[V.sup.2]xM / 480,240 = KE: understanding the take-down power of an arrow's Kinetic energy.

Velocity squared, multiplied by mass. divided by 480,240. The sum equals the Kinetic energy--a.k.a. KE or foot pounds--of an object in motion. In other words: arrow speed times arrow speed times arrow weight divided by the constant equals foot-pounds of delivered energy on target. But what do these numbers really tell you? Why are they important, and how do they affect your bowhunting pursuits? How much KE is enough; and how much is too little to efficiently kill the big game we bowhunt? Misconceptions and halftruths run rampant in this arena, as archery hunters fail to understand the concept or how it factors into the overall scheme of things. KE is important, but does not complete the entire equation, nor can it fill in all the blanks of the picture.

[ILLUSTRATION OMITTED]

Understanding KE

The laws of physics dictate that objects in motion want to stay in motion. The faster that object is moving, or the heavier it is, the more difficult it is to stop. Consider your hunting truck: You're traveling down a country road. A deer runs into your path. You stomp the brake peddle but your truck doesn't stop right away. If you're traveling at speed you'll cover a good distance even after your tires lock up to skid across the abrasive pavement. Even with lightening-quick reflexes you still might hit that deer, simply unable to get that big hunk of iron shut down in time to avoid a collision.

Now think of an arrow launched from a bow after the string release. That arrow doesn't weigh what your truck does, but it's traveling much faster. It collides with that same deer (hopefully not in the truck headlights, nor standing in the middle of a road) and wants to continue its path, to blast right through that deer's vitals, likely penetrate some dirt behind that animal. Put in the simplest terms, how much animal you penetrate, even how much dirt on the opposite side of that animal, is a product of KE.

Other factors come into play. Many elements conspire to bleed off initial launch energy; some more detrimental than others. Obviously, a well-tuned arrow, one slicing the air cleanly, will retain more energy than one yawing and porpoising; especially when ranges grow longer. A well-tuned arrow also penetrates better after impact, the arrow tracking the broadhead through the wound channel, rather than slapping the sides to waste energy.

Minute energy losses are also a product of the fletchings we use to steer our shafts and hunting broadheads. The larger the fletching surfaces, the more energy used. Of course, not enough fletching can allow broadhead-tipped arrows to fly askew. At one extreme, tiny target fletchings provide a speed edge in 3-D tournaments, steering less-critical field points, a speed edge allowing flatter trajectory and compensating for slight range miscalculations. At the opposite end of the spectrum, big 5.5-inch "banana" feathers used by some traditional shooters to steer large, fixed-blade broadheads can create flu-flu effects at longer compound-bow ranges. The debate over feathers verses vanes is easily settled: Feathers are lighter so provide faster launch speeds. Vanes are heavier, yet more aerodynamic, creating less friction when moving through air; providing slower launch speeds, but overtaking feathers at approximately 50 yards; all other factors remaining equal.

[ILLUSTRATION OMITTED]

No matter which fletching you choose, gravity and friction inevitably erode KE. The KE levels determined from launch speeds simply don't apply when range stretches to 30, 40, especially 60 yards. This is important to remember when shooting marginal energy for the animal you're hunting (more later). You may be a whiz-bang long-range shot, but your hunting rig may not be delivering the energy levels you believe at longer ranges. Again, this is a factor with marginal gear, while most adult male rigs produce energy to burn.

[ILLUSTRATION OMITTED]

On a larger scale, broadhead type can affect delivered KE. Energy can't be created or destroyed, only transferred from one object to another. Opening the blades of a mechanical broadhead is a transfer of energy; energy subtracted from driving your arrow deeper. Mechanical designs are becoming more efficient every season, but most still sacrifice some level of energy, some up to 20-percent. This isn't a knock against mechanicals, as they can produce awesome results, just something to keep in mind, especially if you're on the low end of the energy scale. If you're getting passthroughs on every shot; no worries. If not, you may need to look into a more efficient broadhead design. Hand-in-hand, wide cutting diameter heads also shed energy by expending it on cutting instead of driving forward. Cutting-tip designs, or true cut-on-contact heads, are most efficient, sliding through muscle tissue and splitting bone more easily than cone heads or even pyramid tips.

Facets of KE

Kinetic energy is Kinetic energy, right? The numbers don't lie. In theory, yes. Let's take this to extremes. One hundred foot pounds of energy is required to bowhunt African elephant. To make this happen I started with a 29-inch Beman Matrix 300 (discontinued), inserting 28 inches of Beman pul-truded shaft (discontinued, but still available). I added eight inches and 220 grains of steel rod, threaded through the glue-in broadhead adapter and extending back through the pul-truded center. I glued on a single-piece-welded, cut-on-contact broadhead. Finished weight 1,100 grains. My buddy shot that arrow from a 28-inch-draw, 93-pound Hoyt UltraTec. That arrow sailed at a snail-slow 210 fps (I assembled these arrows for a recent Zimbabwe foray). Closer to home, I use a standard Gold Tip XT Hunter 7595 (8.9 gpi)-29 inches-a NAP Spitfire 100, for a finished weight of about 415 grains. I shoot this from a Spiral-Cam UltraTec set at 80 pounds and 30-inch draw, for an actual speed of 340 fps (I own this bow). Both rigs produce 100 foot pounds of energy, or very close. Which would you choose for elephant?

These are two drastically disparate avenues to identical numbers, but with very different results. This is the old speed verses weight debate. Because speed is squared in the KE formula many assume it to be the be-all, end-all. For the bowhunting that the majority of us engage in, this is likely correct. Granted you're maintaining clean arrow flight, producing tight arrow groups at range, there's no such thing as an arrow that's too fast. The two beginning qualifiers are highly important to that statement. Excessive speed can create a world of accuracy and tuning problems, to say nothing of equipment reliability and longevity. That said, today's compound bows make manageable speed easy. All major archery manufactures produce bows that easily break the 300 fps mark today, some slightly faster. Combined with lightweight yet durable carbon arrows and reliable broadhead options, speed is easy today. This makes producing killing energy a slam dunk, even at lower draw weights or short draw lengths.

[ILLUSTRATION OMITTED]

While speed is definitely the craze, there's a lot to be said for old-fashioned "slow-but-sure" thinking. Run the numbers on a selection of setup options and you'll quickly discover that energy increases at a more efficient rate when arrow weight is increased instead of speed. Increase arrow mass and energy increases in five to seven foot-pound jumps. Increase speed and it increases in two- to four-pound jumps. In other words, it's much easier to increase KE by increasing arrow mass than increasing arrow speed. Heavier arrows simply absorb and transfer bow energy more efficiently than light arrows. This is important when tackling larger game such as elk, or even for a youth or woman striving to wring maximum KE from a short draw length and limited draw weight. It becomes imperative should you hold aspirations of bowhunting African nasties such as Cape buffalo.

There are other advantages as well. A heavier arrow, because it absorbs energy more efficiently, also creates a quieter shot. Bows are becoming quieter all the time via built-in vibration-dampening features and all the add-ons now available. But shooting a heavier arrow can make your bow even stealthier. This also translates into longer life for your expensive equipment.

I'm not advocating speed or weight. I pick and choose one or the other according to what game and hunting conditions I expect to encounter. When I go after jumpy Southwest Coues whitetail, or hit the plains for prairie pronghorn, I want all the flat-out, burning speed I can wring out of my equipment (hence the UltraTec setup mentioned earlier). Speed-produced KE also translates into flatter trajectory at long range; less margin of error when something's way out there. This points to carbons in the 6.5 to 8 gpi class and broadheads weighing 75 to 90 grains.

[ILLUSTRATION OMITTED]

When I draw an elk tag, or head north to hunt moose, I lean toward the conservative. I choose shafts weighing 9.5 to 12 gpi, broadheads in the 100- to 125-grain class, giving me more penetration when I need it most, more forgiveness in brush or when nerves are a factor. I might also choose this rig for the intimate ranges offered while installed on stand and bowhunting whitetail-opting for the quietest bow possible and less possibility of a jumped string and missed or wounded animal.

[ILLUSTRATION OMITTED]

KE Minimums

Let's look at some actual KE or foot-pound numbers. I'll throw some figures out and then explain what that might look like. I would call 30 to 35 foot pounds absolute minimum for pronghorn to deer-sized animals, granted an efficient broadhead design is chosen. Before you begin your letter-writing campaign or call me all kinds of names, remember that the traditional bows archers wielded for ages produced little more energy than this. Example: A venerable 2117 at 30 inches, holding a 145-grain Bear RazorHead (about 525 grains finished), shot at about 180 fps from a 50-pound recurve, produces about 35 foot pounds of energy. For beefier Midwestern whitetail I would feel more comfortable with 40 to 45 foot pounds. For elk, moose or bigger African plains game 50 to 55 minimums. African experts recommend 85 foot pounds minimum for Cape buffalo. Add about 10 foot pounds to those numbers if you wish to take less-than-perfectly broadside shots, or want added insurance against accidental bone hits. These are arbitrary numbers, factors such as arrow material, broadhead design, and shot placement all portion to final outcome.

Back to the 30 to 35 foot-pounds: You have a child or wife with a 25-inch draw length, pulling about 40 to 45 pounds. Arrow choices might include Gold Tip's XT 3555 (7.4 gpi), Beman/Easton's Black Max MFX/Axis Full Metal Jacket (FMJ) 500 (8.9 gpi), or Carbon Express' CX 100 (6.5 gpi). Averaged for weight (with an 85- to 100-grain head) and shot at 240 fps, they easily break into the 35 foot-pound range. By comparison a 65-pound bow shooting a 28-inch Gold Tip 5575 (8.2 gpi), Beman MFX 340 (10.4 gpi), Easton FMJ 340 (11.1 gpi), or Carbon Express CX 300 (8.1 gpi)--averaged for weight--with 100-grain tip, shot at 260 fps, produces about 50 to 60 foot pounds of energy. At this draw weight you're in a comfortable range for game such as elk and moose, though, if accuracy doesn't suffer, bumping up those numbers by increasing draw weight gives you added insurance and a flatter-shooting arrow.

[ILLUSTRATION OMITTED]

Assembling dangerous-game rigs is a bit more complicated. As I pointed out earlier, weight is the best way to accomplish this, from a standpoint of efficiency, but more pointedly, to assure the required reliability and durability on such rugged beasts. My buddy's elephant rig I've already described. My Cape buffalo rig consisted of an 885-grain arrow shot from 90 pounds (a long, less energy-efficient finger bow) at 220 fps. That produced a bit more than the recommended 85 foot pounds, and proved a great compromise between required energy, arrow reliability and manageable trajectory. The arrow included a 30-inch, 11 gpi Beman Matrix (now discontinued, Carbon Express 12 gpi 6075 Terminator Hunter Select offering similar characteristics), 14 inches of 10.5 gpi Beman pultruded shaft up front (which helped boost FOC and improve flight), backed by a 3 gpi (yellow) PSE weight tube to hold it forward. My broadhead was a 150-grain cut-on-contact.

As you can see, there are many facets to Kinetic energy. There are a multitude of options to arrive at the same place, but special precautions should be taken any time your quarry owns thick hide or heavy bones. When that quarry can bite back nothing can be left to chance. By understanding KE you can assemble a smart combination for the game and conditions you are most likely to meet this season.