Tissue Resistance to Arrow Penetration

[JACK]

Interesting article from the Alaskan bowhunting site...Physics or Myth?

 

 

Tissue Resistance to Arrow Penetration

Fact: The greater your arrow speed, the greater the resistance to penetration.

Speed in and of itself isn’t bad. Opting for speed at all costs, well, comes at a cost. Here’s one of them. When any object moves through another the object being moved through resists the forward momentum of the other. A good illustration of this is how the wind resists your hand if you put it out the window of a moving vehicle. The faster the vehicle moves, the more the air resists your hand.

There is a formula for determining the degree of resistance various substances have to an object attempting to pass through them. It’s officially called the “Force of Drag” but we generally hear it referred to as wind resistance, or water resistance. This force of drag applies to other mediums as well like animal tissue, including hair, hide, flesh and bone.

The actual formula states:

is the “force of drag”

is the density of the substance being penetrated

is the speed of the object relative to the substance being penetrated

is the reference area.

is the drag coefficient of the moving object. (e.g. 0.25 to 0.45 for a car)

That’s a complex formula but here’s what it means to arrow penetration:
The faster an arrow is moving the more resistance there is to it. (Even in the air.)

Because the velocity is squared in the formula, the resistance increases tremendously as the arrow speed increases.

If you double the speed of the arrow, the resistance factor increases by four. (2 x 2 = 4, velocity squared.) Here’s where it gets scary. If the arrow speed increases by a factor of four, then the resistance factor increases by 16 times! (4 x 4 =16, velocity squared)

What does that mean to bowhunters? It means an arrow traveling at 150 feet per second is met with four times less resistance to penetration than an arrow traveling at 300 feet per second. Another way to say it is; an arrow traveling at 300 fps has to overcome four times the amount of resistance to penetration than an arrow traveling at 150 fps.

The cold hard truth is.... When it comes to penetration, light fast arrows lose the momentum/kinetic energy battle AND when they reach the animal they have to overcome as much as four times the resistance to penetration as slower moving, heavy arrows do.

If you're goal is to gain kinetic energy on paper by shooting lighter arrows faster, science proves it's a lose-lose proposition.

First, you LOSE velocity twice as fast as momentum AND you LOSE penetration potential in what energy you do have left because the medium you're trying to penetrate is resisting that penetration four times more than if you were moving half as fast.

[ready2hunt]

I think that if my life depended on me solving that equation.....I'd be dead by now.

[Pac8541]

^^^LMAO!! Well said!

[mattys281]

Yeah but if we all switch back to slower rigs we'll need pin sight brackets that are 6" tall to be able to sight in out to sixty yards. What's the drag coefficient of punching through an aluminum pin guard? My 380 grain 270ish fps arrows do a pretty good job of blowing through deer.

[AZkiller]

Huh.... You should post this on my bigger gear for bigger game thread in the elk section. I bet that the heavier arrows have a flatter long range trajectory with the reduced drag.

[AverageJoe]

First off, these arent guns and bullets, this is archery. A 60 or 70 yard shot is usually the maximum out here. A small pin gap and flatter trajectory to minimize miscalculations is more important than penetration, not that penetration isnt important.

 

Heavy and slow will win over fast and light once you reach a certain distance. This is my theory in guns 100%. I would take a 200 gn bullet out of a 308 at 2500 fps anyday over a 150 gn bullet at 3000 fps. This is because of the downrange energy and better areodynamics. That being said, archery is very different.

 

Penetration is overrated in the archery world. Why would a guy boast that he shot a deer and buried the arrow 12" in the dirt behind it? What good does the arrow do in the ground behind the animal? If a 400 gn arrow will end up on the ground 2" in the dirt but a 500 gn arrow will end up 12" in the dirt, which arrow made the deer more dead?? Neither...

 

The other factor here is, does the arrow stuck in the chest cavity of an elk kill quicker than an arrow that passed through? The answer...yes. the blood trail might not be as good but it will still suffice. What doesnt kill an elk is a slow heavy arrow with too much trajectory that was misjudged by 5 yards.

 

All this leads up to this conclusion. A 390 gn arrow traveling at 310 fps will have less drop, very ample penetration to kill any animal, and less chance of error in the event you didnt have a rangefinder or time to range the animal, which happens. It has a significant advantage in time in flight which again is a huge factor in the archery world at 60 yds.

[Pac8541]

But I'll bet it sells them a ton of Grizzlystiks...

[rclouse79]

First, you LOSE velocity twice as fast as momentum AND you LOSE penetration potential in what energy you do have left because the medium you're trying to penetrate is resisting that penetration four times more than if you were moving half as fast.

 

Velocity and momentum are directly proportional and assuming the mass is constant must change at the same rate. The second part of this sentence is pure gibberish. I tried to make sense of it for one minute and almost had a stroke. This post is what you get when some one who is extremely bored stumbles upon an old physics text book and a half rack of beer.

[elpepe25]

laws of physics just don't park at the rifle range.

 

Ideal end state for me will always be a pass through.

[TwoGuns]

I think what he's trying to say...is that a bus at 35 mph running you over kills you better than a Ferrari a running you over at 100 mph...

 

Because as anyone knows the Ferrari will obviously be going slower than the bus due to the drag co-efficient thereby only appearing to move faster.

 

It's Einsteins inverse - square law in reverse.

[AZkiller]

Wow!! Physics is physics whether its an arrow or a bullet or a rock or a bus. You must have gone to one of the good public schools. The heavier the better!

[idgaf]

I think what he's trying to say...is that a bus at 35 mph running you over kills you better than a Ferrari a running you over at 100 mph...

 

Because as anyone knows the Ferrari will obviously be going slower than the bus due to the drag co-efficient thereby only appearing to move faster.

 

It's Einsteins inverse - square law in reverse.

Your just as dead either way. What is not taken into account with any of this is bus/ferrari placement. I will take a fast flat shooting arrow with a forty yard pin and a 45 yard animal any day over the same circumstance and the slower looping trajectory heavier arrow. Arrows kill by slicing and creating blood loss in vital areas not by crushing its intended quarry.

[mattys281]

Wow!! Physics is physics whether its an arrow or a bullet or a rock or a bus. You must have gone to one of the good public schools. The heavier the better!

Heavier is better... if you're a medieval archer trying to push an arrow through an armored soldier at a range of 100 yards or more. The technique used here was to launch the arrow in an arch that is more vertical than horizontal and allow gravity to accelerate it on the downward side. Their arrows were actually going faster on impact, due to gravity than they were when they left the bow.

 

The same principle do not necessarily apply to horizontally aimed projectiles where precision aiming is more important than all out kinetic energy. Its a trade off.

 

But at the end of the day, animals fall to all manner of arrows so shoot whatever makes you feel most confident.

[rclouse79]

^{Heavier is better... if you're a medieval archer trying to push an arrow through an armored soldier at a range of 100 yards or more. The technique used here was to launch the arrow in an arch that is more vertical than horizontal and allow gravity to accelerate it on the downward side. Their arrows were actually going faster on impact, due to gravity than they were when they left the bow.}
This would only be true if the arrow was shot from an elevated position. If it is launched from the same height as the target it would strike with the exact same speed it had when it left the bow no matter what angle it is shot at, if you disregard air resistance.

[mattys281]

^{Heavier is better... if you're a medieval archer trying to push an arrow through an armored soldier at a range of 100 yards or more. The technique used here was to launch the arrow in an arch that is more vertical than horizontal and allow gravity to accelerate it on the downward side. Their arrows were actually going faster on impact, due to gravity than they were when they left the bow.}

This would only be true if the arrow was shot from an elevated position. If it is launched from the same height as the target it would strike with the exact same speed it had when it left the bow no matter what angle it is shot at, if you disregard air resistance.

 

You could be right, it's been a while since I heard that little tid bit, thrown out by a hunters ed instructor that is an archery-history enthusiast. Regardless, the arrow would slow down as it reached the apex of the arch & then gravity would cause it to accelerate on the downward stroke, so to speak, thus greatly increasing the kenetic energy on impact. The guy we had talking to us said that the arrows the old English archers used weighed over 2000 grains and would hit with enough force to pass through an armored knight, his horse & still bury in the ground, which is why archers were the most feared of combatants on the battle field. But, when you compare that to modern archers who use a horizontal trajectory for hunting & competition, I thing the heavier arrow trade off very quickly becomes a wasted effort. The name of the game now is accuracy & precision. Penetration is certainly still important, but I'd argue all day long that putting the arrow in the right place far out weighs any penetration concerns.