Arrow Speed vs. Penetration: What Matters For Most Bowhunting?
September 22, 2010
Watching popular outdoor television, it regularly strikes me that complete arrow pass-through has become rarer -- remarkable in light of huge technological advancements in bowhunting equipment. Leaving a single wound channel high on the body automatically spills less trailing blood. Aggressive mechanical broadheads have much to do with this, but I believe something bigger is involved.
Speed dominates the bowhunting industry. The focus of advertising hype is typically not forgiveness or accuracy, but how fast a bow can spit out an underweight arrow. These numbers have little bearing on real-world conditions. Raw speed is derived from harsh draw-curve cycles, but it more directly translates into lighter arrows.
PROS & CONS
Speed is important for 3-D archery when shooting at unknown distances or to provide wider margins of error in wide-open places, but it is less decisive in a whitetail context. Most archery whitetails are killed at less than 30 yards. Range-judging errors are less critical. And, despite the insistence of some, you simply can't beat a string-jumping whitetail off the line. In direct regard to whitetails, the only real speed advantage is the ability to "thread the needle" through tighter spots to reach vitals beyond. Realistically, in tree-stand settings especially, this advantage seldom makes or breaks your hunt.
Conversely, a heavier arrow carries energy more efficiently than a lighter shaft of like deflection. This means deeper penetration and desired pass-through performance, but also a higher degree of dependability. Heavier arrows gain added mass through extra layers of material, directly correlating to increased reliability. In a perfect world, our arrows would find only soft vitals, but in bowhunting stuff happens. Increased ruggedness (and mass) gives errant arrows healthier odds of smashing through bone and reaching vitals. A more efficient transfer of energy also means heavy arrows soak up a higher percentage of a bow's stored power, spelling less string and bow noise.
KINETIC ENERGY MINDSET
Mass (finished arrow weight in grains) divided by 450,240, multiplied by velocity squared (measured arrow speed times itself) equals kinetic energy -- KE or foot-pounds of an object in motion. This is the method most bowhunters use to determine delivered energy because it's reasonably easy to compute and compare. Still, highly variable circumstances can create similar numbers.
KE is customarily measured at launch, not on impact with distant targets. Many elements erode launch energy, from improperly tuned arrows kicking and yawing downrange to friction from fletching. Perfectly tuned arrows not only better maintain initial energy but more closely track broadheads through wound canals to waste minimal energy. The trick is finding a happy medium -- straight flight without undue energy loss.
More dramatically, broadhead style directly affects useful KE -- namely penetration. Energy can't be created or destroyed, only transferred from one object to another.
Opening mechanical broadhead blades is a transfer of energy subtracted from driving your arrow deeper. This isn't a knock against such designs, just food for thought. If you're not seeing pass-throughs, look to a more efficient broadhead design -- something with less cutting diameter or a cutting-tip.
Here's where things become more confused. There are two drastically disparate avenues to identical KE numbers, but with very different results. You can arrive at similar numbers by rocketing a very light arrow (say 300 grains) at blazing speeds (say 330 fps), or driving a heavy arrow (475 grains) much slower (260 fps). This is the old speed versus weight debate. Because speed's squared in the KE formula, many assign it top importance. In truth, energy increases more efficiently when arrow weight is increased instead of speed. In other words, it's much easier to increase KE by increasing arrow mass than by increasing speed.
There are also factors not immediately evident. First, added velocity creates added friction, through basic atmosphere, but most especially following impact with game.
Lighter arrows shed velocity much faster than heavier shafts. In extreme terms, compare throwing a Wiffle ball versus a baseball. Both are released at the same velocity. The heavier baseball maintains its energy much farther than the lighter Wiffle ball. This is a factor of momentum (mass x velocity). In engineering-speak, momentum is always conserved, even in collisions, while KE is not.
For the sake of comparison, let's call "light" anything in the 6.5 to 8.5 grains-per-inch (gpi) class, and broadheads weighing 100 grains or less (85 grains) if shooting a carbon arrow shorter than 28 inches. On the heavy end, consider shafts in the 9.5 to 12 gpi range and the old standard 125-grain broadhead touchstones. This means choosing something along the lines of Easton's Axis Full Metal Jacket (11.3 gpi in 340) instead of the Flatline SuperLite (8.2 gpi in 340); Carbon Tech's new Cougar (9.5 gpi in 55/80) instead of their Cheetah (7.9 in 55/80); or Carbon Express' new PileDriver Hunter (11.3 gpi in 350) as opposed to the Maxima (8.2 in 350); just as examples. Tip weight becomes a function of FOC or Front of Center.
FACETS OF FOC
FOC, expressed in a percentile, is simply a matter of arrow balance. You compute this number by finding the balancing point of your finished arrow (point installed), then the measured center (nock throat to cut-off point) and dividing the difference by overall arrow length (move decimal two points right to convert to percentage). Most experts recommend a minimum of 8 to 9 percent FOC for target/mechanical-tipped arrows and 10 to 12 percent for arrows wearing fixed-blade broadheads. Failing to heed these minimums can lead to difficult tuning, inconsistent flight (especially with fixed-blade broadheads) and loss of overall forgiveness. Maximizing FOC basically means adding tip weight and minimizing rear weight. For example, feathers weigh less than plastic vanes and three 2-inch, high-profile vanes weigh about 8 grains less than three standard 4-inch vanes. Lighted nocks can increase rear weight by 10 to 25 grains.
Earlier I hinted at employing 125-grain broadheads for whitetail hunting instead of newer 100-grain standbys. This instantly provides an accuracy-enhancing FOC boost. While this typically erodes speed by 5 to 7 fps, it also means a more forgiving and stable arrow that might stay the course after a sloppy release induced by buck fever, a nicked branch or gusting wind.
You can also gain an FOC advantage by investing in tapered arrows, like those from Arrow Dynamics, Alaska Bowhunting Supply's GrizzlyStiks and Carbon Tech's Panther.
Carbon Express also offers "Built-In Weight Forward" technology in hunter models like the Aramid KV, Maxima and newer Mayhem and PileDriver, which places heavier material on the front two thirds of the shaft for better broadhead control.
Perhaps more importantly, it has been proven by authorities such as Dr. Ed Ashby that high FOC (15 to 20 percent) helps increase penetration potential, with the same weight-forward function that increases arrow stability actually "pulling" the arrow through the wound channel more cleanly. Ashby has conducted extensive penetration tests on Cape and Asiatic buffalo -- a far cry from whitetail deer -- providing proof positive that the concept holds water.
Smartly employing kinetic energy to its best advantage is the key to gaining additional penetration on the most demanding shots, or simply to assure uncompromising reliability when something goes terribly wrong. Choosing heavier, front-loaded arrows is also the best option when handicapped by a short draw length or low draw weight or when wielding traditional equipment. Sure arrows will prove slower, but they'll also be quieter and ultimately more deadly. That's what better understanding kinetic energy is all about -- more deadly terminal tackle.