I wanted to follow up on some interest from previous threads:
http://belegarth.com/board/viewtopic.php?t=18412
As I say, I'm from Amtgard/HFS with a great deal of experience with LARP crossbows. We went with 450 inch-pounds as safe even for close range use, compared to about 650 ip for a 35#*" handbow. Recently I was wondering how best to equate what is a safe energy, since "half draw" is undefined and means a different pull to different people and thus basically a meaningless term.
Well, what we DO have common to most groups is that fulldraw 35#*" is officially acceptable at >20ft. Therefore, by comparing the speed from an arrow from that arrow at 20 ft versus the exact same arrow out of a crossbow at close range, if the crossbow is slower then it tends to show it's acceptable since there is no difference between the two hits. On top of that, a crossbow bolt will generally have a lighter shaft which certainly helps the situation even more, although there's nothing stopping a person from using a solid fiberglass crossbow bolt shaft which would be even heavier than most 28" graphite arrows.
The data's below, but the short story is that a 450ip (45 lbs at 10" of power stroke, 56 lbs * 8" of power stroke, etc) is about 20%-30% less impact energy than getting fulldrawn at 20ft, even with widely differing examples of arrows. It's possible to go higher, but IMHO this is ok and if you have aerodynamic arrows they can be made light and thus fast and have good range. Unfortunately Bel/Dagorhir have that "protrusion" term that mostly prevents such construction, but that's another story. Anyhow, 450ip is something I've worked with extensively and found to be effective with my gear anyways with no notable complaints and I do my best work at around 20ft. If the Marshal's measurement is off and allows something even 10%-20% over the limit, it's still pretty safe as things go, just kinda stingy at worst.
The Test
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I used 3 different arrows here, the first two were just picked up from the unclaimed junk cleaned up after a large event and I think they're fairly good examples of what people build. "Yellow" a heavy aluminum-shaft design with a 2.77" flat face. It weighs 75.1 grams. "Black" uses a thick Beman 500 carbon shaft with a 2.75" dia head which is somwhat rounded but I believe this would also meet Dag/Belegarth's "protrusion" rule (barely). It weighs 57.2g. MS2K is my own Meatseeker 2000 design, using a thin graphite shaft. It a 2.725" dia ball front and tapered cone backside and weighs 43.9g.
I measured speed on a Chrony F1 chronograph from a 34#*" Beak Kodiak Magnum, 7.25" brace height for a 19" power stroke for 646 ip total (* good bow, btw), shot it right off the bow at fulldraw, then measured the speed with the chrono 20 ft away at fulldraw. Then I put the same arrow on a recurved fiberglass prod crossbow measured at 44.4 lbs, 10" power stroke, for 444 ip. Measurements are averaged from several shots. The (% energy) listed is a calculation of how much of the fulldraw close-range energy for that arrow type is in that shot. Note that energy is velocity squared times mass, mass is constant for these comparisons so it's just a ratio of squared velocities.
Yellow:
Mass: 75.1g
Aerodynamics: poor
Fulldraw close: 93.2 fps
Fulldraw from 20ft: 79.4 fps (72.6% energy)
Crossbow 444ip close: 72.4 fps (60.3% energy)
Black:
Mass: 57.2g
Aerodynamics: fair
Fulldraw close: 107.35 fps
Fulldraw from 20ft: 94.06 fps (76.8% energy)
Crossbow 444ip close: 81.37 fps (57.5% energy)
MS2K:
Mass 43.9g
Aerodynamics: excellent
Fulldraw close: 124.2 fps
Fulldraw from 20ft: 110.3 fps (78.9% energy)
Crossbow 444ip close: 91.625 fps (54.4% energy)
Comments: Distance * ft was pretty hard to get with great precision, trying to both hit a fairly small target area while holding exactly a 28" draw. The MS2K full-length arrow may suffer somewhat from being on the crossbow because its shaft spine is too flexible for a 44 lb draw, the long shaft may be bending more than usual and buffering some of the shot energy.
Conclusion: the absolute flat-face of yellow is an aerodynamic fiasco, despite its slower speed and greater mass it loses the most energy of the bunch. My MS2K I'm proud to say conserves more of its energy despite dramatically lower weight and far higher speeds.
But anyways, the impact energy at 20ft, where LARPs usually allow unrestricted fulldraws, varies by arrow type but was measured as 72.6%-78.9% of fulldraw energy for a fairly comprehensive list of probable arrow types.
In all cases, at 444 ip a crossbow will fire the same projectile with substantially less energy even at pointblank range than a target would get at 20ft from a legal fulldraw. But, you may note, a crossbow bolt is somewhat different. No group allows crossbow heads to be constructed differently than arrows so we will assume the head design has comparable diameter, padding, and mass. A crossbow bolt has less than half the shaft length of an arrow, thus half the shaft mass for a particular shaft type. Buffering and spreading out the shaft's energy is almost the entire safety issue in archery. With half the shaft mass, the arrow will be faster, but the portion of energy in the shaft will always drop significantly and there is significantly less impact as a result. This principle is only true when comparing similar head designs and shaft types. For example, if a builder were to use solid fiberglass rod for crossbow bolts, the mass would be even higher than a graphite arrow and is thus more stress on the padding.