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Discussion Starter #1
I recently took the opportunity to peruse the women's discussion just out of latent curiosity and happened to notice a discussion on the minimum bow requirements for taking elk. I was sorely tempted to enter this discussion, being the inveterate big mouth that I am, but in deference to the ladies I decided to demur. I did this because some of the answers the lady who started the thread was getting have already been posted many times to male forums. I suspect the ladies giving these answers might be repeating what they've heard or read in male discussions. Therefore, I hope some of them will be so kind as to repeat this one too.

In many states the minimum draw weight specified for taking elk is the same as for deer, 40 pounds or so. However, some people have asserted, based evidently on kinetic energy calculations, that this lower limit is too low.

From a traditionalist's perspective such an assertion should be taken with a good deal of skepticism if for no other reason than that native people have harvested elk and even bison, who are a good deal bigger than elk, with bows in the 40-45 pound range. Apparently they did this out of pure ignorance of kinetic energy requirements because they didn't read the right books or magazine articles.

I've been especially critical of kinetic energy analyses in the past because they're frequently raised during typically volatile discussions of pistol cartridge stopping power. In most cases these analyses might reveal a rather weak statistical corrolation but not much else. That they have been subsequently raised in bowhunting discussions is therefore somewhat unfortunate.

I once harvested a bull elk with a 50 pound recurve at around 15 yards, not an amazing feat really, but one I mention because it involves what I regard as details pertinent to the discussion. The most pertinent detail in my view is that I hesitated to take the shot.

The bull was in a classic full profile position making the shot easy enough but a complication arose because there were two cows behind him. I therefore waited for the cows to get clear before taking the shot. Fortunately, the bull remained still and I successfully arrowed him. What's especially pertinent though is the arrow did go completely through his chest cavity. I found it at a location on the other side which left no doubt that my concerns about wounding the cows was entirely merited.

Now if a 50 pound bow can do this, I have little doubt a 40 pound bow could too although the overpenetration might not be as spectacular. There should be absolutely no doubt the 40 pounder would penetrate enough to harvest the bull.

Kinetic energy concerns seem to have been raised during the introduction of compounds when archers frequently used lighter arrows, because they thought they could. This in turn gave them flatter trajectories but apparently at the same time caused some concern about penetration. Kinetic energy savants quickly weighed into the discussions asserting that the lighter arrows had quite enough penetration to do the job as well if not better than the heavier, but slower, traditional arrows.

And there's been disputes going back and forth ever since. I suspect the consensus now is that light arrows aren't especially effective and are most definitely hard on the bow because much of it's energy is transmitted to the limbs not the arrow and it tends to get a good deal noisier as a result.

However that might be, I don't think the discussions have advanced the sport in the least, but rather have imposed another needless complication on something that ought, in principle at least, to be rather simple.

If one doesn't try to drill an elk lengthwise or punch through its shoulder blade, kinetic energy levels or bow poundage should be of little concern provided the poundage reaches the minimum legal requirement. What should be of concern is getting within practical range, waiting for an acceptable shot angle or setting up one, and shooting arrows of sufficient weight with razor sharp broadheads.

I don't think a woman or man for that matter should have to clutter up her or his brain with kinetic energy calculations or doubts regarding the appropriateness of the bow draw weight. There's more than enough to concentrate on without all that and it's of dubious value anyway so why bother with it?
 

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KE

Seymour, my opinion of the KE theory is it's basically marketing for compound bows and arrow manufacturers. Nothing wrong with the shooters just the emphasis of squaring a projectile rather than attention to a sharp broadhead and shot placement. For me penetration and shot placement is my main objective, so mass x velocity plus a good shot is the energy I'm more concerned with.JMHO
Dr. Ashby also had words of wisdom on this issue.
Not to sound prolix I'll move on.

Good luck
Out for now
 

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Seymour, ex-diver -

Interesting thoughts. With a rifle bullet, KE, is an important factor, as it kills (primarily) by impact/shock. Not so with an arrow. So in addition to KE, the design and sharpness of the broadheads are at least an equal factor. A well designed BH, that's honed to a keen edge, will "penetrate" and do more damage from a #35 bow than a poorly designed, and dull one from a #65.

Also, basic physics, KE = Mass x Velocity, so for a given setup, KE can be increased, by raising either the mass (weight) of the arrow, OR it's velocity. But then you are really getting into "effective impact", or the amount of transferance/dissipation of energy, and not pure KE.

I know you guys know this stuff, just stating it for the newer folks.

This is all well and good, but it makes my brain hurt, so I'll just consider a #45 bow with proper broadheads sufficient, for whitetails in the NE, ;).

Perhaps as part of the Bowhunter safety course, we should include a gelatin test for penetration :D .

Viper1 out.
 

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Discussion Starter #5
Just to keep the record straight, because if I don't somebody else probably will, Kinetic Energy or KE is defined by the following equation:

KE = M * V^2/2 where M is Mass and V is Velocity

Please note that the velocity is squared. Viper's equation actually defines Momentum and that illustrates one of the great disputes in terminal ballistics. Which is more important, KE or Momentum?

I'm not even going to get involved in that one except to say that if one asserts it's KE, then Velocity becomes more important than Mass whereas if one asserts Momentum, then the relative importance of Mass and Velocity is essentially equal.

All of which ignores what actually happens on impact and that's what Viper alluded to with the comment about transference or dissipation of energy.

So keeping it relatively simple, we can say with some confidence that arrows kill by cutting. Whether they cut slower or faster is essentially beside the point as long as they cut enough.

Enough then is defined by the amount of penetration and varies depending on the size of the animal, the relative location of the vitals that need to be cut to harvest the animal compared to the point of impact, and the relative tendency of the arrow shaft to slow down from friction or drag as it is penetrating the tissue.

While there really is no formula that will predict with absolute certainty how an arrow will behave upon impact, certain general principles have nevertheless been deduced. Where Kinetic Energy is equal, the respective arrows should penetrate the same, but this isn't necessarily the case.

A lighter arrow, although going faster, will also slow down faster when subjected to drag. Drag coefficients aren't frequently mentioned in these kinds of discussions because their effects are not as simple to deduce as energy or momentum formulas, but they do have a critical effect on penetration.

And probably the most important factor in determining drag in tissue is the broadhead. The more efficiently it cuts, the less the drag. That's why a sharp broadhead launched from a low power bow will often out penetrate a dull broadhead launched from a high power bow coincidentally cutting more blood vessels.

Now, we could argue the merits of all this until the cows come home, or we could simply use what has been shown to work for centuries. One can reinvent the wheel or simply get it rolling.
 

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Discussion Starter #6
One more thing before I let this go for now. Elk, for all practical purposes, are big deer. They have thicker coats and hide as a rule but they're not armor plated.

A ballistic vest that will effectively defeat a magnum pistol bullet developing many times the kinetic energy of any arrow will not defeat a broadhead launched from even a relatively modest powered bow.

In medieval times an arrow launched from what is now considered a very inefficient bow could perforate a coat of chain mail on both sides. A modern broadhead fired at a relatively thin piece of mild steel plate, say one millimeter thick, will perforate the plate enough to cause serious injury to any tissue in contact with the plate on the other side.

From these examples, it should be apparent that penetration is something one can normally count on with arrows so the only real concern should be that the broadhead is very very sharp so it can do the cutting required to harvest the game.

Elk have thick hides but they're not made of kevlar or chain mail. We can calculate the kinetic energy or other mechanical physics quantities involved from empirical data derived from using the tackle that has worked and maybe gain some insight into a general theory of what works on elk, but interestingly enough, what works on deer works on elk and since the empirical data already tells us what tackle works, why not just use that tackle instead of theorizing about it?
 

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momentum or KE?

Just to clarify some facts regarding momentum versus kinetic energy . . .

- momentum (mass x velocity) deals with the Conservation of Energy, NOT the total amount of energy involved. In other words, if you shoot a cue ball at a certain velocity and it hits another ball . . . the other ball absorbs some of the KE of the cue ball; conservation of energy deals with how much of the KE gets transferred to the other ball (based upon impact factors that describe the elasticity/plasticity of the impact).

- Kinetic energy (1/2 mV^2=E) deals with the total amount of energy of a body in motion. It says nothing about how much of that energy will get transferred to another body during an impact. For shooters, the important thing to note here is that a small increase in velocity will cause a large increase in KE; conversely a projectile with most of its KE represented by a high velocity (e.g. a light fast moving arrow) will lose a greater proportion of its KE during flight (to air friction . . . the conservation of energy equation rears its ugly head here) while a heavier slow moving projectile will retain a greater percentage of its initial KE because the mass of a moving projectile doesn't decrease. The only difference between a cannonball, bullet, arrow, or a rock is (assuming equal mass and velocity) the shape and its impact on air resistance. This was proven by Galileo when he dropped those two equal size balls off of the tower and they both hit the ground at the same time.
 

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Kip -

Actually Galileo dropped two balls of UN equal size ...

:D

While your statements are accurate, the point was (at least my point) that, with a bullet, the object is to get the maximum amount of dissipation of energy (shock) to the target, while with an arrow (broadhead), you would want the least, ie most penetration (up to a point, of course). And so, the design and sharpness of the BH, is at least as important a factor as the KE at impact.

:)

Viper1 out.
 

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You're very welcome. Be safe.
Shoot Straight
Derbytown :D
 

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Kit -

Kitsap said:
Viper,
"Point" taken . . .
OK, Just don't take it too seriosly , ;) !!!!

Viper1 oiut!
 

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Discussion Starter #14
Thanks, Derby. I've actually seen that page before relevant to another discussion, but the review opportunity is certainly appreciated.

It's regrettable, perhaps, that I don't favor carbons for reasons quite unrelated to their penetrative properties, but, as is frequently said, to each his own.
 
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