bomtek

Hello all :-)
Just looking for an opinion....an article in our latest South African bowhunter refers to KE being rather over rated and heres why. The author reckons that momentum is a lot more important than KE and when you do the calculations with his formulae it looks pretty impressive.
His example of a skateboard flying 100km/h into a brick wall vs a pick-up travelling at 10km/h makes sense. So the deduction is therefore that as long as the sight pins stay in the window, the heavier the arrow the better, thereby reducing the speed. Anyone got opinions on this????

deer-hunter18

With a heavier arrow you will have more KE anyway right.

Sylvan

It is a fact (not opinion) that a heavier arrow shot from the same bow will result in both more kinetic energy and more momentum and therefore all things being equal will result in greater penetration. If you want to attribute the increased penetration to the increase in KEthat's fine. If you want to attribute it to the increase in momentumthat's fine too. KE and momentum are merely the result of calculations based onthe measureable quantities of arrow velocity and mass. From a given bow, the facts are quite simple. If you put on a heavier arrow, the veolocity goes down and both the KE and momentum increase thereby resulting in greater penetration.The rest is opinion. Along with reduced velocity as a result of the heavier arrow comes increased trajectory. You have to weigh the gain you achieve in penetration against the negative effect of increased trajectory. Where you decide to draw that line is very much a matter of opinion. Personally I like to lean to the heavy side when picking an arrow.

The Rev

Egg-zactly... I would have said that, but you said it much mo betta. I agree with Sylvan... See bro. Sylvan, I am agreeing with you more and more these days.

mobow

Hmmmm./.....I don't buy it. A heavier arrow will ALWAYS produce more KE is just not accurate. I am shooting a 400 gr arrow 290 fps. giving me KE of 75 ft pds. Now, if I increase my arrow weight to 700 gr but the speed slows to 200 fps (which is probably faster than reality) my KE drops to 62 ft. pds.

It's a delicate balance. The trick to KE is to get the heaviest arrow you can flying as fast as you can. There is a happy medium where the optimum for both is achieved. Eventually, the weight of the arrow will slow it down just too much and your KE will drop. Likewise, there is a point where the arrow is just too light and it drops also.

Mementum and KE go hand in hand, but aren't the same thing. I do agree that a heavier arrow will retain more momentum than a light arrow. I mean, which can you throw farther, a light pebble or a heavier rock? But see, I can't throw a boulder at all. Which brings us back to my main point....heavy is not always better. You gotta find the balance.

Arthur P

It is possible to go over the arrow weight that will shoot most efficiently from your bow. If you shoot a variety of arrow weights througha chronographand KE keeps climbing, then drops off when you hit a certain weight, then you know you've gone beyond your bow's peak efficiency range. The ol' bell curve in action. I'd stop at the top of the curve.

KE is definitely over emphasized,but I hesitate to say it's overrated. It's important, but it's only part of the story.

The guys who use light arrows say the KE makes their light, fastarrows penetratesatisfactorily when, in reality, it's momentum. They have to pack a lot of extra speed and KE into their light arrows to do the same job someone with a heavy but slow arrow can do with much less KE.

Like I've posted on here, many times, a 700 gn arrow at 150 fps with 35 ft lbs of energy achieves penetration, mostlybecause of it's momentum. A 350 gn arrowat 300 fps and 70 ft lbs of energyalso achieves penetration, but not because it's carrying twice the KE ofthe 700 gn arrow. If it were due to KE alone, then the light arrow should penetrate exactly twice as far as the heavy one, and it doesn't. In reality, it's because it's carrying exactly the same momentum as the heavy arrow.

That's why those minimum KE charts for hunting various sizes of animals are a joke. They focus strictly on KE, totally ignoring momentum, and momentum is at least equally important. Frankly, for somene using heavy arrows, those minimum KE values couldbe substantially reduced.

I would much rather see minimum momentum charts. Then you could make intelligent and informed choices when it comes to balancing speed and trajectory againsthow muchmomentum you need to put an arrow through an animal's vitals. Make the choice then, whatever the KE winds up being, then it's obviously enough KE to do the job.

Sylvan

A bow with 125 grains of virtual mass that shootsa 400 grain arrow 290 fpswill shoot a 700 grain arrow 231.3 ft/sec which is an increase in dynamic efficiency from 76% to 85% and an increase in KE from 74.7 ft/lbs to 83.2 ft/lbs. These are straight forward calculations. The only number I assumed was 125 grains virtual mass but the result would be the same regardless of what "reasonable" value you choose. If mass goes up so does KE and its true for all reasonable arrow weights.

The only place I can see where this may break down is if you are at extremes where the limbs of the bow are beginning to fail and Hookes laws nolonger apply.But that certainly wouldn't be a reasonable point tobe made here.

Sylvan

The function here is not a bell shapedcurve.Dynamic efficiencyincreases with arrow mass.

Here's the proof.

SE = Stored energy
KE = Kinetic energy
v = velocity
m1 = mass of arrow
m2 = virtual mass

SE = 1/2(m1 + m2) v^2
Therefore:
v=((2SE/(m1+m2))^1/2

and:
KE = 1/2m1v^2
Therefore substituting ((2SE/(m1+m2))^1/2 for v and reducing:
KE=m1SE/(m1+m2)

Now as anyone can see, as m1 is multiplied by SE in the numerator and m1 is only added by m2 in the denominator it isclear that as m1 increases KE increases as well. Now as SE is a constant for a given bow setup as KE increases with arrow mass then so does efficiency.

You can't cheat the physics! Dynamic eff increases linearly with arrow mass. Virtual mass modifies the slope of the function.

Trapper_Hunter

Just like being in algebra all over again LOL

ShadowAce

I agree 100%.