Arrow Penetration Factors

[konrad]

I felt compelled to compose this missive this after receiving a weekly “tech tip” from an archery only web-site containing the prominent statement:

 “Shooting a heavier arrow (shaft + tip) will not necessarily give you more penetration (kinetic energy) over a lighter arrow. Check it for yourself ... take your bow and shoot a heavy arrow and a light arrow through a chronograph, then calculate the kinetic energy. I bet the values aren't much different.”
 
In deference to those espousing (and being led to believe) such an all encompassing statement related to a complex sub-set (archery) in the science of ballistics, I would submit that so much sales hype circulates in the archery market touting arrow speed and really very little discussion time is spent on what I have come to believe is the more critical topic of reliably, humanly dispatching game. After all, the vast majority of shekels spent in the archery industry today are dedicated to deer hunting. Shouldn’t that really be the central issue and not just lauding the ability to launch arrows to 60 yards and beyond? Being able to hit a target at range accurately is important; however, if the pursuit revolves around the ethical killing of game, hitting accurately and killing reliably are not necessarily one and the same topic.

Please bear with me if you’ve heard all this before.

An arrow’s penetration (or lack thereof) is based on a number of variables.
The current rage in archery advertisement centers on velocity. From a simple calculation using an arrow’s weight and it’s velocity at impact, a number, kinetic energy (k.e.) as commonly measured in foot pounds, can be obtained. Basing conclusions about an arrow’s ability to penetrate solely upon velocity, k.e. or a combination of both can be misleading and even cruel. If one were to base their decision on using a particular arrow set up based solely upon k.e., he is missing large parts of the arrow penetration equation.

Killing with an arrow has little to do with actual energy transferred to the target (as is commonly found in the firearms industry). If one stops to think about it, were the arrow to actually drop all of its retained energy into the target at point of impact, the arrow would not penetrate at all (as in the case of a small game “blunt” type arrow). An arrow actually performs better when less of its retained energy is shed in the target and penetration is affected by the slicing action provided by a well designed broadhead that maintains its shape and sharpness while performing its duties of destroying the vital circulatory and/or respiratory systems.

Two of the most overlooked factors in arrow choice for maximizing penetration are inertia and friction imparted to the arrow combination as it passes through a given target.. Inertia is a body’s propensity to maintain speed and direction based primarily upon its mass. Yes, it takes more energy to start a massive projectile moving but it also requires more effort to change its direction or to stop that travel. In other words: We absolutely want that broadhead to penetrate as far as possible into and beyond (if possible) the target. The arrow’s “impact” has little or nothing to do with its intended purpose.

More massive projectiles may or may not strike with more kinetic energy but they tend to retain more of their initial velocity while in flight and during encounters with the target.

Friction related penetration is also affected by:
1.   Diameter of the shaft (smaller shaft diameter equals less surface area drag and resistance while inside the target thus greater penetration)
2.   Number and angle of cutting edges of broadhead (i.e. a more gradual angle to the cutting edge provides less resistance and better penetration, two cutting edges require less energy to penetrate than three edges, three edges require less than four edges)
3.   Smooth transition from broadhead ferrule to arrow shaft promotes penetration (a larger ferrule with smaller O. D. shaft penetrates better than same sized shaft and ferrule while a larger shaft than ferrule penetrates more poorly than either of the above)
4.   Stiffer shaft penetrates better than a more flexible shaft (less energy is consumed in shaft flexing and increased lateral friction after impact and more energy is retained for  forward movement)

Of course, taking any one of the above factors outlined above to an extreme (i.e. very heavy shaft but too slow to even find the target) does our goal of successfully harvesting game no good.

My admonition to everyone is to be aware of the decisions you are making when choosing an arrow/bow combination. Be careful to filter the advertising propaganda possibly spread by the counter salesperson and television/magazine/catalog ads from the physics of what you actually need to do. There is no free lunch and the current focus on speed and kinetic energy as the end-all be-all to our archery problems is short sighted. While desirable for the 3-D course, that intense commercial focus on speed alone has actually produced a dramatically increased level of struck and wounded game escaping because of inadequate penetration. Merely being able to hit our target is insufficient if we can not reliably bring our intended quarry to ground in an ethical and humanely quick fashion.

Not to put too fine a point on it (all puns aside) but the broadhead needs to make it all the way through those vitals and hopefully out the other side producing a wound channel that allows maximum loss of vital fluids and/or gasses. On deer that can be reliably done at 200 feet per second at 20 yards with a sharp broadhead and a reasonably stiff, heavy arrow every time the mark is hit.

Hitting a foam target is not the same as hitting one of God’s creatures.

I hope this gives some food for thought.

[Mudman]

Good speed, good arrow weight and good equipment= quick death.  Good read.

[Sunbkpk]

Good post.

Add also good arrow flight from a well tuned setup. If the arrow hits straight on it will penetrate better than if fishtailing.

I pull 10lbs under what I can comfortably draw to allow for accuracy, good form and an easily tuned setup. That allowed for a clean pass through on a large cow elk this year. The arrow hit 30 yards beyond her and she went less than 10 yards before dropping. Got to love when the arrow hits the exact spot you picked.

[jechicdr]

The reason bows are shooting faster, is that they are more efficient at transferring their stored energy to the arrow.  Easy to measure and advertise velocity (faster must be better), but takes calculations to determine momentum, kinetic energy.  It is probably true that energy has a more direct relationship to killing power with a bullet than it does with an arrow, but energy is still required to penetrate an animal.

I agree with your points though.  If you take a 45 lb bow, shoot a narrow diameter arrow, a gentle sloping cut on contact 2 blade broadhead, you can probably out penetrate a larger diameter arrow with a trochar tipped steep angled 4 blade broadhead shot from a 70 lb bow.   Of course the latter setup would probably create a pass through on most animals you shot at, so any "more" penetration might be overkill.  I kind of like overkill though.  I shoot a Mathews Z7 (a pretty fast bow) @ 70 lbs with a thin diameter Beman MFX (~10 gpi), with a 150 grain cut on contact Muzzy Phantom (OK 4 blades).  Forgot to mention FOC improving penetration.  My setup had complete pass through on a double shoulder, another pass through on second shot quartering away through opposite shoulder on my elk this year.  Sad to say though...never found those two arrows.  I guess they still had enough energy after pass through to bury themselves in the ground somewhere.  So, it might be overkill....but it's still a kill.

[Jonathan_S]

Hello Everyone,
I'm no longer just a lurker on HuntWa. 

With this thread's information in mind, what should someone bow-shopping for the first time be paying close attention to when visiting archery-specialy shops? 

Should I just buy the bow that feels right and then tailor the arrow/broadhead combo to it afterwards?

A fishtailing arrow is caused by overpowering a light arrow right?

I care more about accuracy than FPS, there is hardly any difference in speed between a 60lb and a 70lb bow anyway.  I'm pretty intimidated about buying a compound because I know so little.  I have shot recurve (target only) for years but compound feels like a big change.

[SGTDuffman]

All of that is why I shoot 620 grns. Everyone makes their own choice. My choice is a "heavy" arrow. I say "heavy" because compared to what was shot in the past, 620 gr isn't that heavy at all. I also notice my bow is much quieter than my Dad or brother's bows, and converts a lot more of it's energy more efficiently with heavy arrows. Just for kicks I turned it up from 60lbs to 65lbs, and shot some 390 gr arrows out of it over the chrono. In the case of lighter arrows, the 5 lbs got me 5 fps, for the "heavy" arrows I got a 10fps increase. Shot placement trumps either weight, but I'm of the opinion that light arrows are geared to a perfect shot in perfect conditions, and I don't care about that. Any arrow will work in perfect conditions. I gear my stuff towards minimizing unknown variables. Last minute movements, jumping the string, wind, etc. Anything that may lead to a less than perfect shot. Then your fast and light stuff is in a heap of trouble.

Seems like some people pick lightweight stuff so they can get away with ranging errors too, as if it is a replacement for practice, but to each his own. Too many people worry about speed. You'll never see anything close to your bows advertised speed. The speeds they give you are based on shooting a 350 grain arrow out of a 70lb bow at 30", with nothing on the string, except the arrow. No sights, string silencers, nock point, etc. all of which slows your string and your bow down. So don't worry about the speed. As long as you can set the sight to shoot the arrows you want to shoot, it's fine.

[HUNTINCOUPLE]

 

[konrad]

Hello Everyone,
I'm no longer just a lurker on HuntWa. 

With this thread's information in mind, what should someone bow-shopping for the first time be paying close attention to when visiting archery-specialy shops? 

Should I just buy the bow that feels right and then tailor the arrow/broadhead combo to it afterwards?

A fishtailing arrow is caused by overpowering a light arrow right?

I care more about accuracy than FPS, there is hardly any difference in speed between a 60lb and a 70lb bow anyway.  I'm pretty intimidated about buying a compound because I know so little.  I have shot recurve (target only) for years but compound feels like a big change.

Hello Jonathan,

Welcome aboard!

The following is a list of what I have come to believe is the most critical points to follow when a beginning archer buys his or her first bow. The first is probably the most important closely followed by the second.

1.   Whatever model or brand of bow you decide upon, buy a bow set up for at least ten pounds below what you are shooting at the shop during demonstration. While there, you will be experiencing a slight adrenaline “high” and a draw that seems “easy” there will become burdensome at home. You may even hurt yourself out of a desire to practice too much with a bow of too heavy a draw. Not only will the physical hurdles of a heavy bow be a conditioning hindrance but dealing with a heavy draw will most assuredly promote bad shooting techniques such as short drawing, collapse, or trigger punching (all to be avoided like the plague).
2.   Be sure to be patient when doing your set up of draw length. Invariably, whatever you set it at initially will too long by a half an inch and will produce additional accuracy/technique problems that are easily corrected with draw length.
3.   Don’t buy the most expensive arrows on the shelf. You will be loosing them and hopefully destroying them with your fantastic accuracy within weeks. Unless you have a long range to practice at or multiple targets, you will break those nock ends. In this same vein, I would be cautious of buying a top-of-the-line expensive bow as you will most certainly develop specific tastes about what you like and dislike. Conversely, purchasing top line sights, release and other accouterments is money well spent. You can always move good sights to a new bow and your release will work there as well.
4.   Resist the temptation to buy a bow in a fit of passion. Take you time. Shoot as many brands and models as possible. Just try to make sure they are all set to the same weight and your draw length. If the retailer can not set his demos up for your specs, he will probably not be patient in setting up your bow after you have committed with hard earned shekels.
5.   Get some instruction from a person who knows what they are doing and who can explain how to do it. Pay additional shekels if need be for lessons. The coin will be well a spent investment. Ask about lessons BEFORE you buy. It is a great incentive for a retailer to do a good job on set up if he knows he will be responsible for your success on his range.

Fish-tailing can be caused by improper rest position.
Porpoising can be caused by incorrect nocking point.
Fish-tailing can be caused by under-spined shafting (all of the above the same as in traditional shooting).

Hope this helps.
K

[Snapshot]

There is no free lunch and the current focus on speed and kinetic energy as the end-all be-all to our archery problems is short sighted.

[Jonathan_S]

Konrad, thanks a bunch.  I appreciate your willingness to lend advice to me.

I'm thinking of 60 lb as my max.  I had no trouble pulling 80 lb from a sitting, standing or kneeling position but I like the idea of being well below my threshold, as it were.

I've shot a 57 lb recurve since I was 13 so I think I'll be within my limits with 60.

I went to an archery pro shop last night (not Cabela's)    and learned quite a bit.

[Maybee-R]

Muscle tissue and target material act very different in penetration.
I agree with most points in the thread. But, I also believe there is a point when too much weight takes you down the less efficient road...

[DoubleJ]

Don't be afraid to go lower than you'd think you'd like to.  I shoot a Bear Ledgend set at 49 lbs and do fine with it.  My top pin is at the top of the sight at 20 yards and my bottom pin is at the bottom at 60 yards.  I would never take a shot over 50 anyway.  Been told too many times that a yard per pound of draw weight is about right with the average bow.  I've never taken a shot at an animal over 20 yards.  One of the things I love about bow hunting is how close I get to the animal

[mountainman]

Will addthe importance and differences in broadhead design. Test done years ago (20 maybe) by Emery Loiselle. Tested I believe a Barrie Rocky Mountain Razor (conical tip), a nap Thunderhead (trocar tip) and a Zwickey. Tested how much pressure needed to completely penetrate a peice of tanned elk hide. The Barrie razor needed like 30foot #'S, the Thunderhead low 20's, the zwickey under 5#. So imortance of matching bow poundage/speed/kinetic ALONG with the right broadhead. Wouldnt want a Barrie razor out of a 40# recurve or a zwickey in a 350fps set up either.