/0/2015S_199/_pdf

 

 

*** I realize that the index Ra/Ri in the Table 1 should be Ri/Ra because of typo.

[Brian]

Excellent sources Koichi, thanks.
Some more good reading:
https://sbg-sword-forum.forums.net/thread/36793/stiffest-katana-geometry
And not really related, but since I just found it...worth an unrelated read: https://www.omicsonline.org/open-access/mechanical-properties-of-samurai-swords-carbon-steel-made-using-atraditional-steelmaking-technology-tatara-2169-0022-1000162.pdf

[Ken-Hawaii]

Excellent source, but wrong axis. Calculate it from the ha to mune, not sideways.

[Nobody]

In the cross section of a blade, for each axis (horizontal or vertical), the bending strength decreases if bo-hi is carved. That is clear if you understand the moment of inertia of area.

[Greg F]

Kirill in regards to blade weight couldn't the extra polishes over time contribute to less weight in koto blades too.

Interesting thread.

 

Greg

[SAS]

In the cross section of a blade, for each axis (horizontal or vertical), the bending strength decreases if bo-hi is carved. That is clear if you understand the moment of inertia of area.

 

If this is true, why were they done at all?

[ChrisW]

Aesthetics?

[Guido]

For the same reason I-beams are made: to reduce weight without sacrificing too much integrity.

[Death-Ace]

If this is true, why were they done at all?

Agreed with both. Although I believe it does lend to more bending (seen a bit of blades from differemt cultures with grooves bent, and not all from just poor maintenance or use), I think weight reduction was a goal. Although placement and amount of steel removed can be a factor.

 

As stated, a more solid steel will mostly be stronger than one that has had a bit removed. With I-Beams, I believe it is a way to save material, but lend strength due to shape, although not as much as say if something was made from full on steel beams of the same size. I can see where the argument comes in, and I agreed once upon a time that it does have some structural properties, but saving weight qithout fully compromising the blade. I think of guns sometimes, even though the "trauma" is quite different. Some guns that have been polished down, especially older ones that have had steel when refinished or such, can have drastic consequences when fired, as it can no longer withstand the force projected when fired. Same goes for anything. Wood furniture that can been sanded and refinished theough the ages, even if preserved well will not have the integrity to support the weight of the same person after enough has been removed. But the engineers here know more, I may be mistaken in my thoughts and am open to see results of studies!

 

I believe for practical hi, it was and is meant to be a weight reductor without fully compromising the blade and still give it support. Although blades weren't the main line weapon and fights weren't hours long in a duel as some movies suggest, you still want a good back-up that could give you an advantage to draw quickly and wield shiftly to get home at the end of the day, much like firearms.

 

The weight reduction as studied by one mentioned above brought a memory to mind. I once had a gimei Nobuhide. Very meaty, heavy, but still felt good in the hand. When I practiced Aikido/Iaido for a bit, I could tell how it was well balanced. Cut forward three years when I visited a great collector, Ed, who had shown me a very nice, healthy Ichimonji. It had very familiar similarities in the dimensions of my "Nobuhide", but with bohi, it felt even more balanced and lighter, of course! Even if it had a few polishes more than the Nobuhide, if I were a professional swordsman or modern day practitioner, I may have taken the Ichimonji instead for a quicker draw and faster movements.

 

Of course with nihonto or other blades, grooves be as much about aesthetics/religion as practicality. Some bohi or duel hi only go a certain length of a blade, or one side. One thing we can agree on, sure as hell ain't a fuller for blood!

[SAS]

My question was theoretical; I agree with Lev, but am happy to let the engineers fight it out....us blade smiths will keep making blades

[Death-Ace]

My question was theoretical; I agree with Lev, but am happy to let the engineers fight it out....us blade smiths will keep making blades

Haha, oh yes, I hear what you're saying, Steve! I have become a person who likes to see visual proof anyways when there's a debate like this.

 

But also, would love to see your creations as well!

 

Edit: Ahh, out of my dumba**edness, I did not see your link! Quite the good stuff! I love your guitar as well! I have a love for these hollowbodies, and my dream would be an old 335! However, my heart still goes to Fenders, yet all I have left is my 85 MIJ Squier Bullet. Quite the trooper, though!

[Rivkin]

Kirill in regards to blade weight couldn't the extra polishes over time contribute to less weight in koto blades too.

Interesting thread.

 

Greg

 

That's what I thought, but the lines of weight versus length clearly diverged at Momoyama. Sharp transition. This being said, when I just used Shinto sword that copied old Soshu, their weight was comparable to the earlier ones. While shinshinto very still generally quite heavy.

 

Kirill R.