Thanks Agent Smith, very good answer! I'll clarify some stuff:
You haven't given us a full and proper context for the 'anchor'. What exactly will the 'anchor' be used for?
Is the anchor used for establishing a retrievable abseil system? Or is it for belaying a climber (ie a 'second' climber who is following the 'lead' climber)?
Either an abseil - retrievable or otherwise - or a belay
in principle. I was instruceed in using anchors of this type for both but in practice most likely a belay (because of the reasons you identify in your last point).
1. Your photo hides the end-to-end joining knot which would be located behind the tree trunk.
What joining knot did you use?
It's not my photo, it's from the article I linked (unfortunately I don't have any photos of the actual anchors we were building). We were using pre-made sewn slings (nylon and dyneema,
here's an example), so there is no joining knot. The only knot in the system is the one visible in the picture, which I think is mostly meant to shorten the length (circumference) of the sling going around the anchor object (the tree or 'big rock'. This is especially useful in cases where the a big sling is needed to get over the anchor (probably a rock), but a smaller sling would fit better around the area
ofthat anchor where the sling will actually sit. (cant find an example picture e
at the moment.
It's this knot which I've been taught to use an overhand for, but of course when the sling is loaded (if the climber takes a fall or the belayer just has to lean on te anchor for any time), the overhand jams up terrible and is very difficult to undo in the middle of the Cairgorms with freezing hands in big gloves!
2. The Blimp knot (#582) you refer to is not appropriate for your application and is not required.
Given the above, I was looking for a stopper-type knot which could replace the overhand in that sitatuation (fulfilling the same role), but not jam (or at least not so easily). I immediately thought of thre Blimp knot. I certainly trust that you of all people would know if it was not appropriate, but could you explain why in a bit more detail please?
3. I find myself agreeing with KC that the standard 'wrap 3 - pull 2' is very effective because it is a multi-directional anchor that is jam resistant. However, this is just a best guess because you haven't provided a proper context/application for the 'anchor'. A 'wrap 3 - pull 2' is effective with both webbing and rope. I personally don't use a #1412 Water knot / Ring bend / Tape knot to crate the join. Believe it or not, a simple #1410 offset water knot (offset overhand bend) is just as effective.
Given that I was originally taught with pre-sewn slings I hadn't learned this one, but having now looked into I agree, looks great! Still, not applicable to my situation above unless one happened to have such an enormous presewn sling that it could be used in the bight as effectively one length (not a circle) and tied.
4. I find it interesting that you show a tree - one you are above the tree line, there are no trees! (obviously). Also, a 'big rock' typically exists at the base or the top of a cliff - not part way or half-way up it. More likely scenario is that you would be using a geologic rock feature such as a 'thread' or a chock stone. You would also likely be using wired nuts (eg wired chocks) and maybe hex's. When building an anchor from removable protection devices (eg wired nuts) - a 'cordalette' made from 5mm-7mm diameter high strength accessory cord is best practice.
True, above the tree line there are not trees! I have limited experience of the Scottish mountaineering whence my problem comes, but I will say that every day we went out we did find several large rocks along slopes. We Were'nt really climbing on 'cliffs' as such, but steep rocky slopes. This kind of terrain can also include large 'spikes' of rock which can also be used for a similar purpose (see for example
this video).
You're right, we did also use wires and hexes, and they're great for building anchors, but my question here isn't really about that: I', wondering what the best knot is for the above situatin
given that one is building such an anchor as in that situation (with a sewn sling).
5. If you are building an anchor for a retrievable abseil system, it is implied that the anchor is sacrificed/abandoned. This means your 'anchor' would be construct from material that can be given up...ie it is cheap and not essential for the rest of the route. Usually this means webbing/tape (not rope/cord).
True. (Mostly. Have you heard about Andy Kirkpatrick's upcoming book 'Down'? I've heard there's some interesting stuff about ghosting off anchors in there...). Still, I'm interested in using these anchors for belaying mostly, so it would still be useful to know.
Hope that clears things up a bit!