I am trying to determine the difference in the amount of force required to break multiple boards, depending upon using or not using spacers. At one point on the site, I see that "two boards held closely together" take up to 8 times the force to break vs. with a spacer between them. Is that an indication that with spacers, 8 boards is similar to 2 boards without spacers?
I do not know the exact difference in the force required to break boards with or without spacers, but it is many time easier to break with spacers, that is why people use them. You know how easy it is to break one board; children do it, old Senators do it, and television talk show hosts do it. With spacers, you are breaking one board at a time; each board is no harder to break than the one before it. If you break the first board, you will break every subsequent board as long as you sustain that same amount of force. Therefore, if you could maintain the amount of force required to break one board over the distance of a yard, you could break about 36 boards with spacers. Humans are not able to sustain that amount of force over that great a distance; so they hit the first board with as much force as possible and try to maintain the force until it dwindles to the point it will no longer break a board. To break boards with spacers, your hand or foot must move though all the boards with a sustained, relatively low level of force. In the world of martial art training, self-defense, professional fighting, or street fighting this type of force is useless.
Without spacers, you are basically breaking one board equal to the thickness of all the boards; although it is still easier to break many boards than it is to break one board of the same thickness, due to ability of the many boards to flex and slide against each other. To break boards without spacers, your hand or foot only has to move a relatively short distance through the boards; the flexing will travel all the way through the boards no matter the thickness. To break a single board that is several inches thick, one must strike the board with enough force to flex it a few inches, and then the strike can stop; the flexing force will move thought board and finish the break. The amount of force initially required is very great, but it only needs to be sustained for a few inches. When you reach the limit of the amount of force you may generate, or the amount of force exceeds that which the hand or foot can withstand upon impact, the break will fail. In the world of martial art training, self-defense, professional fighting, or street fighting this is the type of force you want to use; a short, instantaneous, powerful punch that transfers force throughout the target. A long, sustained weak punch does little damage and usually only pushes the target.