Ropes can be divided into two broad categories:
1. Static low stretch ropes
2. Dynamic ropes, which are designed to stretch to absorb the impact of a fall.
Dynamic ropes are further divided into 3 categories:
1. Single ropes. These are thicker ropes (9.1-11mm) used individually to protect a climber in case of a fall. They are the most common for top-roping, sport climbing, and North American trad climbing.
2. Twin ropes. These are the thinnest ropes (7.9-8.1mm) used in pairs to protect a climber in case of a fall. Although there are two ropes, they are treated as a single rope. Both ropes are clipped into every piece of protection and the leader is belayed with both ropes. These are often used in ice climbing.
3. Half ropes (also called double ropes!!!). These are slightly thicker than single ropes (8.1-8.7mm). Each rope is clipped into alternating pieces of protection and the belayer must sometimes be able to give slack/take on one rope, but not the other. These are great for ice, trad, or alpine climbing.
This article of course is covering double ropes, and why you would ever want to use this system. There are quite a few advantages of using doubles.
1. First and foremost is you have two ropes for rappelling which will definitely save you time descending on those multi-pitch climbs.
2. Second you have two ropes for belaying two seconds simultaneously, once again saving time on those multi-pitch climbs.
3. Third you can reduce rope drag by selectively clipping a rope to the left or right of a climb.
4. Fourth you can reduce a potential swing by a second on a traverse. This works on climbs with a traverse and then a vertical climb. You can do this by clipping one of the ropes into all the pro in the traverse. The other rope will now be able to hang down from the anchor to the second.
5. Double ropes often have lower impact forces for those marginal pieces of protection.
6. Double ropes are more redundant than single ropes in case the rope is cut.
7. If you fall while pulling up slack on a double rope, the fall won’t necessarily be any longer since the other rope can stay tight.
There are quite a few disadvantages of climbing with doubles.
1. Belaying and rope management is quite a bit more complex than with singles, requiring a more experienced climber.
2. A pair of doubles certainly weighs more and is bulkier to carry than a single rope.
3. A pair of doubles costs more than a single rope.
4. The additional stretch of a double rope does mean that you will fall further.
Doubles and twin ropes.
A twin rope system gives you the ability to rappel just as far, belay two seconds, and is more redundant than a single rope. It is also lighter than a double rope system. However you have no ability to reduce rope drag, and often the impact forces are the highest with twin ropes.
Thicknesses of double rope systems.
I see two common diameters of ropes used in a doubles system. 8.1mm I see as an ice rope where abrasion is minimal. For rock climbing I would recommend an 8.6 which would provide greater durability when dragged across rocks. There are also ropes like the 9.1mm Beal Joker which is certified as both a single and a double.
Climbing with singles or twins as doubles.
I don’t see any technical problem with using singles in a doubles system. However, I can’t see any reason why you would want to do it. Using 2 singles in a doubles system would be an incredibly heavy system. Never use twins as doubles, the ropes are not designed to be used individually. Never use doubles as twins, this would result in tremendous impact forces.
1. It is best to always alternate which rope you are clipping. This way you have greater protection in case one of the ropes is severed.
2. On wandering climbs minimize drag by clipping one rope to one side, and the other to the other side.
3. Don’t clip both ropes into the first piece. This would result in very high impact forces on that piece as well as a potential point for the rope to burn (as the ropes move at different speeds).