rules of thumb on the optimal bolt weight

So I have done some reading on the threads here about bolt making and I want to get a little feedback from people on my take.

There is a couple rules of thumb on the optimal bolt weight for a given bow...

I want to make bolts for my steel bow:
Draw weight: 250lbs
Physical mass of prod: 806 grams

One rule of thumb is 2 grains per pound of draw weight as a minimum starting point or in this case 500 grains - 32.4 grams. With a suggestion that steel prods like heavier bolts.

The other rule of thumb suggests 1/12th the total mass of the prod as optimal efficiency or 806/12= 67 grams or 1037 grains.

So here is a spread of 500 to 1000 grains.

Does anyone have thoughts on these matters?

Ernest (Hotspur)

I would suggest the figure of 32 grams is probably closer to optimal for a 250 lb. prod than 67 grams. Why? Because 67 grams is a good approximation for weights of medieval bolts in armories, and I suspect those were intended for bows substantially heavier than 250 lb. 32 grams might be a bit light, but I suspect 67 is rather too heavy, even for a steel prod. If you want something heavier, try 40-45 grams.
Its just my gut feeling, but these days I have plenty of gut. Geezer.

Hotspur i also think it has alot to do with how much resistance the string has to th deck of the tiller, my 400lb has quite a bit of string pressure , if i went with 2 grains per lb id need 800, but i find 620 grains is enough, is fairly quiet. Now my other bow 250 was designed with minimal deck pressure from the string and casts the same boltsat about the same speed. Just my 2cents worth.

Thanks, Geezer & AA. Nice to get your ideas and help me think it through (my usual M.O.).

Basically I want to get a reasonable amount of hit for deer hunting. I’m sure it will be adequate for our ¾ size island deer (Columbia Blacktail) nevertheless I want it to be at least 35 or so ft lbs.

My initial idea with this bow was that I thought it would be fun to hunt with a handmade crossbow so with minimal research I ordered a bunch of parts from Alchem (pre-knowledge level zero).

The original 175lbs prod turned in about 169 fps with a stock 425 grain bolt. That was average speed over 50’ not ‘muzzle velocity’. That would create 26 ft lbs of kinetic hit and was too light for me. So I ordered 225lbs from Alchem, waited 6 months, cancelled the order, then ordered a 250lbs from slowbow.

Todd seemed to think you could expect 1 fps per pound of draw with steel prods up to a max of about 230fps regardless of bolt lightness due to the inherent inefficiencies of steel. Although you would gain kinetic energy with ever heavier steel no more speed would be forthcoming.

I have some stock 570 grain bolts, maybe I will start there and just keep adding/ removing weight until I find the sweet spot? I would like to approach 60 ft lbs of kinetic energy. That would mean throwing a 613 grain bolt about 210 fps. Doable?

Just have to put it all together and do some testing.

AA, I have a frictionless set-up - the table is carved away except for the front brace area. You can kind of see it in the avatar.

All the best,
Ernest

Hotspur, don't rely totally on KE to determine if you have a power to hunt with, from my understanding, it works great on very fast light hunting arrows, but they sheds that energy very fast due to a lack of momentum, car vs truck doing same speed thing, work with both of them, my current hunting set up gives me 70 fp KE but .6 slugs of momentum, now with that added to a 2 blade cut on contact broadhead, and this is according to a chart set out but a few of the best SA bow hunters, is enough to hunt buffalo. Example. My PSE X-Force had 90 fp of KE but at 30 m could not blow through 16mm ply, not enough momentum, but my 250 lb crossbow dose it with ease, I do have a 18 percent, foc tho on the bolts which will help greatly. Again, just my 2 cents on a very iffy subject. If u are interested, I can post a link to that chart, its quite interesting to read.

Both empirical testing and physics suggest that a classic shape bow usually has "virtual mass" ca. 1/25th of the bow mass. This reprsesents the energy bow wastes moving itself.

So for 800g prod about 32g is likely be the safe minimum mass - with 50% energy efficiency.
While 67 g should give about 70% efficiency wich is normally considered to provide the best combination of range and penetration.

Anyway, for comparison:
800g lump of yew would make ~100# longbow , with 10grains/lb that would need 1000 grain arrow.

Trial and error is something u could concider. If u have the possibility to use differrent weight bolts.
Try shooting from a fixed distance and see wich weight gives the best penetration and makes nice groups.
I have 32cm long 50 gram bolts on a 350lbs steel prod.
They fly nicely, good penetration, at 50 meters abou 25 cm in to the tree.

As a long time traditional bow shooter....I have always use 10 grains per pound of draw weight when deciding on total arrow weight.

As I understand everything, a prod is about 1/3rd as efficient as a bow. Meaning: 210# draw weight prod * 1/3= the efficiency of a bow drawing 70# at a given draw length. That being said, a 700 grain arrow or bolt "should" be the best choice.....

But, as a new crossbow shooter, does the same hold true?

My crossbow is for hunting, period. Any other shooting will be for fun....

Is there a formula like the above? Meaning based on the draw weight?

Stonedog: It's not that the crossbow prod is 1/3 as efficient as the handbow. Actual efficiency should be pretty close to the same (maybe a bit less for the crossbow, thanks to shorter acceleration time, but stiff crossbows do accelerate to speed quite quickly). The difference is: the Length of actual Power-stroke on typical medieval type crossbows is about 1/3 that of typical handbows. Having said that, your assumptions look pretty reasonable. Geezer

Hey Nerd-

Based upon your calculations......with my prod having a mass (weight) of 810g....then the minimum bolt weight for my prod is roughly 35 grams (540 grains). That being said, the optimal bolt weight would be about:

70 grams (representing 100% efficiency) * .70= 49 grams, which equates to 760 grains

Correct?

I do not know the exact draw weight yet. I ordered for Slobows (awesome guy!) and the draw weight is between 185-200#.....

Like I said earlier, my crossbow is 100% for hunting whitetail and small game and any shooting competition will be for fun!

The Nerd and Geezer, it’s a little like modern physics meets museum data.

So...

African Archer(don't rely KE )40 gm 620 gr
Geezer(Historical extrapolation)40 – 45 gm 600 – 700 gr
Nerd (physics) 67 gm 1034 gr
Ferdinand (on 350 prod) 50 gm 771 gr
Stonedog (10 gr x lb)x.33) 53 gm 825 gr

Thanks for the feedback guys I’ll post my results. I’m waiting for UPS on a couple of things before I reassemble and start testing.

Hotspur

A modern compound of any type is designed for a light arrow/bolt moving FAST...traditional bows (and a crossbow like ours "should" fall under this rule) rely on weight, not speed for penetration....

Just my experience, not trying to argue!

While we are talking bolts... does anyone have thoughts on the FOC percentage with these heavier period style bolts?

Hotspur
I add 10 grams of lead to he front of bolt, the bolt is 30grams including field point or broadhead ,with the lead takes it to 40 with a 18% foc. they fly like darts and hit hard.

Thanks AA,  I will aim for 18% FOC.

Hey, does anyone see potential for catastrophic failure in this idea for adding weight to some off-the-shelf 34 gram carbon bolts.  The hollow inside is about 1/4", so I was thinking of dropping lead split shot fishing weights in there held in place with two lengths of 1/4" wooden dowel.  




As mentioned above I want to mess around with bolt weights from about 40 to 67 grams to see what the sweet spot.  It occurred to me that the carbon bolt may just disintegrate if stuffed with enough lead.

Thoughts?

I think you might be ok with the carbon shaft, but what you might fnd is that if the dowls are not glued in tight , the inersha of the shot might force the insert out eventually, so maybe some epoxy is needed. but hn might be wrong.
Just  thought.

Solid fiberglass rod will make damned near indestructible bolts, but the are heavy as can be.  Some medieval clubs, like SCA use them for their combat-game bolts.  I've seen them stepped on, even stamped on deliberately, with no damage.  Geezer

The following chart shows seven bolts of increasing weight from 515 grains to 1000 grains, released from a 250 lb prod with a 9’ power stroke.  The FPS is the average speed over 50’, not ‘muzzle velocity’.

This is an off the shelf carbon bolt 22 inches with a 170 gn field point (total 515 gn).  The hollow bolt interior just accommodated a .25” split lead shot (25 grains each)  held in place with .25” dowel.   I used the same bolt and just dropped in more lead for each step.




Interesting how the ft. lbf starts to plateau at 900 to 1000 gn, perhaps confirming the Nerds efficiency hypothesis:

903.8 gn               166 fps                 55.32 ft lbf
1000 gn                158 fps                 55.45 ft lbf

However, I think I will settle on a 600 to 700 grain bolt based on this experiment for about 180+fps at 50 ft.lbf and 15% FOC.  Seems like a good compromise of speed and KE for deer hunting and the increased FOC gives the bolts a nice stable flight.

Pardon my ignorance, but what does FOC refer to??

Jeff

Forward of Center

I shortened the commercial bolt to about 14" and 650 - 700 gn and am awaiting UPS for a fetching jig.  The FOC should be about 18% with the shorter bolt.  I am curious to see how a shorter bolt compares.  I will also go from the 3 fletch to a two fletch.  In an initial test with rough fletching the shorter bolt was slower for the comparable weight, but the rough fletching may have made for poor flight, especially with only two feathers.

Jeff, this is a good video explaining FOC:
https://www.youtube.com/watch?v=a0FmMr-QXtU

Further on FOC, Jeff, the more forward the weight on a bolt, the more forgiving it is to aerodynamic vagaries and the more dart-like in performance.  The cost being trajectory and distance.

Hotspur wrote:Further on FOC, Jeff, the more forward the weight on a bolt, the more forgiving it is to aerodynamic vagaries and the more dart-like in performance.  The cost being trajectory and distance.

 Thanks for the video link - that helped immensely.  I'm also into high power rocketry.  In HPR, we use 2 measurements that relate to the stability of a rocket in flight (they may have some bearing here as well).  The first and most well known of those would be Cg (center of gravity) which is used here to calculate the FOC.  Then there's the Cp (center of Pressure)  which is based on some calculations related to the length, distribution of mass, diameter, and profile.  In HPR, the stability is directly related to the difference between the Cg and Cp and how far apart they are, measured in calibers (diameter).  I suspect that FOC may in fact be a crude estimation of the Cp.  The dynamics are amazingly similar.

I got my Grayling Fletching jig and started on a matched set of 15" bolts. 

Just to prove that this collection of wood and steel could make something go 200 plus feet per second I made a 437 grain bolt and clocked it at an average speed of 203 fps. I considered shooting something lighter but didn't want to dry fire my limbs. 

I did a two fletch set of three bolts and will tweak them to just over 600 gn.  That will give me about 188 fps and about 48 foot pounds of force. These will be fixed with 170 gn broad heads.

Will also make a couple blunts for upland game birds. 

Less then a month to archery hunting season here in BC!  Fall is in the air