CEM knot
The so-called CEM knot, also known as the Mexican recoverable anchor, is a releasable and retrievable rigging which requires no gear in addition to the rope itself. This page describes a simplified version of the original knot that foregoes some complicated safety features that prevent inversion of the lowest compression loop in favor of simplicity. (CEM stands for Cañonismo en Mexico.)
Usage
As the CEM does not require any gear such as bolts, webbing, rapide links, carabiners, or pull chords, it is particularly suitable for rigging directly to natural anchors. It can also be rigged to an existing webbing/rapide anchor for reasons such as to avoid difficult pulls, or for quicker retrieval, or to avoid adding rope grooves to a natural fixture.
Like any other retrievable rigging which does not involve a pull chord, this rigging requires twice as much rope as the length of the drop, plus a few feet used for the rigging itself, but avoids having to pull the entire rappel side of the rope around the anchored object (such as a tree) in order to retrieve it. Thus, it helps preserve the natural anchor as well as the rope.
Variants
There are two variants of the CEM Knot out there. The original, which is illustrated at https://canyons.mx/el-nudo-cem, differs from the simplified Southwest variant by adding three additional steps, and is said to be less prone to rope jams caused by the first (lowest) loop becoming too tight. The rest of this article describes the Southwest variant of the CEM Knot.
How to rig
The CEM is formed by wrapping a bight of the rope around, say, the trunk of a tree, or by passing the bight through a rapide link, after having lowered enough rope for the rappel and the retrieval. Several small loops (no fewer than two, preferably three) are formed near the top of the rappel strand. The bight is fed through these loops in such a way that tension on the rappel strand will tightly pinch the two strands of the bight. If pulling on the rappel strand after passing the bight through a loop causes the loop to collapse, the bight was passed through the loop from the wrong direction; reform the loop and try again.
Once the bight has been properly fed through all the loops, the rappel strand can be loaded. It is advisable and common practice to add a safety to the rigging by clipping a carabiner through the end of the bight and around the rappel strand, or alternatively, to use a clove hitch to secure the rappel strand to the carabiner. Needless to say, in order to be able to release and retrieve the rigging, LAPAR will have to remove this safety before beginning their descent.
How to release and retrieve
The CEM is released by pulling the pull side strand from the bottom of the rappel once the rappel strand is no longer loaded. Load on the rappel strand will make the release much more difficult; it is typically necessary to apply roughly the same amount of force to the pull side as is present on the rappel side to achieve release. That is, if the rappel side is weighted with 200lbs, the pull side would need roughly 200lbs applied to it to release the system. This performance varies substantially with the friction around the natural anchor.
The release also sometimes leads immediately to retrieval, as the rope will no longer be attached to the anchor and may simply fall down. That is, the end of the rappel side of the rope does not need to pass back through any anchor components at the top.
Dangers
- This is a releasable rigging. That means it intentionally can detach from the anchor, making it inherently more dangerous than standard riggings which cannot release from the anchor unless the rope is pulled all the way through an anchor ring. If this rigging is released while someone is on rappel, they will free-fall for the remainder of the drop; this is even worse than rappelling on the wrong strand of a blocked rope.
- When rigging the CEM, it is imperative to lead the bight through the loops with the proper orientation – see photos above. Failure to perform this step correctly will cause the rigging to fail catastrophically.
- Note that once released, the rope does not have to pass through any anchor components and often falls down by itself. The person releasing the rope should be prepared for this and also make sure that there are no other canyoneers at the bottom of the drop, and that the falling bight will not catch on any obstructions on the way down.
- Even while the rappel strand is weighted, pulling on the end of the bight can easily invert and collapse the lowermost loop in this version of the CEM. The full CEM adds additional steps to prevent this from happening, but is substantially more complicated. When a loop is collapsed, the rigging should still not fail, but it becomes unretrievable.
- Care should be taken to rappel on only the load bearing side of the rope. You must not rappel double stranded, as the rigging could prematurely release.
- Be careful not to introduce rope twists to the toggle-bight. Additionally, do not make the toggle-bight longer than necessary. Twists in the bight will be amplified during release pull and can prevent the anchor from releasing.
