Remember, when the "gurus' worked out what poundage and arrow weight you need to kill an animal they still used a sundial to estimate the time. Things have change technology wise wrt to bows, arrows and broadheads. If you read the latest bow magazines you will see there is a bigger animals that is now taken with equipment far below the "recommended" requirements. This is possible because shooters and equipment gets better ensuring a clean kill.
I hope this info will help you understanding a little better.
Use the information at you own discretion. This info was drawn up on basis that all things are equal.
Although kinetic energy and momentum are traditionally used as indicators in the hunting community, it has severe shortcomings when applied to low energy weapons as discussed above, and is being replaced by something called momentum density
This equation, SABA applied to archery, not with the purpose to calculate penetration, but for investigating the relationship between speed and weight and its correlation to penetration. The result of investigating penetration into bone tissue, showed a relationship between weight, speed and penetration (Fast Arrow) that exactly mirrored weight x speed squared, or kinetic energy. Penetration into soft tissue was totally different and there the relationship closely resembled momentum: weight x speed to the power (Heavy Arrow) of 1.3, where momentum is of course weight x speed to the power of 1. This clearly demonstrates the reason why different researchers favour different indicators (momentum vs. kinetic energy) as penetration indicators.
NB, The problem with defining penetration ability (PA) recommendations is the great many factors influencing it, and the huge variation of penetration in different tissues by projectiles with marginal abilities. Factors affecting arrow penetration, excluding the tissue type, as found by the studies of for example Dr Ed Ashby to have a high correlation, is momentum, and arrow design. In arrow design are included the number of blades, the angle of the blades, the cutting diameter, surface area, and the diameter of the shaft relative to the hunting point's shaft diameter.
In an effort to quantify all these measurements on the broadhead, Dr Ashby suggests the use of a tissue penetration index (TPI = length/(Blade Count x Cutting diameter/2) ). This index captures most of the listed attributes, but excludes the effect of the general size of the hunting point (see next table). It also fails to provide for modern multi-blade hunting points with so called "bleeder blades."
NB. The chisel tips are very strong, and very robust. There are conical ones, square ones, triangles, and some have the flat areas of the tip machined out. The main advantage of these tips is their strength. You can break bones with them, and they will still have cutting edges to penetrate flesh thereafter.
NB. The cut-on-contact (COC) types are less durable, and may fail on hitting bone. However, their penetration in hide and flesh is by far superior to the chisel nose broadheads.
NB.You must remember that all animals are very heterogeneous, and consists of very hard bones, very tough sinews, muscles that move across the arrow's path, and right at the other end of the scale, tissue that is largely just airbags (lungs and genuine airbags in birds). The ability of an arrow to penetrate these is dependent on the arrow's mass and its speed. The importance of mass vs. speed changes as the hardness of the material changes. For some materials kinetic energy, where speed is much more important than mass, will be indicative. For other materials, speed plays a relatively small role in relation to mass.
NB. In an effort by SABA to define a more comprehensive index, a new formula is suggested. Herein the following assumptions are made about the hunting point:
1. The larger the point size (cutting diameter), the more difficult it will penetrate.
2. The larger the angle of the blades with the shaft, the more difficult it will penetrate (reaching a maximum at 90°),
3. The more blades it has, the more difficult it will penetrate.
As you can see, slower and lighter arrows lose more speed breaking ribs than faster and heaver ones.
Approximation of energy needed to break a rib per animal mass:
Animal weight kg / energy needed
10--------------------- 12
50--------------------- 19
220------------------- 29
1060------------------ 45
5000------------------ 70
Warning: Although tests like these try to prove heavier arrows have more penetration, they rather reflect that the bow is slightly more efficient at higher arrow weights. The medium is not comparable to wet tissue, but is indicative of penetration into bone.
This model works well over a range of arrow weights, but also has its limits. The virtual model predicts that as you increase the arrow weight; the arrow's kinetic energy will always increase, reaching a limit close to the total draw energy. This is a better fit for traditional bows and recurves, but with compounds there is a definite optimum arrow weight, beyond which its kinetic energy decreases as arrow weight increases. A better energy model for compound bows looks like this:
Draw energy = (arrow weight + virtual mass + K (arrow weight)3 ) x arrow speed2 / 450240.