Increase Momentum for More Pass-Throughs

You want more pass-throughs? Then stop focusing solely on the speed of your bow setup.

Bow manufacturers have been touting fast-shooting bows for the last few decades. As a result, consumers might be equating speed with penetration power. So they try to squeeze out every additional foot-per-second from their bow setupsan they choose lighter arrows. I am not a physicist, I have no studies to back me up on this, but I am a bowhunter. I talk to a lot of other bowhunters. My collection of anecdotal evidence leads me to believe there are too many non-lethal or less humane shots because of fast and light arrows.  

Now, before you shoot me down, please read the rest of the article, because I am not saying arrow speed in unimportant. I am not saying that a light arrow will not kill a deer if placed where it need to be. I am saying there is a sweet-spot on the spectrums of kinetic energy (almost only dependent on velocity/speed) and momentum (dependent on both mass and speed). Finding that sweet spot is what will result in more pass-throughs. And you want to your arrow to pass through an animal because an exit hole will help kill it faster and make it easier to find.  So let’s take a look at some calculations.

The Relationship Between Arrow Weight and Speed

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I will use my bow setup as an example to figure out how to find the sweet spot of momentum and kinetic energy outputs.

Step 1. Find out how fast your arrow is shooting

You can use a chronograph, compare arrow drop at different distances or use this online FPS calculator to determine this figure. With my bow set at 62 pounds of draw weight and 28.5 inches of draw length, it shoots my 445-grain arrow at 259 fps.

Step 2. Find the kinetic energy and momentum of your setup. 

Use this handy kinetic energy and momentum calculator.

My results:

Kinetic Energy: 66.27 FOOT-POUNDS

Momentum: 0.512 SLUGS

Step 3. Play Around with the Figures.

Now let’s say, instead of using a 100-grain Wasp Drone, I thread on a 150-grain broadhead like the SharpShooter Traditional (this brings my arrow weight to 495 grains, but my arrow speed to 243 fps):

Results:

Kinetic Energy: 64.89 FOOT-POUNDS

Momentum: 0.534 SLUGS

By adding a heavier broadhead, I lost 1.38 (2.1%) foot-pounds of kinetic energy and gained .0022 (4.3%) slugs of momentum.

For fun, let’s say I added a 50 more grains with a brash insert (now my arrows weighs 545 grains and flies 226 fps):

Results:

Kinetic Energy: 61.8 FOOT-POUNDS

Momentum: 0.547 SLUGS

I lost 3.09(4.8%) foot-pounds of kinetic energy and gained .0013 (2.3%) slugs of momentum. I do not like giving up that much kinetic energy for the little gain of momentum.

Now, let say I went to the other way and picked a lighter arrow. Instead of my 300-spined Easton Axis, I go to the 500-spine. My arrow now weighs 372 grains, and flies 283 fps.

Results:

Kinetic Energy: 66.14 FOOT-POUNDS

Momentum: 0.467 SLUGS

By going to a lighter arrow, I gained 24 fps (9.3%), lost 0.13 (0.2%) foot-pounds of kinetic energy and lost .045 (8.8%) slugs of momentum. If speed were the only factor I cared about and did not research the other factors, I would use this setup – to the detriment of momentum.

Step 4: Hit the Range

Once you have determined the sweet spot on paper, it’s time to sight-in with your new arrow and broadhead setup. A heavier arrow is going to fly slower and not as “flat," but they do have some advantages in addition to increased momentum. They are quieter, less prone to wind drift and more forgiving.

Next week, we will share a video with you that shows how 150-grain broadhead will affect arrow flight and penetration when compared to a 100 grain broadhead shot from the same. Stay tuned, we learned some interesting stuff!

My Unscientific Opinion

It’s a fact that momentum sheds as an arrow travels. It diminishes much faster for light arrows than it does for heavy ones. It’s my opinion that bowhunters see the “pros” on TV take 60-, 70-, 80-yard shots and think they can to. Truth is, you can, but only if you have the right rig. Most don’t, and they realize it – setting their limit at 30 yards. This is bowhunting after all. The point is to get close, and when you are within 30 yards of your target, a heavier arrow at a slower speed will produce cleaner pass-throughs than a lighter arrow at a higher speed.