Number Crunching Archery Specs

Started by gitano, November 20, 2023, 01:41:59 PM

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Since I will be hunting at sakorick's with my bow this year, I've had to do some prep work. "You" know me, there's gotta be some number crunching in that prep somehow.

First, since I hadn't even drawn my bow (Matthews Rebel Solo Cam), in a good 10 years, and the string and cable were at least 20 years old, I thought it prudent that the cable and string be replaced. Got that done at the local Sportsman's Warehouse. Also, I had 18 Easton Gamegetter XX75s that needed new fletching. Got that accomplished at the local archery shop. I thought the $3/arrow was reasonable. I also have 8 Beman 65/80s. Unfortunately, 'parts', (i e. nocks and outserts),  for those arrows are scarce. I'll have to make do with the 5 that are in good repair. Nevertheless, I thought it prudent to buy half-a-dozen new (graphite) arrows. $12 each with nocks, fletching, and outserts. The new-fangled arrows are pretty spiffy. Finally, I bought s new sight. Not sure I like it yet. While I do shoot a compound (due to a a bad spur on my acromium (shoulder)), I shoot it as 'bare' as I can. I REALLY don't like a lot of 'stuff' hanging off my bow. I learned to shoot 'instinctive' when I was 10, and that has always worked better for me. You shouldn't be 'thinking' about 'stuff' when you're shooting - rifle or bow. I'm using the the new sight with just one pin. That's the way I like "it". I'll adjust for range and windage based on my knowledge of my bow.

So, with new string and cable and new, as well as refurbished, 'old', arrows, I knew there was plenty of tweaking that would be required. First thing was getting the draw weight set. 20 years ago I shot at 68 lb draw weight. With my shoulder spur and age, I adjusted the draw weight to 55 lb. I can draw that without 'grunting'.

Next was getting the sight 'on'. For Alaska hunting, I set the pin to be 'on' at 40yd. Moose and caribou are big targets, AND, they're not nearly as jumpy as Missouri whitetails. Missouri whitetails are smaller targets AND prone to 'string jump'. Therefore, I'm setting the one pin for 'on' at 20yd.
It took some fiddling, but everything was dialed in pretty good before too long.

I'd been watching a bunch of YouTube videos, and it seemed like everyone was shooting 'heavy' arrows. Heavy points (125 grains or heavier),  at least. Years ago. When I was hunting regularly with a bow, I did a bunch of chronograph work. The conclusion I drew was very simple: The heavier the arrow, the slower it goes. PERIOD. There's nothing complicated about 'it'. I shot, and shoot, the lightest broadheads I can buy that meet the legal requirements. I wondered what the current tendency to heavy was about? Talking with the local shop owners about it suggested that the belief was that heavier arrows penetrated, especially bone, better. Hmm... Sounded 'good on paper', but inconsistent with my personal experience. Having shot moose, whitetail deer, blacktail deer, dall sheep, and black bear, ALL WITH COMPLETE PASSTHROUGHS, I was a bit skeptical about the NEED for heavy arrows for 'penetration'. Furthermore, in MOST of the YouTube videos I watched, penetration was VERY POOR! Often much less than half the arrow! Hmm... I'd have to look into this further.

I dug out me CED Millennium Infra Red chronograph, and got it set up in my basement . (It's cold outside in Alaska right now.) After some stutter steps, I shot all my arrow types with head weights from 65 to 145 grains. From that, I developed a model of arrow speed as a function of arrow weight. Same result as 30 years ago: The heavier the arrow the slower it goes. LINEARLY, AND VERY PREDICTABLY. BUT.. That's not the whole story. In fact, most archery hunters know that it isn't kinetic energy that kills in arrows - it's 'penetration'. Here's the rub: Most everyone was equating arrow KE with penetration. I was surprised. Furthermore, my arrows with the lightest legal heads, had the  highest KE! So why on earth would you shoot heavy broadheads/arrows?  :huh2:

So, as I have pointed out 'forever' here at THL with respect to bullet penetration, MOMENTUM is what determines penetration, NOT KE! So I calculated the momentum figures of the heaviest and the lightest arrows I shot through the chronograph. Hmm... Same figure. Then the lightbulb went on. There is a LAW of physics called The Law of Conservation of Momentum.  Basically, in the "system" that the bow represents, every arrow, regardless of mass (weight) will get the same momentum from the bow. I calculated the momentum for all the other arrow weights: They were identical.  ;D Had to be.

THEREFORE... Shooting heavier arrows in order to increase penetration is wrong-thinking. If you want to increase momentum (penetration) in a given bow, you have to increase the momentum of the 'system'. The only way to do that with a compound bow is to increase its draw weight. Arrow weight has NOTHING to do with it. Can't do it with a recurve/long bow. What you see is what you get. (There are minor tweaks with string length and draw length, but minor is the operative word.)

Finally, in the context of 'tweaking'/arrow speed/arrow spine/precision, I needed to calculate the balance of the arrows. Just like in bullets, and all projectiles, they travel less erratically if more than 50% of the weight is forward of the center of form. In arrows, this is called "Front of Center %", or FoC. It has been determined by the "experts" that in hunting arrows, an FoC of 10 - 15% is "best". For target shooters, an FoC of about 5% is 'best'. I understand the rationale, and agree - sort of. Anyway, I calculated the FoC of all my arrow types, and found that the new ones had an FoC of 12% with my 82-grain, 4-blade, Wasp broadheads.  ;D Works for me.

Now... ALL OF THAT SAID... I have almost NO confidence that I will even SEE a shootable deer this year, let alone get a decent shot at one. You'll be the second to know!

Be nicer than necessary.