So you decided that you wanted to build a catapult. Now it's finished and ready to fire. Your excitement builds as you pull the release mechanism then, plop. Your ammo sails a few feet and drops to the ground with a disappointing thud. So much for your plans to hurl water balloons at your neighbor on the other side of the fence. You feel the sting of defeat. What went wrong? Why did not it work? Do not despair my friend, for I may have a solution to your problem. The next couple of paragraphs will explain a few things that you can do to increase the firing distance of your siege engine.
Check your construction. Make sure your catapult has been properly built. Improperly fitted parts or loose parts can affect the efficiency of your machine. One of the largest areas of energy loss in a catapult is the pivot points. Anywhere something pivots there will be friction. Make sure the parts move freely, without binding. Depending on your construction lubrication may be needed. If your pivot point is a wooden axle through a hole drilled in another piece of wood make sure it's not too tight. If the fit is too tight, light sand until a fit fit is achieved. If there is too much play replace the piece of wood with the hole in it with a new piece that has a smaller hole. Too much play can also cause energy loss.
More tension! Many catapults are built using springs, bungee cords, or surgical tubing as the source for tension to the firing arm. Beef them up! Replace your existing springs with stronger springs. The same applies to bungee cords rubber bands. Replace the existing ones with something a little stronger. You can also add more springs or cords to the existing ones (double them up). If you have an onager, try winding the cord that is attached to the throwing arm a little tighter. This will increase the tension and force of the throwing arm when it is released. One thing to keep in mind if you are going to increase your spring tension: make sure the framework and firing arm are sturdy enough to handle the increased tension. You do not want your sword engine to blow apart when you fire it. You may need to reinforce the frame and throwing arm to accomplish this.
All hail the trebuchet! Ah, the mighty trebuchet. It is by far my favorite of all the siege engines. It is far more sophisticated in design than other types of catapults. When properly tuned the distance and accuracy are incredible. Getting to this point can be a real pain in the rear end if you're not sure what to do. You can build a trebuchet that works, or build one that works really well. I have a small table top model that can hurl a small wooden cannonball about 20 feet across my basement. If I place a 9 "paper plate on the floor as my target I can land every shot in the plate. I achieved this accuracy by simply making adjustments to the sling (length is critical!). trebuchet.
As I mentioned above, pivot points are areas of friction. Excess friction means energy loss. Make sure the pivot point of the throwing arm moves freely without binding. Adjust as necessary. If your trebuchet has a hinged counterweight make sure this pivot point moves freely as well. While we're on the subject of counterweights, keep in mind that the further the distance the counterweight falls, the greater the energy is to the throwing arm. Raising the height of the main axis (pivot point for throwing arm) and lengthening the throwing arm will allow the counterweight to travel further when released. Increasing the weight of the counterweight will help as well. Note that changing the length and height of the throwing arm will require adjustments to the sling. Slings are a very critical piece of a well tuned trebuchet. Incorrect lengths can cause the sling to release at the wrong time. This can result in a high arc with very short distance or a low arc with a short distance. Experiment with the sling length for greatest distance. Make sure the pouch is not too large for the ammo. This can result in excess wind drag. Use as small a pouch as possible for the size ammo you are using.
I hope this information is helpful to those who may need it. As a final note I want to mention that while catapults are fun, they can be dangerous as well. When making configurations to your catapult make sure the overall structure can handle the changes. Reinforce as necessary. Always use proper safety measures when firing a catapult. Eyewear is always recommended. Maintain a safe distance from the machine when you are firing it. With safety in mind operating a catapult can be a fun experience for people of all ages. Happy Hurling!