Kinetic Energy and Momentum # for Elk

[Bonedalien]

just wondering what numbers some of you aim for concerning KE and momentum specifically for Elk

 

[MattB]

Get an arrow of at least 400 gr., pair it with a strong and efficient broadhead, and then shoot as much weight as you can comfortably handle. No disrespect intended, but I wouldn't take advice from anyone who wants to talk in terms of KE or momentum when recommending archery tackle. That's the fodder of guys who do more honing of their bowhunting skills on the internet than in the field.

 

[weedwacker42]

This is stolen from this website. Pretty good resource.
http://www.huntersfriend.com/carbon_arrows/hunting_arrows_selection_guide_chapter_5.htm

KINETIC ENERGY
Hunting Usage
< 25 ft. lbs.
Small Game (rabbit, groundhog, etc.)
25-41 ft. lbs.
Medium Game (deer, antelope, etc.)
42-65 ft. lbs.
Large Game (elk, black bear, wild boar, etc.)
> 65 ft. lbs.

 

[OR Archer]

Get an arrow of at least 400 gr., pair it with a strong and efficient broadhead, and then shoot as much weight as you can comfortably handle. No disrespect intended, but I wouldn't take advice from anyone who wants to talk in terms of KE or momentum when recommending archery tackle. That's the fodder of guys who do more honing of their bowhunting skills on the internet than in the field.

I agree with most of this. Guys get WAAAYYYY to caught up in crunching numbers. Drives me nuts personally. Like Matt said get a decent weight arrow with a good broadhead. Tune your bow to it. Shoot it religiously. Then go poke an elk behind the shoulder where you should and you'll be eating elk next winter. Simple as that.

 

[kodiakfly]

I think I read a couple decades ago that IBO recommended 55 ft-lbs of energy for a bull elk. That huntersfriend thing says 65. OK. My bows back then at 70 lbs were producing like 75 ft-lbs and my modern bows nowdays are producing 90-95 ft-lbs. My point is, my Hoyt Smoke from 1996 at 70 lbs with an A/C/C is sufficient for elk. I'd reckon that most any bow these days are plenty sufficient. Don't sweat the numbers. Same with speeds. Compound bows were killing elk and moose 30 and 40 years ago...they'll have no problem doing it now.

My only commentary on that, is it seems to me nowdays guys are less disciplined in their shooting and restraint from poor shots than they used to be. And I certainly don't mean you OP or anyone here. Just that more guys blame their bows or their bows' tuning for a missed or wounding shot, than their shot itself. But that's a separate conversation.

 

[5MilesBack]

I don't even know my current numbers, but doesn't really matter at my draw length. I have preferences, but I'd think that most any bow of 50lbs or more should be able to shoot an arrow through an elk with the right arrow and BH.

 

[BackcountryMT]

KE is a useless number and it drives me nuts that the archery industry has adopted it. I like an arrow in the 430-470 range with lots of weight up front and a good reputable broadhead. That will kill every elk it comes in contact with.

I'll be setting up some new Deep Six FMJ's w/two inserts stacked up front for my elk arrow this coming fall.

 

[bhylton]

would you guys say that having more momentum would be beneficial when you dont make that text book perfect behind the shoulder shot. maybe drill a rib or clip the shoulder? or does it just not really matter that much?

 

[Bonedalien]

thanks for the responses guys. i am good with my arrow,and have killed elk with it. its just that time of year ,snowed in,tying flies,building arrows and nerding out on numbers. cabin fever

 

[kodiakfly]

KE is a useless number and it drives me nuts that the archery industry has adopted it.

Why so? You prefer momentum? Just curious.

I agree that the pursuit of a "number" is useless, but the pursuit of a bow that puts as much energy into an arrow and then an animal is a noble cause. If I'm looking at different arrows or weights, I might plug in some numbers and see which arrow is a better combination of speed vs weight.

 

[BackcountryMT]

KE cannot be measured as a direction, only as a potential. Momentum is a true measurable direction.

It's entirely possible for a light, fast arrow to have the same KE as a heavy and slow arrow, yet the heavy arrow will contain more momentum. I'd have to sit down and do the math to give you exact numbers.

 

[GibsonKuenzi]

...

 

[MattB]

If you look at some of the set-ups that trad guys are killing buffalo with, you'd be woefully underwhelmed at their KE. Yet the arrows launched from them generate substantial penetration.

"Using the TPI formula, a 60 pound longbow firing a 788 grain compressed cedar arrow, with a 190 grain Grizzly broadhead, at 148 fps has only 38.34 ft. lbs of K.E., .52 lb.-sec. of momentum, but has a TPI of 1.50. That combination was used to repeatedly shoot through the scapula of a large zebra stallion and through the thorax to the off side, often breaking off-side ribs (never failing to penetrate the scapula and completely through the thorax). This was compared to a compound firing a 555 grain aluminum shafted Black Diamond at 229+ fps. This combination gives 65.21 ft. lbs. of K.E., a momentum of .57 lb.-sec., but a TPI of only 1.27. That compound was, at best, able to penetrate only 5 to 8 inched beyond the scapula, and occasionally failed to penetrate the scapula at all on that same zebra carcass."

http://www.tradgang.com/ashby/Arrow Lethality 5.htm

 

[FishTacos]

I thought this article was interesting when discussing momentum and kinetic energy. http://archeryreport.com/2009/11/arrow-kinetic-energy-momentum-archer/

 

[Beendare]

Delete

 

[Ridge Ghost]

Holy five year old thread resurrection!

 

[FishTacos]

Holy five year old thread resurrection!

Ha you’re right, it didn’t seem like starting a new topic was wise because this thread already existed, so I decided to dust it off instead.

 

[roosiebull]

I think it’s beneficial to NOT get too wrapped up in paper numbers... maybe if you are a really short draw/light poundage person, but besides that, avoid extremes and you will be fine.

I have tried a lot of arrow builds, and have settled in the most boring range, middle of the road, around 525gr with my 28.5” draw, and 73# draw weight.

the heaviest arrows I have hunted with were 642gr, and they were not magic... I have also killed several elk with 380-400gr arrows before I knew those were too light to kill elk... luckily the elk didn’t know either.

I would never recommend an arrow that light these days, because you are giving up a lot doing it for no reason.

I like a mid range arrow, still get some yardage out of the sight tape for off season practice or a follow up shot, quiets the bow down, easy to get good arrow flight (warp speed loses forgiveness) and I’m just happy there

 

[FishTacos]

I think it’s beneficial to NOT get too wrapped up in paper numbers... maybe if you are a really short draw/light poundage person, but besides that, avoid extremes and you will be fine.

I have tried a lot of arrow builds, and have settled in the most boring range, middle of the road, around 525gr with my 28.5” draw, and 73# draw weight.

I’m with you on this. Modern equipment is powerful. I’m shooting 510gr and if it’s well placed it’s no question. I’m not too interested in tinkering with the setup this time of year.

 

[KyleR1985]

I always think it’s funny when different issues in human civilization are on fire and we make fun of people who actually care about details and mechanisms and first principles. Here's an excerpt that I think can close the loop for most people in about ten minutes of sitting still and reading:

*Not my work, don't take credit. Follow link for OP:

Kinetic energy, mass, velocity, momentum, online...

www.archerytalk.com

Let's begin by addressing some common misconceptions that we should eliminate immediately.

Misconception 1.) The one that is going to tick a lot of people of instantaneously—arrow momentum does not determine penetration. This notion is absolutely false. Linear momentum—p, defined in Eqn. 2—is an incredibly useful tool for performing calculations. This stems from momentum being a conserved quantity. In more advanced formulations of classical mechanics momentum is a useful quantity to find the underlying symmetries of nature, so it is a very important quantity—nevertheless, the momentum of an arrow DOES NOT determine its penetration potential.

Now, let's be very precise. An increase in momentum may (and almost assuredly will) correlate with an increase in arrow penetration, but we should be careful about the difference between correlation, when two observations are usually found to occur in tandem, and cause and effect, or causation, where one phenomenon causes another. It is usually the case that an arrow with large momentum will penetrate deeply, but the large momentum does not cause this deep penetration.

In addition, we should be careful about discussing momentum as something the archer actually has control of in their setup. Momentum in a hunting arrow is a dependent parameter. There is no momentum control knob that one may turn on their bow to increase the momentum that their arrow has upon firing. Instead, we must change another independent parameter—also called a control parameter—which will in turn change the momentum of our arrow as it leaves the bow. We will discuss what the appropriate control parameters that we should consider are momentarily.

Misconception 2.) Archery calculators are useful over large ranges of their input parameters, such as arrow mass. To be perfectly honest, most archery calculators are poor, and they tend to give archers very bad information. The most common misconception that archery calculators reinforce is that there is a "sweet spot" of arrow mass that maximizes the kinetic potential obtained from firing the arrow from a particular bow. I have read what seems like countless posts (and shamefully have even posted the same misinformation occasionally myself before actually thinking about the way these calculators work) where the poster thinks that a calculated arrow mass will give the archer the most kinetic energy that they can get out of their bow. This is always incorrect. This will be covered more in misconception 3 and later.

As an example of how archery calculators can give truly horrible predictions, lets take the backcountry bowhunting calculator as an example. I actually like this calculator when used appropriately, but let's see what happens when used incorrectly. Check the default settings on the calculator. The kinetic energy is given as 89.3 ft·lb. Now increase the arrow mass slider to 490 grains; the KE is 96 ft·lb. Above 510 grains the KE begins to decrease again. I assure you, this is NOT correct. This may be demonstrated by going further with the arrow mass slider. An arrow of 800 grains shows a kinetic energy of 74 ft·lb. Take a moment to actually think about this. All of the kinetic energy is transferred to the arrow from the bow, derived from the potential energy stored in the bow at full draw. The calculator is telling us that a 500-grain arrow will absorb 96 ft·lb of energy from the bow. This means that at full draw there must be at least 96 ft·lb of potential energy stored in the bow—in truth, no bow is 100% efficient, so there will actually be more than 96 ft·lb of potential energy stored in the bow. Now shoot an 800-grain arrow out of the same bow—the calculator is telling us that only 74 ft·lbs of kinetic energy will be accounted for in the arrow projection. Energy is a conserved quantity, so that means there is an additional >22 ft·lb of kinetic energy that the bow must be dissipating when it fires the arrow that is not accounted for in the arrow's KE. That much additional energy would result in a sound like a shotgun, and may severely damage the bow itself. We know this is not true in real life—the heavier arrow will result in a quieter bow with less hand shock and generally less energy left over after firing the bow that the bow must dissipate in other ways. The problem with the calculator can be shown in an even more extreme fashion by putting 1500 grains in as the arrow mass. The calculator now predicts the arrow to have 0.1 ft·lb of kinetic energy and to actually leave the bow with negative speed—it predicts that the 1500-grain arrow will be fired backwards. Hopefully this sufficiently demonstrates the pitfalls of using archery calculators to predict the kinetic energy, momentum, etc., of an archery system.

Misconception 3.) Choosing a particular arrow has any significant effect on the kinetic energy that the archery system will produce. If you are trying to achieve a kinetic energy goal by adjusting your arrow mass, you have already lost the battle. *This misconception has a slight caveat, which I will address shortly.*

With these misconceptions addressed, let's take a look at the real physics behind arrow penetration.

We begin with energy. Energy is the capacity of a system to do work. A system without any energy lacks any ability to change or influence other systems with which it interacts. Energy comes in two general forms: potential and kinetic. Arrows in a hunting situation lack any usable potential energy, so an arrow's entire ability to influence other systems—including the living system that is the game animal—depends entirely on its kinetic energy. As such, the first parameter in determining arrow penetration is—regardless of Dr. Ashby's insistence otherwise—the kinetic energy attained by the arrow in the archer's system. Eqn. 1 gives the usual definition of kinetic energy, 1/2·m·v[SUP]2[/SUP]. This definition has led to mass confusion in the archery community. Although the equation is, of course, correct, the general interpretation of the equation is incorrect in the case of an archery system. The mistake is in what is taken to be the control parameter and what is the dependent variable. Most on Archery Talk mistakenly take the kinetic energy of their bow to be determined by the mass and velocity of their arrow. In truth, the velocity of the arrow is determined by the arrow mass and the kinetic energy produced by the bow. Like momentum, velocity is a dependent variable and may not be controlled independently of other control parameters.

Let's readdress misconception 3 above. Many archers think (and have been misled by arrow manufacturers to believe) that one may choose their kinetic energy by choosing a particular arrow setup. This is essentially false. The kinetic energy obtained in an archery setup is almost entirely determined by your choice of bow. It is true that bow efficiency will increase when using heavier arrows, but the effect is minor and may essentially be ignored. A recent example that demonstrates this clearly is this video by DIY Sportsman. The data given spans arrow masses from 379.4 grains to 1163.5 grains. That is an increase in arrow mass of 207%—a tripling of arrow mass. The kinetic energy of the arrow leaving the bow increased from 73.3-77.9 ft·lb., a mere 6% increase. A >200% increase in arrow mass yielded a <10% increase in kinetic energy. For all practical purposes, arrow mass does not affect arrow kinetic energy when the two arrows are fired from the same bow. For newer bows (5 years old or less) this effect is pretty general. Bow manufacturers have gotten so good at producing efficient cam systems with even low gpp arrows that there simply isn't much room for bow efficiency to increase with heavier arrows. As another example, an early review of the Realm SR6 showed a 1.7% increase in arrow kinetic energy for a 45% increase in arrow mass. Even with older bows, the above still holds true generally. The same may not be true of trad bows—finding data to check is more difficult, and I've seen reported that increasing arrow mass increases the efficiency of a trad bow more significantly, though I haven't seen data to support or refute that statement. However, even with a trad bow, it will still be true that it takes a huge increase in arrow mass—doubling or tripling—to see a relatively small 10-20% increase in the arrow kinetic energy produced by the bow.

Your choice of arrow has a minimal effect on the kinetic energy produced by your bow.

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