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A Floating Arm Trebuchet or F.A.T. is a siege machine with a modern twist. Invented in the late 1990's, it uses the same physics involved with a normal trebuchet. The difference involves the weight and fulcrum.

The machine has two rails raised in the air by a frame, usually wooden, anywhere from 6-10 feet (any height can be used, but the biggest I've seen are between 6 and 10 ft.). Halfway down the rails there is a gap. This is where the weight falls down.

The weight is supported by four vertical shafts, two on each side of the gap of each rail. The weight is connected to a throwing arm on a hinge. At 1/3 the distance of the arm from the hinge is where a set of wheels are. These wheels ride on the rails. At the cocked position the weight is suspended at the top of the four shafts with the wheels resting on the rails some distance from the shafts. As the weight is dropped the wheels are pushed back with the arm until the weight gets to the rails.

When this happens, the arm is parallel to the rails. At this point, the weight continues downward and pulls the arm and wheels forward, creating a whip-like action. At the end of the arm is a regular sling that releases at some point before the weight hits the bottom.

The sling is the same kind you would use on a conventional swinging-weight trebuchet. F.A.T.'s can throw much farther with much less weight because all energy is conserved on the throw. With a swinging-weight, some energy is lost because the weight doesn't drop straight down, it drops on an elliptical path. This elliptical path puts force on the structure and axel which would not happen on a F.A.T.

The wheels provide a constantly changing fulcrum that moves as the weight drops. A smooth rail is essential to success. Depending on the amount of counterweight involved, the wheels should be made of a solid material.

I've recently constructed one with my friends and it is a lot of fun. It's 10 ft. long, about 12 ft. high, and it uses over 1000lbs. of counterweight. Our arm is roughly 12-15 ft. long and it will throw a 12lb. bowling ball about 800 ft. I would encourage anyone interested in siege machines or something to get drunk and do on weekends to find some plans and make one. They are relatively simple and cheap, which made it fun for some high school graduates to do in the summer.

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