I wanted a way to tie a short prusik loop that I could attach to a carbiner for an adjustable center "anchor" in a DREC rig. I ended up with a knot derived from a traditional noose knot. It has a single loop on the top for the prusik and two sliding loops on the bottom to tighten on the biner. To make the knot the two ends of the rope wrap around each other 3 turns with the single loop as a core and then cross through and come out through either side of the bottom double loop. The knot is "milked" from the top down to set. Once set it is like a rock. When weight is put on it the double noose tightens on the biner. I also use this in smaller scale to make a prusik for a slack tender.
It is tough to describe how to tie it. If you want to play with it let me know and I'll send you one tied slack tender size which you could take apart and figure out how to make it w/heavier stuff.
Has anyone seen this knot before? I'd guess that some bored sailor has already invented this 200 years ago but one never knows.
-moss