If you are just going to be abseiling less than half a rope length then any rope will do as you simply take the middle of the rope and attach to the belay point then lower or drop the two ends.
If the abseil is longer (up to a rope length) then you will need two ropes. The only restriction here is that both ropes should be the same diameter. Using an 11mm rope with an 8.5mm rope will lead to problems as the two ropes will interact with the abseil device differently with the thicker rope having more friction and passing through the device more slowly. A slight difference in diameter (9mm & 8.5mm) is nothing to worry about.
There is much discussion as to which knot is best to use to join the two ropes together and over the years thoughts have changed.
The table below summarises the various knots that are in common use for tying two ropes together for abseiling. One point to note is that for all the knots, the “tails” should be at least 30cm in length to allow for slippage and tightening of the knot.
|Double Fishermans||Easy to get wrong.||This is the knot by which to judge all others. If tied correctly, it is the strongest knot available for this purpose. If your ropes are of different diameters then it is the only knot to consider. It does have some major drawbacks though: it is very difficult to undo after being loaded; it is also symmetrical meaning that it is likely to snag easily.|
|Figure of Eight||Not as easy as is imagined.||
To tie the figure of eight correctly requires that both ropes travel a particular
path through the knot. This is very difficult to ensure in a stressful situation.
In addition to this, the knot has an alarming tendency to roll along the rope
when stressed at right angles to its main axis.
|Overhand||Fairly easy.||At first the overhand knot looks as if it would be weaker than the figure of eight but in fact it is both stronger and more resilient to pulls across its axis|
|Reef||Simple and easy||The reef knot (square knot) is very simple to tie but, like the double fishermans, is a symmetrical knot so suffers from the same problems of being prone to catch. It is a very poor knot to use for ropes of different diameters. Perhaps its best use is in the centre of a double fishermans knot, using the latter as a safety backup.|
It depends on the situation. But current thinking/best practice indicates one of the following.
See this article at Needlesports for more info on knots for abseiling.
Basically any anchor that is good enough to belay from. Unless you are abseiling from a permament abseil station it is likely that you will have to sacrifice some of your own equipment to make the anchor. Usually this is no more than a couple of wires or a length of sling or cord to replace existing loops (where the condition of the existing loops may have been affected by the elements or excessive use).
Certainly not! Commando style is all well and good when you have solid anchors and an easily impressed audience. Out in the hills it is a different matter and you should aim to stress the system - particularly the anchor as little as possible.
Most stress on the anchor occurs in the first few metres of the abseil so it is at this point that you should be as smooth as possible, creeping over the edge whilst making sure that the ropes do not run over any sharp edges and that they have not snagged beneath you. Keep your feet apart for stability and walk down the rock as smoothly as you can.
There is a simple way to do this. Keep the Figure of Eight device clipped via the large hole. When you come to use it, pull a bight of rope through the large hole as you normally would and pass it round the back of the smaller hole, again as normal. The device is now trapped by the weight of the rope so you can unclip from the larger hole and into the small hole ready for use.
To remove, simply reverse the above procedure.
Tie a knot in the end of each rope to prevent you from sliding off the end. You need one on each rope as just doing it on one rope could cause the system to pull through the belay. This also helps find the end of the rope in those instances when they get caught after being thrown down, either because of wind or because the rock is heavily featured. In order to help remember which rope needs to be pulled at the next stance (or ground) - clip a krab into the end knot of that rope.
Clip a quickdraw between your harness and onto the rope in question. This also has the benefit of keeping the ropes separated when you come to pull them.
Given that in most situations you are totally reliant on the anchor and the ropes and that there is little you can do to back these up, the only other weakness in the system is yourself.
If there is a chance of debris falling on you then it is beneficial to use a French Prussik or Klemheist knot around both ropes. Whether this knot is above or below the abseil device is down to personal preference - I have seen valid arguments for both - my personal preference is to have the knot above the device.
There are rare occasions when you need to abseil a full rope length but only have one good rope available. For example: The other rope has been damaged in some way and you do not want to trust it. Another scenario is if you are climbing lightweight on a single 9mm rope and have brought along something like a length of 6mm cord for emergencies. So how do you make a full 50m abseil?