The "Voodoo Hitch" - a magic tensioning hitch
Let's start with a little honest disclaimer right off the top: This bit of rope wizardry is probably not something you will use in very many climbing situations. However, if you’re an aficionado of #CraftyRopeTricks, well, this is one of the craftiest you’ll see! It's halfway between a knot and a magic trick. The day I learned it, I tied it a dozen times and was still scratching my head as to how it works.
The voodoo hitch is a way to add a moderate amount of easily adjustable tension to a fixed rope. (It's somewhat related to the the trucker’s hitch, an extremely useful knot that just about every outdoors person should know. Here's a nice short video that shows how to tie a trucker’s hitch.)
The voodoo hitch is known by a few different names. Some call it the rather boring “Transport hitch”, others the “PM hitch”, which stands for “Pure Magic”. (I've even heard it referred to as the WTF hitch. =^) The way I learned it is the “voodoo hitch”. You'll learn why it's called a voodoo hitch after you tied a few times and see how it miraculously holds tension when it sure likes like it wouldn’t.
Like the trucker’s hitch, the Voodoo creates a 3 :1 theoretical mechanical advantage - you need to pull 3 meters of rope through the system to move the load 1 meter. But, because of all the carabiners adding friction, the real world mechanical advantage he's pretty much 1:1.
Just get a climbing rope and a few carabiners and try out this rope sorcery. It's cool, magic, and fun.
Here’s what's nifty about the voodoo hitch (and how it differs from the trucker’s hitch:)
You can fine-tune the tension on the rope without untying and re-tying the knot.
There's no lock off half hitch required, the voodoo just magically holds tension by itself.
A few notes:
I was taught this knot in a canyoneering class using all clove hitches. Clove hitches work, but there are several other knots work fine too, such as a butterfly or figure 8 on a bight. I’m liking the butterfly knot, because it's easy to untie after it's been loaded.
You can use any kind of carabiners for this: lockers, non-lockers, large belay lockers or tiny wiregates.
You can use pulleys to make tensioning the system a bit easier, but don't use them everywhere. Friction holds the whole thing together, see below. If you use pulleys at every redirect point, it fails.
How does it work? Opposing tension, or friction, or voodoo, or magic, or something? A clever Alpinesavvy fan on Instagram (@govanathon) shared this: “3 stands on one side of the sliding carabiner, 2 stands on the other side. Moving one way lengthens 3 while shortening 2 so net tension increases. Moving the other way lengthens 2 while shortening 3 so net tension decreases. Friction on at least 1 redirect point is necessary to hold the tension or the rope will just feed through the system and loosen.”
Here’s a good article on the Voodoo hitch, along with this nice graphic. (Note, the end of the rope is fixed at point A.)
As with pretty much anything related to learning knots, this is a better show than a tell. Check out this short (2:20) video that shows you exactly how to tie it.
Finally, if you really want to nerd out on the voodoo hitch, here is a deep-dive video from my pal Ryan at HowNOT2.com.
