Slippage isnt due to your prusik. That hitch is just to keep tension on the other end. (Prusik needs to be made out of a spliced loop, cant tie that with amsteel) 9" is too short for your bury. I tried going short on mine but kept slipping until I got up to a 12-13" finished bury. Has worked fine since for the past 8 years.
Bookmarks