Sunday, 25 April 2010

Trebuchets, chickens and plans

Trebuchets are ancient gravity powered siege weapons that, if corrctly made, can hurl an object some distance with great accuracy. In olden days the projectile of choice included rocks, fire balls and dead people or animals. Nowadays we use, of course, rotten eggs.

The WETF HQ was visited yesterday by a bunch of hopeful egg hurlers under the leadership of a neighbour called "Half Finished". He and his chums have been building a trebuchet for the games in June and requested assistance from the in-house experts with the design of their "egguchet". They explained that the primary problem as that their machine would only throw their golf ball projectile 30 metres and it was travelling way too fast to catch.

In this sport the aim is to hurl an egg to a waiting team member at a starting distance of 30 metres away, three times. The egg has to caught whole for 3 points, if contracted but damaged for 1 point. The distance is then extended by 10 metres and the exercise repeated. Then again and again and again etc.

The trick here is to design a machine that has a variable throw and to use the "lob" technique whenever possible. This involves getting the machine to throwing the egg gently in a suitable arc, so forward momentum is all but lost and the egg falls gently to the waiting hands of the target. When we say gently we mean its accelerating at 9.8 metres per second per second. If one uses a straight throw downward travel speed is limited but the horizontal speed is about 120 mph. For comparison, a bowled cricket ball usually travels at around 95mph.

As the last American team's trebuchet is stored at WETF HQ for this years event (as airlines have problems carrying Weapons of Mess Destruction) this was wheeled out and the problem faced explained. Its not a complicated machine, constructed basically out of a bucket, a piece of stair rail, a section of scaffold pole, two fence posts, a 1953 Morris 1000 exhaust bracket and some string. Size isn't important, nor the counter balance weight, what is vital is the release mechanism. This is a small adjustable pin on the end of the throwing arm that dictates the point of release and thus the angle of the throw.

Watched closely through the fence by the chickens, the device was set up and proceeded to hurl the requested distances. The closer the throw, the higher the lob, the further the distance the more shallow. At 150 metres its an almost straight throw. The height of the lob can be controlled by the release height of the throwing arm and or varying the counter balance mass. A word of caution here, if you over weight the machine it "dry fires", releasing the projectile backwards. That is not a good thing.

Their problem explained "Half Finished" and his team of miscreants depart over the road to re-jig their WMD and take liquid refreshments. Throughout the afternoon cheering could be heard as each shot was released and sent to its, mostly, intended target.

The Beryls wandered off into the woods, clucking their plans amongst themselves, WETF HQ returned to its normal quiet, Mo stalking about keeping watch, doves cooing and in the distance the sound of a woodpecker hammering. At least I think its a woodpecker. We may have to see what the girls are up to.

There are videos and details about how to design a trebuchet on

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