why are katanas curved?

Is there an advantage gained by the slight curve over a straight blade?

-rainy

Katanas are designed for slashing. A curved blade is more efficient for slashing than a straight blade.

Just a guess:
To increase the length of the cutting surface without increasing the straight distance from the hilt to the sword tip?

Indeed, the curve offers more surface area while keeping the overall length of the blade down, thus maintaining balance – which is of particular importance in blades that require more finesse than your average cleaver.

As Johnny said, the proper form with the katana has the cut be a slashing motion, not an axe-like perpendicular “hacking”. The curved slashing blade is also practical for use by mounted combatant riding past one another, or in general any fighting style that favors both parties remaining in motion past each other.

Nonsense. I have one–a replica, actually; the dimensions are, however, historically accurate. The curvature is quite gentle; if the blade were straightened, the difference in length would amount to much less than 1/10th of an inch, if even that much. No, the purpose of the curvature, as it has been explained to me, is to reduce the contact area during a cut, thereby increasing the pressure at the cut. I’m not sure that’s the whole story, but even so, it makes sense to me.

I actually got to pull out those books I have Japanese swords…

“A fourth feature, the distincitve curve away from the edge, owes its origin to another practical demand: the need to draw the sword and strike as quickly as possible and in a continuous motion. Where the sword itself forms part of the approximate cirucmference of a circle with its center at the wearer’s right shoulder and its radius the length of his arm, drawing from a narrow scabbard will naturally be easier and faster than with a straight weapon”

from The Japanese Sword by Kanzan Sato

I’ve got lots more on Japanese swords if you’re ever interested.

As Q.E.D Points out, the curve of your average Katana is not very pronounced.

More importantly, the effect of a curve in cutting performance in a practical fight situation is negligible. You will loose your arm just as readily to a European longsword as to a Katana.

I heard, somewhere lost in the mists of Time, that a curved blade is more effective against laminate armor. Anybody know anything of the sort, or was it just smoke?

I have heard some people claim that it was a side-effect of the tempering process.

When the blade was heated, the edge was treated to increase the carbon content, producing a harder alloy that would hold a sharper edge.

The back of the blade was left untreated, leaving a more softer, more flexible alloy that would make the overall blade less brittle.

When the blade cooled, the low-carbon steel on the back would contract more than the high-carbon steel on the edge. If the sword survived the cooling process without breaking, it ended up with the graceful curve of the katana.
Note: I am neither a blacksmith nor a metallurgist, and cannot vouch for this theory. I got it from either the History Channel or A&E, neither of which is a completely reliable source.

Thought not very well known, the classic curved katana was really more common in the Sengoku and Tokugawa eras. Previously, there was a wider variety of swords. Some were straight, some not, some a little different - but all were cavalry sabers at heart. Its use as a footman’s weapon was mostly comfined to duels and decoration. Samurai much preferred spears, bows, and no-dachi (oversized, katana-like two handers) for real combat. In battle, the katana was a backup, unless you were slashing at unarmored peasants.

Sort of. But the hardness of the metal in different parts of the blade was controlled not by carbon content, which was more or less homogeneous throughout, but by the rate at which various sections were cooled during quenching (rapid immersion in cold water following removal from the forge). Hard steels are made by rapid quenching from high temperature, while softer steels are made to cool less quickly during quenching. To control this cooling rate, layers of clay were used. A very thin layer was painted on what would become the cutting edge, to allow the metal to cool rapidly. A much thicker layer was applied to the heavier body of the blade, to slow the cooling rate, creating a softer, less brittle steel.

The carbon content was controlled through folding: the long billet was pounded flat while red hot, then folded in half, pounded flat again, and re-folded again and again until the original billet had been folded typically between eight and ten times. This process continually exposed fresh layers of metal to the air, where impurities and some of the carbon were burned off. Folding too many times would reduce the carbon content too far, resulting in unacceptably soft steel. Mythical Samurai blades folded dozens or hundreds of times did not actually exist. In practice folding more than about 20 times would not lead to any better result, since the number of layers thus created would begin to exceed the number of atoms in the thickness of the blade.

I’m curious if there’s a relationship here… we sell punches (to create holes for binding) at my work. These have pins that fit through a die and produce a series of cutouts in paper. The pins have slightly different lengths. So slight that unless compared against one another, they look about the same. This is so that when the paper is punched all of the pressure is not applied to the paper at the same time. This makes it easier to punch through the paper. Isn’t it possible that a slightly curved blade would produce the same effect?

Note: I think this may be what is meant by those saying “…reduce the contact area during a cut, thereby increasing the pressure at the cut…” or similar.

Actually I thought the curve in the blade was a side effect of the process in making it. Maybe I’m recalling wrong but I thought when they tempored the edge at a different tempering than the back it caused the blade to curve slightly.

Of course a curved blade is ideal for slashing, especially from horseback, which is what the katana is designed for so…

-XT