Lesser Known Factors of Arrow Penetration

When most bowhunters start the discussion around arrow penetration, talks immediately go to the Ashby studies or subject like total arrow weight, arrow velocity, KE, P, and even sometimes broadheads. While any of those topics certainly do have a great influence of how well your hunting arrow penetrates, there are several other factors to consider. Many of which don't get the attention they deserve.

In this article, we're going to dive into the more less known or lesser talk about factors that have influence over how well your hunting arrow penetrates. 

The Goal

As bowhunters, we need to always remember our goal; 2 holes from a well placed shot in the vitals. To achieve this we need penetration. Momentum at impact has a direct correlation to penetration, not Kinetic Energy. However, without getting into a physics lesson, Kinetic Energy and Momentum are related to one another.  Bowhunting harvests are about creating trauma. To create trauma we need a sharp broadhead on a structural sound arrow and penetration.

Arrow Flight Characteristics

Flight characteristics of your hunting arrow is one of the most overlooked aspects of how well your arrow penetrates. For a extreme generic example, an arrow flying sideways will not penetrate worth a darn, regardless of the KE or P it's carrying. Perfect arrow flight is listed as the 2nd highest priority on the Ashby report of 12 Arrow Factors of Penetration but still not talked about enough in detail. There are several factors to "perfect" arrow flight.

Arrow Recovery - This term simply refers to the time it takes your hunting arrow to reach equilibrium during flight minimizing disturbance. Arrow recovery is important for a few reasons..

1. The faster your hunting arrow stabilizes into tight revolutions, the less disturbance occurs thus, the less velocity deprivation occurs. Velocity deprivation decreases both KE and P. 

2. The faster your hunting arrow recovers the less it is affected by crosswinds. Again, minimizing disturbance is the goal. There's a lot of chatter about micro diameter arrows being less influenced by wind drift but the truth of the matter is that your hunting arrow is impacted more by the cross wind signatures of your vanes and broadhead; meaning the surface area of the shaft is really 3rd priority of influence. To minimize wind drift you need your hunting arrow to be in equilibrium, regardless of shaft diameter. 

If you think about most bowhunting opportunities in the whitetail woods being less than 30 yards, arrow recovery needs to happen fast. In our research and testing multiple different arrow setups, the average distance for an arrow to recover is around 18 yards. In the Ashby reports, Dr. Ashby states this around arrow flight and impacts at close distances;

"Less than perfect flight at extremely close ranges; resulting from arrow paradox; causes a conspicuous penetration decrease; compared to a slightly longer range shot."

A properly tuned bow, properly built hunting arrow, and shot execution all play into arrow recovery. An simple way to test your own setup is to utilize your slo-mo camera on your phone. Have a friend film several shots from different angles and review the footage to see how long it takes your hunting arrow to reach tight revolutions. 

Nock Follow Through/Trajectory - Is your nock consistently following the point of your arrow?

One might argue nock follow through and trajectory are the same as arrow recovery. While that is technically true, arrow equilibrium is only in flight. Once your arrow hits it mark upon impact, the experienced forces upon the arrow change. So, to have perfect arrow flight your hunting arrow's nock needs to be traveling in the same plane or path as your broadhead, we know that. To achieve maximum penetration, at impact when the arrow is no longer in equilibrium we still need nock follow through. A straight line path between your broadhead and nock decreases the likely hood of secondary forces on the arrow shaft itself.

Also, the amount of time or duration those secondary forces have to influence your arrow also play a large factor. Remember, work applied is force x duration. We can limit secondary forces with proper nock follow through and we can limit duration with a quality broadhead powered by proper KE and P.

When we tie in trajectory with nock follow through we are trying to achieve a perfect perpendicular line of impact and follow through on the target animal. Again, referring to Dr. Ashby on arrow flight;

"Poor flight places additional stress on arrow components at impact, and during penetration. On broadside shots, it causes resistance to be oblique to the arrow’s direction of tack, rather than perpendicular. It also increases “shaft flexion” secondary to impact. The resultant oscillation, or ‘noodling’, causes vacillation of the arrow’s force vector during penetration, increasing shaft drag. Decreased penetration secondary to shaft flex is commonly observed. "

So one might think an over spined or overly stiff arrow would lead to better penetration. However, being drastically over spined creates its own problems when trying to achieve perfect arrow flight. 

Rotational Force

Rotational force is hard to quantify in real world applications. If you think about the design and function of single bevel broadheads compared to double bevel broadheads, we can understand and draw some correlations. Double bevel broadheads cut through force without rotation, single bevel heads do cut with rotation matching the bevel.

Any double bevel broadhead cuts and causes trauma through force provided by the arrow. At impact, the broadhead cuts straight with minimal rotation, even though the arrow still wants to rotate. This is why arrows rotating to the left will result in loosening heads at impact. Thus the broadhead is not working with the rotation of the arrow but against it. This results in wasted energy taking away from the overall penetration capabilities of the arrow. 

Take single bevel heads combined with a matched fletching configuration and now you have the broadhead working in unisyn with the additional rotational force of the arrow shaft. Thus, there is less energy wasted during impact and penetration. Single bevel broadheads, by design, cut and cause trauma with rotation. The key here is to MATCH the rotation of the arrow with the bevel of the broadhead. 

Angle of Impact

We briefly mentioned angle of impact when discussing trajectory and nock follow through. Regardless of the angle of impact we want a straight line path between our broadhead and nock but to drastically increase penetration we also want the angle of impact to be as close to perpendicular as possible. 

With a perpendicular angle of impact we can maximize the amount of KE and P that is transfer from the arrow. The further we get from perpendicular the less energy the arrow can transfer to the animal. This is true due to KE being scalar energy (no direction) and P having direction.

If the arrow strikes the target at a glancing angle, much of the kinetic energy and momentum will be dissipated as the arrow deflects off the target rather than penetrating deeply. This can result in reduced penetration, even if the arrow is traveling at a high velocity and has sufficient mass and momentum.

Additionally, the angle of impact can also affect the likelihood of arrow deflection or breakage. If the arrow strikes a hard surface such as bone at too steep of an angle, it may be more likely to deflect or break, further reducing penetration.

Overall, it's important to take into account the angle of impact when selecting a shot placement on an animal, and to try to ensure that the arrow strikes the target at as close to a perpendicular angle as possible. The overarching outlier here is movement from the animal. Regardless of your patience for a perfect broadside shot animals will likely react to your shot, contorting their bodies, moving/shifting internal organs, and ultimately changing the angle of impact as your arrow is zipping through it. We cannot control animal movement as bowhunters, but we can control what our arrows are doing up until the point of impact.

Not Arrow Diameter But Ratio To The Broadhead 

The marketing teams across the industry got a few easy wins when micro diameter arrow shafts hit the market. It's common sense right? Smaller surface areas mean less friction, which would reduce both penetration impediments and wind influence. We've already briefly debunked the wind drift idea, now let's take a look at the penetration. 

Similar to the arrow shaft diameter taking 3rd priority of influence when discussing wind drift, the surface area of the arrow shaft is still not the largest priority over penetration. In the same common sense approach of thinking, consider the broadhead the leader and the shaft simply following (assuming you have perfect arrow flight). The broadhead does the work to pave the way for the shaft, moving tissue and bone while also adding a lubricating factor of blood and other bodily fluids. One that same train of thinking, this is why when shooting "self healing or closing" foam there is a difference in penetration between shaft sizes. No material is moving thus more surface area means more friction. If you have perfect arrow flight, with a broadhead designed well enough to do it's job, the surface area of the shaft becomes negligible. Once again referring to the Ashby studies, arrow shaft diameter has a much closer relationship to the size of the broadhead when considering influences of penetration. Dr. Ashby says this;

"Shaft diameter to ferrule diameter ratio. This relationship is clearly defined in testing. A shaft diameter greater than that of the broadhead’s ferrule averages a 30 percent loss in penetration through fresh, real animal tissues; compared to a shaft diameter equaling the broadhead’s ferrule. A shaft diameter less than that of the ferrule results in a 10 percent average penetration increase. This design feature can change outcome penetration by 40 percent. The 30 percent penetration loss resulting from use of a shaft diameter larger than the broadhead’s ferrule demonstrates the influence of shaft drag on penetration. Reducing resistance is a “free gift”; providing penetration gains equaling very large increases in impact force."

In summary, to maximize penetration you need a broadhead to clear the path for the arrow.

While KE and P get all the buzz, if you are truly looking to maximize penetration out of your hunting arrow setup, these subject matters do deserve some time. Research and experiment with your setup to not only achieve minimum lethality metrics but also to achieve perfect arrow flight, maximum rotational force, remember the impact of shot angles, and know the broadhead to arrow shaft relationship.

Author: Chad Sylvester, Co-Founder/Owner of Exodus