Carbon Arrow Selection Guide. Chapter 5: Arrow Speed and Kinetic Energy

Speed: Oh How We Love It!

Fast cars, fast computers, fast wide receivers, fast-drying paint, fast-acting weed-killer.....if it's fast, we love it. And when it comes to our archery equipment, it's no different. Show most guys a brand new bow, and their first question is likely to be "How fast does it shoot?". Right or wrong, speed is a major consideration for most archers. And it's great to see how advancements in materials and design technologies have made today's compound bows better, faster, and more fun to shoot than ever before.

Of course, there will always be a few dissenters in the crowd, those who'll loftily claim they don't care about speed. But the market trends don't lie. Archery consumers are taking advantage of these innovations, buying high-performance bows and speed-boosting gadgets by the trainload. And nothing yields such a predictable and significant increase in speed like a good set of lightweight carbon arrows. So let's examine the pro's and con's to shooting lightweight arrows. Let's find out where they help, where they hurt, and where they don't really make a difference.

ARROW MASS AFFECTS ARROW VELOCITY

When all other variables are constant, arrow speed has an inverse relationship with arrow mass. Of course there is a point of diminishing returns, but as arrow mass increases, arrow speed decreases. As arrow mass decreases, arrow speed increases. As we learned in Chapter 2, there is a limit to how light you should go, so it is possible to have too much of a good thing. But the informed archer can greatly improve his bow's performance by upgrading to modern carbon/graphite arrow designs. How much performance can be gained? We put the issue to the test.

ARROW MASS VS. ARROW VELOCITY TEST

We prepared 9 arrows, ranging from 250 grains up to 650 grains in precisely 50 grain increments. Each arrow was fired from our test bow (60#/28" Bowtech Patriot) through the chronograph and the results were recorded (table below). Five trials were conducted for each arrow - to achieve a reliable speed measurement (average). Each arrow was fired from the same distance, from the same shooter, and without any modifications to the bow's settings during the test. The test was conducted at our indoor range, where lighting and environmental conditions could be held constant throughout our test.

OK. BUT IS FASTER NECESSARILY BETTER?

Maybe. From a standpoint of accuracy, you may find that lightweight arrows will yield some surprising benefits. From the moment your arrow is released from the bow, it begins to lose trajectory. As it succumbs to the effects of gravity and air resistance, it's flight path (trajectory) changes and the arrow eventually begins to drop back to earth. Arrows which fly more quickly hold their trajectory better than slower arrows. So archers who shoot lighter-faster arrows will have less need to adjust for distance - since the faster arrows will hold a "flatter" trajectory within a given distance. In effect, the faster arrow allows you to make more mistakes in yardage estimation. If you mis-estimate a deer to be 25 yards away, and it is actually 30 yards away, a fast arrow "misses" it's mark by a much smaller margin. Look at the trajectory chart on the right from our Bowtech Patriot experiment. The heavier arrows nosedive much more quickly than the lighter faster arrows. So whether you're a hunter or competition 3D shooter, you may find that faster arrows actually add a little forgiveness into your shooting, allowing you to make an error in yardage estimation without paying such a heavy price.

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 it's muscles to jump and flee. As a result, many bowhunters 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.

So it seems to be a simple choice. For the best possible performance, we just need to shoot the lightest arrow possible, right? In most situations, yes. But there's some 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.

THE DOWNSIDE: MORE SPEED = MORE NOISE

First, 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. If you shoot a high-quality bow that's already pretty quiet, the difference may be subtle. 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 with every shot. String silencers, Limb Savers™, and a good high quality stabilizer will all help, but be advised that added speed will typically come with some added noise too. If you're a hunter, noise is an important issue. But if you're a competition or recreational shooter, noise isn't such a serious issue.

THE DOWNSIDE 2: LOSS OF KINETIC ENERGY

How much penetration, or knock-down power, your arrows will have is a matter 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. And here's where the trade-off comes in. A bow is generally more efficient, when launching a heavier arrow. So mathematically, shooting lightweight carbon arrows will cost you a little Kinetic Energy (penetration) at the target. To illustrate the point, we've converted our Bowtech Patriot experiment from FPS into Kinetic Energy. Look at the results.

As you can see, the heavier arrows manage more kinetic energy, or "knock down" power. But to gain just 4-6 ft-lbs of KE, you'll have to sacrifice perhaps 100 fps of your arrow velocity. Of course, every bow will not perform exactly as our test bow did. However, you can expect similar gains and losses in performance with most modern compound bow models. So this will be another trade-off to consider when ordering your arrows. If you're a hunter, penetration is an important issue. But if you're solely a competition or recreational shooter, don't give penetration another thought.

This issue of KE is a major hot-topic of debate among bowhunters. Some hunters setup their rigs to produce maximum speed, some want maximum KE, and others choose something in the middle. Again, there may not be a right and wrong here. Every hunter seems to have his own opinions on the proper techniques and ethics of big game bowhunting. Like many issues in archery, it's a debate that may never end!

According to Easton's field chart, the amount of KE you'll need varies by the species you intend to hunt. The larger the game, the more KE you'll need. Of course, these recommendations aren't absolutes, nor are they guarantees of success. However, it gives us a place to start:

So would our Bowtech Patriot be capable of harvesting deer and elk? According to the chart, yes. A properly placed arrow - impacting with 50+ ft-lbs of KE - has a very good chance of generating a clean pass-thru on large game like Whitetail Deer or Elk. So with respect to kinetic energy and big-game hunting, there may be no practical difference between the 300 grain arrow impacting with 50 ft-lbs of KE and a 600 grain arrow impacting with 54 ft-lbs (as in our experiment above). Assuming favorable conditions and a good shot, either arrow would provide sufficient energy to make a clean harvest.

MOMENTUM vs. KINETIC ENERGY:

Should you have an unquenchable interest in ballistic physics, you may find it interesting that a few sporting enthusiasts will even dispute the convention that KE is the best measurement for predicting hunting penetration. A number of enthusiasts will debate that MOMENTUM is the better mathematical model. Of course, KE and Momentum aren't the same thing.....

----> Kinetic Energy = Weight X Velocity Squared /2 X Acceleration of Gravity
----> Momentum = Weight X Velocity / Acceleration of Gravity

Since velocity isn't squared in the momentum formula, arrow MASS plays a much larger role. The kinetic energy of a moving body increases as the square of the velocity whereas the momentum increases directly as velocity increases. So if you recomputed our chart to show momentum, then the graph would look much different. The heavier arrows would show a significant improvement in overall momentum, and you could therefore conclude that heavier arrows would indeed yield better penetration.

Right or wrong, the shooting sports have a number of traditions and conventions regarding technical measurement. And the lethality of a projectile (whether from a firearm or bow) is traditionally expressed as a function of KE (ft-lbs). As such, most sporting enthusiasts have some comprehension of this measurement. Unfortunately, a momentum rating in Slug Feet-per-Second would surely leave many of us scratching our heads. Given the dramatic difference in the two methodologies, it seems unlikely that KE has remained the "standard" for so long if it's entirely incorrect. There are a lot of talented engineers in the archery industry. So either they have ALL missed it, or perhaps the momentum theory has a glitch (not to worry - our team of physicists are working on it now).

Should you have an interest in the debate regarding which mathematical model is best applicable to archery, here's some basic argument on the topic (http://www.booneman.com/_terminalarrow.php). For more discussion, Dr. Ed Ashby also has an interesting and exhaustively extensive article on the subject (http://www.datakey.com.au/steve1/Momentum.pdf) you might want to check out (have your scientific calculator handy).

SOME PERSPECTIVE:

Regardless of how you crunch your numbers during pre-season, you can't avoid the elements of chance during the actual hunt. Shooting a live animal in the woods is quite different than shooting a block of ballistics gel in a laboratory. In the field you'll encounter unpredictable and complex variables that limit any mathematical model to just a "best guess". If you consider that your arrow must arrive on target then pass through layers of hair, hide, muscles, bones (perhaps), and a host of other tissues.....AND that all of this is happening in an uncontrolled outdoor environment, it's pretty clear that the issue of hunting penetration cannot truly be distilled into a mathematical puzzle. As many experienced bowhunters can attest, just as it's possible to make mistakes and get lucky, it's also possible to do everything right and come-up empty handed. That's just part of the sport. However, with good equipment, good technique, smart planning, and some good-old common sense, you can surely tip the scales in your favor and maximize your chances of success in the field.

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