A 3:1 “Z” drag, step by step
The “Z” drag (so named because the rope looks like the letter “Z” if you turn your head sideways) which gives you a theoretical 3:1 mechanical advantage, is one of the fundamental setups of crevasse and rock rescue. After you've done it a few times, most people get the hang of it. But if you haven't rigged it in a while, or if you're doing it under the stress of a real rescue situation, setting it up efficiently and correctly can be a challenge. (I’ve seen some quite experienced climbers have a complete brain fade trying to do this if they’re out of practice . . .)
Here's a step-by-step walk-through. Hopefully this will help if you're new to rope rescue, or to dust off this skill if it’s been awhile. So get a rope, 2 prusiks, a few carabiners and a pulley if you have it, and follow along.
Note 1: This shows the basic mechanics of how a Z drag is set up, not all the possible nuances of gear and technique. Prusik minding and progress capturing pulleys, rope grabs, backup knots and releasable hitches, and other fancy rope tricks can be added after you know this foundation inside and out.
Note 2: Don't pull furniture around inside your house as it's tough on the floor and carpet, ask me how I know this . . .
Step 1 - Construct a bomber anchor. Add a locking carabiner. Clip this carabiner to the rope with the load. You now have a 1:1 system (zero mechanical advantage) with the rope redirected.
Step 2 - Add a “capture” prusik on the load strand of the rope, and clip this prusik to the anchor. (This prusik loop “captures” your pulling progress, holding the load if you let go of the rope.)
Step 2A - The way it's set up now, when you haul on the rope, the prusik will pull through your carabiner. Not good. There are a few ways to prevent this. One is to add a quick link, as shown below, which should block the prusik from sliding through. The effectiveness of this depends on the few variables such as size of your quick link, and diameter and grip-tion of rope and prusik cord. Give it a try and see how it works. (You can get quick links that are actually CE rated for climbing from CAMP, discussed here.)
Another way is to have a second person “mind the prusik”, keeping it loose when you're pulling, but letting it go tight on the rope to hold the load when you stop pulling.
If you have a fancy and somewhat expensive “prusik minding pulley”, this is where you’d put it.
(And yes, clever reader, I know the trick of adding a tube belay device here, we're not covering that today.)
Step 3 - Add a second prusik, called a “travelling” prusik, onto the load strand of the rope. It's called the “travelling” prusik, because it moves when you pull. If your prusik cord is a little long, like the one I have here, tie an overhand knot to shorten it up. Shorter is better.
Step 4 - Put the free end of the rope through a pulley, clip a carabiner to the pulley, and clip that carabiner onto the travelling prusik.
If you don't have a pulley, use a carabiner here. A pulley is better. If you have only one pulley, put it on the travelling prusik to increase your hauling efficiency.
Sweet, you now have a 3:1 and you’re ready to pull! Pull on the rope until your load is where you need it, or until the travelling pulley touches the anchor. If this happens and you need to pull some more, set the capture prusik to be sure it can hold the load, and then reset the travelling prusik by sliding it as far as you can down the rope toward the load. Continue pulling.
