Do you always know the exact length of every rappel? At some point in your climbing career, you will probably encounter a rappel that is unknown but looks too long for your measly single line. Instead of tossing the rope, crossing your fingers, and getting to the ends of your rope only to discover that, yes, your rope is too short, there is a simple technique to deal with such a situation.
If you’re concerned about the length of a rappel, send the first climber down to the next anchor by lowering him off the rappel anchor you’re clipped into. Lowering has several advantages: Not only do you now have twice as much rope to get your partner safely to the next anchor, but you also get an accurate measurement of the rappel length. If the middle mark passes through the anchor during the lower, you know your rope isn’t long enough for a standard single-rope rappel.
Warning: Before you start lowering your partner, close and back up the system by tying a figure eight on a bight in the other (brake) end of the rope; clip this to the anchor with a locking biner. This ensures that you cannot accidentally lower him off the end of the rope. You may also want to add a friction-hitch backup (like a prusik or auto-block) to the brake strand, so you can’t lose control of the lower and have your partner plummet straight down. While this step isn’t essential to lowering, your partner will appreciate it!
If the rappel turns out to be longer than half a rope length, you can still make it safely down to the next anchor, even if you don’t have a second rope. The key is rigging a system that makes the rope retrievable.
After your partner is safely clipped into the lower anchor, thread the rope through the top anchor; you should now have one end that reaches the lower anchor, and one end that is short. Have your partner stay tied into the rope at the lower anchor to ensure you don't rap off the end.
Tie a butterfly knot (A) next to the anchor on the short side of the rope and clip the bight back to the long strand with a locking carabiner (B). This essentially “fixes” the longer strand of rope and allows you to safely rappel to the lower anchor on this single strand. The preferred knot to use here is the butterfly because it is less likely to get pulled into the chains or rings and get stuck. (Head to climbing.com/skill/butterfly-knot for instructions.)
Next, set up a single-strand rappel on the longer strand, which is the opposite side of the anchors from the knot (C).
During your rappel, attach untied cordelettes and/or slings to the end of the short rope until it reaches the lower anchors. To join cordelettes with the rope, a flat overhand works well. To join slings with the rope, you can tie a knot in the end of the rope and start clipping or girth-hitching them to each other. It’s not as clean as the cordelette attachments, but it will work. Two standard-length cordelettes for building anchors easily turns your 50-meter rope into a 60, or your 60-meter into a 70. Oftentimes, that extra 10 meters is all you need.
Once you are safely clipped into the lower anchor, pull on the cordelettes or slings to retrieve your rope.