Grains/lb and bow efficiency

[Woolenclad]

Hello Tradgangers.

I was listening to an interview with Dr. Ed Ashby the other day. He was suggesting that 10 grains per pound is the sweet spot for peak efficiency of a selfbow (measured in kinetic energy, I believe), and this is where the old 10g/lb rule of thumb comes from. If my memory serves me correctly, and I don’t mean to misrepresent anything he said, he went on to say that peak efficiency in modern laminated bows occurs with a much heavier arrow (12, 13, or more g/lb maybe). I am curious, has anyone with a chronograph done the math for peak efficiency of laminated glass recurves and longbows?

Also, I am curious about the opinions of some seasoned hunters and shooters. I am experimenting with my set up this coming year and will be trying some variations on the very heavy end of the spectrum (unfortunately I don’t have a chronograph or access to enough variations in equipment to do the math for peak efficiency, thus my experimenting will only yield subjective results). How heavy is too heavy for hunting (for argument sake lets say shots out to 20 yards, although I never end up shooting anywhere near that far)? If I had to choose trajectory vs penetration, I would choose penetration every time as long as I could get my arrow to the target effectively.

Thanks for your thoughts and happy shooting!

[tzolk]

There was some chatter about this very same topic within the last two months on two different media sites. The peak efficiency noted, I believe was 9.5 gpp. 10 gpp was the point where beyond that, you would start loosing return value from your bow. The calculations took into account velocity, energy and momentum. Technically speaking I suppose.

[ChuckC]

I'm not certain you can blanketly say any of that.  Depends on the type of bow and its design as well as its construction.

[Biathlonman]

At one time I read that it was somewhere around 13-14.  Been a few years though, I don't remember the source.

[monterey]

I don't understand the definition of "maximum efficiency".

[Stumpkiller]

Chronograph will only tell you speed . . . always faster with a lighter arrow (that's physics!)

you'd have to then calculate which combination produced the most energy (mass AND speed = energy)

E = 1/2mv^2  

1/2 * mass * velocity squared will give you the knetic energy of the arrow at the chronograph.  In Joules.

So, to get it into 'Merican units . . .

E in ft-lbs = (weight in grains * velocity in f/s * velocity in f/s) all divided by 450,435.

{Velocity is squared}

BUT - the arrow you shoot best may not be the most efficient in absolute energy.

[Woolenclad]

Monterey,

Maximum efficiency, to be best of my understanding, is the point at which a specific bow provides the most of its energy to the arrow.

Here is an example with some made up numbers:
Grains/Speed/KE
300/210/29.37
350/200/31.8
400/190/32.06
450/180/32.37
500/170/32.08
550/160/31.26

The idea is to find the point where you maximize the potential of the bow, as arrow weight and velocity are a trade-off.  As you add weight, you put more of the power of the bow into the arrow, but only to a point, after which adding weight reduces speed so much that the energy carried by the arrow starts to drop off again. In my fictional example this would be around a 450 grain arrow at 180fps with 32.37 ft lbs.

I hope this is a reasonable explanation... I am not always the best at trying to explain stuff.

ChuckC is correct, this would be a calculation applicable only to one specific bow. Each bow would need its own calculation. I was just curious if anyone had any numbers for their glass bows.

[Stumpkiller]

I do not.  Don't own a chronograph.

I'm shooting about a 600 to 620 gr arrow from 47 to 58# bows.

No idea if it's near efficient; but I won't go lower than the 125 gr heads I use and I like to hope I'm getting the best speed I can with my Douglas fir shafts.    

[Friend]

A subject extensively broached countless times yet will continue to be brought again and again.

Resist the propensity to grasp merely what you wisht to hear, thus promoting real growth.

Remember to bring your GPS and compass as back up, as you attempt to navigate thru this wilderness.

[JR Williams]

For me it  usually ends up with the heaviest arrow that I am comfortable with the trajectory. My personal preference in most of my hunting bows is around 11 GPP.  This is usually roughly 600 grain arrows out of my 55 lb hunting bow.

Works well for me.

I like to shoot enough weight so the bow does not feel like it being dry fired.  That is less than ten grains per pound on my bows with my draw length and I still get pass through penetration on deer.  Now, if I was trying to kill a cape buffalo, I might change a few things.

[Mike Bolin]

For several years I went with 12.5 gpp (620 gr total arrow weight) and had good success using a 3 blade broadhead on deer, caribou and hogs. Due to some shoulder issues I dropped 5# in bow weight and my old arrow set up was a bit too stiff.
Being thrifty by nature, I added more weight up front until the flight straightened out and ended up with nearly 14 gpp. Arrow flight was good but the trajectory was, lets just say I put the arch back in my archery.
I dropped down to the next lowest spine carbon and with the same 250 grain heads and a standard aluminum insert ended up with 510 grains. These arrows bare shafted near perfect. Trajectory was MUCH flatter and the 2 deer I killed this year were complete pass throughs.
Not 100% sure how this set up will work on bigger game, but shooting into a new foam block with both arrow set ups using broadheads, both arrows penetrate within a 1/2" of each other with the lighter arrow having a slight advantage. Not at all what I expected. At this point I feel like have come up with a very efficient set up for this bow.

[Rob DiStefano]

killing critters very small to very large with a coupla sticks and a string doesn't need to be rocket science.  

i suggest putting aside the calculators and chronographs, and first and foremost find out what arrow type and weight both yer bow and you likes best.  a well flying arrow is half the battle for killing game. it will matter far less if it's 8gpp or 12gpp, or if it's FOC is 9% or 30%. trust me.

make sure that arrow's front end will be commensurate for the type of game you wish to kill.  unless it's a small game blunt, it should be a cut-on-contact super sharp broadhead.  a c-o-c broadhead can be 2, 3 or 4 bladed - don't much matter which one, pick whatever makes you happiest.  single bevel or double bevel 2-blade?  whatever, just pick one.  if the blade's aren't razor sharp, keep at it 'til they are.  the arrow's front end is the part of yer tackle that does the deed after you do your part.  make sure it's all done right.  

then take into consideration your personal shooting distance limits for consistent accurate hits under hunting conditions.  this has lots to do with holding weight at full draw and yer overall shooting skills.  be honest with yerself, please.

see, not rocket science - nor does it need to be.  use common sense - a commodity that sometimes can be both scarce and elusive here in the new millennium.    

[Woolenclad]

I totally agree with you guys that this sort of stuff is totally just a thought exercise and not that useful in the real world. Still, I can't help but be curious. Clearly the arrow setup of choice has to be the one that flies best and the one a guy is most comfortable shooting.

I am curious by nature and the idea of finding out where my equipment is working most efficiently is intriguing, if not almost entirely useless.

[Rob DiStefano]

Originally posted by Woolenclad:
... I am curious by nature and the idea of finding out where my equipment is working most efficiently is intriguing, if not almost entirely useless.

experiment, test, repeat.  

[Crash]

You would need to define peak efficiency and what it means to you.  As the arrow weight goes up, the efficiency of the bow goes up, it's physics.  Never reaches 100% because of losses in the system, but steadily increases.  

For me, peak efficiency is when the bow feels good at the shot, there is reduced noise because a major portion of the energy is imparted to the arrow, and the trajectory is such that I am comfortable with it.  I normally shoot around 12 to 13 GPP.  It's what I am used to, I like the way my bows feel at this arrow weight, and I am comfortable with the arrow speed, thus, the trajectory.

Experimentation is a wonderful thing, it can also be the devil.  You have to try enough different aspects to determine what you like, just don't let it get away from you and rob you of the joy and simplicity that is Classical Archery.

Originally posted by Crash:
Experimentation is a wonderful thing, it can also be the devil.  You have to try enough different aspects to determine what you like, just don't let it get away from you and rob you of the joy and simplicity that is Classical Archery.

My thoughts exactly...
   

[monterey]

Thanks for the clarifications on efficiency.

I'm inclined to define it more around momentum, but still energy numbers should tell the story as far as efficiency goes.

I have a chronograph and test every bow I build or get my hands on.  But, I don't have the array of arrow weights needed to test with.

I'm all for keeping it simple when it comes to the practical and everyday aspects of archery and hunting but OTOH, curiosity needs to be satisfied.   For me, simplification and experimentation are two different aspects of archery and I enjoy both.

[McDave]

I think efficiency may be sort of a red herring.  The effectiveness of a bow/arrow combination for a particular purpose may take several factors into account.  

For hunting, the first would be the ability to kill the game.  Dr. Ashby has shown in exhaustive tests that the most effective measurement for this purpose is the weight of the broadhead.

Next would be the ability to accurately hit the game you want to kill.  The most effective arrow for this purpose would be a well-tuned arrow with the flattest trajectory, i.e. the fastest well-tuned arrow you can find for your bow.

Since neither of these factors relate to efficiency, and they are mutually opposed, one has to determine the most effective arrow by experimenting to find the best compromise between the two, as has been mentioned.

[ChuckC]

Bow efficiency and arrow efficiency or lethality are not the same.