Arrange two twelve inch pieces and two five inch pieces as shown. The exact location of the five inch pieces will become clear in the next picture.
Now add the triangular pieces and either nail, screw or glue them down just as shown. Note that these two pieces you have made are not identical. They are actually mirror images. This is why I had you lay them out exactly this way.
Stand the two pieces you just made up on two five inch pieces as shown. Then screw, nail or glue them down. The best would be screw and glue. And if you have a drill you should drill thin pilot holes. This will help prevent cracking of the wood. A pilot hole is just a thin hole , usually around 1/8 inch that goes right through both pieces of wood where you are going to put the screw or nail.
Now install the crossbar at the top of the uprights. Use your preference of screws, nails but no glue! This crossbar is something that you can adjust in the future to change the trajectory of the projectile. You can move it down, up a little, or even use a thinner crossbar. All of these things will change the performance of your mangonel. Keep this in mind because you may want to experiment with this piece.
Now we need to drill three holes. The picture below shows where to drill.
From top to bottom here are the sizes and locations of the three holes:
Top Hole: It is 2 1/4 inches (Two and a quarter inches) from the left edge of the mangonel and it is drilled all the way through with a 9/32 drill bit. This size drill can be fudged a bit. Just try to get the hole as big as possible without weakening the catapult. The bigger the hole the more wraps of string you will get through it and the stronger your mangonel will be.
Center Hole: This hole is in the other upright and is identical in every way to the first hole you drilled. 9/32nds and 2 1/4 inches from the left edge.
Bottom hole: This hole is drilled in the ten inch swing arm and this hole doesn't have to be as big as the previous holes I used a 15/64 drill but you can go smaller if you want. You will only pass two or three loops of string through it.
Lets Start the Hard Part - Stringing up the Mangonel
This can take some time, go slow and don't hurry. You also don't need to pull the strings very tight at this point. They can sag a little bit. When you tighten it up to fire it off it will take out all the slack.
Cut yourself a very long piece of your string. If your string is about like mine then use about twenty feet of it. Now attach a wire to the end of it so you can use the wire like a sewing needle and sew your way through the various holes. You have to use a wire because as the holes get more and more string it will be impossible to do it without the wire. You want to get as many wraps as possible in there.
Tie one end of the string to the center of your small dowel (This is the place where you could use a nail or a pencil if you don't have dowels). run the strng through one side of the catapult, then through the hole in the throw arm, then through the other side of the catapult and through the other hole. This was all from right to left. Now continue on. with the string go over the peg on the left then under it and back through the hole. This only does one revolution around the peg. Do not do multiple wraps around the peg, Just once over and under then back through the hole.
Go back and forth like this three times and each of these three times you pass through the hole in the throw arm. This will insure the arm stays in position and will not fly off when you release the projectile.
Now you are going to do a bunch of wraps in a figure eight pattern. Just keep going back and forth. But now you dont go through the hole you go either above or below the swing arm. The picture below shows the wrapping is completed. See how some wraps are on one side and some wraps on the other. I did about ten wraps per side. And for best performance alternate them, one over, one under, one over etc...
When you are done wrapping and you can't get any more string through you are done. Tie off the end of the string on a peg. The picture shown below is a little different than what you have right now. Your strings are a little saggy. You can wind it right up and it will turn out like my picture. Make sure you wind both pegs in the same direction - Forward!
 Okay! Lets finish building this mangonel catapult
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