Slinging a boulder for an anchor - two cautions

A common anchor on alpine routes is the simple sling around a boulder or rock spike.  Even though the boulder itself may be super solid, there are some things to watch for when using this method.

1) Check the boulder carefully all the way around for any sharp edges.  A new sling that can hold 20kN can cut very easily under tension combined with the sharp edge of rock or a crystal.

2) A short sling around a large boulder may make a wide angle in the sling that put a larger-than-ideal load on the sling material.  (An angle of 90 degrees or less is the rule of thumb, and 60 degrees or less is ideal). Solution: Use a longer sling to make the anchor angle smaller.  The diagram below shows how a small change in sling angle and greatly increase the forces on your anchor.

The diagram is from the excellent book "The Complete Guide to Climbing and Mountaineering" by Pete Hill.