How to Build a Trebuchet. A trebuchet was a weapon used in medieval times to hurl stones and boulders at castle walls. It is a kind of modified, huge catapult mounted on a sturdy support. Trebuchets vary in size. A contemporary model, named T-Wrecks, is capable of hurling a 500-pound piano across 500 feet. Science Olympiad, a US-based organization, organizes an event called "Storm the Castle," requiring participants to build small trebuchets. Instructions given here are for a reasonable tabletop trebuchet.
Setting up the Base
Use a long piece of wood and cut it in eight lengths. One of these should be the longest, a pair few inches shorter, another pair a few inches shorter than the last pair and then three of the same size.
Cut a square wooden piece diagonally to obtain two wooden triangles. To get two same-sized triangles, draw a line from one point to the opposite diagonal point before cutting the wood.
Join two pairs of wooden pieces such that the shorter piece is attached at three-quarters of the length of the longer wood. You should have two such identical attachments.
Tie a triangular wooden piece to one of the attachments made in the last step. Glue the triangle such that the slanting side is toward the longer side of the base.
Paste the other triangle to the second side. Join two pieces from the remaining three smallest ones to the sides assembled earlier. Make sure you get a rectangular setup with triangles facing outside and two upright arms. Stick the third smallest piece at the junction of the base and upright arms.
Secure all wooden pieces by hammering in nails once they have been glued together.
Use a pen/pencil to mark the location of holes in the upright arms. The holes should be marked at least an inch from the top. This is where the axle will go.
Making the Swinging Arm
Choose a cylindrical piece of metal to be used as an axle, the component that carries the counterweight. The axel should be the same length as is the distance between two upright arms of the base.
Drill holes in the upright arms. Make sure the holes are big enough to allow the axle to move comfortably.
Make the swinging arm of the trebuchet by marking three holes on the longest piece of wood. The first hole should be a quarter of the total length away from the edge. The other two should be one inch and two inches apart from the first hole.
Drill the marked holes. Insert the axle in one of the holes and swing the wood around it.
Fix a hoop hook at the edge near the drilled holes. This is where the counterweight is to be loaded.
Hammer in a nail on the opposite side. Once it is secure, cut its head. Bend it slightly so a loop can stay there until released. On the same side of the swinging arm, fix another hoop hook away from the nail toward one side of the wooden piece. This is the end where the projectile to be hurled is fastened.
Assembling Swinging Arm with the Base
Insert the axle through an upright arm of the base--one of the holes drilled in the swinging arm and through the other upright arm. Make sure the hoop hook fixed on one side of the edge faces down.
Test for correct assembly by swinging the arm. Make changes if it fails to swing smoothly.
Hanging the Counterweight
Collect materials to be used as the counterweight. It can be molding clay or discharged batteries. Small sized stones are a good option.
Wrap the collected materials in plastic and tape them to make a counterweight of the size of a golf ball.
Use thick yarn or twist ties to hang the counterweight from the hoop hook fixed at one end of the swinging arm.
Assembling the Sling
Cut two lengths of about 20 inches of strong yarn. Take a rectangular piece of fabric and tie one end with one yarn length and the second end with other piece of yarn. The middle part of the fabric can hold a projectile.
Twist a piece of thin metal wire into a small loop. Secure the loop to one of the free ends of the yarn with which the sling is tied.
Tie the other free end of the yarn to the hoop hook fixed on the far side of the swinging arm.
Slide the thin metal wire loop over the nail. The slight bend in the nail will help keep the loop in place.
Firing the Projectile
Provide cleared space from where the sling can slide off by fixing a piece of cardboard or plywood on the inside of the base.
Bring the edge of the swinging arm down to where the sling is attached.
Stretch the sling on the base as far as it will go. At this time, the sling is placed toward the counterweight.
Load the projectile into the sling.
Release the swinging arm.
Attach a piece of sand paper to a block to easily sand small wooden pieces. Take care that you don't get hurt from sharp metal edges by filing down all pieces of metal you cut. Try hurling the same object from different holes drilled in the swinging arm. Observe variations in the speed. Experiment with different types of projectiles to determine the one showing the best results. Bigger trebuchets may be used in large open areas. Inform the spectators in advance so no one gets hurt when a projectile is fired.