This is a scale model of the Trebuchet I hope to build with my son's scout troop this year. It is made with 6, 4ft dowels 5/8 inch in diameter. Using 1/8 inch jute twine for all the lashings, Only the launching plank is glued, all lashing are standard boy scout square lashings 3 wraps and 2-3 fraps each . There are many sites to show you how, look up the inside outside rule, this will make the lash as tight as possible. Each lashing should take about 4ft of twine,

It consists of 2 A Frames with dual support along with t buttresses / stanchions on each side to keep it from racking.

The throwing arm is 25 inches with a hole drilled at the 5 inch mark for a 1/4 inch steel bar, this arm currently support a counterweight of 10lbs. The throwing basket is duct tape reinforced with a piece of fiberglass cloth. The escape mechanism is made of a coat hanger and a key chain ring. As of right now I do not have a release mechanism (works fine by hand the full size trebuchet will have to have one).

Enough prep, onto the build

Step 1: Materials and Cutting

Materials:
6 5/8" x 4' dowels (there is some waste about 2ft when I was done.

Cutting:

6 18" - 3 for each A frame, 2 for the frame and one for the lower support
6 12" - 2 for the upper A frame supports and 4 for the stanchions.
2 24" - Support base to hold up A frame and provide a base for plank
1 1 1/2" x 14" x 1/4" launching plank (mine was a scrap piece of molding)
1 25" - throwing arm

Jute Twine:
100' of 1/8 inch jute twine or alike each lash should take about 4 ft +

Head of Arm Release

4" from coat hanger
1 Old key chain loop about 1/2 inch in diameter.

Throwing Axel
~ 9" 1/4" steel rod
Drinking straw
4 Nylon Washers
2 1/4" end caps

Basket

Duct tape multiple pieces:
5" x 2 in.
Fiberglass cloth ~ 5" x 2" to strengthen the tape.

Zip tie and electrical tape to hold Arm release.

Tools
Hand or Chop Saw
Pliers and Cutters
1/4 inch drill

Step 2: Lashing Aframe

Each Aframe requires 3 18" and 1 12" dowl.

Take 2 18" dowels and form an X with the cross about 2 inches from the top. Lash them together at a right angle.
Take the third 18" dowel and lash it about 3" up from the bottom on the other two. The distance between the two poles you are lashing two should be about 11 1/2" on center.
This one is slightly higher due to the bottom supports to be added later.
The picture above shows the top of the A frame lashed, below it is the lower support and the upper support (sorry that is a scrap below that).

Next lash the upper support (this is support for the stanchions) about 5 1/4 inch down from the upper A frame lashing on both sides.
Now to attach the A frames to the support beams.

Step 3: Lash a Frame to Support Beam

The support beams are both 24 inches high and are lashed to the bottom of the A frame. In my case I didn't care about A Frame orientation.
Lash the support beams 8" from the end of each beam about 1 1/2 " up on the bottom of the A frame.
When you are done each A frame should support itself.

Step 4: Lash the Stansions

Lash the 12" stanchions about 1 1/2" in from the end of the upper A frame support and about 1" down the stanchion dowel.
Lash the bottom such that it can help support the structure when placed on a flat surface.
Repeat for all the stanchions.

Your structure should have 8 points of contact with the ground, try to level it as much as possible.
Note the picture here shows that some adjustments must be made to make the A frames line up. These dowels are well sanded and sometimes the lashing slips no matter how tight you make it.

Step 5: Throwing Arm Part A

Take the 25" inch thowing arm and drill a clearance hole for the coat hanger wire 3/4" below the top.
The wire should be about 3" long. Bend it at a right angel using pliers about 1 1/2". Pass it though the hole.
Push it up against the dowel mark the point with a pencil on do it on both sides. Using a utility knife carve a slot the dept of the wire.
Place it through again and bend the other side such that it falls into the groove. It should not prodtrude beyond the top on one side and about 3/4 inch on the other. Use pliers to hold force it into place, I used tie wraps to hold it near the top and bottom.
This comes in handy a s tie off place for the trowing arm rope. Once you have the tie wraps in place bend the exposed wire to about 30 degrees.

Step 6: Throwing Arm Part B

Drill a 1/4" hole 5" from the bottom to hold the axle. About 1" from the bottom drill a 1/8" hole to accommodate wire to hold the counterweight.
Center the axle, I cut drinking straws to use as spacers and nylon washers to hold it in place, but this is optional. Lastly add the end caps using a hammer to drive them in.

Step 7: Basket / Trowing Rope

The trowing rope should be such that the bottom of the basket is at the axle.
To do this first make the basket:
I used 4 pieces of duct tape 5" long.
Place the first sticky side up on a flat surface.
Cut a piece of fiberglass cloth just smaller than the duct tape and lay it on the sticky side .
Place a second piece over the fiberglass cloth.
Seal the top and bottom using 2 more pieces of duct tape.
Next fold each end of the basket such that it makes the whole thing cup.
Duct tape the ends to retain the shape.
Use an awl to make holes for the twine.
Attach the twine leaving plenty for adjustment.
(Still looking for a picture of this)
Attach the twine such that the bottom of the basket is at the axle.

Step 8: Glue the Ramp Attach Throwing Arm

Like it says glue the ramp to the base at the center of the support beams . It should also be centered between the A frames.

Attach throwning arm such that it is centered over the ramp.

Step 9: Test the Trebuchet

I have tested mine up to 10lbs it can toss a rubber ball about 24'
Of course it would go better if the base were on wheels.

Pinch the ball between two finger in the basket pull back such that the basket is a little past the center of the ramp (just past the axle).
Release in one movement do not pull back. I will post some videos after I upload it.

I will put a few snapshots in the next day or two.
Thanks