Questions about very heavy composite bow

[Dane]

Hi, everyone.

I have been planning a gastrophetes for a while, and figure this is a good year to get started. That is an ancient Greek arrow shooting machine, and uses a very heavy bow, something like a crossbow prod (or whatever you call the bow on a crossbow).

I'm thinking this machine should pull about 200 lbs. or perhaps a bit more, and a composite bow is the best kind of design for a machine like this.

For those who have built composites, what basic dimensions do you recommend or suggest? Length is not an issue, as these kinds of machines were probably used from static positions, for defense of forts and encampments, maybe even marine use for ship to ship combat.

In other words, the bow can be fairly long, but probably doesnt have to be. My thoughts are that the arrows / bolts will be fairly short, 12" to 14", and tipped with iron bodkin style heads.

It will be the standard construction, i.e. wood core, horn belly, and sinew back. Not sure if it will have sirahs (spelling?) like a "horse bow" or not, and there is no archeological evidence one way or another for how the bow actually looked.

Thanks,

Dane

PS Just to clarify how one of these machines works and how a single shooter can draw such a weight, the stock of the machine has a grooved slider dovetailed into the stock of the machine. At the end of the slider is a trigger mechanism with a pawl that hooks onto the bow string. At the distal end of the machine is a crescent shaped wooden belly rest.

You slide the slider forward toward the bow, hook the pawl onto the string, place the end of the slider on the ground, and lean your body on the belly rest, pushing mightily. That spans / draws the machine, and it is locked back for shooting by two linear ratchets on the sides of the stock. You place the arrow / bolt into the groove, aim, and make a widow.

That is why they are called belly bows, or gastrophetes. The main point is to be able to outrange enemy archers and slingers.

I can post a photo of an early 20th century recreation later, if anyone wants to see the weapon.

[Badger]

   Dane, how long will the draw length on ths machine be. At 200# it would need to be probably at least 24" to keep up with the heavy elbs. You could proably do it with an all wood design composite, sinew back would be helpful. Steve

[Dane]

Hi Steve. The arrows I will put through this machine will be probably around 12" to 15", fairly short.

Keep in mind these machines predate by ELB by at least 1000 years. It is strictly a military weapon. The heavy draw weight is critical to defeating shields and armor of the ancient period. I would also consider it a terror weapon when it first appeared on the scene.

I seem to recall some formula used by crossbow makers, but dont recall the details.

Dane

[Badger]

Dane, the formula is actually very simple. You measure the draw weight at each 1 inch interval, add up all the numbers from start to full draw and then divide that by 12. This will give you foot#'s of stored energy. Most crossbow users do not divide by 12 and simply gives us inch pounds of stored energy once they add up the numbers. You can assume roughly 70% efficiency. Once you know the energy storage you can figure how heavy of an arrow and how far it will go approx. Typical a crossbow will store about 40% of the energy of a standard bow so needs to be about 2 1/2 times heavier just to match the power. The big advanatge was less training to become accurate with these weapons. Steve

[Dane]

Simple eh? I will keep that formula tucked away.

I wonder though how heavy a bow to shoot for before I build it, and what dimensions will get me in the right ballpark? I actually have all the materials at hand (sinew, Asian water buffalo horn, wood, and the stock is already completed).

There is a limit to how much a typical man can span one of these by leaning his weight on the machine, and I think 200# is about right. I do have to keep in mind the enemies of the Romans and Greeks 2000+ years ago will not have bows nearly as powerful or armor as well developed and effective as in the early medieval period. The Nydam bows prove that, and in 250 BC, probably even less developed. I may be wrong in some assumptions, however. The weapon has to be appropriate in strength to the threat it faced at that time.

Dane

[Tsalagi]

Dane, I'd love to see a picture of a re-make of the gastrophetes! Alexander's army used those, as I recall.

[Dane]

Thanks. I will of course post photos, etc. No one since Schramm has tried to recreate this weapon, and he got it wrong with a metal bow. I will get a photo of his up later today.

I am not actually sure if it survived WWII. More than one of his machines perished in the allied bombing campaigns. The gastrophetes is almost totally forgotten. Alexander I believe did use them.

BTW, I havent forgotten your project. I'll begin it this weekend. Clients from hell swooped down on me, a really rough week.

Dane

[Dane]

Here is an incomplete SketchUp drawing of a small scorpion I am building, to give you some idea of the basic function of a gastrophetes.

Following are two shots of the same small scropion, so you can see the machine "in the flesh."

Imagine the spring frame at the front of the machine being replaced with a heavy composite bow.

The question I have is around what dimensions I should shoot for for the bow. I was thinking 24" to 32" long, a composite sinew, wood and horn bow. The bolt or arrow will be between 12" and 15" long. Would that lenght bow of sufficient width and depth give me the 200 - 250 pounds of pull I am aiming at? Based on the size of the case (the wooden stock of the machine), width can be no more than 3" wide.

Any suggestions are very appreciated.

Dane 

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[hermitking]

I would suggest that you start with an osage or hickory core with a heavy sinew backing.  If you do not actually hold the bow in the hand but rather it is placed in some sort of frame then it does not need to be narrow enough for a human grip.  This opens up a new design possibility that may increase effeciency and durablity.

I would start with a stave about 2 inches wide, 36 inches long(double the projectile length).  The center should be 2"inches wide and then taper to a narrow tip in an almost straight line.  I would also add some sort of short stiffener at the tips (2-3 inches) such as an antler/horn, bone laminiation.  At 200 hundred pounds pull the string nocks on the tips need to be very strong.  A wide flat limbed design is probably more durable and will shoot a little faster.  Since you are not actually pulling with finger tips as in a normal shot, you probably do not need to recurve it.  Power comes from the shear brute strength of the draw weight, rather than using a recurve to increase energy storage.  It seems that crossbow prods were very rarely recurved.  I would also allow a very slight bend in the center.

Since a 3 foot sinew backed wood bow can be made very easily and quickly I would do that first and then try one with a horn belly after experimenting with a wooden one first.

But then again I know absolutely nothing about this particular weapon you are trying to recreate. 

[Jude]

Seems like the draw weight would be limited to the weight of the shooter with this design.   Some of the medieval designs had a hand crank to allow for really heavy draw weights.  Can't wait to see it.  Anyone know about Greek/Macedonian bow designs in general?  That might provide some clues as to how they would have designed a crossbow.  If they used recurves, they may have designed them into a crossbow, since they do provide more energy storage for a given draw weight/length, due to the heavier initial draw weight.

[Dane]

Great advice, Hermit. Thank you. I think making a self bow, sinew backed, is good first step.

The one glitch is that both osage and hickory were unavailable to the Greeks. The woods they would have used would have been fir, maple, alder, birch,box, hornbeam, chestnut, cedar, sanderac, cornel / dogwood, hazel, cypress, beech, fig, ash, ebony, holm oak, walnut, juniper, larch, olive, palm, pine, poplar, oak, willow, yew, terebinth, linden, and elm. Hickory I think would be best, and I think I have enough of that in stave form.

Jude, yes, the limit to the draw weight, I think, would have been how much a single man could span the machine with his body. As the weapons evolved, mechanical means were developed to allow a crew to draw back the bow, and then the arms in the torsion machines.

Does anyone know about ancient Greek bow design? I have not read or heard anything about that. Enemies of the Greeks would have used composite designs, though, so they would have been familiar with them.

Dane