Real world effect of 4 ft-lbs KE difference when hunting?
 

Last season a guy I know who is an experienced hunter but new to bow hunting just like me - and one of the very few people I allow to hunt my farm because I know he'll only take a broadside or quartering away shot - had a deer move on him at the last minute and wound up hitting the deer in the shoulder blade at around 35 yards or so and lost the deer. One of the local bow whisperers where he lives told us we should try to shoot the heaviest arrow possible within the limits of our equipment which might allow us to salvage a clean kill if everything went wrong at the last minute. And reading on the various forums last year it seamed the general consensus was that heavier is better, as long as you're still keeping up the speed.

So I ran some numbers based on a new bow purchase for next season, which will likely be a Bear Agenda, PSE DNA, an Obsession Evo, or something similar. Something hitting close to 350 IBO. Using one of the lightest arrows and one of the heaviest arrows from CE I show a 65/29/350 bow throwing the Mutiny Slasher @ 359gr at 321fp and 82 ft-lbs. The same bow shoots the Piledriver Hunter @ 535gr at 269fps and 86 ft-lbs.

I don't remember much from my physics classes in college so many years ago, but it seems a 4 ft-lb difference isn't much energy gained for the longer time in flight. In the real world, does it make a lot more difference than it seems to? Or am I not fully appreciating what 4 ft lbs really amounts to? Would the difference be worth the considerable speed loss?

Kinetic Energy isn't as meaningful as we'd all like it to be. It makes more sense for bows than does for rifles, because it corresponds more directly to our draw weight and length, but it doesn't mean as much in terms of terminal ballistics as you want.

Newton's law is about the conservation of MOMENTUM, not Kinetic Energy. MOMENTUM is where the difference is. (Momentum = mass x velocity, I use arrow weight in grns times velocity in ft/s, then divide by 7000grn per pound to get to lb ft/s).

359grns @ 321fps = 16.5lb ft/s Momentum
535grns @ 268fps = 20.6lb ft/s momentum

So that's about 25% more penetrating power on the heavier arrow than on the lighter, even though the Kinetic Energy is about the same.

In your scenerio IMO the 4 lb of KE is not much of a difference. That said however there is another factor to consider and that is momentum (.511 vs. .638). The heavier arrow will have more momentum and therefore will penetrate more. Now is this factor enough to make you switch to the heavier arrow by losing 52 fps?
Then too you have the fact that these IBO speeds are a manufacturing standard of shooting a 350 gr arrow at 70 lb draw weight and 30" draw length. A shorter DL or heavier arrow will NOT give you this rated speed.

Not only is momentum to be considered, but what type of broadhead being used. If it was a mechanical then it's common knowledge that a certain amount of he KE and momentum is used to penetrate the hide and deploy the blades. Depending on the draw weight of the bow there may have been insufficient energy left to get proper penetration when bone is encountered.

I have this problem in that due to limited physical abilities my setups usually produce less than 40# of KE. Because of this I have to be very selective with my shots and most often get complete pass throughs. I also use a replaceable blade head with a narrow 1 1/8" cut.

Printed ... saved.

Thanks VERY much guys! :cool2:

So I hearing two things:

- the difference in momentum IS significant, and might be worth the speed loss. I used my 29/65 setup and a 350 IBO since all the bows I'm looking at are around that, but may or may not make it based on tune, real IBO rating, etc. Would it be worth the loss in speed?

- a mechanical BH might not be the best choice with a heavier arrow.

Am I hearing correctly?

Momentum is what matters. KE is just what they publish. KE corresponds to a bow efficiency, since the potential energy you draw into the bow is what gets conserved as you release the bow. That's a different conversation entirely, but arrow KE vs. draw power curve speaks to bow efficiency.

Momentum is what penetrates. A .30-06 and a .45-70 can have roughly the same energy, but the momentum is considerably higher for the .45-70, which is why it earns a reputation as a heavier hitter. Similarly, heavier arrows, even with the same KE as a lighter arrow, will hit harder and penetrate better.

You have this backwards. Mechanical broadheads need a minimum degree of KE or momentum to deploy. A heavier arrow will deploy a mechanical broadhead better (read: more reliably) than a lighter arrow, and provide better penetration thereafter.

The opening action of a mech robs some momentum and KE from your arrow. When you're shooting a 40ft.lbs. bow, that stolen power might be significant enough to be detrimental to your arrows killing power. Where KE is the same, the added momentum of the heavier arrow will drive it open and penetrate better.

Limited KE broadheads often have smaller blades, so essentially the stolen deployment power is the same, but the arrow is able to penetrate better due to less blade drag in the wound channel. Overall, this is a negative effect as compared to a fixed blade broadhead, since that means less damage along the wound tract, but a small blade that exits is better than a big blade that doesn't in terms of tracking and blood loss (killing speed), so it's a bit of a compromise.

82-86ft.lbs. and 269fps is more than enough to deploy any mech on the market. In fact, you might find that it's even over powered for many mech models, so you may actually break certain broadheads even without hitting bone.

That's not to say that we should all be shooting 700grn arrows just to kill deer. I've killed plenty of deer with ~300grn arrows around 250fps (I can't recall what my recurve puts out, but I'm pretty confident that it's less than that!). They're no more dead these days when I put 450grn arrows through them at 330fps. But I CAN kill them a lot further out there than I used to be able, with a higher rate of recovery.

I've set up some 750-1100grn arrows, tried them on deer, but ultimately, it's a mess. Way too slow with way too much drop to be practical, and my hunting pack got really heavy hauling around a handy-man jack to pull them out of the ground after they exited the deer...

But with a 350+ IBO bow at 29/65, tuned properly, this isn't a concern, correct?

Shouldn't be a problem at that level. Mechanicals typically get in trouble under 50-60ft.lbs. hence the creation of the 40ke rages. Your bow, in general should be in the 84ft.lb. ballpark, as you noted with your two arrows. Fun math, bows are typically somewhere in the 45-55% ballpark of ideal case: 29" / 12"per foot x 65lb draw x 53% = ~84ft.lbs. If you change your draw weight or length, you can use that offset to approximate what you should expect on your arrow, and track where your bow gets more or less efficient.

And again, the heavier the arrow, the better the deployment, regardless of what bow it comes out of.