Speed... Curiosity only

[Tracker0721]

So I got a chronograph to test my air rifle and build up some hunting data for different pellets and I decided to shoot my 55@27 Selfbow through it. With my 485+- Doug fir arrows I'm getting 162fps for an average. With my 770 grain ocean spray arrows(what I plan to hunt with) I'm getting 124fps. 88fps is 60mph so I figure my arrow with a sharp obsidian tip will kill something deader then dead for sure. Anyone else have some numbers to compare? This isn't a speed thing really, more of an efficiency thing.

[bow101]

Keep them light. Personally I dont care as long as I come close to the bulls eye and I enjoy Archery and the camaraderie for what its worth.   

Today I was shooting arrows with 125gr tips and new ones with 145gr tips seems like the 125gr were better Flying.  All shafts are 40# spine. And I  shoot them from 35# bow and a 43# bow.  I give up on trying to rationalize everything.  I found out years ago that been a perfectionaist drove me nuts and discovering that I had obsessive compulsive disorder behaviour to a certain degree. 
Enjoy the shoot forget the technicalities 

[bradsmith2010]

well i have alot of numbers going back 20 years or so,,,I think 30, wow it goes by fast(no pun intended)
your bow seems to have nice cast,, 162 fps with the 485 grain arrow is a good hunting combo,, fast enough to get to a deer before it jumps too far,, but heavy enough to do the job,, the slower arrow will work for sure,,,,
I just finished an osage bow,raw hide backed,,55 ntn ,50@25 inches 157fps with a 500 grain sitka spruce  arrow,, 12 strand fast flight with fur silencers,, it is nice cast for me with a hunting set up,, I plan to shoot 500 to 700 grain arrows to hunt deer,, I like the fast 500 grain arrow,, that being said I shot a deer last year with a 700 grain birch slow arrow,,,, so I guess what shoots best for you out of your bow is the way to go,,

[Del the cat]

You can compare the energy using 1/2 m times v squared... don't worry about units as long as you are consistent.
So the 485gn at 162 fps gives 6364170 units of energy
The 770gn at 124fps gives 5919760 units of energy which is less, so I'd say you are going a bit too heavy at 770 gn as you are loosing speed and energy.
Del

[Badger]

  I think your test was flawed, I would recheck. The 770 arrrow should have more energy instead of less. Maybe you didn't draw them the same. Bows are always more efficient with heavy arrows. The 770 arrow should have been closer to about 135 fps or so. I think you should select an arrow weight that gives you at least about 150 fps for easier aiming.

[JW_Halverson]

You can compare the energy using 1/2 m times v squared... don't worry about units as long as you are consistent.
So the 485gn at 162 fps gives 6364170 units of energy
The 770gn at 124fps gives 5919760 units of energy which is less, so I'd say you are going a bit too heavy at 770 gn as you are loosing speed and energy.
Del

Thanks for making it blatantly obvious which is the better choice!  You see, I was about to chime in and say to go with the heavier arrow since sheer weight leads to improved penetration, but that would have been wrong.  You made me stop and think about the thru-and thru I got with a mere puny 425 grain arrow in a large heavy bodied mule deer years ago. 

With the drop in arrow speed, you are looking at a lot more drop in point of contact, so the faster arrow means less futzing around with calculating or compensating for drop. 

Good luck with your setup!

[bradsmith2010]

Del that looks right on paper for sure, but the 700 grain arrow will give great results on game, and usually better penetration,, there is something in the calculation that does not account for the amount of energy that goes into the arrow,, the way I understand it ,, is that the heavier arrow is able to take more energy from the bow than the lighter arrow,, I am not good at math ,, so someone might be able to explain it better or in a more clear way,,,,but I do have alot of experience shooting hogs with different weight arrows,, and even though heavy might be slower,, they have had better penetration in my experience,, I do like the lighter arrow for deer,, as they do move at the sound of the arrow,, and I feel the lighter arrow gives me just a bit of an advantage,,  this debate had gone on forever,, and I just have my personal experience to go on,,    the deer I shot last year was with a 700 grain arrow through the top of the shoulder and came out the other side toward the bottom of the deer,, I was shooting a bow in the range of the 55# bow mentioned from above,, if a lighter arrow would have done better,, it would not have been noticeable at the range I was shooting,,

[Badger]

  Brad, I believe you are correct. Del is also correct but it appears to me that the arrow speed test was off. I have tested arrows from 100 grains all the way up to 4,000 grains. Heavier arrows always take more energy from the bow. Not as good fro trajectory but for penetration they will do better. I believe that he need to retest both arrows, one of them was a bad test.

[Mac43560]

Brad I get your point.  Sometimes lighter arrows don't seem to work as well.  But Del is right.  The velocity of the arrow directly relates to its energy.  The transfer from the bow is more about limb efficiency and spline but in the end it's 1/2 mv squared.  To your point, a stiffer arrow transfers that kinetic energy more to the prey of choice, via the pointy end, versus more vibration and flex.
Side note: I've met many a hunter, that partakes in variety of hunting methods, who gets drawn into the "heavier is always better" group from a misplaced idea about the takedown power of their projectile.  No offense intended to anyone but the velocity required to achieve this physical phenomenon is unattainable by a bow, crossbow,  and many a firearm.   
Congrats on 162 fps.   That will get the job done.  Happy hunting.

[bradsmith2010]

thank you Steve for explaining that so clearly,, 

[Del the cat]

Yes, I was only going by the figures given.
Here is a post where I increased the weight of a crossbow bolt and plotted the energy.
The heavier the bolt the higher the energy up to the point where you can only get out the energy you put in.
http://bowyersdiary.blogspot.co.uk/2011/11/mass-velocity-and-energy.html
There is a point on the graph where increasing weight doesn't make much increase in energy, but does start loosing speed. It shows there is an optimum trade off point.
No point having a quarter pound arrow for maximum energy if it is so slow it can't reach the deer!
I've attached the graph, Red line is energy. Blue line is speed.
You can see that at about 20 grams (308 grains) the the energy is pretty much up to full and heavier than that the speed drops off without much gain in energy. Note this is only a low power crossbow pistol 50# at short draw (about 6") so the optimum weight for this isn't the same as the optimum weight for the bow we were discussing.
On this graph, compare the arrow speed at the 2 weight we've been discussing (approx 30 grams and 50 grams) you'll see the energy is similar but the velocity is way down on the heavier arrow.
I'd guess that the speed figures in the original post are just a bit out (as Badger suggested) and that the two energies are probably similar. There is plenty of experimental error in my graph and that is with a crossbow, it shows how hard it is to get good accurate results!
Del

[Tracker0721]

I'll have to test again. My ocean sprays fletchings were a little too tall and I only shot 3 times. I think that and I was shooting from about 2-3 yards. I wonder if the obsidian tip on the OS played a part too since I had a steel point on the Doug fir shaft. So I guess I'll go back and cut the fletchings the same, paint my obsidian tip black and shooting all from a measured 6 feet. Didn't know we could go all high speed, mathematical, awesome on this. I'll get my chronograph and notepad! Haha

[joachimM]

Thanks for the explanation Derek. A picture says more than a thousand words (if you can read what's on the axes  )

[Del the cat]

I'll have to test again. My ocean sprays fletchings were a little too tall and I only shot 3 times. I think that and I was shooting from about 2-3 yards. I wonder if the obsidian tip on the OS played a part too since I had a steel point on the Doug fir shaft. So I guess I'll go back and cut the fletchings the same, paint my obsidian tip black and shooting all from a measured 6 feet. Didn't know we could go all high speed, mathematical, awesome on this. I'll get my chronograph and notepad! Haha

At 2 or 3 yards, the fletching and point shape won't have time to make much difference as their effect will be aerodynamic and it's not in the air very long... on a flight shot they will make a big difference.
Del

[LittleBen]

I think that 162fps is very respectable given the bow specs.

I find that my very best selfbows shoot just about (100 + draw weight) FPS at my 25" draw with an arrow of just under 500gr. So my best 50# selfbow I might expect to get just about 150fps with 480-500gr.

Lighter bows seem to do slightly better, for example my 36# selfbow shot 138fps, 51# bow shot 150fps, so this is just a rough guideline.

This probably happens because of the fact that the (100+draw weight) is a linear approximation of what would probably be a logarithmic curve.

Tri-laminate wood bows seem to do better. They tend to run just under about (100 +  draw weight (1.1-1.2))  or so.

So a 50# tr-lam I would expect to shoot just about 155-160 FPS in a best case.

My 62# tri-lam recurve shot 174fps, for example .... Slightly better than expected
My 46# tri-lam longbow shot 150fps ... Slightly lower than expected according to the formula.

These are just my results, with my draw and release. My main use of the chronograph is to
1: compare my bows to each other on the same day with the same arrows.
2: practice a perfectly consistent anchor point and release. If you can shoot repeatedly within 1 or 2 FPS, you are consistent.