A trebuchet is a medieval siege engine, a weapon employed either to smash masonry walls or to throw projectiles over them. It is sometimes called a "counterweight trebuchet" in order to distinguish it from an earlier weapon that has come to be called the "traction trebuchet." Sometimes the word trebuchet is used to refer ambiguously to either weapon, but originally it referred to the counterweighted weapon. All trebuchets were made from wood.
The counterweight trebuchet appeared in both Christian and Muslim lands around the Mediterranean in the twelfth century. It could fling three hundred pound (140 kg) projectiles at high speeds into enemy fortifications, at times including corpses infected with various diseases including the black plague, in an attempt to infect the people under siege, as a medieval variant of biological warfare. Trebuchets were far more accurate than other forms of medieval catapults.
The invention of the trebuchet derives from the ancient sling. A variation of the sling contained a short piece of wood to extend the arm and provide greater leverage. This was evolved into the traction trebuchet by the Chinese, in which a number of people pull on ropes attached to the short arm of a lever that has a sling on the long arm. This type of trebuchet is smaller and has a shorter range but is a more portable machine and has a faster rate of fire than a larger counterweight powered one. The smallest traction trebuchets could be powered by the weight and pulling strength of one person using a single rope; but most were designed and sized to utilize from 15 to 45 men, generally two per rope. These teams would sometimes be local citizens assisting in the siege or in the defense of their town. Traction trebuchets had a range of from 2000 to well over 3000 feet when casting weights up to 750 pounds (340 kg). It is believed that the first traction trebuchets were used in China as early as in the 5th century BC, descriptions of which can be found in the 5th century BC Mozi.
The traction trebuchet next appears in Byzantium. The Strategikon of Emperor Maurice, composed in 539, calls for "ballistae revolving in both directions," ('Βαλλίςτρας έκατηρωθεν στρεφόμενας), probably traction trebuchets. (Dennis 1998, p. 99) The Miracles of St. Demetrius, composed by John I, archbishop of Thessalonike, clearly describe traction trebuchets in the Avaro-Slav artillery: "Hanging from the back sides of these pieces of timber were slings and from the front strong ropes, by which, pulling down and releasing the sling, they propel the stones up high and with a loud noise." (John I 597 1:154, ed. Lemerle 1979)
19th century French three-quarter drawing of a medieval trebuchet.There is some doubt as to the exact period in which traction trebuchets or knowledge of them reached Scandinavia. The Vikings may have known them at a very early stage, as the monk Abbo de St. Germain reports on the siege of Paris in his epic De bello Parisiaco dated about 890 AD that engines of war were used. Another source mentions that Nordic people or "the Norsemen" used engines of war at the siege of Angers as early as 873 AD.
Our first clear written record of a counterweight trebuchet comes from an Islamic scholar, Mardi al-Tarsusi, who wrote in 1187, "Trebuchets are machines invented by unbelieving devils." (Al-Tarsusi, Bodleian MS 264) This by itself suggests that by the time of Saladin, Muslims were acquainted with counterweight engines, but did not believe that Muslims had invented them. Al-Tarsusi does not specifically say that the "unbelieving devils" were Christian Europeans, but Saladin was in fact actively engaged against Crusaders for much of his career, and the manuscript predates the Chinese and Mongol weapons. (Needham p. 218)
At the Siege of Acre in 1191, Richard the Lionheart assembled two trebuchets which he named "God's Own Catapult" and "Bad Neighbor". During a siege of Stirling Castle in 1304, Edward Longshanks ordered his engineers to make a giant trebuchet for the English army, named "Warwolf". Range and size of the weapons varied. In 1421 the future Charles VII of France commissioned a trebuchet (coyllar) that could shoot a stone of 800 kg, while in 1188 at Ashyun balls up to 1,500 kg were used. Average weight of the projectiles was probably around 50-100 kg, with a range of ca. 300 meters. Rate of fire could be noteworthy: at the siege of Lisbon (1147), two engines were capable of launching a stone every 15 seconds. Also human corpses could be used in special occasion: in 1422 Prince Korybut, for example, in the siege of Karl?tejn shot men and manure within the enemy walls, apparently managing to spread infection among the defenders.
Counterweight trebuchets do not appear with certainty in Chinese historical records until about 1268 AD, when the Mongols laid siege to Fancheng and Xiangyang, although Joseph Needham has propounded the view that Qiang Shen, a Chinese commander of the Jurchen Jin Dynasty, 1115-1234, may have invented an early counterweight engine independently in 1232 AD. (Needham, Volume 4, p. 30) At the Siege of Fancheng and Xiangyang, the Mongol army, unable to capture the cities despite besieging the Song defenders for years, brought in two Persian engineers who built hinged counterweight trebuchets and soon reduced the cities to rubble, forcing the surrender of the garrison. These engines were called by the Chinese historians the Huihui Pao (回回砲) or Xiangyang Pao (襄陽砲) ("huihui" means Muslim), because they were first encountered in that battle.
The largest trebuchets needed exceptional quantities of timber: at the siege of Damietta, in 1249, Louis IX of France was able to build a stockade for the whole Crusade camp with the wood from 24 captured Egyptian trebuchets.
With the introduction of gunpowder, the trebuchet lost its place as the siege engine of choice to the cannon. Trebuchets were used both at the siege of Burgos (1475-1476) and siege of Rhodes (1480). The last recorded military use was by Hern?n Cort?s, at the 1521 siege of the Aztec capital Tenochtitl?n. Accounts of the attack note that its use was motivated by the limited supply of gunpowder. The attempt was reportedly unsuccessful: the first projectile landed on the trebuchet itself, destroying it.
In 1779 British forces defending Gibraltar, finding that their cannons were unable to fire far enough for some purposes, constructed a trebuchet. It is unknown how successful this was: the Spanish attackers were eventually defeated, but this was largely due to a sortie.
A traction trebuchet functions in the same way as a counterweight trebuchet, except that instead of a hoisted weight, the hurling arm is powered by a crew of men, pulling on ropes attached to the short lever arm. A counterweight trebuchet is powered by a very heavy counterweight, acting on a lever arm. The fulcrum of the lever (usually an axle) is supported by a high frame, and the counterweight is suspended from the short arm of the lever. The sling is attached to the end of the long arm of the lever. One end of the sling is captive, while the other end is hooked to the long arm in such a way as to release when the arm and sling reach the optimal hurling angles. The trebuchet is energized by lowering the long arm and raising the weighted short arm, usually with a winch, and is locked into the charged state by a trigger mechanism (cocked). With the long arm lowered near ground level, the sling is loaded with the projectile, and laid out on the ground, with the captive and hooked ends away from the target, and the load and pouch laid on the ground toward the target, under the trebuchet. When the trigger is released, the weighted short arm is driven by gravity into an accelerating pendulum motion, causing the lighter, long arm of the lever to revolve around the fulcrum at the opposite arc, which in turn, pulls the sling and its contents into a whipping motion at the end of the long arm. As the arm continues to swing past the vertical position, the counterweight rises, causing the lever motion to begin to slow down, while the sling continues to whip forward around the end of the long arm. When the sling reaches its launch angle, one end slips from its hook, releasing the projectile toward the target.
Each of the components of the trebuchet affect its performance. The lever is critical. It must be as light as possible, for maximum acceleration, yet strong enough not to break under the stress. The ratio of the length of the long to the short arms of the lever, and to the sling length, are important factors in determining the range of the projectile. The object of a good design is to transfer as much energy as possible from the falling counterweight into the projectile. The maximum range for a hypothetical 100% energy transfer, Rmax, of the projectile can be shown to be Rmax = 2hmc / mp, where h is the distance the counterweight falls, and mc and mp are the mass of the counterweight and projectile, respectively. The efficiency of a real trebuchet is then easily determined as the ratio of the actual range achieved to the calculated maximum range.
Medieval designs were probably carried out by making variations on a scale model and determined empirically. There are no really detailed descriptions of medieval or earlier trebuchets that give, for example, the dimensions or shape of the beam, the ratio of its long arm to its short arm, and so on. No specimens or models from medieval times survive. The few extant contemporary drawings of them are highly schematic and even sometimes show physically impossible proportions. Methods used for optimizing their performance and design were apparently closely held military secrets, and are not available to present-day reconstructors.
Emplacing and aiming the trebuchet was also, no doubt, done by empirical trials. Small adjustments can be made by changing the angle of the hook holding the free end of the sling, a process which requires a heated forge on a full-scale engine. For larger, quicker adjustments, the length of the sling can be altered. Small adjustments from side-to-side can also be made by moving the channel in which the missile and sling slide in the base of the frame. The trebuchet itself could be moved as well, but with larger trebuchets, this would have been difficult; the largest trebuchets could weigh many tons.
Because of the time required to load the sling and to raise the counterweight, a large trebuchet's rate of fire is slow, often not more than a couple of shots an hour. This was due both to their sheer size and the massive weight of their counterweights. Smaller trebuchets can fire a couple of times a minute. The payload of a trebuchet was usually a large rounded stone, although other projectiles were occasionally used: dead animals, beehives, the severed heads of captured enemies, small stones burned into clay balls which would explode on impact like grapeshot, barrels of burning tar or oil, Greek fire, or even unsuccessful negotiators, prisoners of war, hostages, and captured spies. Pots of burning lime were also used as projectiles.
Trebuchets were formidably powerful weapons, with a range of up to about 300 yards/ 270 m. Castle designers often built their fortifications with trebuchets in mind; for instance, Caerphilly Castle in Wales was surrounded by artificial lakes to keep besiegers and their siege weapons at a distance. The range of many trebuchets was in fact shorter than that of an English longbow in skilled hands, making it somewhat dangerous to be a trebuchet operator during a siege.
A trebuchet can increase its efficiency (and hence either range or payload) by allowing the counterweight to take the straightest possible downward path. This maximizes the transfer of the counterweight's potential energy to the projectile rather than to stressing the frame. Mounting the counterweight on a pivot (below top) straightens the path of its fall, increasing its effectiveness. A fixed counterweight trebuchet in particular can therefore be made more efficient by the addition of wheels to allow the frame to move freely back and forth (below bottom). This also allowed the trebuchet to fire farther.
The addition of wheels also makes the trebuchet more stable as part of the forward momentum of the falling counterweight is transferred to the forward motion of the trebuchet instead of a tilting action of the vertical frame, possibly tipping over of the machine or severely damaging the structure. The velocity of the trebuchet frame is added to that of the item being thrown, increasing its velocity and range by up to 33 percent. The wheeled trebuchet can effectively employ a fixed counterweight, mounted to the short end of the throwing arm, rather than the pendulum weight described above. The weights were usually stones and rubble, since lead was far too expensive and could be used for better purposes in a siege.
Today, many hobbyists build and experiment with trebuchets. Trebuchets are also used in classrooms to illustrate mechanical and physical principles. Trebuchets range in size from table-top to huge machines weighing many tons.
The largest working trebuchet of medieval design today is at Warwick Castle, which is used as a tourist attraction and is fired by members of the public under professional supervision. It stands 19 m tall and uses a 6 tonne counterweight to fire 15 kg stone balls distances exceeding several hundred feet. T-Wrecks, a trebuchet with a more contemporary design, can throw pianos weighing 230 kg (500 lbs) over 150 m (500 ft). In England a group of farmers threw a car close to 120 m (400 ft) and a 250 litre (55 Imperial gallon, 66 US gallons) drum filled with gasoline over 300 m (1000 ft).
Recent modern developments include: the floating arm trebuchet, where the counterweight is constrained to drop down vertically, while the fixed axle is replaced by rollers; a variation is the F2K trebuchet. Other variations are the "scissor-jack" , "whipper" . Modern hobbyist trebuchets sometimes replace the counterweight with banks of springs.
Another notable variation on the classical trebuchet involves one that props the counterweight in its initial configuration. Since the center of mass is higher than the conventional one in which the counterweight hangs from the end of the short end of the beam, more energy is available for throwing the projectile. This is a little more complex, though, and few have been built. A propped counterweight trebuchet named "King Arthur" and built by the team led by Christopher Gerow, was a Punkin' Chunkin' contest champion for four years running. Its reign was ended in 2004 by the huge "Yankee Siege", video, with a record throw of 424.9 m (1394 ft).
Another trebuchet type is the multi-rotational trebuchet, which is powered by a weight turning a wheel. Although the wheel allows the counterweight to fall straight down and enables the arm to complete multiple rotations, it creates an important difference that separates multi-rotational trebuchets from others. Whereas the high efficiency of a floating arm trebuchet comes from the constrained motion of the arm in addition to the sling, the high efficiency of a multi-rotational trebuchet comes from the sling alone. Due to the behavior of the sling under multiple rotations, small changes in sling length result in large changes in performance.
A United States organization, Science Olympiad, hosts a "Storm the Castle" event for middle and high school students involving the trebuchet. The competitors build a small trebuchet that is less than 1 meter on a side (exact dimensions vary from year to year) and are scored based on the distance and accuracy of their throws.
A trebuchet operated by Roloff Farms (featured on the TV show Little People, Big World) malfunctioned in October 2006, resulting in injuries to two people.
Trebuchet is Old French, from trebucher "to throw over" < tres "over, beyond" and buc "torso" < Latin trans and a Germanic word.
Trebuchets are often referred to as a variety of catapult, though this word is today generally reserved for a device powered by elastic energy.
Other names for counterweight (or counterpoise) trebuchets include bricole or brigola.
Descriptive terms, such as a "witch with ropes for hair" were used by some sources to describe how a traction trebuchet looks.