Ed AshbyMemberDecember 14, 2016 at 6:06 pmPost count: 816
Watch the short film clip below. There is much here that can help one understand the terminal performance of a hunting arrow.
What can be gleaned from this film clip? It tells us a lot about how a projectile applies whatever force it carries. First, a few calculation. The military .30-06 load, circa 1961 was a 172 grain, non-expanding bullet at 2700 fps from the M1 Garand. That indicates 2783.70 ft. lbs of KE (the energy carried by the projectile) and 2.062 slugs (lb-feet/sec.) of momentum (the projectile’s ‘force’). For the arrow we have to do a little assuming. The ‘typical’ arrow, circa 1961, would be about 550 grains at a maximum of 170 fps from a period recurve bow. This indicates 35.29 ft. lbs of KE and 0.4152 slugs of momentum. That means the .30-06 has 78.9 times as much KE as the arrow and 4.97 times as much momentum. Remember that this is a non-expanding bullet of .308 inch diameter – smaller than the typical arrow shaft of 1961, which would likely have been 11/32 inch (0 .34375 inch, or 11.6% greater diameter than the non-expanding bullet from the .30-06).
First, how well did the KE predict the penetration? Not very well at all. While the difference in momentum does not directly predict the penetration either it does come far closer to doing so than does the KE. Why does the arrow’s lower momentum out-penetrate the non-expanding bullet so drastically? Because all momentum is not equal in predicting a projectile’s penetration capability. The arrow derives most of its momentum from the arrow’s mass whereas the .30-06 derives most of its momentum from the bullet’s velocity.
Now, how does this relate to the study’s data? First, it explains why the Heavy Bone Threshold is related to the arrow’s mass rather than the force or velocity the arrow impacts with. The heavier a (structurally intact) projectile is the slower it loses its momentum, which results in a longer TIME OF IMPULSE … it pushes forward for a greater period of time; exactly why the Heavy Bone Threshold is related to the arrow’s mass, rather than the amount of KE or momentum the arrow carries. The heavier arrow ‘pushes’ on the bone longer … long enough to overcome the bone’s mobility and flexion.
A lighter arrow might well “hit harder” (have more KE and/or momentum at the time of impact) but it loses its forward impulse before the bone reaches its limit of mobility and flexion. The arrow can’t even BEGIN to penetrate the bone until the bone’s limit of movement and flex is reached. This is a graphic illustration why the arrow’s mass is the determining factor in the Heavy Bone Threshold; it allows the arrow to apply whatever force (momentum) it does carry to the bone for a longer TIME OF IMPULSE. Whenever that TIME OF IMPULSE is greater than the TIME required for the bone to reach the limit of its movement AND flexion the arrow will begin to penetrate the bone. Whenever the arrow’s retained force, after exceeding the bones movement and flexion, is sufficient the bone will be breached. And that why the mass of your arrow is THE important factor in the arrow’s ability to breach bone, assuming the arrow maintains its structural integrity throughout penetration. It’s also why it is important to design your arrow so that it maximized its potential for conserving as much of the arrow’s force as possible throughout penetration (through maximum utilization of the other arrow-force conserving design features) – the arrow’s remaining force at the time the bone reaches the limit of its mobility and flexion will determine whether the bone is breached or the arrow stops without penetrating the bone.
LongtradDecember 15, 2016 at 12:38 amPost count: 8
Very interesting points. Is there ever an amount of FoC that can break bone with a lighter arrow or does that not have as much affect as the 650 grains.
What is the highest FoC arrow anyone has worked out to use for hunting? I see a few 31% arrow builds but not really anything higher than that.
Ed AshbyMemberDecember 15, 2016 at 1:25 amPost count: 816
The initial testing of EFOC/UEFOC effect on the Heavy Bone Threshold showed no effect on the bone-breaching rate. However, when compared to matched-set arrows of normal/high FOC they DID show a very marked increase it the total outcome penetration in all instances where the arrow did breach the bone (with arrow sets both above and below 650 grains total mass). They DID NOT show any increase in the bone-breaching rate in all matched-set testing of the Heavy Bone Threshold with arrows having below-threshold mass (below 650 grains total weight). Above threshold weight all sets tested showed a 100% bone-breaching rate, regardless of the degree of FOC.
I must note that this testing was done during the last year I was able to do testing. Though the data is limited the frequency and magnitude of the increase in post-breaching penetration demonstrated was easily sufficient to say that having EFOC/UEFOC is a great benefit once the bone is breached.
There are a number of folks who are hunting with arrows in the range of 31% to 32% FOC, and a few using slightly higher FOC. That, of course, does not take into account those natives in Papua New Guinea who are using arrows all the way up to well over 40% FOC.
Col MikeMemberDecember 18, 2016 at 1:01 amPost count: 906
Thanks Doc for showing that one again. Think I may have mentioned this before if so disregard.
When I was still knocking about in tanks M60A-1 (rise,passive) the primary armor defeating round was a 105mm (they are 120mm now) fin stabilized discarding sabot.
After the sabot discard the penetrator was about oh 2+ feet long weighing about 35#. With most of the weight in the first 6″ in a depleted uranium tip and about 40mm in diameter traveling at about 1500m/sec. So what you have here is maximum KE with maximum mass of the penetrator and most of that up front. In tests and real life it penetrated a Russian T-70 through the armored front glaceous and after going through the crew compartment and through the engine block exited the rear. Nice fire works.:D
Point is heavy weight forward gives momentum as the Doc stated and maximum penetration.
Of course we are not gonna launch arrows at 1500m/s but—IMHO if you tune your bow to the heaviest arrow it can shoot accurately at you distance (mine is 20-25yds) and put as much weight up forward (mine is EFOC %31) you will take just about any animal in N.America even when bone is hit.
smiley1MemberDecember 18, 2016 at 11:19 pmPost count: 92
Interesting information Doc. A few years ago I was hunting bears in Saskatchewan. I was shooting a 50# longbow with 660 gr carbon arrows and Tuffhead broadheads. UEFOC was 31.7%. I shot a bear at 15 yards quartering away. My arrow penetrated the last rib, exited between the front legs and the broadhead buried into the dirt. The bear went 9 yards. One of the other guys in camp was shooting a 74# high tech compound with 4 blade replaceable low MA broadheads. He considered his arrows heavy at 480 grains. He kept talking about his KE being so high. He shot a bear with very poor penetration 4″ and couldn’t figure out why he didn’t get better penetration. They found the bear the next morning 200 yards from the shot. His hit was high but the only bones hit were ribs. My KE was about 1/2 of what his was but my arrow significantly out performed his. Steve
LongtradDecember 19, 2016 at 4:18 amPost count: 8
I was thinking about this while pondering over the old english longbow and the way they used those weapons. They often just lobbed the arrows over into their enemies but the shafts and points are very heavy. I guess that mass is what gave them the momentum to do the job when it came to poking through armor and whatnot.
This post really helped that click in my head.
Thanks for sharing, Doc, I am a big fan of your work.
Robin ConradsAdminDecember 28, 2016 at 11:49 amPost count: 835
Dr. Ed Ashby wrote:
Longtrad, the minimum force to breach heavy bone is a lot lower than I had anticipated. In fact, I never reached it in the testing. If you will look at the 2008 Update, Part One, in the section on testing with the 40# recurve you will see that 100% of the above threshold weight arrows breached the bone. The lowest force arrow(s) in that testing carried only 0.368 Slugs of momentum (and only 22.09 ft.-lbs of KE). With a well tuned, structurally intact arrow having high overall arrow efficiency it’s far more about the Time of Impulse than it is about the arrow’s force.
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