Question for you physics experts.

[Jock Whisky]

I'm putting up a backstop behind one of our new targets on the club range. It's to stop arrows that might reach a road if the shooter happens to miss high. The material is "drier felt" from a paper mill. It's a plastic mesh, very tough. I'm going to hang two big pieces of it, one behind the other. While I was enjoying a cold beer and contemplating how to hang it a thought occurred to me. Now since this doesn't happen very often I figgered it must be important.

 So here's my question. I can hang the material 4 inches apart or 12 inches apart. At 12 inches apart the arrow will be moving slower than it is at 4 inches apart because of friction. So at 12 inches it will have less energy/momentum (take yer pick) than at 4 inches. This should result in less penetration in the second layer and a better chance of stopping the arrow rather than just slowing it down.

For me this is what I call a 2 beer problem. But I only had one beer in the fridge and I'm too tired to go to the beer store and buy another 6 pak. Now I may be overthinking this but be that as it may, it'll nag me like like a wife because I'm just plain interested. So this is where you come in. That's my theory. Is it valid?

[McDave]

I’m sure you’re right. 12” separation will stop more arrows than 4”. There’s a world of difference, however, between trad and compound. We’ve found that compound arrows will blast right through a lot of things that will stop trad arrows. So if there will be compound arrows, you might want to test your backstop with them before you get too comfortable with it.

[Jock Whisky]

Good idea McDave. Our main problem is compounds. There are one or two people that like to shoot 80 lb +. I'll see if I can arrange a test.

[Trenton G.]

I have 3 squares of carpet behind my target spaced about 4 inches apart. We shot my dad's compound at it and it stopped them fine. You definitely want them to hang loosely so that each layer absorbs as much energy as possible.

[Al Dente]

Space definitely makes a difference.  Carpet sure does the trick.  I would also think that a welding blanket, since it is an aramid fiber, similar to Kevlar might work as well.
Many years ago, when I shot a compound, I tested how powerful it was.  I had about 6" of penetration in 1/2" plywood and another on through a heavy milk crate.  Albeit, I was testing with broadheads, but there is no comparison between compound and trad in my opinion.

[Burnsie]

I saw a similar concept shown on YouTube about bullets, but same theory holds true for arrows.
The had something like 8-9 metal trash can lids nested tightly together - bullet blazed right through all of them.  Then they hung the metal lids about a foot apart in row. When they shot this time the bullet it made through the first 4-5 lids and then ran out of gas. Part of it was due to the bullet expanding and creating more surface area is it passed through the lids - but it was a pretty interesting demonstration.

[Even]

Not a physics expert here, by any means, but if you do some (hard to find!) physics reading on primitive, slow speed projectiles, the wider spacing makes sense. 

Most of the stuff I've found over the years (aside from the Ashby broadhead stuff) has been in relation to archaeology studies on primitive hunting effectiveness.

Lots of terminal ballistics stuff out there for high speed projectiles, but for various reasons (hydrostatic shock, mushrooming, etc) a bunch of it doesn't apply too well to slow speed projectiles.

One of the common themes I've seen on the slow speed stuff, has been a big increase in penetration if, at the joint between cutting/piercing head, the shaft diameter decreases.

Shaft friction, in arrow, spear, atlatl, etc, is a HUGE source of friction if it drags on tissue, reducing velocity.

I'd think that the longer the dryer felt hung onto the arrow shaft, dragging along it, before it hit the next layer, the more the velocity would be pulled downwards, leading to better stoppage on the second layer. 

Kinda explains too, how the stone point guys get such good results.  A stone point has more diameter at the mount, and pushes the tissue wide, so the shaft drags less, giving more penetration than you would think.

[BAK]

I like your thinking...