I had someone ask me how to tie a "Prusik Hitch", Its actually a climbers knot that grips a climbing rope, But it works VERY well to attach a loop on a boom. The use of glass or carbon fibre limits the way you fasten things to them. The Prusik Hitch grips all the way around and can be moved if required to change pulley positions.
The first step to tie a Prusik knot is to take the loop of cord and place it behind the main climbing rope.
The second step to tie a Prusik knot is to take the loop of cord behind the climbing rope and bring half of the loop through the other half of the loop and form a girth hitch. A girth hitch is a basic knot for attaching a sling or cord to any object, including a tree, piece of climbing gear, or, in this case, the climbing rope. Note that the knot in the small cord is on the outside of the hitch.
The third step to tie a Prusik knot is to bring the loop of cord back through the girth hitch on the climbing rope two to three more times, forming a barrel with the tail of the cord hanging out from the middle. This is simply done by wrapping the loop of cord through the inside of each previous wrap. After you're done wrapping the rope, tighten the knot and dress it by carefully arranging all the wraps of cord so they're next to each other and not crossed.
How many wraps of cord you put on the knot is up to you. Usually three is sufficient. The more wraps you put on, the more the Prusik knot will cinch onto the climbing rope. It's best, especially if you haven't used a Prusik knot much, to test the knot by weighting it. If it slips, add another wrap. If it's too hard to push up the rope, take away a wrap. If you leave the knot a bit loose, it is easier to slide up the rope.
www.liveabout.com/tie-and-use-a-prusik-knot-755674