How can I "reload" a slingshot?

I'm working on a 2D scroller that has an slingshot mechanic (a boat in the main plane shooting at other boats on the background, kinda parallax), but I'm having a few issues on instantiating the projectiles while keeping their Physics and Rigidbody values. I have tried using prefabs, with no success. This is how the slingshot works. Can someone give me any pointers or ideas?

 void OnMouseDrag()
     {
         bola.isKinematic = false;
         Vector3 curScreenPoint = new Vector3 (Input.mousePosition.x, Input.mousePosition.y, 1f);
         Vector3 curPosition = Camera.main.ScreenToWorldPoint (curScreenPoint);
         transform.position = curPosition;
     }
     void OnMouseUp ()
     {
         dir = sling.transform.position - transform.position;
         dir = dir.normalized;
         bola.AddForce (dir * 1250f);
         spring.enabled = false;![alt text][1]
 
 

[1]: /storage/temp/89638-potakingunityhelps.png