When you declare war on the neighbouring dorm, make sure you have a weapon of mass hydration!

At short notice today we decided to build a machine to launch water balloons at the neighbouring building. With the aid of Levi, we designed, acquired parts, and assembled in a few hours a lethal water balloon launching sling shot.

The weapon consisted of the following parts:

• 4 metres of bungee cord @ 1.25/mtr
• 2 sections of scrap wood @ $1 ea
• 4 180mm long 10mm dia bolts, and nuts to match @ ~ $2 ea
• left over piece of shade cloth

and of course 1200 water balloons @ $2 per 200.

We eventually got the weapon capable of firing water balloons from the upstairs balcony, over the neighbouring building, and into the car park on the other side.

The construction:

• A frame was made with the wood, cut so we could place one piece on either side of the railing on the balcony. Held together with the bolts, formed the frame and supports for our catapult.
• two holes in each of the supports to feed the elastic cord through
• A roughly 200x120mm square of shade cloth, a few layers thick, with small holes placed in the corners to run the cord through
• The cord threaded through the holes in the supports with the shade cloth pocket threaded onto the cord between the supports
• Attach the supports about 1m apart

A few lessons learned:

• Water balloons burst very easily, if the sling is too tight, and places too much force on the balloon, it will burst on launch.
• the elastic we bought was probably too heavy (it was about 10mm dia), and put too much force on the balloon.
• To counter this, we found placing the supports only 1m apart, rather than keeping it tight at 2m worked better. It accelerated the balloons more gently.
• A single piece of elastic, or much smaller gauge elastic would have worked better. Weaker elastic with a longer stretching distance would give the balloon a much gentler acceleration, hopefully allowing a more powerful, or longer throw
• I don't recommend putting anything other than water balloons in the sling. Balloons don't do too much damage. With this sort of power, they do hurt if they hit you, but anything more solid would be downright dangerous.

Elsbeth, Sarah, Rob, Sarah and Carey, preparing amunition.

What a way to waste an afternoon. Enjoy kids.