Kids and I built not just 1, but 2 catapults today! Second one is better, made some modifications to the plans from what we learned with the first one.

Total size is 36" long by 15" wide, and 15" high. Arm is 30" long. Tension is a nylon rope twisted using 2 wooden dowels. I found my old leftover rebar in the garage this evening from a project a few years ago, so I may take the 2nd one apart and use rebar instead to twist the rope. Just to see how much further I can get it to launch the water balloons.

(Plans and instructions came from stormthecastle.com.)

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Update:

I went out tonight after the kids were in bed and replaced the small wooden dowels which had started to crack with metal rebar. This allowed me to put much more tension on the rope than before. Had we been 2 people, I probably could have continued to put even more tension. As it stands now, I launched a small stone approximately 52 feet (15,8 meters) with this catapult.

This is no longer a kid's toy to throw water balloons. I'm going to have to reduce the tension before someone gets hurt!