Mechanical Properties of Wood for Building Bows and Arrows

[willie]

What draw length are you looking at,-

and how many pounds do you want it to pull?

[Badger]

   I don't have my paperwork handy, but I think it was about 15 to 20,000# at 32 feet and about 120,000# stored energy. I may redo the plans for 60 ft total length.

[willie]

20,000# pull using doug fir
60 ' long elb drawing 30'

6 and 1/8 in thick at handle, but it's gotta be 96 inches wide.  will store just under 300,000 ft lbs, though

[joachimM]

   I have an odd question, if I were building a bow say 80 ft long, intuitively I am thinking I should go to a less dense wood, less dense than what we normally use for bow woods. How would you guys look at this?  Looking at about 18" wide by 6" thick.

I don't see why they would have to be less dense. Basically, you scale up everything.
Bows of different dimensions but of the same design, shot at 10 gpp and with a draw length scaled to the bow's length (DL = bow length/2.5) will be strained the same amount, and will shoot equally fast when shot in a vacuum (air density is the only thing we cannot scale up or down, so a lighter arrow will experience more drag than a heavier one).

[Badger]

  You don't really use the grains per pound on a very large bow. You would scale up the grains per pound of stored energy. Just by coincidence a 28" draw bow stores about the same number of units as its peak draw force. So to do this you have to scale up the size of all the units as well.  I am actually planning on using an almost dry fire situation of about 1 grain per pound of stored energy. Maybe 10 grains per pound to demonstrate destructive force.

[willie]

Interesting project, Steve. One that certainly requires a bit of number crunching to execute.

I hope we have not dissuaded PNWarcher from posting further, with all our meanderings. Seems like the PA way sometimes.

I have found a database that has some values collected from a few different sources. It includes the works of George Garratt from 1931.

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http://www.matweb.com/search/PropertySearch.aspx
Also if interest might be this discussion from a few years back at PP.

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https://paleoplanet69529.yuku.com/picking-a-wood-for-a-bow-t28706.html#p310687
for those that wish to learn how to make adjustments from the published data for varying moisture contents and densities, this pub from back when airplane were built from wood may be useful. (also other pubs at westcoastpiet.com)

http://westcoastpiet.com/images/Construction/ANC-18%20Part%201%20of%203.pdf

[joachimM]

   I don't have my paperwork handy, but I think it was about 15 to 20,000# at 32 feet and about 120,000# stored energy. I may redo the plans for 60 ft total length.

For a bow that's quite a lot of poundage to pull! But still short of the heavier Roman ballista ;-) (replica shown here https://www.youtube.com/watch?v=jn2wbZow8cw) that pulled an astounding 870 000 pounds (give or take a few thousand)

[Badger]

   I don't have my paperwork handy, but I think it was about 15 to 20,000# at 32 feet and about 120,000# stored energy. I may redo the plans for 60 ft total length.

For a bow that's quite a lot of poundage to pull! But still short of the heavier Roman ballista ;-) (replica shown here https://www.youtube.com/watch?v=jn2wbZow8cw) that pulled an astounding 870 000 pounds (give or take a few thousand)

      I think they measured the 870,000# at some point that was not reflective of stored energy. I know the ballista's did store a lot of energy but I doubt they stored quite that much. A 1,000# crossbow may store as little as 100# energy depending on the draw length. Where you read the stress it not so important as much as how much is being applied to the projectile. I am still thinking a much lighter wood might be in order for a giant bow as the cell structure of wood may need to be considered when scaling something like this up.

     I don't remember the exact numbers as it has been a while since I played with it but I think on my mass calculator it calls for limbs that weight about 500# where in reality they would weight closer to 2,000# likely even more than that.

[pnwarcher]

Willie: on the contrary! I'm delighted this topic sparked so much conversation. Keep it coming!
Aussie: thanks for all your time collecting and sharing your knowledge. Valuable stuff.
Badger: At the risk of making an arrows post in the bows forum, see attached PDF for some arrow wood properties. To compare overall suitability for flight arrows, I made up something I'm calling the "ballistic index", which is a measure of density and stiffness relative to POC as a reference material. Lighter, stiffer woods get a higher score. In terms of pure stiffness (MoE), Greenheart is the highest I've seen. Bulletwood, Ipe, and purpleheart are up there, too. I'll make a post over in the arrows section to avoid getting too far off topic over here.