Picture of Easy Backyard Catapult for Hero Dads
This is a catapult (ala torsion style) I built inspired from the video a contributor named schoondogs posted.  His catapult looked simple enough so I thought I'd make a step-by-step with some improvements (that's the American way right? - and the Japanese way for that matter).  I am not a master craftsman, just trying to be a Hero Dad* for my boys so look past the low grade finishing and more so on the end result.  The design is not pretty but it is very functional and that's really all a 5 year old cares about anyway.  It took about three hours to build (not counting the trips to Home Depot) and does not include any difficult cuts (all straight cuts) or elaborate tools.

2 - 8' 2x4s
25 or so 3" brass wood screws
24" 1/2 PVC pipe (SCH40)
6 feet of paracord cord
2 small wood dowels (I used a old drum stick)
some type of cup to hold ammo (with hardware for attaching to arm)
Approx 10 big nails (2 1/2 inch honkers)

Power drill
9" Japanese pull saw (this is great for silent late night projects)
7/64 drill bit
1" drill bit
tape measure (or yard stick)

Range: after all modifications this baby can chuck a tennis ball easily 60 feet

*Hero Dad - any dad trying to impress his kids with some type of act or service - in this case (in the steps of my dad) it's building something really cool

Step 1: Building the base

Picture of Building the base
Cut the 2x4s into 14" and 30" lengths.  Drill the 3" screws into each end to create a base for the catapult
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SymanthaP3 months ago
I was woundering how far it whent becuse we are makeing a school project and i what to win 1 2 3 4or 5 place could you git back to me as soon as you can thank you
bgepp1 (author)  SymanthaP3 months ago

it really depends on what kind of "ammo" you are using. I shot a wiffle ball about 30 feet. It might be good to experiment with different materials for the arm. You might try something with less flexibility like a broom handle. Remember - the tighter you can wrap that rope the more tension on the arm. hope that helps.

DrevMaster4 years ago
Very simple wooden structure! My son will be delighted.
Try to do with him and a catapult to storm the sand castle.
To adjust the range to add a movable plank-limiter at the top.
I would love to see a photo of what you mean by a movable plank limiter.
bgepp1 (author)  DrevMaster4 years ago
ya I did a very crude limiter on the top - see the last step under the photo section. It worked okay. I need to make one that doesn't move when the arm hits it. thanx!
simply fabulous elegant even. Remove the extra blocks and add angle butress' then two blocks of wood on the cross beam and stretch a bicycle inner tube. The launch arm will hit that first . Using a cup gives better aim if you made it more spoonish shaped you could eliminate the cross beam and just use an inner tube. I did this with cub scouts in very small scale they could toss a red wirenut 20 feet or tootsie rolls about the same. Had 6 kids doing a free fire shower of tootsies at pack meeting one year. they fired as fast aspossible whilste the other kids scrambled for the tootsie rolls.

this is a hoot, use water balloons. I will see if my son still has his where I can find it and post a picture. But sadly mine is rubber powered!
I would be very interested in seeing a photo of the cup you used.
howarthe6 months ago
I love how small this is. I want to build eight of them for Summerfest. Set them up on s firing line and let kids shoot at targets. I think I need more info on how to make them so that the kids can change the aim of the thing. I'm thinking of setting up targets at 20, 30, and 40 feet. Something like that. What do you think?
MajJ7 months ago

I was wondering if you could use a wooden catapult arm instead of a PVC pipe? What would you reccomend for the length and spacing of the drillbits for that. Also, would the force be too much for the stopper to handle?

bgepp1 (author)  MajJ7 months ago
I think the PVC is best because it is lightweight, I guess you might try a wooden dowel, that might work too. I am not sure on the spacing, you'll have to do a little trial and error. :)
Ginny Head3 years ago
nice project, i'm building one for my wood shop class. i just wanted to say thanks for the cord and band ideas, i think thier going to push my catapult over the top.
thatcub3 years ago
How did you apply the surgical tubing?
stupidninja4 years ago
i was wondering how to make the larger catapult
(please reply)
same thing…Bigger wood
Dr.Paj4 years ago
Now that you've got the basics of a catapult down, I suggest you look into building something slightly more complicated... like a trebuchet. Strictly for educational purposes of course...

I made a trebuchet a few years ago for a physics project and it was quite a fun experience, especially with all of the moving parts and the way the sling works. It's definitely a thing of beauty when you get it calibrated.
bgepp1 (author)  Dr.Paj4 years ago
is the range of the trebuchet (pronounced tray-bu-shay???) better than that of a comparable catapult? I've seen them on youtube (one guy in England made one that could throw cars) but never in person.
rjnerd bgepp14 years ago
The current distance records as kept by the Punkin Chunkin association, have the best ever Trebuchet shot at just over 2,000 feet, while a torsion catapult has managed over 3,000 feet at the same event.
Hey Rjnerd:
You a Scadian or Scouter?

Ever year we have a catupult contest at our scout camp, they lauch flour filled paper bags at each other.
It is a site to see. Look up Rovent Camp OPE
Dr.Paj bgepp14 years ago
It depends what the catapult uses for its power. A 4 foot tall catapult with 3 lines of that super elastic medical tubing can launch a tennis ball over 200 feet. I know because I've seen it first hand, although it did take a 230 lb wrestler to cock it back all of the way. I've built an approximately 1-1.5 foot tall trebuchet (pronounced ˌtrebyəˈ sh et, or treh-bu-shey) and it managed to launch a golf ball around 40+ feet with a couple dollars worth of pennies in the counterweight. In a smaller scale, it won't launch balls as far as a catapult due to the advantages of elastics in smaller models. Now if you were to build a few foot tall trebuchet, it may eventually be able to compete.

As for range, that varies with the size of the throwing arm, the amount of weight, the release angle, etc. A good sight to go to for that kind of information is THIS LINK. He even has a link near the top of the page to a trebuchet calculator that will calculate the range, but it requires quite a bit of information. Anyway, with a trebuchet, it is less about the distance and more about how it all works together. I made one and enjoyed messing around with it, I would post pictures of it but I'm in a dorm and not at the house.
rjnerd Dr.Paj4 years ago
Trebuchet took over from the torsion catapults because they are easier to build and tune. (yes, I have built both sorts of machine).

Quick history:

They started throwing rocks with things that looked (and functioned) like an ordinary crossbow. (ca:8-500 BC) There is only so big you can build a wooden bow, so ca: 400 BC they invented the torsion ballista, with rigid arms, and rope springs for power. Around 100BC they discovered that if the turned it inside out, (springs on the outside, arms sticking inward) they worked better (one example, found near Hatra in Persia) because you wound the springs further.

By 100 AD, they invented the Onager, (the machine described here), which only needed one bundle, and one arm, but performed at least as well. They stopped making ballista.

Around 1000 AD, somewhere in Asia, the Trebuchet was developed, and it spread to Europe within 200 years. They had fewer parts than an Onager, and required fewer special parts get hauled around. They stopped building Onagers.

Eventually gunpowder ended the use of mechanical artillery.

Oh yea, for the most part catapults were built as/where needed. The engineer would show up with a cartload of the special bits, and some stonemasons, The grunts would be sent out to cut down and drag the nearest forest to just outside of bowshot. The engineer would direct the grunts to get things built, the masons would make ammo, and then the castle would suffer granite rain.

Treb's were faster to build, and since they didn't need a lot of expensive and bulky to transport rope, won the logistical battle.
chris152524 years ago
This is great! I want to be a hero dad for my son when he reaches that age (hes 4 months old now). I considered making a replica Da Vinci catapult for him eventually. I bought a desktop version of it and your instructable inspired me to build it. Cheers!
fly2sky4 years ago
Love this project! Can't wait to get started. Just a few questions:

- So you pass the paracord only once through the throwing arm and then just alternate between the two sides correct?
-There should be 2 dowels and you ended up using twice as much paracord to up the power (8 yards)?
- Once you crank it up the pegs snug up and hold themselves flush against the wood?

Thank you in advance, appologize to all for my ignorance in catapult matters. My daughter is going to love it.
bgepp1 (author)  fly2sky4 years ago
Actually I only used 6 ft of paracord. I originally said 8 yards but that was incorrect (not sure where that came from - sometimes I go too fast and mess things up). This part is kinda hard to explain so it's really a bit of trial and error. You have the right idea - you'll know you're doing it right when the arm starts to rise and rest up against the blocker. As you crank the pegs they will stay flush against the wood. The tension is very strong so you don't have to worry about that.

Look at my last step on this project. I highly recommend getting some type of elastic to add power to the throwing arm. I hope this helps.
fly2sky bgepp14 years ago
Thanks so much! Will add the surgical tubing and reply with results when completed.
ncaplan4 years ago
Built it with slight modifications -despite my one goofup of my extra hole which you'll notice in the picture. My one question is what is paracord, what is it's (approximate) dimensions, and where can I get some? This will be fabulous with my students. Thanks!
Look for your nearest Army Surplus store or wilderness/ camping outfitters. Then you can go to the whole section of Instructables of paracord and stay busy all summer! :-P
Paracord (also known as 550 cord), is the cord used in parachutes. There are multiple (6 i think) strings within a woven sheath, and it will hold a surprising amount of weight ( i.e. 550 lbs) for it's dimensions.

It is commonly available at Mil-Surplus stores or online.
bgepp1 (author)  ncaplan4 years ago
wow looks awesome, way better than mine. but alas it's all about performance, not looks (5 year olds are not as concerned with aesthetics). Paracord can be purchased at sporting good stores that have a decent size hunting accessories section. you can probably use other rope (anyone have suggestions?) as long as it's nylon or some other poly based material.
riff raff4 years ago
Very cool. Now we need a video of your dad's larger catapult in action!
bgepp1 (author)  riff raff4 years ago
ya I wish he would have made a video. he said he might make another one this summer - I'll be sure to get video.
stormthirst4 years ago
*Hero Dad - building stuff that Momma really doesn't approve of?
bgepp1 (author)  stormthirst4 years ago
Hero Dad's are usually to Worried-to-Death Mom's. :)

actually my wife is a good sport about this kinda stuff. she knows boys will be boys. I am a lucky dad.
This GrandMa loves building stuff like this with the grandson - occasionally get in trouble with the Moma, but, life is still fun. Can't wait to try this one.
rjnerd4 years ago
Replace the cup on the throwing arm with a sling, to get much more range (without any other changes). The cup on the throwing arm style is mostly a creation of the movie industry. The sling acts like a much longer arm, without the weight of a rigid beam.

Good choice of power lever for a kids machine, enough to be interesting, but not enough to do serious injury. You should be able to skip drilling the holes in the throwing arm, the bundle will hold the arm just fine.

If you are building this for a "big kid" (one of beer drinking age) you will naturally want more power. Power comes from the size of the bundle. Add strands, till you get something where the length is 8-10 times the diameter. You will need to add a brace near the bundle to keep the frame from crushing inwards. You would also need a more durable material in the arm, and some sturdier material for the bits at the ends of the bundles (the greeks called them "epizigus", Mista Ballista uses steel 5 inches thick on its bundles)

Twisted rope can exert tremendous force, so if you scale things up, do your research first. The machines you see on the Punkin Chunkin show, with their bundles a foot in diameter, exert as much as 100 tons of crushing force just from the pre-tension. Our machine has bent heavy steel structure, sheared off 1 inch steel pins, and crushes the 2x6" joists that we use as "padding" on the arm stops.

Oh yea, the most difficult bit when you start to scale things up is the trigger.
bgepp1 (author)  rjnerd4 years ago
thanks! I'd love to build a more power one someday. At the end of my Instructable I have a photo of a torsion catapult my dad build out of logs. he was chucking zuccinis from his garden
neddup984 years ago
I built a smaller version of a catapult for my grandson from a kit. We tried it out using a marble for a load. All I can say is "Don't try firing anything solid inside the house." However, I found that model rocks rolled up from 'Polyfilla' and painted grey flew realistically but wouldn't crack picture glass. We had some fun demolishing a wood block castle until my wife put her foot down.

But seriously, just remember this a WEAPON and even a model can accelerate a projectile to an alarming velocity.
smeeves4 years ago
I made a trebuchet as well, it was made out of steel, so I dont know how much of a backyard project it is. I made it at school. Heres the link: (yes I know that this is shameless self promotion)

and7barton4 years ago
A few years ago I built a trebuchet with an 8 foot throwing arm. The counterweight weighed 250 pounds.
It could throw a 1 pound water-washed pebble around 300 feet in a low trajectory. Or an egg around 400 feet in a high arc.

bgepp1 (author)  and7barton4 years ago
pretty awesome. did you u see the guy in England who can throw old junker cars with his?
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