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Arrow Speed and Kinetic Energy
Hunters may also find that a faster arrow gives game animals less opportunity move out of position. When you fire an arrow at a game animal, the sound of the bow travels much faster (about 1100 fps) than the arrow. So the game animal will certainly hear the sound of the bow before the arrow arrives. During that time, the animal has a brief opportunity to lunge, jump, squat, or otherwise get out of the way. For example, an alarmed deer will often "crouch" in preparation to jump. This crouching motion makes it appear the deer has attempted to "duck" the arrow, when in fact the deer is just loading its muscles to jump and flee. As a result, many bow hunters tend to miss high, shooting the arrow over the deer's back or impacting above the kill-zone, resulting in an unfortunate wound. Of course, there are several factors that come into play (the alertness and athleticism of the deer, your distance and angle from the target, the amount of noise from your bow, etc.) which can affect how quickly a deer responds to the sound of your shot. But one thing is for sure. The faster your arrow arrives at the target, the less time the deer will have to react. So let's take a look at how quickly an arrow traverses it's flight path, given a known distance and launch speed.

Arrow Speed and Kinetic Energy
Click here for larger image.

So it might seem to be a simple choice. For the best possible performance, we just need to shoot the lightest arrow possible, right? Well, before you hit the turbo button, there are a few other things to consider. Most everything in archery is a trade-off; you have to give up some of this in order to get some of that. The issue of arrow weight and speed is no different. Shooting an ultra-light arrow will indeed give you eye-popping speed, but at some expense.

More Speed = More Noise

Shooting a lightweight arrow may result in a notable increase in noise from your bow. It's to be expected, as the faster moving string makes a bigger disturbance in the surrounding air and bows transfer energy into lightweight arrows slightly less efficiently than with heavier arrows. On your particular bow, the difference may be subtle - or quite significant. If you shoot a high-quality modern bow that's already pretty quiet, the difference may be minor. But if your bow is already noisy even when shooting your heavyweight aluminum arrows, shooting ultra-light carbon arrows will guarantee you a virtual harmonic train-wreck.

As with any bow rig, maintaining your fastener torques, along with a compliment of string silencers, dampeners, and a good high quality stabilizer will all help, but be advised that added speed will still come with some added noise too. If you're a hunter, noise is an important issue, and you'll have to weigh the advantages of flatter trajectory against increased noise. For what it's worth, most modern equipment bow hunters tend not to choose the lightest shafts, but rather a mid-weighted shaft (Easton Axis, Gold Tip Hunter, Beman ICS Hunter, etc.) which yields a moderate finished arrow weight.

And while there is no single formula which fits every preference and bow hunting application, our average custom arrow order (for men's hunting bows anyway) yields a finished mass of around 400 grains. Now ... if you're not a hunter, bow noise isn't really a concern. For competition 3D shooting, where noise and penetration really don't make a difference, lighter arrows with flatter trajectories are definitely better. Most competition shooters want arrows which are at the league minimums or speeds at the league maximums.

Speed vs. Kinetic Energy (Knock-Down Power)

Here's where the debate gets heated. With all other variables constant, your bow will have more knock-down power when shooting heavier arrows. So some bow hunting enthusiasts will dismiss the benefits of faster arrow speeds and flatter trajectories, and claim that heavier and more deeply penetrating arrows are more responsible choices. But we have to keep this issue in perspective. Mathematically, there is a difference ... but how much? Does arrow mass affect knock-down power significantly? Does it actually make a difference in the field? Or is this debate just academic?
To understand this issue, let's look at the basic physics. How much penetration, or knock-down power, a projectile has in the shooting sports is generally expressed in ft-lbs of kinetic energy. Arrows which impact the target with more kinetic energy will penetrate the target more deeply than arrows with less kinetic energy. Kinetic energy is the energy of motion. Any object which has motion has kinetic energy. Total kinetic energy depends upon two variables: the mass of the object and the speed of the object. Since we had mass and speed data from our earlier chronograph test, we simply converted those numbers into Kinetic Energy in the table below. Notice that as the arrow gets heavier, the amount of kinetic energy increases.

Arrow Speed and Kinetic Energy
Click here for larger image.

In our example, the computed kinetic energy of the (5 gr/lb) 300 grain arrow was 49.66 ft-lbs. But when a 600 grain arrow was fired from the same bow, its computed kinetic energy was 54.16 ft-lbs. So doubling the arrow mass yielded another 9% in kinetic energy, or "knock down power." Obviously, all bows won't perform exactly as our test bow did, but you get the general idea - heavier arrows absorb more of the bow's energy (and carry more energy downrange). So if you want the added kinetic energy, you can shoot arrows at a heavy 10 gr/lb, just don't forget that to get the extra 9% in KE, you'll give up over 25% of your arrow velocity.

So before you make your decision in the speed vs. kinetic energy debate, you should consider your target - and the energy is takes to ethically harvest that animal with a bow. If you're hunting smaller athletic animals, say Pronghorn Antelope, which are particularly alert and skittish, a faster arrow would surely be best. Hunting smaller game doesn't require as much knock-down power, so getting the arrow to the target quickly will increase your chances of success. Other the other hand, if you're pursuing a 600 lb. Elk, you'll be less concerned about the animal "jumping your string" and more concerned about getting optimal penetration. So if you hunt large heavy game, a heavier arrow may increase your chances of success.

Like many issues in archery, the speed vs. KE debate is one that may never end! Every hunter seems to have his own opinions on the proper techniques, strategies, and ethics of big game bow hunting - and what applies to one hunter may not necessarily apply to another. Ultimately, the choice is up to you.

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