I emailed NAP about the kinetic energy minimums required for their spitfires and shockwaves awhile back. I believe I received a well thought out and honest reply from their tech. Here' s the reply

We publish a minimum specification for all of our mechanicals of 50 FT/LBS.
Depending on your set up and arrow weight you may be able to use a much
lower KE than this. However that is a personal choice.

Few people realize that broadhead penetration has very little to do with
broadheads. Rather, penetration has almost everything to do with arrow
flight. A well tuned 50 pound bow with arrows that fly without wobble
penetrates better than an 80 pound bow with aberrant arrow flight. Probably
the greatest disadvantage of mechanicals in general is that as a group they
tend to be much more critical of arrow spine than replaceable or fixed
bladed broadheads. For instance, because there is a loss of energy of about
1 to 3 ft/lbs. when a Spitfire contacts a target, and because this energy
loss is directly opposite the direction of travel, an improperly spined
arrow will have an additive effect. Generally an improperly spined arrow
fails to penetrate well regardless of broadhead style because the energy is
partially lost in some direction other than straight forward when the arrow
strikes an animal. A standard broadhead in this scenario would most likely
penetrate and stick in the animal with enough of the shaft inside to lodge
it until the animal does something that causes the shaft to break or
possibly get pulled out during escape. Spitfires along with most other
mechanicals will almost instantly back out because of the additive effect of
the spine related energy loss, reverse energy of opening the blades and the
fact the blades can close easily preventing any anchoring effect. Arrow
flight is especially important on high angle shots. If the arrow is
wobbling back and forth or up and down, then when it hits the target/animal
the energy continues to the side rather than straight forward. On a
mechanical this would have the effect of pivoting the broadhead and reducing
penetration. Keep in mind that if your arrow flight is good then increased
angle has NO effect.

Another thing to consider is momentum. Momentum is often disregarded in
archery because people tend to concentrate on kinetic energy. Let me
explain the difference. Kinetic energy is the impact energy of the
projectile. It is a very good way to determine how efficient your bow
transfers stored energy in the limbs to dynamic energy in the arrow. For
the most part kinetic energy in the 50 ft/lbs. to 60 ft/lbs. range is ample
for taking animals in the deer to elk range. Kinetic energy in the 60
ft/lbs. to 70 ft/lbs. will effectively take any animal in North America.
For example, Chuck Adams took his 1400 pound Kodiak Grizzly Bear with about
65 ft/lbs. of kinetic energy. I believe he used an arrow in the 630 grain
range that traveled about 220 fps. While there are several arrow/speed
combinations that would produce 65 ft/lbs. of kinetic energy, Chuck opted
for a heavier slower arrow because of the momentum. Momentum is the ability
of the projectile to continue in motion. Momentum is a linear function of
mass. The idea is the greater the mass the more resistance is required to
slow or stop the object. Think of it this way if someone were to pitch a
baseball and a bowling ball to a batter, and both balls had the identical
amount of kinetic energy even though one is traveling very fast and the
other is traveling very slow. The batter would be able to hit the baseball
and move it in the opposite direction very easily while the bowling ball
would most likely break the bat. The bowling ball has tremendously greater
momentum and therefore is much more difficult to stop or slow down.

Hope this helps.


Cary J. Pickands
Technical Support Specialist
New Archery Products, Corp.
7500 Industrial Drive
Forest Park, IL 60130
800-323-1279