Launching Operations - Tuning/Debug

A trebuchet is a machine where making a change in any one area has an effect on the final result: the fling. Several variables affect the fling: the weight of the flung object, counterweight weight, and sling length among others, so only change one variable at a time, and take notes!

All Systems Go

You've built your trebuchet and you want your very first launch to be a perfect toss. It might come to pass that this is your experience - high fives, hooting and yelling, break out the Bud - but it wasn't mine.

I built my machine with an eye towards experimentation, and a good thing, too - it took some tuning and modifications to get consistent throws and to increase the tossing distance.

The Flung Object

Flinging things is what a trebuchet is all about, but a given treb is going to have better flinging success with some objects than with others. Here is a list of general behaviors that I've experienced with the War Weasel:

Everything else being equal,

So, what to do?

The War Weasel (90 lb counterweight) can fling a variety of objects from a softball to a coconut to waterballoons weighing around two pounds with a fine, soaring flight consistently to distances over 160 feet. I recommend tossing objects like these, since they are round, compact, and don't have more than a 1:45 ratio of object mass to counterweight mass.

A trebuchet can fling big, heavy things if the counterweight is heavy enough; if you want to fling a pumpkin 150 feet, you will need more than a 90lb counterweight on your trebuchet.


Everything else being equal, the heavier the counterweight the flatter the trajectory. The lighter the counterweight the higher the trajectory of the thrown object, and if the counterweight is border-line too light, the toss appears to be almost tentative and a bit soft.

The War Weasel started out with 60lbs of counterweight. It could fling water balloons about 120 feet and was fun, but when I added the additional weight and boosted the total to 90 lbs., the treb started tossing with more authority and yielded higher trajectories once I lengthened the sling to compensate for the additional weight.

Sling Length

Everything else being equal, the longer the sling is the flatter the trajectory. The shorter it is, the higher the trajectory. This is because of later/earlier releases from the prong on the throwing arm.

Typically the sling length is how you tune the treb to fling for the optimal flight and distance for a given object. I favor a high trajectory because it looks more impressive than a flat one, and if the trajectory is really flat it seems to me that the flight gets less consistent.

I recommed that if you are showing off your treb to other people, settle on a consistent weight of the tossed objects and avoid having to dither with sling length adjustments. Having a bucket of softballs or a bag of water balloons of very similar weight will allow you to show off how consistent the tosses are.

Indeed, after one balloon toss you can dare people to stand in the middle of the wet spot the water balloon made and see if you can land the next water balloon on them. If you have tuned the treb for a high trajectory, the odds are very good that water balloons of the same weight will land pretty close to them.

If you use softballs, you can send kids out with fielder's mitts to shag the flung fly balls - this is hardly any different than catching a softball hit from a bat. The difference is catching a treb-flung ball is a lot cooler.