Opie Gets Another Complete Pass-Through on a Nice Doe
It is no secret that the way we do things is steeped in tradition. It takes years and years to create a paradigm shift in any direction. For years now archery has consider an arrows force and power by terms of Kinetic Energy. Archery got this measurement from the world of firearms. Grumblings have been growing for some time now because many disagree with the idea that kinetic energy is a good way to measure the amount of force an arrow has. Many are arguing that momentum is a much better way to represent what an arrow can do. Well with any argument let’s get down to the facts.
The kinetic energy formula is the speed squared multiplied by the mass divided by 450,240. This formula gives speed of the greatest importance. In a firearm situation the bullet is traveling so fast that the speed is a greater factor because at speeds of 2,000 to 3,000 feet per second a bullet will shatter bones and continue to travel through the animal. The bullet is going faster than the “heavy bone threshold”. Arrows do not travel fast enough to shatter bone and continue. No matter how fast your arrows they are not going fast enough to overcome this threshold. What affects the arrow’s ability to break through this threshold, because of its lower speed, is mass. This brings us to momentum.
The momentum formula is the mass multiplied by the speed. It gives equal value to both speed and weight. The difference between the two is not as evident as when you crunch the numbers and check out what the formulas tell us. I ran some calculators on my bow and while I realize they are not perfect they do show some interesting results. I currently shoot a Mathews Reezen set at 70 pounds with a 29 inch pull. With a 350 grain arrow I should be getting about 321 fps, which would give a kinetic energy of 80 and a momentum of 112,350. As you increase arrow weight they both continue to grow until you reach 510 grains, at which time the KE starts to fall. Oddly enough momentum continues to grow until you reach 720 grains.
What does this mean?
Well I think it means that kinetic energy is not a good way to represent what an arrow can do. Speed while important is not everything in archery. Weight should be much higher value then it currently is getting in the kinetic energy formula. Momentum better represents arrow force because it gives more value to the weight of the arrow, which can make a big difference, especially with shots that aren’t perfect, such as when a bone is struck or a bad angle shot. A lighter arrow while traveling faster decelerates much faster then a heavy arrow traveling only slightly slower.
Like many things, it is all about finding the right ratio for your set up. A super light arrow, while flying flat will not always have the mass to punch through bone or tough hide and muscle. A super heavy arrow will have a poor trajectory and require large pin gaps which can make aiming difficult. The answer is a middle weight arrow, one that flies flat and still hits hard with enough weight to carry it through the target. Middle weight arrows range from 390 to 475 grains. I personally shoot a 426 grain arrow that flies great and hits hard. My advice, get out there shoot different arrows, see what effects your penetration and arrow flight and find the perfect arrow for you.
Shoot Straight,
The Rocket