HOW MUCH KINETIC ENERGY IS NEEDED TO BOW HUNT WHITETAIL DEER
Part of ethical hunting is knowing the energy your weapon puts out and how much energy you need to successfully harvest the animal you're hunting. In the bow hunting industry, the last 25 years have put a huge emphasis on bow speed. What does that have to do with kinetic energy? How much kinetic energy do you need for bow hunting Whitetail deer?
The minimum kinetic energy needed to bow hunt whitetail deer is 25 ft-lbs. This is the absolute minimum. Compound bows with a draw weight of 40 lb generally put out a 240-grain arrow with 30 ft-lbs of kinetic energy. To increase kinetic energy you can increase draw weight or arrow weight.
This article talks about what arrow kinetic energy is, how to calculate arrow kinetic energy (KE), kinetic energy vs momentum, KE’s impact on boad-head selection, items that eat KE on impact, and how to increase KE.
What Is Arrow Kinetic Energy?
Kinetic energy is the amount of power in the arrow at the time of impact. It also could be described as the amount of energy an arrow has during flight. The factors that impact arrow kinetic energy are arrow velocity and weight. After a shot, arrow velocity continually decreases. Related to that, the kinetic energy continually decreases in flight. More on that later.
How to Calculate Arrow Kinetic Energy
To calculate arrow kinetic energy, you need to have accurate measurements of the speed of your bow with the arrow it is paired with, and the weight of the arrow in grains. Using different weighted arrows will change the speed and kinetic energy.
Square the speed first, then multiply that number by the weight of the arrow, then divide by 450,240 to get the total KE.
Fps2 X Weight of Arrow / 450,240 = Arrow’s Kinetic Energy
Example of calculating KE
As an example. We will use a 40lb compound bow with an average speed of 240 fps. Arrow weight 240gr.
240 x 240= 57,600 | 57,600 x 240 = 13,824,000 | 13,824,000/450,240 = 30.7
By increasing the weight of the arrow to 300 grains and deducting speed loss (1-1.5 fps for every additional 5 grains) we get a KE of 35 ft-lbs.
Arrow Kinetic Energy vs Momentum
What about momentum? Kinetic energy is the energy on impact from motion. Momentum is the amount of force the arrow will have pushing forward after impact. Momentum is mass (for simplicity we will go with weight) multiplied by velocity. Momentum helps with pass-throughs on deer.
Arrow Momentum Formula:
Momentum (arrow) = mass of arrow (grains) x arrow speed (fps) / 225,400.
For comparison in our example, the KE of a 300-grain arrow at 230 fps is 35 ft-lbs. The momentum is .306 slug fps. To go back to our original example, the arrow weighing 240 grains would have a KE 15% less and the momentum would drop 17% from the heavier arrow. You can see, the heavier an arrow is the more momentum and KE it will have.
What does this mean to us? Heavier arrows give greater KE and momentum which leads to more pass-throughs and better blood trails. Choosing the right arrow is a big issue.
Kinetic Energy and Momentum: Scalar vs Vector Force
Kinetic energy is a scalar force. That means that KE is a measurement of the magnitude (amount) of energy stored at a given time. It does not indicate how much power is moving in a direction.
Momentum is a vector force. While momentum contains the complete magnitude of energy contained in an object, it also measures the force in the direction it is traveling.
What does this mean to us bow hunters? Bow companies are pushing for faster speeds. Speeds look really good when measured with kinetic energy. It looks like the best way to improve performance is to make the arrow lighter and make it fly faster. But momentum is the most accurate measurement of the power an arrow has in the direction of its flight.
Does an Arrow’s Kinetic Energy Change During Flight?
As stated earlier in this article, velocity changes during the flight of an arrow. When the arrow is launched from the bow it is at its maximum velocity and therefore its maximum kinetic energy. The farther the arrow gets downrange the slower the arrow flies and the lower the kinetic energy becomes. Arrow tune, fletchings, arrow spine, and broadhead or tip selection are all factors in the loss of velocity due to the influence these factors have in arrow recovery. In short, the faster an arrow recovers the more efficient it is down range. This is why arrow builds are such an important part of the lethality equation of your hunting arrows.
To make this ultra-simplified, we will assume the arrow being shot loses five feet per second every 10 yds. This is the average energy loss of an arrow. Your arrow may be different, but this is a good baseline.
A 40 lb bow that originally shot a 300-grain arrow at 230 ft per second at 0 yards, will be shooting 205 ft per second at 50 yards. The kinetic energy drops from 35 ft-lbs to 28 ft-lbs in 50 yards. That's a big difference. A bow that could easily be used for whitetail deer at short distances could be underpowered at longer ranges. Arrows shot with large fletchings and a fixed blade broadhead could see further reduction in velocity.
Among other reasons, this is why it is important to use the correct arrow spine and tune your arrows. An untuned arrow or an arrow with an incorrect spine will not fly correctly. This will create more friction in flight, force the arrow to work harder to recover, and will cause further speed reduction.
Kinetic Energy Impact on Broadhead Selection
To retain the best speed in flight a mechanical broadhead could be used. I think this is one of the strongest arguments for a mechanical head. These broadheads are more aerodynamic due to their minimal profile. This means that speed reduction is kept to a minimum.
Some fixed blade broadheads are significantly more aerodynamic than others. Generally, a tip that whistles in flight is creating lots of drag and is slowing down rapidly. So, when choosing a broadhead, think about its effect on flight and the ability to penetrate. A big and nasty looking broadhead will likely be nasty on your arrow speed.
The other factor is weight. A heavier broadhead will always give you more kinetic energy as long as the arrow is recovery properly. Other things can be done to make your arrows heavier, but a heavier broadhead when paired with the correct arrow spine will have a huge positive effect on KE. It also increases your arrow FOC. To a point, this will help with momentum.
KE drops off exponentially faster than momentum on impact. This is why a heavy broadhead or heavier arrow weight is a good thing...to a certain point. With total arrow weight there is a point of diminished returns. Keep this in mind. Shooting a hunting arrow on the extremely heavy side gives you awesome lethality metrics but the velocity and trajectory loss usually isn't worth it.
Items That Eat Arrow Kinetic Energy Upon Shot Impact
Huge cutting diameters make for great blood trails, but they smash the arrow's kinetic energy after impact. Remember a factor of KE is velocity multiplied by itself. A large cutting diameter rapidly slows down the arrow and exponentially reduces the amount of kinetic energy the arrow has.
A small cutting diameter, on the other hand, allows the arrow to keep more of its kinetic energy for a longer period. This leads to better and more consistent pass-throughs.
Another factor that will eat your kinetic energy is a dull broadhead. A dull broadhead will slow down your arrow on impact very quickly. Make sure to use sharp broadheads. This will lead to better penetration, more pass-throughs, and better blood trails.
Mechanical Broadhead Deployment Effect on Kinetic Energy
While a mechanical broadhead may be the most energy efficient in flight, deploying the blades on contact can zap KE fast. Mechanical broadheads use resistance to deploy their blades. This resistance ranges from a small amount of pressure, close to 2 lbs, to almost 12 lbs of pressure to just deploy the blades.
For a bow shooting 35 ft-lbs of kinetic energy, you can see why the pressure needed to deploy blades would be a problem. It could take your KE to 25 ft-lbs before the arrow even gets the chance to partially penetrate.
Some mechanical broadheads do not have a cut on contact tip. This is like trying to stab a deer with a table knife instead of a skinning knife. Before the blades even deploy, in this case, the arrow is rapidly decelerating, your KE is exponentially decreasing, and your penetration will suffer.
All this goes to say: calculate the KE of your setup. Use the measurements you have to determine if you should shoot a mechanical broadhead. Manufacturers will specify what KE an arrow needs to shoot their broadhead. Compare your measurement with their numbers to make your decision.
How to Increase Arrow Kinetic Energy
I'm sure you've caught on to some of the ongoing themes in this article. The two primary ways to increase the kinetic energy of an arrow are to increase its weight or to increase the speed the arrow flies at. The fastest way to achieve higher kinetic energy is to shoot a faster bow or shoot a heavier arrow.
Arrow Weight
Using a heavy arrow increases KE but also helps with momentum. This is another huge positive attached with added weight. The more momentum an arrow has the better penetration you will get.
Conversely, shooting a light arrow will increase the speed and therefore its kinetic energy but you trade off momentum. The arrow will lose its KE very rapidly and will not penetrate as far. It’s important to use the correct arrow weight for deer hunting.
Bow Speed
Shooting a faster bow or cranking up the weight on your bow will help with kinetic energy. This will exponentially increase the KE delivered to your target. This will also increase momentum significantly. Note: Do not turn the weight up too far. A bow you can't handle is an inaccurate bow. The kinetic energy you want may cause great pass-throughs but also will sink arrows into the ground.
Make sure you are fitted correctly to your bow. If you are still growing, you may be shooting a draw length that is too short for you. The longer your draw length is the faster your bow will shoot. 1 inch of draw length gives an 8-12 fps speed increase. Note: adjusting a bow for too long of a draw length will decrease accuracy.
Summary
Multiple factors impact arrow speed and kinetic energy. Knowing that the absolute minimum of 25 ft-lbs of kinetic energy is needed to bow hunt whitetail deer gives us a baseline for our bow setup. Building in margin for your hunt is a good move. The more KE we can get out of our bows without hurting our accuracy the better off we will be.
The two most basic factors in kinetic energy are arrow weight and speed. Fine-tuning our setups can give us better performance. It is good to measure the actual performance of your bow and think about making investments in quality items. With everything set up correctly, you can have complete confidence when that big buck appears.
AUTHOR: Evan Grimm, Exodus Black Hat Team Member
