Onager Catapult design

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Hi! I'm making a onager for a project and so far I'm not having much luck! My Onager has to fit inside a 0.5m x 0.5m x 0.5m cube, and it will be throwing tennis balls.

I've been browsing around a few nights already and found this topic, which has helped me the most by far: .physicsforums.com/archive/index.php/t-155437 (please add "www" in front and ".html" at the end, I can't post links :cry:)

But there are a few things I still don't quite grasp.

1. Long thin ropes vs Short thick ropes, which one would give me more torque?

I'm guessing it's the long thin ropes because most pictures I've seen use those.
I'm using really thick natural fibre ropes right now I can only have two pass-bys on the arm with 3 full-revolution twists on each side.​

2. How would I get the sling to release the at 45 degrees like the one in this picture?
.onager.net/pict12 (please add "www" in front and ".jpg" at the end, I can't post links :cry:)

This just befuddles me

3. How would I make a locking mechanism and where do I place it?
I attempted a few ones but only to end in failure --it was either too weak or I couldn't pull it to release it.

4. Round arm vs Rectangular arm
Would it be better to have a round arm like most of the Onagers i've seen? I'm not sure if it would help with anything.

I'm currently using a rectangular arm with two nails on the side to secure it in the rope​


Any help would be MUCH appreciated!
I'm getting a headache from trying to figure these things out​
 
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