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Varieties of Gennou: Ryouguchi Gennou
There are several types of gennou. The most popular is the standard, double-faced symmetrical gennou called the “ryouguchi gennou” 両口玄翁 pronounced ryou-guchi-gen-nouh. “Ryou” 両 translates to “both” and ”kuchi” 口 means mouth, so a ryoguchi gennou is one with a striking face on both ends. This category includes its stumpy brother the daruma gennou, which is a shorter, stubbier version of the ryouguchi gennou. One face of ryouguichi gennou hammer is flat, and the opposite face is domed. The flat face is used for striking chisels and nails, with the domed face is used for the last couple of hits on a nailhead to recess it below the wood’s surface. It can also be used for something called kigoroshi (“wood killing”木殺) which we will touch on in a future post.
This most popular of gennou is symmetrical in all axis, an extremely stable shape making it well-suited for using at many different angles and at different swing velocities to make powerful hits where stability during the swing is important. And stability is often not just important but critical because a hammer that easily wiggles or twists out of alignment during the swing, or jinks upon impact, will make you look like a child.
Varieties of Gennou: Kataguchi Gennou
Besides the ryoguchi, the other common variety of gennou is called a “kataguchi” 片口 or single-face gennou. “Kata” 片 in Japanese means “one” or “half” and “kuchi” 口 means “mouth” but for some reason unknown to me is used to mean “striking face” in the case of hammers. It has a slightly domed face on one end with the opposing end tapering to a small square face for setting nails. Besides setting nails, the tapered end is handy for “tapping-out” (uradashi) the hollow faces of Japanese plane blades. The domed face of the kataguchi gennou is shallow enough to be used for striking chisels, but is not as good for kigoroshi. Kataguchi genno include the yamakichi style common to Kyushu Island, the funate or Iwakuni style common to Western Honshu Island and Hokkaido way up north, and several variations thereof.
The hammer pictured immediately below is the “Funate gennou” 船手 which translates to “boat hand” gennou. It is especially suited to driving nails, while it’s tapered tail can be used to make a starting hole for nails, a capability especially suited to ship building.
The gennou pictured immediately below is called the Yamakichi gennou 山吉, with Yamakichi meaning “lucky mountain.” This was the brandname of the blacksmith on Japan’s Kyushu Island who developed this style of hammer. It is a stubbier, heavier hybrid of the ryouguchi and funate styles, better suited to chisel work while still being well-suited to driving nails. I am told that Kosaburo received permission from Yamakichi and modified the design slightly to better meet the requests of his customers in the Tokyo area. If you can only have one hammer with you in the field or when doing installations at the Client’s home or facilities, the Yamakichi gennou is hard to beat. It’s unusual and sexy-looking.
The Varieties of Gennou: Daruma Gennou
The daruma gennou (dah-ru-mah) 達磨玄翁 is a variation of the double-faced ryouguchi genno, but at the same weight, it is shorter and fatter. It is named after Bohdi Dharma, a Buddhist Monk who was the founder of the Zen (Chan) sect of Buddhism in China, as well as an important person in the history of the Shaolin Temple made famous in Hong Kong Kung Fu movies. You will remember seeing Shaolin Priests in Hong Kong movies dressed in saffron robes, and with rows of dots on their bald pates, jumping around thwarting evil warlords with long mustaches.
There are many legends about the Enlightened Dharma, but one story says that while meditating for nine years in a cave near the Shaolin Temple, his atrophied arms and legs fell off leaving just his trunk and head. Because of this legend, in Japan he is portrayed as an oval-shaped figure without any limbs, and with bushy eyebrows glaring out from inside a red hood. He has come to symbolize wisdom and victory through persistence and endurance. This image has deep roots in Japanese culture.
The daruma genno is named after him because, like the buddhist priest, it is short, stubby, and round. Religious matters aside, at any given weight, the daruma is not as physically stable as the standard genno due to its reduced Moment of Inertia.
The Moment of Inertia refers to the tendency of a body to resist changes in position. Quoting from Wikipedia (which is no doubt a quote from some physics textbook): “It is the moment of inertia of the pole carried by a tight-rope walker that resists rotation and helps the walker maintain balance. In the same way the long axis of a dragster resists turning forces which helps to keep it moving in a straight line.”
It is the increased Moment of Inertia that makes a steel I-beam so much stiffer and stronger than a plain steel rod of the same length and weight.
Like these three examples, the standard genno head has its mass spread out from the center, making it more resistant to movement than if the same mass were concentrated in a solid ball.
The math for a rod about a center, which is a close approximation of a hammer head, is I= (1/12) x ML2, where I equals the Moment of Inertia, M equals the mass of the rod, and L equals the length of the rod. As you can see from the equation, the Moment of Inertia varies with the square of the object’s length, so that a ball has the lowest possible Moment of Inertia for a given mass, and is the easiest shape to get moving, while a hammer head with its mass moved away from the center will have a much higher Moment of Inertia, and will therefore be more resistant to changes in direction.
For any given mass, the daruma gennou head has less length than the standard genno head, and therefore has a reduced Moment of Inertia, and so is less stable.
Why is this important? Because you are not a machine, and when you swing a hammer several contradictory forces act on the hammer. Sometimes those forces are large and problematic and sometimes they are small and insignificant, but often some of those forces work to drive the hammer off course so it misses the target, and others tend to cause it to twist during the swing so that a line drawn through center of the hammer’s face and the center of its mass is not aligned with the target causing a glancing blow wasting time and energy. But since a longer hammer head has a higher Moment of Inertia, it will tend to not twist out of alignment as easily as the shorter daruma will during a swing, and is more likely to impart more of its energy into the chisel even if the hit ends up being a bit off-center.
In comparison to the shorter daruma, the longer standard ryouguchi gennou head, or even Yamakichi gennou, will tend to rebound straight back, instead of twisting, helping the user to maintain a steady rhythm thereby saving time. Of course, with practice, the daruma can perform just as well as the standard ryouguchi gennou head, but if you intend to make a lot of fast, hard strokes at various angles, which is common in carpentry and timber framing, a standard ryouguchi gennou with its higher stability is a superior choice.
The daruma gennou has traditionally been the preferred primary hammer for two trades: Joiners (tategushi), who use the daruma to their advantage in a specific way, and sculptors, who don’t require stability but do appreciate a large face. Cabinetmakers, tategushi and tansu makers often have a heavy daruma on hand for assembly work because the high face area/weight ratio is convenient for knocking joints together.
I learned about daruma genno from a retired joiner in Tokyo who was kind enough to instruct me occasionally over a period of several years in the making of Japanese tategu, especially wooden doors, shoji, ranma, and free-standing screens (tsuitate). Nowadays, commercial joiners (tategushi) cut mortises mostly by machine, but traditionally, all joints were cut by hand, so the old boys were required to do very precise work, very quickly, frequently cutting hundreds of small mortises for a single screen or door. The daruma gennou exceeds at this precise, repetitive, speedy work where the chisel is almost always oriented vertically in the cut, the workpiece is almost always located at an unchanging height from joint to joint, and the hammer is not so much swung at the chisel as dropped on it to ensure a very predictable depth of cut with stability not being a significant problem making the daruma suited for very precise cuts in narrower workpieces such as door and furniture parts.
For example, when cutting joints in shoji, the material remaining at the bottom of a mortise cut in a stile to receive a rail may be only be 1/4 millimeter thick, almost translucent, so if care is not taken, the chisel will cut all the way through ruining the stile. To avoid this, the joiner needs to be able to control the depth of cut very precisely, and rather than swinging the hammer, it is more-or-less allowed to drop imparting uniform impact forces than would be more difficult to achieve by swinging the hammer. The hammer should not rebound from the chisel but transfer all its energy for smooth, consistent cuts. When used properly, a daruma genno feels like it is sucked towards the chisel, and when it strikes, it feels like it sticks to the chisel for a fraction of second with little or no rebound providing excellent control and more precise control of the depth of cut. This technique takes lots of practice to master.
I have seen carpenters in Japan laugh at a fellow that brought a daruma gennou to a jobsite because the stumpy things are thought by many carpenters to appear clumsy. I must agree. Also, they assume that a fellow that uses a hammer with a face as big as a daruma does so because he has a hard time finding the end of his chisel with a standard hammer. They may have a point.
For reasons unclear to me, Americans and Europeans have an illogical affinity for the daruma gennou. That said, when I need to cut a lot of small, precise mortises, I use a daruma. When I need to cut bigger or deeper mortises, or mortises at angles, however, I bring out a standard gennou of the appropriate weight for the relatively greater stability they provide. If you only have one gennou, the standard ryouguchi style head or even yamakichi style would be a good choice.
In the next chapter in this bodice-ripping yarn of romance and intrigue we will examine a more sinister application of the gennou hammer, namely kigoroshi, or “wood-killing.” Please use the bathroom before reading it to avoid embarrassing accidents.