I recently received a PM from a BH forum member asking about arrow weight and KE for elk. It was timely as I had completed some numbers from a recent archery shoot. I own a chronograph, grain scale and bow scale and take this setup to archery clubs to offer K.E. and efficiency testing to bowhunters. We also run this K.E. testing station at the WBH annual broadhead shoot and will have several hundredth bowhunters run though the test. Below are the numbers from the hunters I tested at my own archery club a few weekends ago. Most archers were shooting modern bows with carbon arrows. There were a few with older bows (More than 10 years old) with Aluminum arrows. I asked each person what their broadhead of choice was. Most (I estimate 75%) did not know and had not purchased (let alone flight tested) any broadheads and were open to suggestions. Many thought they would try expandables (mostly Rage heads) Those that had their mind made up had used and will continue to use Muzzy or one of the 3 blade fixed heads like Montec, Magnus Snuffer and NAP Hellrazor. It was interesting working with the youth and women. I had my broadhead collection and the Cabelas archery catalog on hand and steered them towards 2 fixed blade heads like the Muzzy Phantom (w/o bleeder) and Magnus stingers. A woman with a 39 pound Bowtech that developed 27.8 Ft/Lbs of K.E. had won some 2 blade Rage and planned to use them on deer. I steered here towards a 2 blade Magnus stinger and only broadside shots no longer than 20 yards. Most all archers were very open to suggestions and were largely misinformed about arrow lethality and seemed very subject to current marketing drives. Several were not at all ready for bowhunting, including the guy that hit my chronograph with his first 2 shots (only 3 feet away) Another fellow who was in his late 40's to early 50's (shooting the 49 pound Bear compound and had shot through my chrono last year) seemed as though he was not at all familiar with his gear. He curled the index finger of his bow hand around the shaft of the arrow prior to the draw (like a trad shooter preventing the arrow from falling of the shelf) When he drew the bow, he had the arrow held so tightly that his draw pulled the nock out of the back of the carbon arrow and was aiming at full draw before I realized that the only thing on his string was the nock and the arrow itself was still sticking out and forward but with the tip pointing towards the ground. I was able to stop him in time to keep him from dry firing the bow. When he did it a 2nd time, I explained that his drop away rest with an arrow clip would hold his arrow and that his finger was safer if it was wrapped around the grip of the bow and that he would be less likely to cut himself with his broadhead while drawing. Most did not know what grain field tips they were using but were certain they had to shoot 100 grain broadheads. As expected, most archers are subject to the speed fad which is fine for 3D tourney shooting but they neglect to consider the effects of these light arrows in hunting conditions. Light and fast is great for 3D where it compensates for errors in yardage estimation and where one does not care how deep they penetrate foam but in a hunting situation, they do themselves a disservice. The goal of a hunting archer should be to have both an entrance and exit hole to maximize blood loss and ease in tracking. Leaving an arrow in a deer is not a desired outcome. The running average for efficiency as tested by hundreds of participants at the annual WBH broadhead shoot is .843, that is rapidly changing (increasing) as modern bows get better. Below are the numbers from the archers I recently tested. In my humble opinion, a hunting archer would do well to stay above 7 grains per pound of draw weight (FYI, the IBO numbers used in ads that sell speed is 5 grains per pound ) The suggested minimum K.E. for deer sized game is 40 but many deer have taken with K.E. as low as 25 with a well placed and close shot using a simple 2 blade broadhead. The suggested minimum K.E. for elk sized game is 50. (remember, the goal is a pass through) If your planning to use a Mech head, Make sure you have the K.E. to deploy the blades and pass through both sides of the animal. Many light weight setups using mech heads will not get a pass through. This prompted Rage to release a 40 K.E. broadhead (which is just a narrower version of the regular Rage head) Understanding your gear makes you a more lethal hunter. Something we should all strive for.
Very good post. I noticed that in your chart the rated efficiency was obtained by dividing the KE by the draw weight. I always thought that efficiency was rated by dividing stored energy (potential) by KE. ( PE/KE ). I used to make Draw Force Curves for all my bows by recording draw weights for every inch of draw length, then measure the area under the curve and arive at PE. I haven't done this in a long time, but back when I did, not many bows would shoot over 100% efficiency. There were too many losses due to friction, histerisis and such. Not that it matters as todays bows are way more efficient than yesterdays bows.
One can structure it as he sees fit so long as you remain consistent rather than using one formula for one bow and another formula for other bows. The formula I use comes from fellow WBH member and former WBH Director Norb Mullaney. Norb has run the Chronograph at the WBH broadhead shoot for many years and has provided bow reports for a few bowhunting magazines (such as bowhunting world) for many years. He has authored over 250 published articles and papers dealing with the technical side of archery and bowhunting. See the attached bow report by Norb. http://www.grandviewoutdoors.com/assets/documents/Bow Reports/PSE X-Force Dream Season.pdf The formula I used is that of Norb. I had breakfast with Norb a few weeks ago during the WBH broadhead shoot and he asked if I would take over his duties running the speed, K.E. and efficiency testing station at the WBH annual event. I was honored and agreed. I spoke to him about getting he prior years test data so that I can compile it electronically. Currently it is all hand written and resides in boxes in his basement.
Mr. Crabtree, not trying to be arguementitive but you cannot structure formulas as one sees fit. Bean counters are famous for doing that in order to come up with numbers that are more favorable to them. There is only one formula to determine bow efficiency and it always includes Stored Energy as did Norb's article that you provided the link to. Simply taking the KE from a bow and dividing it by it's draw weight is not a good way to determine the bows efficiency. You have to consider draw length. DL greatly increases stored energy. It is where speed and KE comes from. This is the reason that some archery contests have speed limits. It gives the short DL people a chance to compete with the faster, longer DL people. You can take two 60 lb bows shooting the same projectile and the one with the longer DL will produce more energy. Again, I maintain that Efficiency = PE/KE. I too got my information from Norb M. Except I was at Racine Instinctive Bowmen at the time. Along with reading almost everything that he has written. BTW, I've been to Sherwood Forest many times, great club.
As I sit here reading, I pulled from my wallet a piece of paper handed to me by Norb. It is the WBH bow/arrow measurement and speed test data for a bow I shot using Norb's set up several years ago. What I labeled as efficiency (in my graphic above) Norb has labeled as "Power Factor" My use of the word efficiency may be improperly used. For the "Power Factor" Norb has K.E. divided by peak wgt. in ft. lb / lb. I find it easier to discuss the results of the tests using the term efficiency rather than "Power Factor" when dealing with those people who run their bows through the test but Power factor is how Norb labels it.
