I would think that with all the dialogue associated with the subject of arrow momentum, someone would come up with a guide for minimum arrow momentum requirements. For example, say I want to go Elk Hunting. Minimum KE recommended for elk is 42lbs. My setup generates a little more than that. But I get the practically the same KE whether I shoot a 300gr arrow or a 500gr arrow (approx. 2lbs KE difference). Common sense tells me the heavier but by how much? There are no guidelines from what I can see..
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Bowtech 82nd, 26.5"draw, 54lbs
HHA Optimizer Lite
QAD Hunter
Maxima 250's
300fps, 59lbs KE
RE: Why no "standard" recommendations for Momentum?
In all honesty, I think it may be because MOMENTUM is measured in "pound/sec" and the measurement is usually SMALL numbers.
Doegirl,
If you're shooting a 315gr arrow 271fps you get 51ft/#'s and .3785 "pound/sec" of momentum, off the riser, at 30yds, you're down to 258fps with 46 ft/#'s of KE and .36 "#/sec" of momentum.
Up that to a 450gr arrow at 231fps, you have 53 ft/#'s of KE and .461 "pound/sec" of momentum, off the riser, and at 30yds, you've slowed down to 223fps with 50ft/#'s of KE and .446 "pound/sec" of momentum.
Those are numbers guestimated off of your set-up. So considering those numbers, you'd maybe be more well served to work on achieving moderate speeds while not sacrificing KE or Momentum. But it's just not "catchy" to say I'm getting .461 "POUND/Sec" of momentum out of my bow. It sound so much more "cool" to say I have a set up getting 70ft/#'s of KE.
RE: Why no "standard" recommendations for Momentum?
Its not written down anywhere, but this is what I gather:
1) The firearms industry used ft/lbs of energy, combined with speed/tradjectory (see ballistics tables) to rationalise "effective killing power" - which in my opinion is a load of crap, but lets not get personal - Long and short of it, is that its is a uniform away to compare the speed/energy/weight/performance of different cartridges accross the board. A bullet is a projectile, an arrow is a projectile...seems like a decent enough reason to me.
2) Ironically, arrow KE are fairlywhole numbers compared to some of the momentum readings I've seen. Its easy for me to remember that if I have 60ft/#s of kinetic energy, the author of a magazine article says its safe for me to shoot deer with any kind of broadhead I want. I might get confused if he said at least 2.13750498 kg/(m/s). The simplicity of the answer makes it easier to understand.
3) There are no conversions needed to figure KE. All you need to remember is 450,240. If you can remember that number, read a grain scale and a chronograph, you'll have KE in a quick second.
For momentum, the forumla is easier to remember p=MV.... no big deal right? Lets start with mass.In order to calculate arrow mass, you need to roll that decimal left a few places on your arrow weight, to get a true reading in kilograms. Then, you have to switch feet per second to meters per second toharmonize everything for your velocity measurement. When you are done, you'll probably end up with a number somewhere in the 1.xxxxxxxxxx to the 3.xxxxxxxxxxxxx range. To totally grasp this concept and see exactly what sort of net change you have with one arrow/speed with another, you need to divide those out using a % forumla to see your increase vs decrease % wise.
4) Finally, I don't think either KE or Momentum is a perfect fit with regards to potential penetration, especially on game where the variables are numerous. HOWEVER (I think this is the big one) There are certain cases, both with bullets and with arrows, where lighter arrows at much faster speeds, have been shown to out perform heavier/slower arrows, as well as arrows that have a decent balance of the two. These being conducted on ballistics gel, which is an analgous testing medium. When momentum says the heavier arrow should penetrate the best, it does not. KE says the lighter, faster arrow should... and it does. Thats why they go with KE!
__________________ You get what you put in, and people get what they deserve. - Kid Rock
RE: Why no "standard" recommendations for Momentum?
It's a holdover from guns, bullets have always been shown with KE, the move to archery, it makes sense to give people a number/title they can relate two. KE and M are related since they are both based on velocity.
RE: Why no "standard" recommendations for Momentum?
Quote:
It's a holdover from guns, bullets have always been shown with KE, the move to archery, it makes sense to give people a number/title they can relate two. KE and M are related since they are both based on velocity.
Yep. And switchover hunters have brought magnumitis into archery. KE vs momentum is a really old argument among gun hunters. The debate had been going on for years when Shooters Bible ran a pair of articles, one by Roy Weatherby and the other byElmer Keith, side by side as a debate. Weatherby liked light bullets going fast and Keith was a self proclaimed'punkin roller'.
I thought Keith's point of viewmade a lot more sense than Weatherby's. Still do. Compare two rounds, the .223 and the old .45-70. The light, fast40 gn .223 bullethas a muzzle energy up to around 1450 ft lbs and the slow300 gn 45-70runs aboutthe same KEin factory loads. Most people wouldn't consider using the .223 even on somethingas small and puny asa deer while the 45-70 was one of thefavorite guns of the old buffalo hunters.So, even in guns the fixation on KE doesn't make a bit of sense.
Anyway, doegirl, your bow puts out less KE than my 50 pound longbow does. So, you and all others like you that shoot bows of similar power levels, need to select arrows more like a traditional shooter does. I know some of the yayhoos on this forum are sick of hearing about Ed Ashby, but he really has developed a huge amount of information on how various arrows and broadheads perform in actual animal tissue - not blocks of foam or ballistics gel.
It's very technical reading, but well worth the effort to read, sift through and get enough good, real world information to help you make educated choices on arrow setups for your needs.
Why no "standard" recommendations for Momentum?
