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Is a 40 lb bow REALLY enough for bowhunting?


If you’re new to archery, or getting back into it after many years (like me), trying to pick a bow for hunting with can be a bit overwhelming with so many excellent options on the market.  As I’m gearing up for my first bow hunting season, I have a lot of questions.

One question that has been in the back of my mind regularly is this: can you ethically harvest a deer with 40 lb bow?  Many jurisdictions specify in their hunting regulations that the minimum draw weight to hunt with is 40 lbs, but this really enough?

When I’m hunting big game, I want to know for certain that I have the right tool to be able to ethically harvest the animal and quickly.  So I did a little research.

A 40 lb is, in fact, adequate for bowhunting and can provide enough energy to drive the arrow through the vital organs, so long as you can place your shot in the vital zone under stress.  It’s best to try and use heavier arrows since they will carry more momentum to cut through bone and achieve better penetration. 

Try 500 gr arrows with your 40 lb bow.  That means it is necessary to practice with the heavier arrows and to simulate stress so that, when the adrenaline is pumping, you don’t miss your shot. 

But what are the best hunting arrows? I went and found out what the top Professional Archers are using when they go bowhuning.  I recommending checking out this article to find out what are the Top 4 Best Hunting Arrows Used by the Pros.

Now, let’s dive in to our topic.

Kinetic Energy: What Drives the Arrow

This is reaching back a bit for me, but let’s review some basic physics principles.  Kinetic energy is the energy that moving things have.  In order to start moving, they had to get that energy from somewhere.

When your arrow is sitting on the table, it’s not moving and, therefore, has no kinetic energy.  If the arrow stays motionless, it’s pretty harmless (unless you cut yourself on the sharp broadhead, but minor details); something needs to get the arrow moving for it to be effective.

Kinetic energy is measured in units of foot-pounds (ft-lbs) and can be calculated using the formula:

KE = m x v2/450,240

where,

m is the mass of the arrow (shaft + broadhead) in grains (gr)

v is the velocity of the arrow in feet per second (fps)

KE is kinetic energy in foot-pounds (ft-lbs)

450,240 is the conversion factor for grains to pounds

For example,

For an arrow with a mass of 350 gr travelling at 350 fps, kinetic energy would be:

K = 350 x 3502/450,240

K = 95.22 ft-lbs

Great!

So what does this mean for us?

It means that in order to get the arrow moving fast enough with enough kinetic energy in the right direction, pointy end first, you need something to supply the energy.

You need a bow.

Energy Transfer: How the Arrow Does Damage

All bows, whether recurve or compound, are rated according to draw weight and is measured in pounds (lbs).  The draw weight is basically, “How strong do I need to be in order to pull this thing back?”

For a recurve bow, it’s measured by pulling back to 28 inches.  So for a 40 lb bow, it means that it takes 40 lbs of force to pull back the string 28 inches for a recurve.

But how does 40 lbs of draw weight translate into kinetic energy for the arrow?

Energy transfer.

When you’ve got the string pulled back to your maximum draw length, that means there is 40 lbs worth of potential energy being held in the string.  The nock attaches to the string and when you release the string, those 40 lbs are set free.

The string launches the arrow forward and transfers the energy from the string to the arrow, which is now travelling at around 240+ fps.

This is where the broadhead comes in.

So our arrow is clipping along at 240 fps.  Let’s say you’re shooting a heavier arrow that weighs 500 gr.  That means it has about 64 ft-lbs of kinetic energy when it leaves the bow.  During the flight, some of that energy will be lost to wind resistance.

When the arrow makes the impact with the deer, all of the remaining energy is transferred to the razor sharp edges of the broadhead.  All of the force behind the arrow is applied over the tiny, tiny area of the razor edge and the arrow penetrates the hide of the deer.

The arrow is also sinning in flight so the rotating blades do massive damage when penetrating the hide, flesh and vital organs.

But what if something gets in the way, like bones?

Momentum: What Makes the Arrow Unstoppable

Ideally, you don’t want to hit any bones when you make your shot since that will decrease arrow penetration.  The danger is that if the arrow does not penetrate far enough, it may not cause enough damage to successfully harvest a deer.  You could be in for a long day tracking the animal.

So what’s the solution?

Heavier arrows.

Here we need to talk about momentum (and here we mean linear momentum).  Momentum can be thought of as how hard is it to stop the thing.  It can be calculated using the following formula:

p = m x v/225,120

where,

m is the mass of the arrow (shaft + broadhead) in grains (gr)

v is the velocity of the arrow in feet per second (fps)

p is momentum in slug-feet per second (slug-ft/s)

225,120 is the conversion factor to convert grains to pounds

So how do lighter arrows compare against heavier arrows when it comes to hard-to-stop-ness?  Let’s do an example.

If we take an arrow that weighs 250 gr with a velocity of 300 fps we get:

Kinetic Energy = 50 ft-lbs

Momentum = 0.33 slug-ft/s

If we take a heavier arrow that weighs 500 gr with a slower velocity of 200 fps we get:

Kinetic Energy = 44.4 ft-lbs

Momentum = 0.44 slug-ft/s

We can see from the example that the heavier arrow while travelling slower, has more ability to break through ribs and other obstructions.  What that means is that there will also be a greater arch to the arrow flight path and this requires practice to achieve consistent accuracy and precision.

Put the Arrow on Target WHEN It Matters

So you’ve got your 40 lb bow with some 500 gr arrows.  You’ve done the math (because why not?) and you know your setup can deliver the required energy and momentum to drop trophy buck you’ve been dreaming of.

You’ve also spent some time in some casual target practice and you’re pretty confident you can hit accurately out to 50 yards.  You head out on your hunt.

You’ve been waiting….and waiting….and waiting…all day.  There have been signs of him everywhere, but he’s clever.  You have to wait longer.

And then there he is.

The perfect trophy buck.

Your heart rate rises when he makes his closer to your stand.  You think, “I’ll let him come closer”.

Your heart rate rises more and you begin to sweat a little bit as he gets closer and closer.

You notice your vision gets a little hazy; it’s hard to focus.

He’s in range!

You draw and notice your hands are shaking!  This isn’t what you practiced.  No time; take the shot!

Miss.

And he’s gone.

Welcome to the Adrenaline Dump.

If you’re not prepared to deal with the excitement that comes with finally spotting your trophy buck, you are risking a miss or injuring and losing him.  So how do you prepare?

Try this:

Using the same weight field arrows as your hunting arrows, after you set up your target for some practice, leave your bow and arrows and all your gear on the ground and walk about 25 to 50 yards away.

Give yourself 1 minute on a stopwatch and then sprint to your bow.  This will get your heart rate up.

For added difficulty, do 10 push-ups when you get there (or however many you can comfortably do).  Gear up as quick as you can and take 1 accurate shot.

The goal is to get 1 accurate shot off in 1 minute with your heart rate jacked up and being able to control your breathing.

What about tunnel vision?

When you get an adrenaline dump in a stressful situation, your vision can tunnel to about the area of a tennis ball held at arm’s length.  With this in mind, practice looking around and limiting yourself to that tennis ball sized area.

Hint:

If you use a sight, get used to only looking through the circular area to scan the area and find your target.

So that’s just one example of how you might train yourself to deal with adrenaline.  The basic principle is to increase your heart rate, control your breathing and get used to aiming with shaky hands and limited vision.

Peter Sontrop

I live in Alberta, Canada where I enjoy field archery and target shooting with traditional bows and compound bows. On this site, I share everything I’ve learned about archery along the way.

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