Are You Using Appropriate Gear?
What size animal is your bow capable of killing? In regards to the growing ranks of youth and women bowhunters, the question of how much arrow energy is enough often poses a real sticking point. Bowhunting equipment is becoming better every year — more efficient, storing more energy with less draw weight and length. But is a 40-pound bow drawn to only 25 or 26 inches too much to ask, even when faced with just a 100-pound whitetail? Is your current whitetail rig up to stuff for the big elk hunt? What about the biggest, most dangerous game on earth like Cape buffalo and elephant?
Measuring Arrow Energy
Kinetic energy equals velocity squared, multiplied by mass, divided by 450,240. The sum equals the kinetic energy — a.k.a. KE in foot pounds — of an object in motion. This is how the vast majority of bowhunters determine usable arrow energy. But what do these numbers actually tell us? Not much in my estimation.
There are glaring problems with KE as an indication of killing potential. First, these numbers are normally gained at muzzle velocity, right off the arrow rest. It's a measurement of available energy at a given moment, but really tells you nothing of how arrows perform downrange. Drag created by fletching and atmospheric friction bleeds initial energy quickly. In other words, KE off the blocks doesn't accurately reflect energy available on target 20, 30 or 60 yards away. Nor does it tell you anything about how well a given projectile will perform after meeting living flesh and blood.
Since velocity is squared in the KE formula, relatively small jumps in speed significantly boost KE numbers. Yet fast, light arrows bleed initial energy much faster than heavier arrows traveling at slower speeds. Gaining speed often means subtracting arrow/point weight, directly affecting projectile reliability. Finally, the faster an object impacts the target the greater the drag while penetrating a given medium.
I've long held KE's essentially worthless to bowhunters, and can't fathom why its use persists industry-wide. It's useful for apple-to-apple comparisons, but doesn't reveal a thing about real-world performance potential. A more accurate and useful indicator of available energy is momentum (momentum equals mass times velocity), but again, while more indicative than KE, it leaves a few gaps in the overall picture.
That said, not every bowhunt is all about penetration. Speed certainly holds merit, as it seems that the thinnest-skinned, lightest-boned animals are also generally those most difficult to approach closely and most prone to string-jumping — pronghorn and Coues whitetail, for instance. It's all about balancing equipment with likely scenarios.
Getting Better Penetration
Other elements besides kinetic energy and momentum also factor in outright penetration.
- Tuned Arrows: Obviously a well-tuned arrow, one slicing air cleanly, retains more energy than one that's fishtailing, especially at long range. A well-tuned arrow also penetrates better following impact, tracking the broadhead through wound channels more precisely, rather than slapping wound edges to waste energy. Arrow material is also part of this. Carbon arrows recover from impact flex faster than aluminum or wood, so they more cleanly track broadheads through wound canals.
- Broadhead Design: Broadhead design directly affects energy delivery. Energy can't be created or destroyed, only transferred from one object to another. The opening of aggressive mechanical broadhead blades is a transfer of energy. Mechanical designs are becoming more efficient every season, but most still sacrifice some small level of energy. If you're using them and getting a pass-through on every shot, no worries. If not, look for a more efficient broadhead design — two holes are always better than one!
Wide cutting diameters also consume energy cutting instead of driving forward (especially if they chop instead of slice). Cutting-tip designs, or true cut-on-contact heads, slide through muscle tissue and split bone more efficiently than cone or even faceted pyramid tips.
Arrows from Turkey to Elk
I've long advocated a flexible approach to various classes of game we bowhunt. Instead of a one size fits all mindset, the bowhunting outfits I assemble reflect the size and tenacity of game pursued, likely conditions faced, and especially predominant ranges anticipated. Again, equipment choices involve compromises. Gaining in one area means sacrificing something in another — the deepest penetration verses the flattest trajectory is a prime example.
Lightweight arrows (6.5 to 8 grains per inch), 85- to 100-grain broadheads and especially mechanical broadheads do have their place, especially on the smallest, most delicate game regularly bowhunted. Turkeys are the most obvious example, as recovering them after a hit often depends on flattening the bird on the spot.
Combine small vital areas with long-range shooting and a strong case can be made for mechanical broadheads. With average equipment, penetration shouldn't pose an issue on such game. Here we're looking at a 350- to 400-grain finished arrow with a minimum 9 percent Front of Center (F.O.C.) pushed at about 300 fps (+/-10 fps) as an example. (Determine F.O.C. by finding the measured center of the arrow, then the balance point with tip installed; measure the distance between these points and divide by overall arrow length [nock throat to cut-off]. Convert to a percentile by moving the decimal point two spaces right.)
The game sparking serious penetration debates are critters such as elk and moose. These animals are equipped with thick hide and sturdy bones capable of stopping poorly-made broadheads. Just as you can cleanly kill deer with a .22 Hornet, you can also kill elk with wide-cutting mechanicals and light arrows — if everything proceeds according to script. Worst-case scenario is a clichéd term, but it is more likely to rear its ugly head on bigger animals. This is when you want nail-tough, cutting-tip broadheads leading the way and heavy arrows to stand up to abuse while driving broadheads home despite bone impact or quartering angles.
Heavy spells shafts in the 12 to 15 gpi class and broadheads from 125 to 145 grains in modern equipment. The idea is not only to boost overall strength, but to bump F.O.C. percentages so forward weight literally drags the trailing arrow through bone and massive flesh. Instead of standard percentiles in the high single digits, seek numbers in the 12 to 15 percent range.
What ideal might look like is a 450- to 500-grain finished arrow with 13 percent F.O.C., pushed to around 250 fps (+/-10 fps).
Most of us sit somewhere in the middle of these extremes. There's no such thing as overkill, and shooting an elk arrow certainly promises a quieter bow, but a middle-ground compromise allows ample penetration potential combined with versatility to confidently thread the needle on tricky brush shots, or stretch range a tad when a buck treads across an open corn field. What I have in mind are arrows in the 8.5 to 10 gpi range, tipped with a 125-grain head (or 100-grain, so long as F.O.C. doesn't suffer). These numbers describe 75 percent of the shafts currently offered in the industry.
Dangerous Game Arrows
Assembling dangerous-game arrows used to involve ingenuity, such as stuffing one arrow into another to create needed weight or gluing heavy steel inserts into traditional broadheads to boost forward mass. As already mentioned, heavier, slower-moving objects retain momentum better than lighter, faster-moving objects.
Today, assembling a super heavy-weight arrow has become easier. Manufacturers offer special shafts with the biggest game in mind, with finished weights of 650 to 850 grains. Prime examples include Alaska Bowhunting Supply's (ABS) continuous-taper GrizzlyStik Safari, Easton's Dangerous Game FMJ, PSE/Carbon Force's Black Mamba or Gold Tip's 100+ Big Game. Broadheads weighing from 190 to more than 300 grains are added to these (Steel Force and ABS both offer several extra-heavy broadheads) to create F.O.C. percentages of 19 percent or more.
When dangerous game is involved, you're looking for a minimum of 650-grain finished arrow weight combined with minimum 19 percent F.O.C., delivered at 200 to 225 fps. Anything more is certainly welcomed.
Dr. Ed Ashby has spent the past twenty-some years studying aspects affecting arrow penetration on game animals, using actual animals (already dead), first in Africa, more recently Australian water buffalo, as test medium for various arrow/broadhead combinations. Most interesting are his findings on extreme F.O.C. in relation to penetration. His tests show that high F.O.C. offers a definitive edge in penetration on the biggest game and even formidable bone. By shooting arrows weighing more than 650 grains, high F.O.C. created by a solid, single-beveled broadhead (sharpened steeply on only one side of each edge, creating a mechanical twisting on impact to crack bone easier), Ashby has demonstrated that even average bow setups are capable of blasting through sturdy water buffalo ribs.
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Perhaps more importantly, Ashby's studies pave the way to better understanding how compound shooters plying below-average kinetic energy — women, youth and elderly archers owning short draw lengths or pulling low draw weight — can achieve better penetration through increased F.O.C. This also includes average draw-weight traditional gear (45 to 65 pounds), as recurve and longbows just can't touch even light-pulling compounds for energy transfer.
Conventional thinking has long held that kinetically-challenged bowhunters choose light broadheads to match lighter/shorter arrows. Ashby's studies reveal that just the opposite is true. Choosing a true cut-on-contact, as well as a much heavier head, sacrifices a bit of speed but provides added penetration potential such shooters need to succeed on bigger game.
I'm always uncomfortable with etched-in-stone rules. I offer this information only as food for thought. If your current rig's blasting through everything you shoot, wonderful. Don't change a thing. If, on the other hand, you regularly lose animals due to lack of penetration or terminal-tackle failure, another approach is obviously in order. Using these baseline examples assures you keep wounding loss to a minimum. The animals we bowhunt deserve no less.
Editor's Note: This was originally published on November 15, 2010.
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