With so much talk about the speeds of crossbows these days, it is also a good time to discuss an arrow’s kinetic energy measurement and why it is important to consider its significance in the context of crossbow 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.
The kinetic energy of your arrow is how much energy the arrow carries in flight. In other words, kinetic energy is a measurement of how much force is exerted by the arrow onto its target. A high amount of kinetic energy means that the arrow will hit the target harder and will penetrate farther 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 slower, 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 versus a heavy arrow.
To calculate your arrow’s kinetic energy, multiply the arrow speed by 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 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 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 crossbow itself endure less stress over the life of the 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 20-inch arrow, you should hold the end of the crossbow 20 inches from the chronograph sensor gate to get an accurate reading. Your local archery dealer will likely have 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 that would give you greater arrow penetration.
Now that you know how to get the most power out of your hunting crossbow, you are ready to take on even the biggest of bucks this fall hunting season. Good luck!
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