The string pouch didn't seem to be much of a problem. I thought that I would have problems making something that releases well but it went smoothly and there were no issues so here is what I did. I used a piece of cloth, some masking tape and a pair of shoelaces from an old pair of workboots as the strings. To assemble the bowling ball I used some rope, masking tape and a small piece of shoelace. It took a little time and patience to get the ball nice and secure but it came out great.

I cut a piece of cloth 10 inches by 4 inches and I securely taped the top and bottom edges with masking tape.You apply the tape and then fold it over.

Now fold the cloth in half like this. You can now tie a string at each end. The strings I used are 36 inches long.

Here is the string pouch assembly completed. You have to also make a little loop for one end of one of the strings. I used a loop from a spiral bound notebook. Just tie one end of one of the strings to it. This loop will be looped onto the headless nail on the end of the trebuchet swing arm and it will come free allowing the pouch to open and the projectile to fly.

Now just tie the string without the ring to the hoop on the trebuchet swing arm. When you pull down this arm and put it in the ready to fire position. You will put the hooped string onto the nail shown here. This will release as it fires - opening the pouch. This nail is important. It should be headless and it should be pointed at a slight angle to the front. Adjusting this angle will adjust the trajectory of your projectile. Pointing it more forward will release later giving you a lower arc. Pointing it more toward the back will release it sooner causing it to arc higher in the air.

About the Bowling Ball Counterweight. I used rope very tightly tied on the ball then taped it down. I left just a small spot open at the top with no tape and through this spot I ran a piece of shoelace as a loop - It goes under both pieces of rope. When you make your counterweight for your trebuchet make sure the little loop of string is long enough so the ball will be not touch the swingarm when in full vertical position. And also make sure your counterweight does not touch the base of the trebuchet when it swings. It needs a little bit of clearance. So you will need to tinker with the size of this small string that connects ball to swing arm.

Lets Fire off the Trebuchet and get some tips and hints on the building