stoneagebowyer wrote: Quick question on the strings...how much stretch can I expect out of a sinew skein? I used your guide...25 strands of artificial sinew, etc. I was thinking about using fast flight for my next string, and wonder if total non stretchiness is a good or bad thing.
I used artificial sinew mostly because that was what I had one hand, and didn't know where to get quality hemp like that used in original skeins. One of the tricks I learned was to prestretch the sinew, in length of 30' at a time, then wrap the loops around a board with nails in it to start making the string. Then I form the end loops in the same knot style as shown in Egon Harmuth's DIE ARMBRÜST. Using steel rods, even screwdrivers, the eyes are stretched apart to set the eye knots and even out the tension in the strands. Now for the cool trick. before you start to serve the eyes, put the unfinished string in the microwave oven on high setting and heat it until the wax begins melting, the using the steel rods, stretch the string as tight as you can. The water content in the string is what causes it to heat up. Excess wax that oozes out is smoothed back over the string. Stretching it hot will really even out the tensions in all the fibers, even in the eyes. This is so there are no high stresses in only a few fibers such that they may tear. Overall, the skein will stretch in length about one inch or so, depending on how many turns or strands (how thick) you made the skein. When you are satisfied that the skein is set, and will stretch no further, then you proceed to serve the eyes and center portion using coxcombing with your favorite fray-resistant cord. I use plain old brown waxed linen. Some people use Dacron, which is much stronger, but I stick with natural materials and live with the drawbacks of the old traditional materials.
You want the string to be as non-stretch as possible. You don't want the string to be stretchy like a rubber band or you lose power. It's the prod that does the work. Of course, if you have an extremely string prod and you use hemp (the most non-stretch natural fiber) and the string is too thin in diameter, it will eventually break with a snap at one of the eyes. The eyes are the weakest point in the string because the skein fibers are bent around a small radius at the nocks of the prod which adds more stress to the strands in the outermost diameter away from the nocks. Since the stress is not even, these fibers may exceed their tensile strength if the string is too thin, and there are not enough fibers in the skein to share the load. If the skein is thick and strong enough for the prod, it should withstand the force of snapping taut when firing the crossbow. You will note that since hemp is strong but non-stretchy, the skeins on heavy medieval bows are rather thick, sometimes 1/2" in diameter. I imagine that the size may also be due partially to poor quality of the hemp strands themselves, but this is just speculation. The huge bars of steel that I have seen on heavy sporting crossbows certainly merit a thick, heavy skein. For the light crossbows we typically make today, a thinner skein is desirable because it is faster (less weight and table friction) and thus results in more velocity of the bolt, as Iolo (Geezer here) will attest to from his experience. You want the skein to be as thin as the materials will allow it to be without reducing its lifespan.
You will note that on heavy German sporting bows, the spanning distance (draw) is as low as 6". The effect is that the bolt is practically slapped into flight. It would be interesting to study why the Medieval bowmakers had such short draw lengths combined with thick and heavy steel prods versus longer draw lengths with lighter prods. Perhaps the heavy prods were too stiff to allow a long draw length. No wonder they often needed the crannequin!