Faster rappelling with a group - fix each strand
Note - This post discusses techniques and methods used in vertical rope work. If you do them wrong, you could die. Always practice vertical rope techniques under the supervision of a qualified instructor, and ideally in a progression: from flat ground, to staircase, to vertical close to the ground before you ever try them in a real climbing situation.
Sometimes, a well moving climbing team can come to a screeching halt when making a rappel (or a few of them.) The logistics of setting up and completing a rap seems to gobble up the clock like no other part of climbing, especially if you have a group of newer climbers. Add to this approaching darkness or a building storm, and reasons to rap quickly can become even more pressing.
The next time you’re stuck in a crowd at a rap station, try this trick:
Feed the rope through the rap anchor as for a normal rap.
If the anchor has a single master point, clip a locking carabiner to the masterpoint. If the anchor is twin chains, clip a locking carabiner to each bolt hanger.
Tie a butterfly knot in each strand. Clip the butterfly loops to the carabiner(s).
You now have two fixed single lines, allowing one climber to rappel while another is rigging their rope. (I call this being in the "on deck circle" like in baseball.) The second person should be ready to rappel the moment the climber ahead of them is on the ground or at the next anchor.
This can really speed up your downward progress, because there's no downtime waiting for anyone to rig. Someone should be moving down the rope at all times.
The last climber unties the butterfly loops (which are used because they are fairly easy to untie even after being loaded), cleans the carabiner(s), and raps normally.
Note: Be sure that all climbers are comfortable rapping on a single strand of rope. If the rap rope is free hanging, and/or the rope is skinny or wet, climbers may descend faster than they are used to. If this happens, climbers can use various techniques for adding friction to a rappel - read about a few methods at this tip.
There are lots of different ways you can fix two strands of the rope. Here are two more.
The methods shown above are visually easy to check, and use knots and anchor components that are clear and obvious.
However, there are certainly a few other options.
You could tie a double strand bight knot, which effectively blocks both strands of the rope as well. Here's a double strand butterfly. (A figure 8 or an overhand would also work, but they are a bit harder to untie after being loaded.)
Another option is the Stone hitch. This is commonly used in canyoneering, not so much in climbing, but it is a rather simple and elegant way to isolate two strands of rope. Here's an entire article on this knot. Definitely practice this with a qualified instructor before you use it for real!
Another way to expedite a rappel - the “pre-rig” with an extension
Another approach to expediting a group rappel is to have everyone use an extended rappel, and do what's called a “pre-rig”. This means that everyone in your group has their rappel device on a shoulder length runner, and they are all attached to the rope at one time. By extending the rappel, the climbers waiting at the top will not be yanked around when the rope is under tension.
Doing this expedites the rappel, because the moment the first person gets to the ground or the next anchor and creates a little slack in the rope, the next person can immediately head down. This is the same idea as the fixed single strands Illustrated above, but allows each person to rap on double strands of rope, which gives a little more friction and is probably more comfortable for beginners. Having a pre-rig speeds up the entire process, because once again there should be someone moving down the rope pretty much constantly.
The number of people in your party, and the size of the ledge around the rappel anchor will dictate the method you use. Lots of people, and maybe a small stance? Use The “fix separate strands” method mentioned at the top. Small team, and perhaps a larger ledge around the anchor? Then use the pre-rig method described here.