The skeins are made of 300 foot of 1/4 inch twisted, braided nylon anchor line.  I believe that another 200 - 300 foot of line would ease the load on the skeins, allowing it to remain consistent for a longer period before tightening the skeins.
 
 
 
 
 

The sling is the most important part of the engine.  It is attached to the arm of the onager by a rope fed through a hole drilled transverse, then knoted on both ends after being passed through the leather sling pouch.  The other side of the pouch is fitted with a rope that is tied off into a loop at the end, the loop fitting over the pin at the end of the arm.  Adjustment of the length and balance of the sling is critical to the trajectory and range of the projectile.
 
 
 
 
 
 
 
 

A hand-forged sliphook acts as a release, it fits into a steel ring on the bottom of the arm.  A rope connected to the lever end of the hook (the end with the hole) is the simple trigger.