Craig
60X Strings's Link
So many good choices out there!!
Yeah, my preference would be a heavier arrow but you can increase the performance of your existing arrow with a very efficient BH- read; 2 blade
Momentum has no DIRECT connection to penetration. Additional arrow mass will help get more KE from the bow and help retain the KE over distance by reducing the velocity which reduces the drag.
Because KE and momentum share the same components, momentum will increase as arrow mass increases and is therefore mistakenly given the credit for increased penetration. However, it has nothing to do with penetration. KE is what propels the arrow and is what stops the arrow. KE and the average resisting force are what determine penetration.
If momentum was truly a factor in penetration, it wouldn't matter whether the momentum came from velocity or mass. Look at these two combinations of weight and velocity that produce equal amounts of momentum ( 0.5713 slug-ft/sec) : 740 gr @ 174fps = 49.7 ft-lb KE and 350 gr @ 368 fps = 104.7 ft-lb KE
Do you really think that an arrow with 50 foot-pounds of KE will have the same penetration as one with 105 foot-pounds of KE ? So how good of a predictor of penetration can arrow weight or momentum be? Try it.
DISCLAIMER: This can be a very boring post, so if you do not want to get nauseated by details and my misguided understanding of some Newtonian Mechanics, do not read.
Question: Why does an arrow that weighs 391 grains traveling at 306 fps penetrate deeper when tipped with a field point vs. a blunt tip? They both have the same KE AND Momentum.
KE is the result of the work the bow does to get the object of mass (arrow) to a prescribed velocity. The bow system cannot change. Mass, in this case, is the independent variable. Velocity is the dependent variable.
Now let's take an example of likeness. A 180 gr bullet fired out of a .30-06 traveling at 2650 fps has a muzzle KE of 2,800 ft-lbs. A 130 gr bullet fired from a .270 traveling at 2900 fps has a muzzle KE of 2,430 ft-lbs. Interesting. A lighter bullet traveling faster has less KE than a heavier bullet traveling slower. Yet, when a deer at 150 yds gets hit, the '06 will likely knock the deer off it's feet. This tells me that mass has more to do directly with penetration than the concepts of KE or momentum do. They are both related to each other, have something to do with conservation of energy, and each a function of mass (and velocity), we can do the proof, but why. Just google it.
Now, let's look at an example that is related, an arrow of likeness. Remember, the bow cannot change - it is set. So, what variables can change? Remember, velocity is a result of the work the bow does to move the arrow of mass. Arrow weight at 350 gr traveling at 320 fps has a KE of 79.6 ft-lbs. A 430 gr arrow traveling at 290 fps has a KE of 80 ft-lbs. Huh?!? Pretty much the same! How come?
In the bullet scenario, the delta mass didn't change much relative to the delta in velocity. With the arrows the delta in mass doubled relative to the velocity (delta of 80 vs 30). The bigger a number gets and increases by a power function (graphically it looks like a U), the more a bigger direct multiplier has on the outcome (bullet and arrow is the direct multiplier) when comparing systems of likeness. So, which one penetrates deeper, slightly heavier arrow traveling slightly slower, or the lighter faster one? (assuming the same broadhead - for argument sakes, a razor sharp broadhead from the ACME Broadhead Company).
Shoot the heaviest arrow that gets close to the best speed your bow can do at your draw/weight.
This way you blend the performance of the bow from an absolute (speed) and relative (forgiveness/comfort shooting) sense.
This also will help you retain the most KE at the critter - which is where it's most important.
I'll take a stab.
"Question: Why does an arrow that weighs 391 grains traveling at 306 fps penetrate deeper when tipped with a field point vs. a blunt tip? They both have the same KE AND Momentum."
The blunt tip creates a far greater average resisting force for the same reason the flat side of an ax penetrates less than the sharp side.
Your 30-06 vs .270 comparison is bogus because the rifles are not equal. They have different case sizes and may also be shooting a different powder. The barrel lengths may have been different too, which will effect the data. The initial explosions produce different amounts of pressure on bullets of different diameter. It's an apples and oranges comparison.
Change only one variable, the bullet weight, and see what you get. Even that may give bogus result due to the fact that powders can produce different pressures with different weight bullets.
"So, which one penetrates deeper, slightly heavier arrow traveling slightly slower, or the lighter faster one? (assuming the same broadhead - for argument sakes, a razor sharp broadhead from the ACME Broadhead Company)."
The heavier arrow will usually penetrate slightly deeper, especially at longer ranges because it gets a little more KE from the bow and it's slower speed retains that KE because the slower arrow has less drag. Be sure the slight advantage in penetration is worth the trade-off in trajectory. This is especially critical for those shooting with a short draw length and/or low poundage.
Purdue,
I appreciate your concerns and dialog, but, drag is a function of surface area and crazy things happening to the air movement on the end of the object in forward motion. Two objects of different mass AND size will free fall at the same rate in a vacuum. Again, let me define KE for you. It is the work required by the bow to move and arrow of a given mass at a certain velocity. Penetration is directly related to momentum because of the laws of conservation of energy. An object at rest will remain at rest until acted on by an opposing force. KE is energy, not force. To really simplify it, let's discuss a suspended load. The force applied to the rope holding it is calculated by F=mg. Same principle used to calculate momentum, p(or rho) = mv.
When an arrow strikes its target at rest, it tries to transfer its "energy" into the target. Penetration happens because the target cannot absorb it. If it did, you would have an elastic collision (principles of momentum). You don't want that. You want the collision to be as inelastic as you can get it (again the momentum thing). Play with some balls of different mass to see what happens when banged together. That's why the blunt tip doesn't penetrate as deep, the target can absorb more of the arrows "energy". Granted, yes, the arrow does have KE, but only at impact. Work is expended into the object to get it to move. Sometimes the work dissipated is in the form of sound or heat as well.
As far as the rifle thing, no different than comparing a long bow of slower velocity and very high arrow weight to a compound of lighter arrow weight and higher velocity, that's apples to oranges. Comparing like arrow weights and like velocities, that's closer, at least apples to pears. The only thing that makes it apples to apples is the same arrow but the bow changes by increasing draw weight. Bringing up different powder and barrel lengths is completely irrelevant. All the propellant is used for is to get the projectile of mass down range safely and to achieve favorable results. Bullets or slugs fired from a muzzleloader travel pretty slow, but they can knock the fire out of something on impact. The propellant does the work. The propellant has KE in the form of a chemical reaction.
We can go around and around if you wish, but we don't need to bore everyone to death. They all have better things to do. Don't overwork it dude - it ain't that complicated.
Mooseguide - a heavier arrow will give you better penetration due to it's ability to better retain its forward momentum and not transfer this into the animal. You will sacrifice speed, but should be able to drive the arrow in deeper. A sharp broadhead that can begin the cut as soon as possible is a big help as well.
Wrong. The arrow has KE as long as it is moving. That includes inside the target.
If penitration is determined by momentum, then you should be able to produce a formula for determining that distance. Can you? Can you even produce a question from a physics book or website that asks you to find the distance due to an impact from a given momentum? No you can't. It can only be done by breaking momentum down into its components and using velocity. Not so with KE. Distance can be calculated directly from KE, if the average resisting force is known, without knowing either velocity or mass.
Also PE = KE so, the arrow does have KE at rest...
KE = 1/2 mv^2. p = mv.
see the similarities. You said momentum is not related to penetration. Looks like it is since both quantities use the same variables.
Good grief...give it up
Wish I was one of the lucky many sitting back laughing at two idiots arguing back and forth...
Since an arrow at rest has zero velocity it also has zero KE. It will only have PE if other forces are acting upon it such as gravity or a spring force.
"You said momentum is not related to penetration."
What i said : "Because KE and momentum share the same components, momentum will increase as arrow mass increases and is therefore mistakenly given the credit for increased penetration."
Still waiting on your formula to calculate distance using momentum.
Forget the speed go heavier in general so its has the power to punch through the bones if you hit them. Think Gold Tip Kinetic, Carbon Express PTX, Easton Axis, VAP etc. You don't need long draw, heavy draw weight or blazin' speed for penetration.
