First of all, while I have done some rappelling, it has always been with a doubled rope or a single top-anchored rope, not with joined ropes, so my comments are not based on experience.
The article that Allene links to places a lot of emphasis on ease of retrieval. and less on the absolute strength of the knots being investigated. This makes sense, since most ropes used by climbers and cavers have very large factors of safety, and a snagged rope could be a serious problem. However, the knot Drohan finally recommends doesn't have much going for it but decent lead. (I note that the Wikipedia article on the Overhand Bend says that American climbers have referred to this as the European Death Knot.)
Since lead is considered so important, I wonder if the Tucked Sheet Bend, aka Binder Twine Knot, is worth investigating. That's the knot this link
http://www.hudson-family.net/knots/knots.html calls the Locking Sheet Bend.
Pros:
- near-perfect lead in one direction, and presents a fairly slim face to any possible snags.
- stronger than the Overhand Bend
- probably more secure than the regular Sheet Bend under jerking loads, but it should be backed up by tying overhands with the ends around the standing part.
- easier to untie than an Overhand Bend or a Double Fisherman
Cons:
- more complicated to tie than the Overhand
- must be tied facing the right way
Anyone want to do some testing? In the lab or on the rock face, your call.
