11 – Penetration

One of the things that all archers must learn is how their arrow will hit the target and how far they will penetrate. Many archers get into primitive archery and find they have problems with good flight and penetration. The problem is that they have always used the arrows their local shop owner or maybe the Easton chart told them to use. They found an arrow that was spined for their bow. It might have told you if the spine was correct but did nothing to tell you if the arrow had enough power to penetrate very far. Using the arrows you are told to is all well and good but now if you are making your own arrows who is going to tell you what spine to use and if you have enough kinetic energy to have proper penetration? You are, right after you get finished reading this book.
Let us start right in on kinetic energy. Kinetic energy is the energy that the arrow has when it is in motion. It is the energy imparted to the arrow from the bow. So from that we can say that the bow is one of the major pieces in making strong kinetic energy. The other factors are the speed of the arrow, which is determined by the force of the bow and the weight of the arrow. We can easily figure this energy by using a simple math formula. It is arrow weight times the arrow speed squared and then both divided by 450240. So the arrow weight would be measured in grains and the arrow speed would be measured in feet per second. The formula would look like this:

(Arrow Weight) x (Arrow Speed) ²  = Kinetic Energy

If we are to use this formula we can figure the energy that any arrow will have when it will hit an animal. We can use this to make sure we have the proper energy to be able to put down our prey humanely and quickly and prevent any suffering due to poor equipment choices. Now with this I can find that an arrow that is 500 grains and shot from a bow at 150 feet per second or fps that will yield 24.99 foot/pounds or Ft/Lbs. I can also see that if I built the bow better and made the arrow and got and arrow speed that was 170 fps that the kinetic energy would increase to 32.09 Ft/Lbs. That is a big increase just by making a faster bow. By raising our speed 20 Fps we were able to increase our Ft/Lbs by 8 pounds and increase energy by almost 30%.
Now, if I took that fast bow and added a 650 grain arrow to it I might get more energy even though I would loose speed. Using the standard drop of 1 fps for every 5 grains added we would lose 30 fps. My speed might drop to 140 Fps but my energy would increase to 28.30 Ft/Lbs. That is almost a 5 pound jump and a 16% increase just by increasing the grain weight by 150 grains even though we lost 10 Fps. So you can see tat by building or buying a faster bow and increasing our arrow weight we would be able to increase our kinetic energy. The examples shown before show almost 12 pounds and a 48% increase in energy. I am sure that you can see it will make a huge difference in how far you arrow will penetrate. Revise its wrong

11 - energy chart

One of the other things that will affect your penetration that you might not of even considered. Your arrow spine and the arrow flight can affect how your arrow will penetrate. Arrow flight will affect your arrows by making them enter a target or animal at an angle. If your arrow is porpoising, spiraling or doing some other erratic flight the back of the shaft will not enter a wound directly behind the point. This will cause unwanted friction and drag on the shaft, slow it down and reduce penetration.
Improperly matched arrow spine can also reduce the amount of penetration that you are getting when you are shooting your arrows. If you have an arrow that is under spined it will naturally bend more when it is shot than a stiffer arrow will. The weaker in spine an arrow is, the more it will bend when shot from a bow. Likewise, the more it bends when shot, the longer it takes for the arrow to recover and fly straight and true. If an arrow is still trying to recover from the Archers paradox and comes in contact with an animal or the target it will not penetrate as well as a recovered arrow. While the arrow is still trying to recover the shaft will still be flexing and will cause drag during penetration. This increase friction will naturally slow the arrow down and reduce how far it will travel through the target.
All things consider you will need to evaluate the equipment you are using or making if you are hunting with it. It is ideal is to use a heavy arrow with the fastest bow you can make. There are of course limitations to every thing and an arrow can be too heavy and a bow can be too fast. I know the last statement just made you about drop this book, right? A bow should be fast but you might have notice that static Recurves and other really fast bows are not as forgiving as a standard Recurve or flat bow. The may yield 10 fps but become less stable and more unforgiving of bad releases and the like. A stable bow might be just slightly slower but more likely to shot an arrow with good flight.
An arrow can be too light or too heavy. A very light arrow does not absorb most of the energy of the bow and leaves left over energy in the bow. This left over energy is not imparted to the arrow and resides in the bow as sound energy and id damaging to the bow if the left over energy is too high. A heavy arrow absorbs most of the energy of the bow but is slower and drops of faster.
You need to consider all aspects of your bow and arrow combinations to get the best setup that will work for you.

This is all part of the Archer’s paradox which is explained in the spine chapter.

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