Momentum Beats Speed for Lethal Arrow Hits

Momentum Beats Speed for Lethal Arrow Hits

Lethal shots without wounding deer are every ethical hunter’s top priority, and shot placement and an understanding of anatomy are paramount. However, momentum should be the most important part of arrow selection if we want to achieve every ethical bowhunter’s top priority.

The archery industry and too many bowhunters have a fascination with the speed of an arrow. If 3-D shooting is your only quest, speed is the major factor, and you don’t need to be concerned with momentum. If you are a deer hunter striving for quick, clean, humane kills, then it’s all about momentum.

Much has been written about this subject, but little of it seems to be put into practice. I’m sure someone out there will argue that they kill every deer they shoot at with their ultra-light, super-fast arrows. And I’m also sure someone out there could kill a deer with a jackknife. I just want hunters to understand the physics of momentum and be aware of this critical aspect of bowhunting.

In simple terms, speed is what gets an arrow to its target. Momentum is what happens after it hits the target.

Yes, speed figures into the equation of momentum, but weight has more relevance than speed in the momentum equation. Kinetic energy is a firearms equation and not relevant to archery. Here are a couple non-hunting examples to help you grasp what momentum is.

• A baseball going 100 mph has less momentum than a bowling ball going 10 mph.

• A basketball coming down a hill at 20 mph is easily stopped by a healthy adult, but that same adult would do well to get out of the way of a Cadillac rolling down the same hill at 5 mph.

The same is true for arrows. A heavy arrow has greater momentum and it is much harder to stop after it meets resistance. That can be especially important with a marginal hit or a hit that encounters bone.

Why then do we strive for speed by going to lighter and lighter arrows? I think because we have been convinced that speed means power and flat trajectory. Both are true to a point, but for bowhunting we need to go a step further and ask “Compared to what?” If more power means the ability to penetrate after meeting resistance, then it’s not completely true that speed produces power. If flatter trajectory means like a bullet, then it’s not true at all. The fastest hunting arrows are extremely slow compared to even the slowest bullet. These fast arrows are much slower than the speed of sound, which is 1,085 feet per second. Let the 3-D and firearms folks have speed; we bowhunters need to concentrate on momentum. 

The good news is we can use all this modern bow technology the industry is hyping as speed and convert it to the more important task of momentum. All that’s required is that we shoot a heavier arrow. Sure, speed will go down, and yes trajectory will not be as flat. However what we give up is minor compared to what we gain in quick, humane killing power. For those of you who are shooting high-poundage bows, let’s say above 65 pounds, even the lighter arrows should have enough momentum for whitetails at reasonable distances, however heavier arrows will perform much better for you at longer distances, on marginal hits, and when you hit bone. You may also want to recalculate if you plan a hunt for moose, elk, or bear. Remember, momentum is diminishing as an arrow travels. And it diminishes much faster for light arrows than it does for heavy ones. Don’t blame me, it’s simple physics.

As you can see, this fast-arrow thing is luring us into a false sense of security. Because we can hit a target farther out, we do it, even though the light arrow that allowed the easier shot may not have the momentum to kill quickly and humanely. Hitting an animal at 40 or 50 yards is no consolation if it runs away with only one punctured lung. Where we really get into trouble is with the bows of lighter poundage, say below 60 pounds.

This article is mostly about the killing power of heavier vs. lighter arrows, but there are other advantages as well to the heavier arrow. The shot will be quieter, more forgiving, and less prone to deflection or wind drift. Yes, a well-placed light arrow is certainly better than a poorly placed heavy one. However, all things being equal, the heavy ones allow for a greater margin of error.

Here are some guidelines if you want to quickly kill deer and track wounded ones less.

Only go after the “speed you need.” Let the rest of the bow’s power be converted to momentum. Here in the northeast we rarely shoot at deer beyond 20 yards. When we teach the Quality Deer Hunter course (a 4-part training course focusing on fine-tuning archery and firearms equipment as well as anatomy and shot placement) , we ask our participants to fill out a questionnaire, and one of the questions is, “What is the average distance deer are from you when you shoot?” The answer for several years of data is about 16 yards. My own average is closer to 12. So, the speed we need is most likely far less than what we have. Again, I’m sure we have this speed because we have been led to believe that speed is power. Hopefully you have already been convinced that it’s not true. So if my 64-lb. bow shoots a 520-grain arrow at 220 fps (very slow by today’s standards) I can sight it in to hit dead on at 16 to 17 yards and not have to worry about any other sight pins from 0 to 20 yards. The trajectory will be slightly higher closer up and slightly lower further back, but not enough to cause a miss at the 8- to 9-inch vital area we are shooting at. With this setup I can be confident of clean pass-through shots, even if I shoot a sharp angle where the arrow enters behind the last rib and exits in front of the off-side leg. Please don’t ever try this shot with overly light arrows or you will be tracking for a long time. What should have been a dead-in-10-seconds deer will become a maybe-dead-in-several-hours one.

We all know the exit hole is more conducive to retrieving a deer than the entrance hole. Momentum is your best friend if you want exit holes nearly every time.

For those of you worried about the deer jumping the string because of slow arrows, let me assure you at 12 yards even 150 fps will get an arrow to the deer before they can react. Where we get into trouble with deer jumping the string because of “arrow noise” is when the distance gets beyond 20 yards. Remember, sound is traveling 1,085 fps, so even your super-fast 300 fps arrows are not going to beat a deer’s ears at longer distances. Movement and bow noises before the shot are the more likely culprits at the shorter distances.

Perhaps someone someday will be able to tell us how much momentum we need to cleanly kill any given game animal at any given distance. Until that day let me leave you with my unscientific advice that has served me well for whitetails. I’ll start with the premise that the bow setup I’m using leaves nothing to be desired. It is simple, lethal, has never let me down, and has killed 43 deer to date. Based on that, let’s just say we need our arrows’ total weight to be about 8 grains per pound of draw weight if our bow is in the 60-lb. range. If you are shooting a bow in the 70-lb. range, we can drop that back to 6 grains per pound of draw weight. And lastly and most importantly you might want to think about 10 grains of arrow weight per pound of draw weight if you get down in the 45- to 50-lb. range. Yes these heavier arrows out of these low-poundage bows will be slower, but faster arrows that wound are of little use to an ethical hunter. If that means the hunters with lower-poundage bows can only shoot out to 15 yards, then so be it. I have a feeling, however, that even these slower arrows could be sighted in for a couple of inches high at 10 yards and still be in an 8-inch vital zone at 20 yards. At least when these arrows hit the deer they have a decent and more lethal amount of momentum.

Let’s review my advice:
6 grains of arrow weight per pound of draw weight for bows in the 70-lb. range
8 grains of arrow weight per pound of draw weight for bows in the 60-lb. range
10 grains of arrow weight per pound of draw weight for bows in the 50-lb. range

In summary, set up your archery equipment understanding that momentum is more important than speed. If you need more speed, it should come from a higher poundage bow, not a lighter arrow.

by Bill Badgley
on June 18, 2013