Hello everyone.
My reenactment group is trying to build a ballista, with a single piece bow.
The draw length would be either 1m or 1,9m. Question is how big the bow will be.
I'm sure there is some kind of formula to figure this out.
Anyone?
Plus, what kind of wood we need to use? Any particular cross section for the bow (rectangular, D-shaped, oval...)?

Thanks in advance.

All the ballista builds I've done have used twisted skeins and twin arms.

Hi Augusto,

which period is the weapon from, based on what? Usually 'ballista' in the modern context refers to the roman or greek versions so as mentioned above, are usually powered by simple arms fed inbtween the arms. I use ash for this but holly would work well, very tough. When you go into the medieval era then great or rampart crossbows are the more favored term. They would be using a one piece bow arm unless you are going for the springald/espringal where they tried to copy the roman versions, and didn't do that well. I have one we built sitting in the Tower of London which works ok but needs some refining. There is little evidence for any of them but a good source (as well as the Osprey books by David Nicolle) is ' Springalds and Great Crossbows' by Jean liebel, a Royal Armouries monograph. It does have a few errors though.

The period would be the second half of the XIVth century. We already have in mind to do the two-piece springald, but it will be bigger than this one.
The one we are trying to build now wold be a great crossbow, to use in siege defenses on walls, with a wooden support on which it can be rotated.

Augusto,

Mark is right- You need to look up Springalds and Great Crossbows by Jean Leibel. The Royal Armouries in Leeds has an English version for sale pretty cheap for about 10l. But the idea there is little evidence for them is actually pretty far from the truth. Every major town in Europe likely had them. These were looked at in England as being pretty much the norm for war preparations in the later 13th and into the 15th. Edward II before his St. Sardos campaign starts demanding towns and castles all over England to send them to him for his use there. Southampton seems to have had somewhere between 12-20 and London around 15-20 at least, likely many more. These numbers seems to be fairly low to many continental towns as well. John Fastoff seems to have had a large number of them in his castles as well if Caister castle is any indication into the mid 15th century.

Basically if you are making a great crossbow or springald you are likely looking at something that is about 1.6-2m. All the documents I have seen point to a bow made of either composite make: horn, sinew, bone and wood or yew. I'd suspect they were pretty thick either way. I actually have a photo of what is likely a great crossbow in fact that a member of the forum here sent me. It clearly is of composite make and looks to be very, very thick.

I have a ton of info on these.... sort of one of my things. In primary Latin sources these are indeed often called ballista so you are fine there. Issue is they often call crossbows by the same name as well often.... luckily when listed with crossbows they often get into good details like one and two foot crossbows (I am with Liebel on this that they refer to 1' and 2' bolts and the crossbows needed for the size of bolt used) we also have the size of springald bolts being 24-40".

Good luck and if you make one make sure to make one for me.... or at the least take pictures!

RPM