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Questions and Answers.

Bowhunters of today constantly try to expand their knowledge as they search for information that will help them better understand their equipment and the game animals they pursue. The questions I receive today are much more complex and technical than those that came my way just 5 short years ago. Can there be any doubt? Bowhunters today are more knowledgeable than ever before. The great questions in this column are proof of that fact:

Dear Dave,

I was wondering if the following article contains more fiction than fact. The thrust of the article is that momentum is the key ingredient in penetration, but I have read conflicting reports on this matter. Can you clear things up?

Michael Schirer

Dear Michael,

From your use of "fact and fiction," I'll assume you read my previous columns using that title. So first let me say that the basis for good scientific test work is to remove all preconceived ideas, personal prejudices, and unwanted variables. It is surprising to me how few people can get past this first crucial step.

Now to your question. Momentum and kinetic energy are related, but the foremost test work strongly indicates that kinetic energy is the most accurate measure relating to penetration. Momentum, which is the function of the first power of velocity is not as closely related to penetration as is kinetic energy, which is a function of velocity squared.

Norb Mullaney, a registered engineer from Milwaukee, Wisconsin, who consistently performs excellent scientific test work, has conducted a thorough investigation of this subject. His test results show that kinetic energy is the most accurate measurement to predict arrow penetration.

I have also conducted scientific tests that strongly support Mullaney's findings. In addition, all of the true ballistics experts I have talked with agree with this principle.

Still, there are plenty of folks who espouse the theory that momentum is the primary factor relating to arrow penetration. To my knowledge, however, no scientific test work supports this hypothesis.

Dear Dave,

I use helical fletch on my hunting arrows to stabilize them quickly. I use two-blade broadheads and get more than adequate penetration with my longbow. However, some people insist that you get 20-percent less penetration using helical than you do with straight fletch. Do you know of any serious tests that have been conducted and might support this -- to me a very unlikely -- opinion?

J.P. Coudert, France

Dear J.P.,

Several years ago, in conjunction with Easton, I conducted extensive tests regarding the down-range decay of arrow velocity. I set a chronograph at 1 yard and a second at 61 yards. With a shooting machine, I shot many arrows through both chronographs. In other words, each arrow passed through both chronographs as it traveled downrange. Test #1 in that series specifically addresses your question regarding fletching.

Shooting arrows of identical weight, diameter, length, and spine, I varied the configuration of the fletching. The helical and straight fletched arrows passed through the first chronograph at the same velocity. At 61 yards, there was less than 1 foot per second (fps) difference in their velocity.

These test data strongly indicate that helical fletching, when compared to an equal-sized straight fletching, does not have a large effect on arrow velocity or, consequently, on penetration. But do keep in mind that the above-mentioned test was conducted with arrows weighing in excess of 500 grains. Lighter arrows may yield more of a variance between fletching styles, but I seriously doubt that the difference would be anywhere near the 20 percent you mentioned -- especially at normal bowhunting distances.

Dear Dave,

I shoot a Mathews Z-Max set at 64 pounds and Beman Carbon 90/100 arrows at 31.5 inches with 125-grain heads (I shoot with fingers). The problem is that my broadheads and field points do not fly the same. I have both line-tuned and paper-tuned my bow. Accuracy is not the problem. It is just that I have to adjust my sight for broadheads when hunting season comes. Would it help if I changed from 125-grain to 100-grain broadheads? I use four, 4-inch fletches. Any help would be appreciated. Thank you.

Steve Lucas

Dear Steve,

First, I'm delighted to see that you are most concerned with accuracy. Although having the same impact point for both head types is convenient, broadhead accuracy should always be a higher priority. Each setup is different and many variables come into play -- particularly with a fingers release.

Possibly shaft spine contributes to your problem. Your suggestion to try a lighter broadhead indicates you feel your shaft spine is too weak. I'm inclined to think your shafts are too stiff.

Try adjusting your draw weight and use different weight heads. And try the next weaker shaft size. Also be sure to try moving your rest in and out. Other remedies could include: eliminating all fletching contact with the arrow rest; fletching with feathers instead of vanes; using larger fletchings; and increasing the helical twist of your fletchings. Be sure, when experimenting, not to change more than one component at a time.

Torque introduced by improper hand placement can increase many tuning problems. And even something as seemingly minor as nock fit can affect bow tuning and accuracy.

Again, many factors enter into the bow-tuning equation, and you may need to experiment with several of the above-mentioned elements to achieve your goal. But, remember, some bow and shooter combinations simply will not shoot broadheads and field points to exactly the same point of impact. Again, these situations may be inconvenient, but arrow flight and broadhead accuracy are far more important.

Dear Dave,

I shoot a Hoyt Impulse bow set at 68 pounds with 65-percent letoff. My full-length, 33-inch, XX75 2117s are tipped with 100-grain Thunderheads. I shoot with a release aid. How can I shoot faster, shorter-arrows without going to an overdraw? What would the ideal arrows be for me?

Randy Wilder

Dear Randy,

First, the possibility is strong that your 2117 shafts are underspined for your setup. The Easton shaft-selection chart recommends a 2514, among others, at your 33-inch arrow length. The recommended shafts are considerably stiffer than your 2117s.

Second, I would suspect that your draw length is too long. Over 90 percent of the folks I coach are able to improve their shooting form by shortening their draw lengths. It is best to shoot with a slight bend in your bow arm. I recommend that you consider adjusting your form to shorten your draw length.

Arrow velocity is directly related to arrow weight. Changing to an all-carbon arrow or an aluminum/carbon arrow such as the Easton A/C/C 3-71 will reduce your total arrow weight. In addition, most of today's compound bows come with offset risers that permit the broadhead to be drawn inside the handle to the front of the rest. Many quality rests, such as models offered by Golden Key-Futura and New Archery Products, serve as mini-overdraws.

Most of these options will permit you to shoot shorter, lighter arrows, which will increase arrow velocity. Obviously, shortening your draw length can reduce arrow velocity slightly, but shooting form and accuracy are far more important than all-out velocity.

Dear Dave,

The kinetic energy formula I know is: (1/2 mass) x (velocity squared). I saw in Bowhunter that archers use the formula (grains) x (ft./sec. squared))/450,240. I understand where the 450,240 comes from, but I am confused: Why are both called kinetic energy formulas? Why do we use one over the other?

Drew Dorman

Dear Drew,

These formulas are equivalent, except the first [(1/2 mass) x (velocity squared)] employs the term mass rather than "weight" as it relates to the arrow. It is the normal "physics" expression for kinetic energy. To use the weight of the arrow, certain conversions must be applied. The second formula -- arrow weight in grains times arrow velocity squared divided by 450,240 -- includes those conversion factors and will accept the weight of the arrow in grains, which is the normal use in archery.

The second formula is the correct one to use when figuring kinetic energy of an arrow. Example: 240 fps x 240 fps - 57,600.00 x 500 grains = 28,800,000 divided by 450,240 = 63.965 foot pounds of kinetic energy.

Starting with the second issue of Bowhunter back in 1971, I have enjoyed reading "Ask Bowhunter." It has always been a great way to expand my knowledge and keep up on the latest information. Back in the early days, I never dreamed I would be answering some of these questions myself. The experience has taught me that it's often just as educational to answer a question as to ask one, especially when I know several thousand sharp-eyed readers will be reviewing my answers. So be sure to keep those questions coming. They help us all learn and share knowledge.

In the April/May '01 issue Dave Holt will discuss "The Complex Arrow...."
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Author:Holt, Dave
Publication:Bowhunter
Date:Dec 1, 2000
Words:1493
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