Definitely NOT another desktop trebuchet experiment

My big Floating Arm trebuchet (FAT)

Here is my first trebuchet not made from lollipop sticks.  It was originally a kit bought from America, but I had to discard most of the wood.  I bought fresh wood and some MDF for the base and cut and drilled the base and the legs.  Once the eight legs were attached to the base and made to be perpendicular the rest of the wood was cut and drilled to fit to ensure that the whole frame was as square and true as I could possibly make it.  When completed the first dry fire with empty counter weight boxes left me grinning like a mad fool.  It had an impressive action.

After the glue had set, for which I gave it 48 hours just to be on the safe side, I tried my first shots.  With only 8 1/2 pounds of counter weight I achieved a 108 foot throw.

I've since been playing with the machine with some office colleagues during a lunchtime and it was firing a golf ball quite some distance.  I was using 9 1/2 pounds of counter weight, and based on what I know about the weight of the golf ball and the efficiency of the machine so far I estimate that we were throwing the golf ball some 150 to 160 feet.  We were targeting a small castle made of boxes used to hold printer paper, we managed to scatter the boxes across the grass several times.

Here are some stats.

And here are the results of the best throw that I've measured:-

The efficiency is calculated as the ratio of actual throw distance to the theoretical maximum, expressed as a percentage.  The theoretical maximum can be approximated as 2 * counter weight * drop of counter weight divided by projectile weight.  For the 108 feet throw we get the theoretical maximum of 2 * 136 oz * 26 inches divided by 2 oz = 3536 inches.

The photos

A test firing sequence

The sequence below shows the treb during a dry firing, mainly to show the action involved.  The counter weight boxes are empty and there is no projectile in the sling, so the arm thrashes about a bit after the treb is triggered.  When everything is normal the arm is much less vigorous after the counter weights have dropped fully since the energy in the weight has been expended by throwing the projectile.