I built this catapult (actually a trebuchet) based on an article in Make Magazine by William Gurstelle. It worked extremely well! Was able to launch basketballs 80 - 90 feet, and small pie pumpkins over 100 feet. I think fine tuning of the sling release could further enhance the performance.
It is made of plywood, a few 2 x 6's, and a lot of plumbing parts. The folding design is great for storage, and in my case, the catapult actually went up on the roof of a church for the main launching event. It fit up a ladder through a roof hatch with no trouble. (Well, other than the fact that it is bulky and heavy).
The counterweight is two 60 pound bags of concrete, which gave about a 100:1 ratio for the pie pumpkins and basketballs. After the pictures were taken I also added several bungee cords (2 x 18" and 2 x 24") to assist with pulling the weight down, and it added another 20 feet or so to overall distance.
Just something fun about catapults!
Step 1: Building the platform
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The base is made of 3/4" plywood. It uses about a full 4 x 8 sheet, which I cut up into four 2 x 4 sections to make it easy to work with. The uprights are two 28" long pieces of 2 x 6 lumber glued and screwed together.
When you cut the plywood sections to size there are three pieces of wood per section. The platform piece itself, and then two narrow reinforcing pieces that go along the edges.
Cut the plywood and 2 x 6s to size, and glue and screw together.
Step 2: Assembling the folding base
Lay the assembled plywood sections out as they will be in the final assembly. The sections are attached together with door hinges so they can easily fold. When I was attaching the door hinges, I set one hinge at each end of the gap between sections, with the hinge "bulge down" to make sure the spacing between boards was correct.
Step 3: Attaching the uprights to the base
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Next step is to attach the uprights to the base. They are only attached with a pair of hinges, so they can fold flush with the platform for transport or storage. Wire cable (1/8") and turnbuckles are used to tension the arms and keep the solidly upright for launching.
Also in this step, attach 1" flanges to the uprights, which will hold the throwing arm assembly.
Two screw eyes go in the top of each upright, and then matching screw eyes go on the base platform, in line with the uprights. This is where the wire will attach to hold things in place.
Step 4: Wire rigging for uprights
Tensioned wire rope is used to add rigidity and stability to the uprights. Use S hooks and turnbuckles to connect wire between the screw eyes on the uprights and the screw eyes on the platform.
Step 5: The counterpoise - concrete holder
The counterpoise platform is a 16" x 20" piece of plywood with 1" pipe flanges attached. The rest of the platform is made up of pipe nipples, 90 degree elbows, and a tee fitting. This will swivel at the tee fitting, so I lubricated with grease. Make sure it is not so tight that the tee will not swivel freely.
Step 6: Assembling the throwing arm
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The throwing arm is made of 1" pipe. Six foot long section connected to a 1" cross fitting. (Had a lot of trouble sourcing this part. Eventually had to order it from Amazon). The throwing arm is also tensioned with wire rope and turnbuckle to help with rigidity.
Drill a hole through the cross to insert a long eye bolt. Two other eye bolts are also inserted into the throwing arm, one at the end, and one at midpoint.
A cap is attached at the end of the throwing arm, with a hole drilled through to allow insertion of a bolt. This will be what the free end of the sling attaches to.
The counterpoise assembly connects to the opposite end of the cross fitting, and two 8" nipples are attached to the sides of the cross fitting.
The arm assembly is attached to the uprights with close nipples and 1" pipe unions. Thread the close nipples into the flanges on the uprights, and then thread the non-removable side of the union onto the close nipple.
Step 7: Attach throwing arm to uprights.
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Assemble the throwing arm to the uprights using the unions. Also attach wire rope to eyebolts on arm, and tension with the turnbuckle. Don't over tighten. You don't want to bend the eye bolts or the arm.
Step 8: Quick release mechanism
The catapult release is made with chain and an equestrian panic snap. The panic snap releases with a downward tug, allowing the arm to quickly rise. The panic snap is connected to a steel ring on the arm, which is attached with a hose clamp. The other end of the panic snap is attached to a chain, which is attached to the platform with a conduit clamp.
In order to be able to remotely trigger the panic snap, I lashed some cord around the body, and liberally soaked with thin CA glue. The cord then goes through an eye attached to the deck of the catapult.
Step 9: Sling and final assembly
The sling I used is a modified veterinary sling that I had from our dog's recent hospital visit. It was the perfect size for cradling a pumpkin, football, or basketball. One end of the sling is attached to a ring on the throwing arm, held in place with a hose clamp. The other end of the sling has a ring attached, and it slides onto the bolt that is protruding from the pipe cap at the end of the throwing arm.
I wrapped some duct tape around the bolt to allow the ring to slide off more easily.
Step 10: Final notes
Catapult ready for launch. The weight is provided by two 60 pound bags of concrete. In the final version, I also added some bungee cords that run from under the counterpoise platform to the turnbuckles at the rear of the platform, adding more force and speed to the falling concrete weight.
Also during testing, the sling would drag slightly on the plywood platform, so I added a piece of horse butt leather for testing. This was later replaced by a piece of slick MDO plywood.
Basketballs were launching from 60 - 80 feet, and footballs about the same. It actually throws a football in a spiral, which is cool to see. The furthest pie pumpkin (1 - 1.5 pounds) went over 100 feet.