Project #22 in the Daddy Handbook: A Catapult

As-built catapult

Because she saw one at her school’s science fair, last week our youngest daughter asked me to help her build a catapult. Feeling intrigued by the idea of putting together something that shoots something else but not wanting to go overboard (you never know what kids do with these), I came up with this design. As you can see in the images on this page, the catapult requires only two different wood items, a rubber band and a cup as raw materials. It can be manufactured with only few tools and everything is pretty much self-aligning. The outrigger-type supports can be used to either mount wheels or to hold the catapult down with bricks.

First tests with marshmallows (apparently a very popular projectile) showed that this thing shoots approximately 10 feet at the given size. Just to mention this again: Firepower was not in my best interest, therefore the size is completely sufficient for kids. Within limits, this design may be conducive to scaling.

The rendering below shows the how one could use bricks to weigh the catapult down.

Catapult rendering (using Twilight renderer)

So how was this made? Here are the details:

Cutting and assembly diagram (click to enlarge)

A quick SketchUp model (that made good use of SketchUp Pro’s Trim tool to cut all holes) shows what is needed:

  • 2 pieces of 1/2″ x 1-1/2″ trim, cut to 9-3/4″ length
  • 2 pieces of 1/2″ x 1-1/2″ trim, cut to 1′ 3-5/8″ length (keeping the first two items at these lengths creates a right-angled support – vary the numbers to modify steepness of projectile path)
  • 3 pieces of 1/2″ x 1-1/2″ trim, cut to 1′ 6″ length
  • 2 5/8″ wood dowels, cut to 8-1/2″ length
  • 1 5/8″ wood dowel, cut to 5-1/2″ length
  • A rubber band (I used the tie-down rubber shown in the image because it has convenient hooks)
  • A plastic cup, cut shorter in height
  • 2 screws to attach the plastic cup
  • Wood glue to fix all non-moving parts (not absolutely necessary because everything will friction-fit pretty well already)

After cutting everything to length, use a 5/8″ drill bit to cut holes 1″ from all ends into the wood pieces. Make sure you center these as accurately as possible. I used a notch to attach the rubber band to the moving arm. Alternatively, you could drill a hole there and run the rubber through it. Finally, screw the plastic cup with two small screws into the top of the moving arm.

Assemble everything in the order as shown on this page. If you want to, glue all of the non-moving parts (everything except the arm) to give the catapult more rigidity.

Now you are ready to try it out. As always, it’s a good idea to give the kids some basic instructions like “always stand behind the thing” and “never shoot rocks or anything that’s alive” but I’ll leave the details of this up to you.


Interactive 3D Viewer


Front view

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