It is sometimes necessary to use a mechanical advantage to haul a load. If a load releasing hitch slips and loads the figure eight on a bight back up, a haul can be used to return to a load releasing hitch and then into a belay. Hauling becomes important in many rescue situations. Litters must be raised, high lines must be tensioned. A haul can be used to help a participant free themselves from a stuck rappel, or through a particularly difficult spot on a climb. Such assistance can be especially useful in multi-pitch climbing.
Each of the systems illustrated offers some mechanical advantage to the person or persons doing the pulling. The force of each pull is increased in exchange for a decrease in the effect of each pull. With a mechanical advantage of three to one, three feet must be pulled for every foot that the load is actually raised.
The systems are shown with carabiners at the points where the rope bends. These carabiners can be replaced with pulleys to achieve greater efficiency. (In some situations this added efficiency can make the difference between being able to raise someone and having to leave them hanging.)
It is not recommended that the prusiks be replaced with mechanical ascending devices because of the large forces which can develop. Under a large load, a mechanical device can cut a rope or break. Even with single person loads, excessive forces can be generated when the hauled object snags on something and the haulers continue to pull.
In large rescue situations, where there are a lot of people present, the more simple hauls offer the most efficiency. For example, the four to one composed of a C on a C or a simple Z drag .
When hauling alone, the greater advantage of an eight to one C on a C on a C or a six to one Z drag with a C might be necessary.
The belay assist is not very efficient, but it is quick and can make enough of a difference to help somebody past a section of rock that is holding them back. (Sometimes failure is valuable learning experience, but sometimes you have to get a participant up a climb.)
The simple Z drag is often performed on the belay rope itself. This is less than ideal as the belay may be compromised. It may be preferable to leave the belay intact and use a C on a C. Tie an occasional figure eight on a bight back up into the belay and attach it to the anchor. The C on a C can be set up with the tail of the rope, webbing, or a piece of extra cord. (A cordelette--25-30 feet of 7 or 8 mm cord--is a useful tool to keep on hand.) Alternately, the Z drag can be set on a C in order to maintain the belay.
There are many ways to get yourself in trouble using haul systems. Enthusiastic haulers have killed people when they failed to notice that the person they were lifting had become caught under a lip; they just pulled harder. It is also possible to simply overload a single line if hauling a large load with bends in the rope that increase the force required to pull the load. (Even a strong wind on a long line can increase the load.) The hazards of hauling are covered with more detail in full scale rescue, but be advised that hauling in any situation requires care and a knowledge of many other techniques in order to prevent mishaps from becoming disasters.
Learn how to attach the keeper prusik to the anchor with a built in and backed up load releasing hitch. It is preferable, although not absolutely essential, to use a load releasing hitch which can lower a load a short distance, not just release it. A cordellete can be used as the prusik and a munter with a mule hitch, all in one. This preventative measure can provide critical salvation if the load is hauled up against a lip or if something becomes pinched and immovable in the system.
Consider carrying one of the new mini prusik minding pulleys produced by Rock Exotic. The use of a prusik minding pulley at the upper point with the keeper prusik will create a self minding, one way ratchet that will make hauling much more simpler. Without a prusik minding pulley or a bachman in place of the keeper prusik it is easy for the keeper prusik to pull through the upper point. Use the full size prusik minding pulley for full scale rescue loads.
Practice moving from a belay to a Z drag and then back into a belay. Use a realistic load and do not let the load ever be supported by a single prusik that is not backed up with either a full Z drag assembly, a belay or a figure eight on a bight clipped into the anchor. You will find that the initial inclusion of a load releasing hitch in the keeper prusik assembly will greatly facilitate the process. However, the load releasing hitch can be added to the system after you have begun hauling. Find solutions to both situations.
The first of the illustrations above is a simple assist will not provide much mechanical advantage, but it may be enough to help a student through a climb without taking them off belay.
The second is a single "C" which might be enough to help a climber move short distance, or temporarily take a rappeller off of the load line to get something out of an eight, 2-1 advantage
The third is called a "C" on a "C" .It can be set up with separate rope lengths or a single line, the part in between the "C's" being uninvolved. Additional "C's" can be added, each doubling the mechanical advantage of the system. Some adjustment of the length of each "C" is necessary to achieve maximum efficiency. 4-1 advantage
On the left above is a Z DRAG(3:1), on the right a Z DRAG WITH A "C" (6:1)
keeper prusik-(This can be replaced with a bachman or supplemented with a prusik minding pulley at the upper carabiner point to prevent the keeper prusik from pulling through.)