[Michael Curl]

Hello all,

I am doing some research for a hypothetical pre-gunpowder weapon (fiction work) and was wondering how fast something like an arrow or javelin might be travelling when it impacts its target. For reasons I don't want to go into, the initial velocity doesn't matter, but I am looking for final velocities right before impact by various missile weapons. I have a reconstruction of a sharp weapon which would hit the target at 32mph and I need to now if that is fast enough to actually do anything. It weighs 75-110g by the way.

Thank you all for your input.

[Lin Robinson]

If you want actual data, try using a modern chronograph setup to register the speed of the missile at the point of impact. That will be difficult unless the weapon is extremely accurate or you can fire it accurately enough to put it through the field measured by the instrument. Chronographs used to be terribly expensive but over the years cost has come down dramatically. I am also uncertain as to the ability of the chronograph, ordinarily used to record the speed of bullets, to pick up other kinds of projectiles. If you have a trusting friend who will loan you a chronograph to try before you buy, that would be helpful.

[Tom King]

you're talking a ~1100-1700 grain projectile going 47 feet per second /14 meters per second for a grand total of 5-8 ft lbs of kinetic energy

a standard ~300-320 grain arrow from a longbow will go about 150fps and have about 15 ft lbs of KE

http://archerycalculator.com/archery-kinetic-...alculator/

in comparison a modern hunting crossbow throws bolts with somewhere between 80-150 ft lbs of KE and a compound bow throws arrows with 60-80 ft lbs of KE.

[Timo Nieminen]

[Michael Curl]

Damn, sounds like it is barely within the range of usability. For those interested, I'm writing a novel and the story is set in a stone age aquatic setting. The weapon is powered by buoyancy, and is basically a buoyant plumbata that's about two feet long. I have found some buoyancy calculations and got the weapon travelling at 32mph. One of the neat things about it is that buoyancy doesn't run out like the force of an arrow or javelin would, so the "torpedo" will travel at 32 mph over its whole trajectory. (Technically the buoyancy will get better as it approaches the surface as the water pressure weakens and the internal air expands to displace more water).

However this does sound like it is just barely acceptable as a lethal weapon. Admittidly armor is rare-ish (think aztec instead of medieval) and metal is non-existant, so the thresholds I need are lower, but I'm not sure if this is enough. I am going to try to calculate another weapon which has more internal area for displacement, but now I have to find the volume of things like bowling pins and juggling bats! Thank you all for your help.

[Timo Nieminen]

I'll be more dangerous than a thrown object of that speed and weight (and sharpness). As you say, it won't "run out" of force, while a thrown object is limited to the energy it has at impact. Your floater will keep exerting a force (as long as it's still underwater), even after it slows down after impact. This might not be enough to do anything useful. Try the experiment! Simulated point against simulated target (just in air, point above target). Put a weight on the point to push it down with a force equal to the buoyancy driving the "real" weapon upwards. See what happens.

[Michael Curl]

Interesting idea, I wish I had like a deep pool or something which I could try it in, I'd need like a scuba kit to do so, but I once got hit by a kick board that really took the wind out of me (we were holding them underwater and then releasing them horizontally, and the surface area prevented them from just rising so they launched underwater like a wing and would ram into people. (Dumb kid stuff).

Anyway, anecdote aside, I'm not sure how the continued pressure would do much, other than perhaps sustaining the penetration for longer during the initial push. The momentum would still be lost even if the propellant is still at work.

[Peter Lyon]

This is an interesting gag for the book (and I mean gag in the movie sense here).

A couple of considerations come to mind.

Streamlining will increase the speed; you don't need more bouyancy if the extra bulk becomes self limiting; a long, streamlined bag with a weight at the base to keep it travelling perfectly upright, and the spear or whatever at the upwards end. I don't know how fast it could go, but energy increases with the square (or cube?) of velocity, so a little extra speed makes for more punch. The deeper it is released the better, up to the point where bouyancy balances out with drag.

The weapon itself might injure or kill the opponent on its own, but there is another thing that could do the job. In humans, rapid depth changes by scuba divers cause air embolisms that can cause permanent damage or even death, so dragging the opponent to the surface could be nasty too. Technically it is nitrogen narcosis, where the nitrogen from air dissolved in the blood stream comes out of solution, so relies on air atmospheric mix and doesn't apply to all animals - whales don't have this issue and dive deep then come up fast. It may be related to continually breathing from a pressurised tank, so could be specific to humans doing scuba. Would these factors work in your world?

[Michael Curl]

Humans aren't around in my world but different species of fish are and the bends is something that happens with swim bladders, cuz rapid depth changes will cause them to rupture. So is your idea to have a buoyant net which will drag people to the surface?


My main problem with that idea (which I already am using) is that most sea life lives in shallow waters and so do most cities and so on, so deep water fights are going to be more like fighting in a dungeon. Happens all the time in roleplaying games, but pretty rare in real life. It's fiction so whatever the plot needs it'll get, but I am trying to work out as many reasonable weapons as I can in an aquatic setting. Besides for nets which are powered by weights/floats, I also have the torpedo (mentioned above), and bows (which are rare hunting weapons rather than proper combat weapons, and that's it besides melee weapons. However I am all ears.

[Peter Lyon]

A net would do it, but I had been thinking about the weapon itself (spear or whatever) hitting the opponent, sticking in, and also dragging them to the surface. Even if bends don't happen, your opponent has lost their ability to dive and avoid, and is in a lot of pain, so at a big disadvantage even if the weapon didn't disable them.

[Michael Curl]

Ah, so a harpoon pilum hybrid. Head will be barbed obviously. Evil plan sir.

[Gregory T Kallok]

I have tested this formula and it seems somewhat accurate for a longbow You add 115 fps "feet per second" to the bows draw weight. This is for an arrow length of between 28-30 inches. So I take my 90 longbow shooting a 30 inch arrow and add the 115 fps so.. 90+115 = 205 fps plus or minus 10 percent. This is a reasonable figure. Bare in mind the modern compound bows and crossbows are much different and generally shoot much lighter arrows faster then your typical longbow shooting a wooden arrow with a bodkin style step ahead..

[Eirik R. F.]

You could use the data from The Great Warbow on page 411. It's the only scientific experiment I know of measuring arrow speed at max distance. They used a Doppler radar.

I KE = Initial kinetic energy
MD KE = Max distance kinetic energy.

150 lbs bow, 33", 95.9 gram arrow
53 m/s = I KE 134.69 Joules
43.3 m/s = MD KE 89.9 Joules
Retained KE = 66.74 % ~ 67 %
Retained Velocity = 81.69 % ~ 82 %
Average range = 237.7 meters

150 lbs bow, 33", 86.6 gram arrow
53.55 m/s = I KE 124.167 Joules
43 m/s = MD KE 80.1 Joules
Retained KE = 64.5 % ~ 65 %
Retained Velocity = 80.29 % ~ 80 %
Average range = 233.6 meters

150 lbs bow, 33", 74.4 gram arrow
57.8 m/s = I KE 124.279 Joules
44.9 m/s = MD KE 75 Joules
Retained KE = 60.34 % ~ 60 %
Retained Velocity = 77.68 % ~ 78 %
Average range = 259.1 meters

150 lbs bow, 33", 57.8 gram arrow
62.25 m/s = I KE 111.989 Joules
48.3 m/s = MD KE 67.4 Joules
Retained KE = 60.18 % ~ 60 %
Retained Velocity = 77.59 % ~ 78 %
Average range = 297.5 meters

150 lbs bow, 33", 53.6 gram arrow
64.3 m/s = I KE 110.804 Joules
48.9 m/s = MD KE 64.1 Joules
Retained KE = 57.84 % ~ 58 %
Retained Velocity = 76.04 % ~ 76 %
Average range = 318 meters

150 lbs bow, 33", 108 gram arrow, 52 m/s = 146 Joules

If you make a graph of this you'll see that the 86.6 gram arrow is outside the graph because it was a "blunt" arrowhead. 53.55 m/s is too slow with a sharp arrowhead. It should be around 54 to 55.1 m/s or there about and 126.26 to 131.45 Joules and maybe around 44 to 44.5 m/s at max distance and around 83.82 to 85.74 Joules. This will give retained KE = between 65.22 and 66.38 % and retained velocity between 80.76 and 81.48 %.

60 Joules at max distance does not seem to be much, but it's equal to lifting a 6.8 kg (15 lb) hop bar/digging bar 0.9 meters above ground and letting it drop. This will crush ice and go into soft ground enough to not fall over.

mgh
6.8 * 9.81 * 0.9 = 60 Joules.

The bar will strike the ground at
Sqrt 2gh
Sqrt 2 * 9.81 * 0.9 = sqrt 17.658 = 4.2 m/s

It's also equivalent to a 20 oz (0.566 kg) hammer at 14.6 m/s.

Both the digging bar and hammer have more momentum, but the kinetic energy is the same.

[Jean Thibodeau]