When making an anchor with static rope around a boulder, what knot should I use
If I'm using a natural feature such as a boulder or a lump of rock as part of an anchor by tying a static rope around it, what are the best knots to use? I'm thinking that there would be different knots: one for when the feature is used at the end of the rope, and one where the feature is tied using a loop of rope
Currently I use a figure-eight at the end of the rope and an alpine butterfly if I'm tying from the middle.
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Your current choices are good. For a fixed loop in the middle of a rope, an alpine butterfly is a solid choice. I can think of alternatives, but no really better ones.
For a loop at the end of the rope, a figure eight (assuming a trace eight) is plenty strong even in that configuration, even if it is not optimal for a tight loop around a large object like a boulder. Just don't clip a carabiner to the loop itself though! fig eights can roll. And for the same reason, make sure that the free strand comes out in the same direction as your loaded strand, or the load will try to roll the knot.
i.e like this
___
/###\
|###|
\ /
8
||
|
|
(you)
Not like this
__
|##\ /
|## 8
|##/ \____ (you)
--
Alternatives might be to simply tie a bowline or a clove hitch (with a generous tail) around the load bearing strand. This will make your loop into a noose, which might or might not be a good thing depending on circumstances. It will also make it more obvious that clipping into the loop is not a good idea. (Though there have been tragic accidents where someone has clipped the loop formed by a larksfooted sling)
Anyway I would be more concerned by the boulder itself, whether it was solid enough, it there were sharp edges and so on than by which knot you used.
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I agree, these are good answers. If I tie in to my harness with a figure-8, it must be solid!
I'll add one more - My personal preference in this case would be a tubular webbing sling, or pre-sewn runner. I like how they distribute the force, and if you girth hitch with a runner, there's no knot to worry about getting wrong, or untie after it's been loaded!
And a backup. Always a backup.
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