How to build a Trebuchet (Catapult)
A Trebuchet (pronounced treb-oo-shay) is a medieval siege engine that transfers gravitational energy into kinetic energy – similar to a playground see-saw. In ancient times, it was used to launch (throw) projectiles including boulders, dead horses, and diseased bodies into or over castle walls to soften the defenses in preparation for invasion.
On one end of the trebuchet is a projectile you want to fire and on the other end is a counterweight that falls quickly to raise the throwing arm. In physics terms, raising the counterweight above the ground produces a build up of potential energy. When you release this counterweight and allow it to fall the trebuchet pivots on the fulcrum and the other end (with the projectile) receives the energy. This force of energy can be quite substantial and it is enough to send a projectile flying through the air for an unbelievably long distance.
Building a trebuchet is less complicated than you would think. You can begin with plans or pre-made kits but regardless, there are dozens of different trebuchet designs and each kind always requires “tuning” after they are completed. For that reason, the easiest way is to just build one from a picture and tune it as you go. Besides, most trebuchet enthusiasts, the fun of “hurling” is also in the designing, constructing and modifying their creation.
Basic parts and important design guidelines
The base of the Trebuchet supports the glide track and the tower support frame. The base must be sturdy and stable to support the “whipping” transfer of energy when the projectile is launched. The glide track, upon which the sling harness will slide at the beginning of the throw, is attached to the base. Likewise, the support frame is also attached to the base. In addition, if the design incorporates wheels, which not only make the trebuchet portable but also improve performance, then they too are attached to the base.
A general rule of thumb regarding base size is to use a base that is 75% of the length of the swing arm. The width should be about 1/3 of its length. Since the counterweight must swing through the primary support beams of the frame, the base must be wide enough to allow the counterweight to pass through unimpeded.
The glide track (or glide board) is the surface upon which the swing harness slides after the trebuchet trigger is activated. The glide track may have raised or curved sides to guide the swing harness rig along the correct path. It goes without saying that the glide board should be smooth in order to lessen the amount of resistance as the swing harness slides over it.
The glide track should be about 3/ the length of the base.
Tower Support Frame
The tower support frame is what the swing arm lever is attached to. It consists of two primary support beams, attached to the base, and ancillary support structures for added strength and durability. A typical A-Frame construction is most common.
The tower frame height should be about 3/4 the length of the base. Keeping in mind that the counterweight should be from 75-100 times the weight of the projectile, allow enough width between the primary support beams to allow both the projectile and counterweight to pass through unimpeded.
Swing Arm Lever
The swing arm lever may also be referred to as the “primary swing board”, “long arm lever”, “throwing arm”, “swing arm”, or “long arm beam”. The swing arm lever will have a counterweight attached on one end and the projectile holder on the other end. It will be attached to the tower support frame via an axle located between the two primary support beams, providing a pivot point for the arm to swing around. A lot of energy can be wasted on the pivot point so make sure the arm can swing with as little resistance as possible. If you want to really go wild, try incorporating a bearing mechanism such as a bicycle wheel hub to make a very efficient pivot point for the swing arm lever.
The end that the projectile holder (i.e. the sling) is attached to is called the “long end” while the end that the counterweight is attached to is the “short end”. When cocked, the arm should be about 45 degrees from horizontal.
The swing arm lever should be about 1 1/2 times the length of the base. The long end should be about 4-5 times longer than the short end (4:1 or 5:1 ratio) and must be constructed to be strong but light. Lightweight Ash wood can be used on smaller machines but larger machines require Oak, Aluminum, or even carbon fiber. The long arm can be tapered to lighten it up a bit but the short arm should be strong and stout in order to support the heavy counterweight.
Making the arm longer in order to increase leverage is not desirable since our primary goal is to enable the counterweight to swing through quickly. A longer arm slows the arm’s movement.
The counterweight is a major factor in how high and far the projectile will travel. In many designs, the counterweight is simply a hopper that is filled with heavy objects (dirt was commonly used in ancient times). Some trebuchet designs incorporate mechanisms, such as dual pulleys, to assist with locking the heavy counterweight into the firing position.
The counterweight can be “fixed” or ‘free” (i.e.”hinged”). A free or hinged counterweight provides a swinging bucket that allows the weight to fall more vertically and thus is more efficient in converting that vertical motion into the rotational motion of the throw (see photo).
A fixed counterweight is simply a heavy weight on the end of the short arm. Fixed counterweight designs tend to try an pull the treb over. Wheels are often used on trebuchet designs that use fixed counterweights to counter the tipping motion.
The counterweight should be 75 to 100 times the weight of the projectile. Never “dry fire” the trebuchet – always load a projectile to balance the firing mechanism.
The sling is attached to the long end of the swing arm lever and holds the projectile that is to be thrown. It consists of a pouch to hold the object, that is connected to the swing arm lever via rope or cable.
The sling harness must be designed to release the projectile when it reaches the precipice of the launch path. There are many “release prong” (or release hook or release pin) designs and may be as simple as end of the sling rope permanently attached to the long end of the swing arm lever and the other end looped over a release prong (or hook) protruding from the end of the swing arm lever.
Sling length and release prong angle affect the angle of release. Longer length slings have a lower angle of release, good for line drives (if you need a direct hit against the castle gate, for instance). Shorter lengths will result in a higher angle of release, best for lobbing projectiles over tall objects.
A more hooked release prong will hold the sling loop longer (and release the projectile later) while a more straight prong will release the projectile earlier. If the projectile is released from the sling too early,the trajectory will be too high. If the projectile is released too late the projectile will slam into the ground.
Some sling release designs utilize a “slot” in which a knotted rope is placed (the rope slides out of the slot to release the sling). This is not as efficient but may be used if you do not have the materials required to construct a hooked release prong.
Also note that a heavier projectile will tend to pull the sling loop off sooner than a lighter sling loop. For this reason, the sling prong should be made adjustable in order to compensate for varying projectile weights.
The sling harness length should be about 3/4 the length of the glide track, slightly shorter than the length of the short arm, or 80% the length of the long arm.
There are many ways to design the trigger (or “release”) of your trebuchet. The trigger design must allow the trebuchet to be loaded and the trigger safely engaged. It should also allow you to release the trigger using a rope or string from a distance. You don’t want to be standing close to a trebuchet if something goes wrong.
One trigger release design utilizes a eye hook (or pin) on the end of the long arm with a swingable, free-moving hook secured to the base. When the long arm is set, the hook is inserted through the eye hook to lock the arm into place. A rope attached to the hook allows the operator to pull the hook from the eye holder (or slide it off the pin if a pin is used) and thus release the swing arm.
Another design uses a stationary eye hook on the end of the long arm and another stationary eye hook on the base arranged so the two eye hooks align when the treb is loaded and ready to fire. A pin with a rope attached can be slid through the two eyehooks to keep the arm locked into place. Pulling the rope disengages the pin from the two eye hooks.
A third option is a hook trigger. A hook trigger is sturdy but sometimes difficult to get right (see below). it is basically a large hook that secures the entire long arm. The hook is pulled off the long arm to release the treb. A sturdy piece of metal is used for the “hook”. It must allow for free movement so it can be pulled to swivel away and off the long arm. A rope tied to the end can be pulled to release the hook off the end of the long arm.
Below is a design for a very complex, but very safe, trigger mechanism (image courtesy thehurl.org).
Commercial off-the-shelf release mechanisms can also be used. These include the archer’s arrow release, the sailor’s pelican hook, Sea Catch toggle release, and the horse trainer’s panic snap.
Some trebuchet construction (and physics) tips and tricks
- To attain the optimal transfer of energy, the counterweight should be allowed to fall as vertically as possible to gain the most momentum. Wheels under the trebuchet actually help the weight fall straight down by shifting the mechanism forward as the weight falls.
- To achieve maximum range, the release point should be between 40 and 45 degrees. A lighter projectile will release later and hence, have more speed. The angle and adjustment of the release hook can be adjusted forward to attain good results with a heavier object.
Below is a photo montage of a basic trebuchet design. The second photo montage demonstrates a more complicated design, closer to the design used in medieval trebuchet designs.