Penetration (for the purposes of this thread) is being compared for different arrow sizes, not broadhead designs.

Oh no MNRut you've done it now!!! You've challenged a guru.

You think you are superior
You are pompous
you think you are more knolegeable than someone who spent years studying this
You are arrogant
Your flaunting your education...

Just thought I'd save BobCo19-65 some time! LOL

ME and wife are just rolling over that one

You seem to think that I disagree with Ashby. I thought I made it clear in an earlier post that after reading his paper I take issue only on a minor point and that is with regard to the "energies" he includes in KE and his analogy with a tuning fork in that I believe KE as archers use it should be considered to be a vector quantity for practical reasons.

Unless I've misread it Ashby does not agree with those who say that lighter arrows carry more momentum/KE when shot from the same bow. He says the same thing I do. Am I missing something?

Sorry Bob - I meant you read more into his statement on whatever forum you got it from.

Good article for those who are interested:

http://www.bowhunters.org.au/results...Ballistics.pdf

It gives some credit at the end to OL Adcock whose Longbow Designs are famous (they actually appreciate in value). Lucky I got a custom one before he stopped taking orders!

I believe it was copyrighted 2005.

No. Perhaps I am missing something. I refer to your repeated references to Ashby as a 'guru.' Perhaps you don't mean it to be as disrespectful as it sounds?

As to the tuning fork, Ashby is exactly on target. But you are not. According to physical law, you cannot refer to KE as a vector quantity. That's momentum's realm. KE is, by definition, a scalar quantity. It says how much, momentum says in what direction that KE is going to be used and how much time it will take KE to spend itself in a medium.

Low momentum means the energy will be used up very quickly - which, in our case, means you better hope you have enough speed and energy to keep momentum going long enough to make it through the deer's chest (although increased speed also means increased friction and resistance, which eats up KE even faster!) High momentum with the same amount of energy means that energy will take longer to be spent, so the arrow will pass through the deer's chest long before all the energy is used up.

It also means that raising momentum allows arrows with lower KE to keep going long enough to go through the deer's chest. If you need the arrow to keep going for 1/100th of a second after it contacts the animal to get a total passthrough, and it stops at 99/1000ths of a second, you get no passthrough. A little more momentum, a little more time for energy to work and the arrow would have been stuck in the ground on the other side of the deer.

That is the nature of the close interrelationship between KE and momentum. KE is simply how much energy is available. Momentum controls how that energy is going to be used. Again, physics 101.

Ok, my eyes just started bleeding, and there is an immense pressure in my head. That's usually a sign that I have learned enough for today.

I havent followed all of the bickering closely, but I think you guys need to squash it....no need to fight about stuff like this.

My thoughts on bowhunting are to keep it simple.

Simple bow set up. low poundage (mid 50's to 60#), perfect draw length...one or two pins, catwhiskers, etc...(thats perfect for me anyways...you may be different)

Relatively heavy arrows, with sharp replaceable blade broadheads, tune it to fly as true as I can, and then make any adjustments, if any, to make the BH's group better.

Make the final sight in adjustments with my bh's and then practice. Practice, practice, practice. Just a few arrows a day though, so it more closely resembles hunting. Practice awkward positions and such as well.

What does all this have to do with KE you ask?

Nothing, but it has to do with this thread in the fact that, like I said before, most hunters dont need to worry about this stuff. So why argue about it?

Unless your shooting cape buffalo, or you only draw 35 pounds, you really dont need to worry about this stuff. Sure its interesting, and good to understand, but to fight about it. Come on guys....spit on your hands, shake, and make up!

As long as you take into consideration what game you're after, and whether or not your bow can humanely harvest them, not much else matters. Just make sure you're up to the task, thats all! We're bowhunters, not physists!

Have a good one---Matt

I absolutely did not mean any disrespect for him. I was using the term in a pejorative manner but not intended to be directed at Ashby but with regard to my little disagreement with BobCo19-65. Sorry if you were offended, again it was not my intent. His work cleary demonstrates he is a tireless and dedicated individual who obviously loves archery.

I understand completely that KE is not correctly termed a vector quantity. You missed my point entirely. I thought I was specific about it. The point is however that when talking about an arrows KE energy, ALL the energy that is referenced has, without question direction. KE is correctly termed a scalar quantity because it does not have to have direction. So you are making a semantic issue here nothing more. I agree he is correct but it is also correct to think about KE in an arrow as vectored becuase in fact it is vectored. It has direction and quantity. Again, he is correct about the tuning fork. The fork vibrating can contain a lot of KE and of course that is technically correct but for practical reasons it is misleading and an extremely poor analogy to arrows. Like I said if you held the tuning fork over the chronograph it would read 0 ft/sec and regardless of its mass the KE and momentum caculated as we do arrows would both be 0. Still this is a minor point. Besides, we all know what the definitions are, I was talking about how we think of things. The practical approach rather than a purest.

Technically if we were all purests it is incorrect to say that an arrows kinetic energy can be calculated as KE = 1/2mv^2 because that calculates only the portion of the total KE that has direction toward the target. It's one of those things that we all know is wrong but we kind of ignore that it is technically wrong for practical reasons.

On the point about arrow KE "including" rotaional, vibrational etc. again he is technically correct with regard to a pure physics definition of total KE but he is absolutely incorrect to say that the KE values he reports in his paper included them because he calculates it the same way everybody else does. KE = 1/2mv^2. Not one term in this equation considers any energy other than forward movement of the arrow. Again, fine points.

The reason I get so primed up about this KE business, Mattiac is the repeated calls I see from uneducated folks, ones I call KE snobs, calling for states to set minimum KE levels for hunting, rather than simply setting minimum draw weights, or distance the bow can shoot a broadhead hunting arrow. I think setting minimum KE levels would endanger bowhunting, especially the segments I truly love, traditional and primitive. The ORIGINAL bowhunting. KE levels could be set so high that only he-man mongo types could draw enough bow to achieve it!

If they did anything along those lines, I'd be more in favor of setting minimum momentum levels. Or maybe using a power index like some gun writers do [(mass X speed)/1000] and setting a minimum power index.

They say a good selfbow would shoot an arrow at 10 grains per pound of draw weight at a speed of 100 + draw weight. A 40 pound bow is legal for deer hunting here in Texas. So, a 40 pound selfbow should shoot a 400 gn arrow at 140 fps. Using the power index, that would work out to a PI of 56. My longbow cooks up an 89.1 with a 540 gn arrow. My round wheel 60 pound bow at 33.5" draw does 118.8 with the same arrow. That mythical 350 gn/300 fps arrow does a 105. Increase the arrow weight from that bow to 400 gns and slow the speed down to 290 (which is feasable) and it's PI would jump up to 116. Might not make any difference on deer, but it could mean a great deal on an elk or moose. And, really, there's not a hair's difference in trajectory between 290 fps and 300 fps.

I need some time to sit and study Ashby's article that Bobco posted the link to. Looks very interesting, especially force and impulse. Thanks for that link, Bob!