Easy Backyard Catapult for Hero Dads

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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)
hammer


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

Cut the 2x4s into 14" and 30" lengths.  Drill the 3" screws into each end to create a base for the catapult

Step 2: Add the Side Supports

Approx 17" from the back of the catapult nail in the upright supports.  These supports are 13" tall

Step 3: Install the Block Supports

Most catapults use a triangular support for their uprights but since I just wanted to make a functional (not pretty) catapult I just opted for easy horizontal blocks. 

I did not show this step but you will also need to add the horizontal (stopper) support - this is 14" wide.

Step 4: Drill Side Holes

I used a 1" drill bit to make these side holes - this is where the rope will be fed through.  Placement is about 1" down and 1" over.  Notice that this hold is not in the middle of the 2x4 - this will allow for the arm to swing and not scrap on the ground.  I made sure that these allowed for the pvc pipe to stand vertical, flush with the cross supports

Step 5: Drill Catapult Arm Holes

I used the 7/64 drill bit to make these holes.  The great thing about pvc is you can easily carve out the holes so they are a larger than your bit.  This is nice if you don't have a larger bit.   When drilling these holes make sure the end of the arm does not drag on the ground when the arm is in action.

Step 6: Threading the Para Cord Rope

This is a crucial step and not easy to explain so I took a lot of photos.  I ended up using about 4 yards of rope to string up the arm.  I found that if you tie down one side with the peg that helps in threading this baby up.  I first threaded the rope through the two drilled holes and then alternated front and back until I ran out of rope. 

Step 7: Crank It

Now it's time to crank.  Alternate from each side and crank the pegs clockwise until the cord is really tight.  The arm should be firmly standing at a 90 degree angle, tight against the stopper bar.

Step 8: Ammo Cup

This was my first ammo cup - I changed it later.  My only recommendation is to not use a bowl or a curved cup, the ammo get's thrown to the ground.  I used a old clay container - it has straight sides so that worked pretty good (until it broke due to the force of the arm hitting the stopper too many times).  Depending on what ammo you use - we used mostly wiffle balls and tennis balls.  Later on I made a more of squared platform with a cut out for the balls - see "Modifications for more detail"

Step 9: First Test

http://www.youtube.com/watch?v=k4u8DrwyosQ

after this test is when I decided to double the rope



 

Step 10: Modifications & Action Videos

After some field testing I realized the catapult needed some modifications.  So I added a foot on the front and on the cross bar to be able to change the trajectory.  I tried extending the arm to see if that had any baring on the power.  I also added a stripe of surgical tubing that gave more tension to the arm.  This was a big help and really gave the catapult more power.
http://www.youtube.com/watch?v=US97RQglQzA

http://www.youtube.com/watch?v=j-LiVs2BVOo

this can also be placed atop any skateboard so you can easily drag it to family picnics or the local park

here's an old video of one of my dad's catapults
http://www.youtube.com/watch?v=bTaiYhzCaDg

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    56 Discussions

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    rach2005

    3 months ago on Step 3

    What are the dimensions for this? We are doing a catapult for a class project and it needs to be 18x12x12 to fit in a box and needs to be able to launch a small rock 20ft. And the rope is double wrapped right?

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    shelby221345

    2 years ago

    hey how do you make the ammo cup . I have been having trouble with

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    bgepp1shelby221345

    Reply 2 years ago

    I would use a small plastic cup. An old Tupperware container might be your best bet because the plastic is flexible (i.e. non brittle.) Just drill a hole in it and through the pvc pipe. Attach it with a bolt and nut- make sure to use washers cuz you don't want the cup to fly off. hope this helps.

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    SymanthaP

    3 years 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

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    bgepp1SymanthaP

    Reply 3 years ago on Introduction

    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.

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    DrevMaster

    7 years ago on Introduction

    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.

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    howartheDrevMaster

    Reply 3 years ago

    I would love to see a photo of what you mean by a movable plank limiter.

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    bgepp1DrevMaster

    Reply 7 years ago on Introduction

    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!

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    spark master

    7 years ago on Introduction

    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!

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    howarthespark master

    Reply 3 years ago

    I would be very interested in seeing a photo of the cup you used.

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    howarthe

    3 years 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?

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    MajJ

    3 years ago on Introduction

    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?

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    bgepp1MajJ

    Reply 3 years ago on Introduction

    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. :)

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    Ginny Head

    7 years ago on Introduction

    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.

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    thatcub

    7 years ago on Step 10

    How did you apply the surgical tubing?

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    Dr.Paj

    7 years ago on Introduction

    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.

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    bgepp1Dr.Paj

    Reply 7 years ago on Introduction

    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.