You can't isolate penetration down to only one or the other, plus there are so many variables beyond these two parameters, this question is pretty much worthless IMO.

 

yep

false dichotomy

 

Interesting, thanks for posting! I think this is a nice tidbit in favor of "you probably want the heaviest arrows you can still shoot fast". Of course, "still shoot" fast is going to be really bow dependent so hard to give general advice on. Thanks again!

 

I read the above!? I’m not good at math so my head hurts! Buy in a nutshell, to me, kinetic energy is how hard it hit the side of the animal. Momentum is what push’s the arrow through the animal after the initial contact.
They work together, but I’m in the camp of “momentum“ wins the day.

 

He is showing from his testing and using equations that it is KE that gives us penetration, not momentum. KE is how hard to stop. Momentum is how hard to change direction.

It's easy to look at. Take a 650gr at .57momentum being shot from a trad bow. Then a 450gr from a compound bow with the same .57 momentum. Which one will outperform? The compound bow. For the compound bow to outperform a trad bow with two arrows of unequal mass but equal momentum shows that momentum is not the predictor.

 

KE and Momentum are both functions of mass and velocity. In my mind, trying to differentiate the two is like picking fly poop out of chili. There are too many other factors at play when considering penetration on an animal in a hunting context.

 

the reason to look at this is because of what is being taught. what is being taught is KE has nothing todo with penetration and momentum built on mass vs velocity will always outperform. Both are incorrect.

 

A 30-30 rifle with a 150 grn bullet has a ton more KE than my 125 grn Magnus Snuffer SS on a 385 grn shaft shuffling along at 300 fps.

But if I fill a 5 gallon bucket full of sand and shoot into it, only my arrow will poke out the other side. Which has better penetration? Or KE?

 

A 30-30 rifle with a 150 grn bullet has a ton more KE than my 125 grn Magnus Snuffer SS on a 385 grn shaft shuffling along at 300 fps.

But if I fill a 5 gallon bucket full of sand and shoot into it, only my arrow will poke out the other side. Which has better penetration? Or KE?

Which projectile has a higher sectional density in your scenario?

 

Kinetic energy is how much mechanical energy is the in the system. Momentum is a directional measurement of mass and velocity or the kinetic energy with a vector.

If X is my bows launch point and Y is the deer. The Momentum at Y is a function of how much energy was put into the system at X minus the effects of gravity and air resistance as it reaches Y.

I can get 52 foot lbs (roughly skull breaking territory) of energy with a heavy arrow and slow speed or a lighter arrow and fast speed. i don’t think mass matters so much as the resilience of the materials and the complex mechanical forces on impact.

theoretically a bodkin on an arrow with two fletches should penetrate further than a 2D broad head - it doesn’t because the broad head cuts a path for the vanes while the vanes are pure drag (just in limited tests I’ve seen).

i think you’d use momentum on impact to describe penetration because it gives directional information can tell you as you measure it in the forming wound if there’s enough directional energy to break bone or the arrow gets deflected.

This is just my instinct. I actually don’t think mass matters for projectiles that are roughly the same shape and size. It maybe surface area for friction matters - larger object with more surface area at the same speed at a smaller object with less surface area in context for friction but that’s a prediction and it may simply be relative.

This is all speculation from a lay person. All that said mass, size, and materials probably all matter at some point let’s say we accelerated a car to X KE and a feather to X KE, which would you rather get hit by? Intuitively it seems the feather. Even if it accelerates fast enough to shoot through me it’s a smaller point of contact or maybe it’s that to get them equal the car has to be moving 1mph and the feather the speed of light. I’d take the car. (This is hypothetical bit mathematical)

feel free to correct me. I’m many years past physics classes and was never very good.

 

I think the math as always, dictates that with the same energy (same bow, DL and DW) going into an arrow weighing 450grs with the same exact arrow with the same broadheads at 600grs is NOT drastic as some people are saying.

Of course this can't be compared to a 600grn arrow on a bow generating more energy sending that arrow at the same velocities as lighter arrows from lesser energies. This of course illustrates the same basic flaw by so many on AT comparing, a bowling ball to a baseball, and then asking what would you rather get hit by.

You see the same thing eco'd by many Western guides or PHs and top Archers how have all put down more animals than some preaching 600+ grn arrows or you'll bounce off the animal. Certainly many of theses top hunters, have also gone heavier and returned to a system they feel gives them the most chances for success.

Basicly the majority of the anitotal evidence suggests that the solvers and phyics are correct.

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I think the math as always, dictates that with the same energy (same bow, DL and DW) going into an arrow weighing 450grs with the same exact arrow with the same broadheads at 600grs is NOT drastic as some people are saying.

Of course this can't be compared to a 600grn arrow on a bow generating more energy sending that arrow at the same velocities as lighter arrows from lesser energies. This of course illustrates the same basic flaw by so many on AT comparing, a bowling ball to a baseball, and then asking what would you rather get hit by.

You see the same thing eco'd by many Western guides or PHs and top Archers how have all put down more animals than some preaching 600+ grn arrows or you'll bounce off the animal. Certainly many of theses top hunters, have also gone heavier and returned to a system they feel gives them the most chances for success.

Basicly the majority of the anitotal evidence suggests that the solvers and phyics are correct.

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I saw a video of someone killing a boar with a steambow stinger 2 loaded with the Steam bow broadheads (those suckers are no joke). Sure it was from a stand at most at 12 - 18 yards but I think people underestimate just how powerful bows really are. The stinger 2 is a pistol crossbow with a magazine you can hand cock limbs top out at 90lbs with a very short power stroke.

In my state if your bow hits 30lbs by 28 inches you can hunt with it. Whether or not you find a shot worth taking with such a low lb bow is a whole other question i can’t answer but I have no doubt in my mind from ballistics gel tests I’ve seen and my own shooting a 30lb bow is a lethal weapon. I don’t think at a sub 20 yard range you’d have trouble killing a Turkey or even a deer.

i think the combination of guaranteeing success in the hunt and hunting ethically as well as “this is how I was taught” really factor into the belief of what is needed. More than anything it comes to your skill. You could take the best compound on the market with maxed 80lb limbs, a quality sight, stabilizer setup, and drop away rest and still utterly miss or wound the animal in a way you didn’t intend.

it’s consistency with the tool you use that matters as much as the kinetic energy it has.

 

All of this math and science leads me back to where I've been for the last few years....after being in the 5gpp camp for a few years, followed by 2 years in the 8-10gpp group. I've settled somewhere in the middle 6-7gpp range and have been completely happy with the results.

My advice to anyone who asks is simple "Shoot the heaviest arrow that gives you an acceptable trajectory". For the overwhelming majority of hunters NOT on AT, this usually leads to arrow weights in the neighborhood of 6-7 gpp.

 

I like the 6-7 range. at this time I'm just under 7

 

I use 9 gpp it is hard to stop it

 

I like the 60mps graph, it makes my 10.5gpp 220 chrono look like a sledgehammer at 30yds...

 

My son is at 14 gpp

 

Here is a test/demonstration some might find interesting.... It also shows the math in the OP works out...

 

My 08 82nd Airborne is hard to beat for KE, momentum/speed. 420 grain arrow is fine, no need for a heavy arrow IMHO.

 

Should gpp even be a metric considering a 60lb 300fps ibo bow @ 24" is considerably different than 60lb 350 ibo @ 30"?
Seems weight per fps or vice versa would be a better metric.

 

The Shape of the penetrator....will have a HUGE impact on the shock wave thru the sand and clay matrix.

So, using the Poncelet equations are probably not a good predictor for how a projectile shape will penetrate thru ballistic gel, especially if the TEST subject was a conical point penetrator fired into sand and clay. A Saunders field point shape will have much better penetration performance cuz of the stepped point shape, that long leading nose, with the much smaller initial OD.

KE is defined as (1/2) times the (mass) times the (velocity) times the (velocity).
If you want to understand penetration physics, we are talking subsonic fluid dynamics.

Another Sandia National Labs report "Projectile Penetration into Representative Targets" by George W Stone, October 1994.

Projectile penetration into representative targets (Technical Report) | OSTI.GOV

The U.S. Department of Energy's Office of Scientific and Technical Information

www.osti.gov

Abstract

The differential equation representing the penetration of a "hard" projectile into semi-infinite, homogeneous target materials is solved for several generic combinations of the target material / projectile characteristics. A "hard" projectile is defined as one that does not change size or shape and does not lose mass during the penetration process. The target materials evaluated range from the structurally "soft" materials (liquids) to structurally "hard" materials (armor plate) with viscous and fluid dynamic drag considered. The solutions to the differential equation(s) are expanded in series form to demonstrate the underlying parameters governing projectile penetration and the way they interact to limit penetration in a given target material.

It is shown that the fundamental parameter governing projectile penetration into structurally "firm" materials is the initial kinetic energy of the projectile divided by the frontal area of the projectile and the inherent structural characteristic of the target.

Experimental data on the penetration of steel spheres into ballistic gelatin and for armor piercing bullets into armor plate materials are used to verify the characteristics of the solutions to the equation of motion for the projectile and to demonstrate how penetration can vary with projectile size and target characteristics. The penetration equation for a single "hard" target material is used to develop a solution for the penetration of multi-layered "hard" target materials.

So, if you have REALLY REALLY high KE, combined with a really SMALL diameter projectile
and the SMALL diameter projectile does not change shape, and does not lose mass during penetration (no mushrooming on the bullet or arrow), HIGH KE and TINY diameter wins for penetration.

 

The Shape of the penetrator....will have a HUGE impact on the shock wave thru the sand and clay matrix.

View attachment 7686654

So, using the Poncelet equations are probably not a good predictor for how a projectile shape will penetrate thru ballistic gel, especially if the TEST subject was a conical point penetrator fired into sand and clay. A Saunders field point shape will have much better penetration performance cuz of the stepped point shape, that long leading nose, with the much smaller initial OD.

KE is defined as (1/2) times the (mass) times the (velocity) times the (velocity).
If you want to understand penetration physics, we are talking subsonic fluid dynamics.

Another Sandia National Labs report "Projectile Penetration into Representative Targets" by George W Stone, October 1994.

Projectile penetration into representative targets (Technical Report) | OSTI.GOV

The U.S. Department of Energy's Office of Scientific and Technical Information

www.osti.gov

Abstract

The differential equation representing the penetration of a "hard" projectile into semi-infinite, homogeneous target materials is solved for several generic combinations of the target material / projectile characteristics. A "hard" projectile is defined as one that does not change size or shape and does not lose mass during the penetration process. The target materials evaluated range from the structurally "soft" materials (liquids) to structurally "hard" materials (armor plate) with viscous and fluid dynamic drag considered. The solutions to the differential equation(s) are expanded in series form to demonstrate the underlying parameters governing projectile penetration and the way they interact to limit penetration in a given target material.

It is shown that the fundamental parameter governing projectile penetration into structurally "firm" materials is the initial kinetic energy of the projectile divided by the frontal area of the projectile and the inherent structural characteristic of the target.

Experimental data on the penetration of steel spheres into ballistic gelatin and for armor piercing bullets into armor plate materials are used to verify the characteristics of the solutions to the equation of motion for the projectile and to demonstrate how penetration can vary with projectile size and target characteristics. The penetration equation for a single "hard" target material is used to develop a solution for the penetration of multi-layered "hard" target materials.

So, if you have REALLY REALLY high KE, combined with a really SMALL diameter projectile
and the SMALL diameter projectile does not change shape, and does not lose mass during penetration (no mushrooming on the bullet or arrow), HIGH KE and TINY diameter wins for penetration.

i looked up this guy because he was the one the ranch fairy and rocket man with the THP used to discredit a test looking at bow effienincy test by Joel maxfield and myself.

I posted what he wrote not because of his paper on penetration on sand, although very interesting. I shared it because he stated that momentum is not what we look at for penetration.

My purpose in all of what I have been doing in the last year is to show people what KE does for us.

 

My head is spinning, I am sticking with my 500 grain arrows and not worry about this stuff.
Ches.

 

I am not sure but I think it has everything to do with speed. Then broadhead selection.

 

A well tuned arrow in the pump house gets it done every time.

 

I seems so weird to me that people struggle with simple engineering concepts.

Take a yew branch and some sinew, bend the branch, string it together as much you can without braking at full draw. The energy in that branch while high can only return back to it’s static state so fast. The efficiency window is predominantly predetermined by these materials and technology. To best take advantage of the rate of return you do not want something so light that the yew didn’t have resistance. No real efficiency numbers were known. Just bigger heavier yews, maybe with 200lbs of DW and really heavy arrows worked best.

Fast forward to 2024 — that’s where engineering is looking at a minimum today at the major bow companies. The efficiency numbers, the design parameters are engineered to have the highest efficiency at IBO specs they can achieve.

What does this mean (other than I had too much beer)? The range of arrow weights that are the most efficient (translate the energy to KE and momentum) from these bows is in a relatively narrow window. While heavier may get a fraction more efficiency, nothing is what it was when long bows were used. The RF is out of his expertise comparing and contrasting the long bow studies of Ashby to a 2022 compound hunting bow. Even worse discounting tests, he has zero idea how to interpret.

Sorry for the Siri rant.. to many IPAs and no great signs today hunting..

Just weird calf poop.

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I think people are wayyyyyyyyyyyyyyyyy over thinking this archery hunting thing. Its being wayyyyyyyy over engineered too.

 

You do realize the bow you hunt with has tons of "engineering" and "overthinking" in its design, by way of extensive R&D, right? Including your arrows, broadheads...everything. How do you think a 60lb bow of today, can outperform a 70lb bow of a decade ago?

If you don't want "overengineered", go TRAD. But you'll need to change your name to 150fps 😃

 

I am currently shooting a 470 grain arrow out of a 60# compound bow. That’s 7.5 grains per pound. Trajectory and speed is still good. The question is how far does the arrow have to go after passing through the deer in order to kill it?

 

I look at arrow penetration through the lens of inertia.

 

Did anybody else notice that the conclusion from the charts was that maximum penetration EFFICIENCY was about 60 meters per second (about 197 fps). Me thinks Ashby found 650 grains at 190 to 200 fps was the magic formula! Very interesting.

 

don’t forget that was in sand With a 1604gr projectile

ashby 650 was only if bone was encountered.

but it was very interesting

 

The problem I have with using KE as the determining factor is that it erodes over distance. The lighter the arrow the faster it erodes and speed along with it. We all hear the argument about needing lighter faster arrows for western hunting and elk hunting. Where 50/60 yard shots are "normal" right? Like I said above those lighter arrows lose alot of KE, speed and momentum. They will not penetrate like a heavier arrow will. At treestand or blind distance (18-25yds) speed or trajectory shouldnt be a huge concern. There really isnt a great argument for light arrows when there are so many cons to them like noise, vibration and usually less durability. Too each their own.

 

When you look at two arrows of different weight, being shot from the same bow. You are correct.no one is doubting this nor trying to say any different.

When we shoot from same bow and depending on how efficient the bow is, KE will be around 2% different. At 50 yards it may still be less than 6% and possibly closer to 5%. So the gain by shooting a heavier arrow from same bow is their. It’s just not as big as it was years ago.

the reason we look at this is because no one, well some of us, are testing to see if momentum is the predictor. I have showed it’s not. The test is very easy, I don’t care what you shoot. Make it a animal. But take a trad bow with a 650gr arrow and a compound with a 450gr. Try to get the momentum as close to each other as posible or even greater for the trad bow at impact.make sure both arrows have same tip. Then see which one will out penetrate. That will show you if momentum is the predictor or not.