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Bowhunting - Kinetic Energy is King

By Jeff Smith
August 2002

How fast does my bow need to shoot to harvest a deer? That’s a question I’ve heard many times at hunting camps, pro shops and just about anywhere hunters congregate. There’s a lot of factors that deal with an arrows ability to cleanly harvest a deer, however the standard in today’s world of bowhunting is Kinetic Energy, KE. KE is a factor of the velocity and weight of the arrow and is the baseline for harvesting an animal. You might say, “Kinetic Energy is King”.

A decade ago, bowhunters struggled to achieve the correct balance between arrow speed and weight. It seemed if an arrow was light enough to shoot “flat”, it just didn’t have the weight behind it for deep penetration. A heavy arrow would penetrate better, however the resulting arrow flight was such that a 5-yard miscalculation in distance could result in a miss or worse; bad arrow placement in the animal. With today’s high tech bows, finding the correct arrow balance is not nearly as difficult and the KE values are much higher than just a few years ago.

Calculating Kinetic Energy:

Kinetic Energy is a factor of two variables – velocity and weight. KE can be measured for any object that contains mass, however it would only make sense that the object be in motion. While in college obtaining a degree in physics, energy transformation was one of my focal points and an intriguing area of study.

The arrow velocity can be determined by measuring the arrow speed. Most pro shops have a chronograph to determine arrow speed or you can purchases one for around $100.00. I purchased one and it works well for rifles, pistols and archery equipment.

The arrow weight is the overall weight of the arrow ready to shoot. Once again, pro shops should have a scale to use or you can purchase one. I have an RCBS digital scale that I use for reloading rifle and pistol rounds. You can also call the arrow manufacture and they can give you the approximate arrow weight based off of the arrow components.

Kinetic Energy Formula:

 KE = (weight / 450240) x (velocity x velocity)

Examples:

My compound bow shoots a 460gr arrow at 270 ft/sec

KE = (460gr/450240) x (270 x 270)
KE = 0.001021677 x 72900
KE = 74.48 ft/lbs

My recurve shoots a 530gr arrow at 200 ft/sec

KE = (530gr/450240) x (200 x 200)
KE = 0.001177149 x 40000
KE = 47.08 ft/lbs

Note: You will need to carry the weight calculation out to several decimal places. This calculation converts the arrow weight from grains to pounds. The arrow weight is the overall arrow weight ready to shoot including the broadhead.

How much KE is needed:

According to arrow and broadhead manufactures, a minimum of 45 ft/lbs of KE for whitetail deer and 55 ft/lbs of KE for larger game. For the last seven bow seasons, I’ve shot a bow producing 62 ft/lbs of KE. In my opinion that was plenty of kinetic energy. I shot several dozen deer at that KE range and it did an outstanding job. On a compound bow, I’d recommend a minimum of 55 ft/lbs of KE, but would strive for 60 ft/lbs. I shot several deer in the late 1980’s with a plastic wheel compound shooting a 514gr arrow at 220 ft/sec. This setup equates to 55 ft/lbs of KE. I could see the arrow fly through the arrow with a large arc from 30 yards, however that big heavy arrow would shoot through a deer like a knife through butter. I feel you can get away with less KE with a heavier arrow. Had you rather get hit with a ping-pong ball traveling 60 ft/sec or a golf ball at 30 ft/sec? I’d take the ping-pong ball - It’s the same concept.

Kinetic energy is in some ways the same and some very different when comparing a bow to a firearm. A firearm delivers hydro shock through projectile velocity and kinetic energy. When you skin a big game animal that has been shot with a rifle, you’ll find a large area of hydro shock damage just under the hide. The massive force of the projectile hitting the animal causes the blood vessels to rupture and in turn delivers a jolting blow to the objective. The perfect amount of rifle penetration is just enough to shoot through the opposite side of the animal and fall to the ground. This provides two important events; almost all the KE is delivered to the animal and you have two holes to provide blood flow for tracking. I had some major issues with a 7mm Remington Magnum round several years ago. The weapon produced a very high velocity rate while firing a slow expansion bullet. I shot a dozen or so deer with that bullet and the animal would run for a hundred or so yards after the shot. The exit hole was not much larger than the entry hole; it was almost like shooting the animal with a full metal jacket. I was able to recover the animals, however I prefer the objective to fall on contact with the projectile. I switched to a different style of bullet and alleviated the problem. The KE produced by a bow works much differently. An arrow doesn’t provide near the amount of hydro shock as a bullet. If you skin a bow shot deer and find a large blue area of hydro shock the size of a volleyball, let me know because I want to shoot that same setup. An arrow strike kills through hemorrhaging of the animal as the broadhead cuts through the animal severing blood vessels.  Would an arrow that shoots completely through the animal and sticks in the ground 6 inches on the opposite side harvest the animal better than an arrow that shoots through the animal and falls to the ground on the opposite side? No. I like to have as much KE as possible within the parameters I decide upon for arrow characteristics. The increased KE can help out on shots that strike bone or heavy hide on larger game animals such as bear, moose and caribou.

Arrow Speed:

With today’s compound bows, high arrow velocity can be attained at a reasonable draw weight. In the old days, only a few bows would shoot over 260 ft/sec and to achieve these speeds required 75+ lbs. of draw weight.

 Higher Arrow Velocity Characteristics:

Have a flatter trajectory resulting in less critical yardage estimation
Tend to make more noise coming off the bow
Can be harder to tune
Is normally less forgiving on shooting form mistakes

 Arrow Weight:

Arrow weight is very important in a hunting situation. International Bowhunting Organization, IBO, rates bow speeds at 5 grains per pound of draw weight. In my opinion, five grains per pound in not nearly enough for a good hunting arrow. Most experts agree that 400 to 425 grains is the minimum arrow weight for hunting.

Arrow Speed vs. Weight:

Kinetic energy is a factor of two variables, velocity and weight. If all other factors stay constant, a decrease in one of the two variables should result in an increase of the other. When dealing with KE, velocity and weight are inversely proportional. What I’ve found through testing, KE is directly related to the bow characteristics. For example, if I set my bow on 70 lbs at 31” draw; I produce relatively the same amount of KE with a light arrow, heavy arrow or any where in between. This makes sense as KE is a measure of force and the arrow doesn’t produce the force, as the bow is the mechanism used to produce the force. 

I received a very good tip this year from a PSE sponsored shooter. With today’s flat shooting bows, shoot the arrow that groups the best with broadheads and don’t let arrow weight be the only determining factor. I’ll remember this statement for a long time.

Draw weight is one of the biggest problems I see with today’s hunters. I see hunters shooting bows that they can barely draw in “summer” clothes on a bow range. If you get one thing out of the article it should be “Arrow placement is much more important that arrow speed”. Shoot the draw weight that you are very comfortable shooting. Ted Nugent, Spirit of the Wild TV host, shoots full-length arrows at 55 lbs draw weight. I’ve watched him shoot many animals with almost all complete pass through shots. Shoot what your comfortable with, your buddies will be more impressed with your tight groups than you pulling some huge draw weight.

Key Notes:

Shoot the arrow that flies the best with your broadhead setup
Practice with broadheads and not just field points
Practice from an elevated stand – 10 feet high is better than ground level
Shoot a comfortable draw weight
I tend to lean toward a heavier arrow – at least 425 grains
Practice at least some in full hunting gear
Practice in low light situations
Make up games that will keep the shooting fun – shoot 2 arrows from 20, 30, 40, … out to the range you are comfortable with
Practice a lot from long range – when you move in close it seems like a chip shot
Practice drawing the bow as smooth and free of motion as possible
Get some place quiet and draw listening for any noise from the bow – get another person to listen as well
Really work on shooting good form – bad form hurts more with broadheads and from long range
Number your practice arrows and take out the ones that shoot the best to use for hunting
Shoot on windy days to see how it affects your arrow flight
Don’t be tweaking with your bow all the time – get it setup and leave it that way
There’s no substitute for practice

 In Conclusion:

Practice often to get your bow set up correctly and to build confidence that you hit what you shoot at. A lot of things in life that come down to success and failure deal with confidence, if you believe you can … normally you will. To me, it’s a great feeling to be at full draw with the sight buried in the good stuff. Just knowing that the hard part is over and the deer’s chances are next to nothing. All hunters owe it to the game to be as prepared as possible to humanely harvest the animal. Get prepared and confident!

Have a safe hunting season.