These days, speed seems to be the primary selling feature of most hunting crossbows. With so much emphasis being put on speed, it is a good time to discuss an arrow’s kinetic energy and why it is important to consider its significance in the context of crossbow shooting and hunting. A lighter arrow that shoots faster out of your crossbow may be a great arrow to use for competitive target shooting, but it may not be the best one to use for hunting. If you understand what kinetic energy is and why it is important, you will be able to make the best crossbow arrow selection for hunting with your crossbow.
The kinetic energy of your arrow is how much energy the arrow carries due to its being in flight. In other words, kinetic energy is a measurement of how much force is exerted by the arrow onto its target.
Having a high amount of kinetic energy means that the arrow will hit the target harder and will penetrate further than an arrow that carries a lower amount. The amount of kinetic energy is determined by both the weight and the speed of the arrow. Because the force of the string accelerating the arrow down the flight rail is, generally, constant, a lighter arrow will accelerate faster down the flight rail and have a faster speed-at-muzzle than a heavier arrow will. Thus, a lighter arrow moves at a faster speed than a heavier arrow does. But which is better? A heavier arrow that hits harder but flies more slowly or a lighter arrow that flies fast but doesn’t hit with as much force? To best answer this question, let’s examine how to calculate the kinetic energy of an arrow and compare the energy calculations for a light-weight versus a heavy-weight arrow.
To calculate your arrow’s kinetic energy, multiply the arrow speed times itself, then multiply that total by the arrow weight, then divide that by 450,800. The resulting number is the kinetic energy measurement of the arrow in foot-pounds of force. For example, let’s say that you have a 370 grain arrow that shoots at 440 fps. Plugging the arrow weight and speed into the equation yields a kinetic energy amount of 158.89 ft lbs. Next, let’s compare that energy to the energy of a 445 grain arrow that shoots at 410 fps out of the same crossbow. While this heavier arrow is traveling at 30 fps slower than the lighter one, it has a kinetic energy amount of 165.93 ft lbs – which is a full 7 ft lbs more than the lighter arrow. For this reason, most crossbow hunters choose the heavier arrow and are willing to sacrifice some arrow speed because of the increased amount of penetration power that the heavier arrow will inflict upon the animal.
Shooting a heavier arrow has other advantages as well as increased penetration. A heavier arrow will carry more energy away from the crossbow and leaves less energy left in the crossbow to be dissipated, which means there is less vibration and your crossbow will shoot more quietly. This also means that the parts of the bow assembly endure less stress over the life of your crossbow. Heavier arrow shafts tend to have stronger spines, which seem to shoot tighter groups at longer distances than lighter arrows with less dense shafts and weaker spines.
A good rule to follow is to shoot the heaviest crossbow arrow that you can while still increasing the kinetic energy of the arrow. The easiest and surest way of determining the kinetic energy is to weigh the arrow and then shoot it through a chronograph to get a nearly exact measurement of the speed. If you know the speed and the weight of the arrow, you can calculate the kinetic energy using the formula above. Remember: when using a chronograph to determine the speed of an arrow shot out of your crossbow, be sure to keep the end of the flight rail the same distance from the entrance gate of the chronograph as the length of your arrow. If you are shooting a twenty-inch-long arrow, you should hold the end of the crossbow twenty inches from the chronograph sensor gate closest to you to get an accurate reading. Your local dealer will likely have both a scale that you can use to weigh your arrows and a chronograph that you can use to determine the arrow speed. They can also recommend other ways to increase the weight of your arrow or even recommend better, heavier crossbow arrows that are available on the market, which would give you greater arrow penetration out of your crossbow.
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