WARNING: Rappelling is a dangerous sport which may cause death. This page is for entertainment purposes only. If you wish to try this, you must get instruction from a trained and qualified instructor first.
The following is a summary of rappelling techniques as taught by One Axe Pursuits. This page has five sections:
The safety line should be approximately 30 ft of 8 mm rope with a 6 ft 6 mm Prusik cord. One end of the safety line should be tied with a figure-eight knot secured with a double overhand knot as a safety while the other end should be secured with a double overhand knot. The Prusik rope is tied into a loop with a double fishman knot. Both these are shown in Figure 1. These lines are then attached as shown in Figure 2 using a Prusik knot.
Figure 1. Knots in the safety line and the Prusik cord.
Figure 2. The Prusik cord attached to the safety line using a Prusik knot.
The standard means of securing a line is by connecting the line to a pair of carabiners which are then secured to two different stationary objects using one-inch webbing. The webbing is cheaper and wears less quickly than rope would over sharp edges.
The webbing is secured to trees by wrapping the webbing around the trees (using tree huggers, of course) and using water knots, as shown in Figures 3 and 4.
Figure 3. A water knot securing webbing to a tree.
Figure 4. A second image showing a water knot being used to secure webbing to a tree.
Attaching the two webbings to the two central carabiners is also done with two water knots, and both webbings pass through both carabiners. The line is attached using, in this example, a figure-eight knot. These are shown in Figures 5 and 6.
Figure 5. Attaching webbing to the twin carabiners.
Figure 6. Attaching the line to the twin carabiners.
The standard means of securing a rappel line is by tying the dynamic rope or a static rope to two overlapping (twin) carabiners. Two lengths of webbing are then run to two stationary objects to secure the line. The webbings and the line form a Y, the centre being the twin carabiners.
This serves the following purposes:
The greater the angle, the greater the stability, however, the greater the angle, the less the security: the longitudinal and lateral forces on the carabiner go to infinity as the angle between the webbings goes to 180o. Also, while for rappelling, the force on the line is increased by at the very most a factor of two, a serious fall during a climb could result in siginificantly greater lateral forces on the carabiner.
If we do a free-body diagram of this system, we get the result in Figure 7 where simple trigonometry is used find the lengths of the two sides. The line experiences a force of magnitude F.
Figure 7. A free-body diagram.
Assuming that each webbing is tied off separately onto the central carabiner, the lateral force on the carabiner grows much more quickly (∼ t + t3/12) than the longitudinal force (∼ 1 + t2/8). This is more serious, as all carabiners are much stronger in the longitudinal direction.
Figures 8 through 13 present the forces for angles of 20o, 40o, 60o, 80o, 100o, and 120o assuming that a force of 1000 N (the force of gravity applied to a person weighing approximately over 102 kg) is applied on the line. The percentage of the force on the force on the webbing is given at the top, while the actual forces are given in kN. You will note that in each figure, the Pythagorean theorem holds. For example, using Figure 8, 5082 ≈ 5002 + 882.
Figure 8. Forces with a 20o angle of separation.
Figure 9. Forces with a 40o angle of separation.
Figure 10. Forces with a 60o angle of separation.
Figure 11. Forces with a 80o angle of separation.
Figure 12. Forces with a 100o angle of separation.
Figure 13. Forces with a 120o angle of separation.
An angle of 67o will result in a 60% load on each of the two ropes, however, this will also result in a 33% lateral load. Keeping the angle less than 50o will keep the longitudinal load under 55% and the lateral load under 23%.
An alternate means of securing a line is to wrap it numerous times around a tree (using a tree hugger). Three wraps is secure on a large tree (10"), while more should be used for smaller trees, up to six wraps for a tree of 6". Trees less than six inches in diameter should not be used for rappel anchors. The friction of the wrappings secures the lines, however, for safety, the line is secured to itself using a figure-eight knot. This is shown in Figure 14.
Figure 14. Wrapping a line around a tree.
A second line shown on the left of Figure 15 is a security line using a Prusik knot. The connection is shown in Figure 16 and securing the safety to the tree using webbing and two water knots is shown in Figure 17.
Figure 15. Wrapping a line around a tree.
Figure 16. Securing the safety to the line using a Prusik knot.
Figure 17. Securing the safety to a tree.
Figure 18 focuses on the connection.
Figure 18. The alternate top-rope configuration.
Figure 19 shows the load on each webbing as a percentage of the line load for angles up to 160o. The horizontal line represents a 60% load.
Figure 19. The load in relation to the angle between the webbings as a percentage of the force on the line.
You will note that for an angle of 160o, the force is almost 300%, and in this case, both in the lateral and longitudinal directions on the carabiner.
For simplicity, it is recommended the colours of the webbings be related to the length. The easiest to use are the rainbow colours with the darker brown and black for the shortest and the brighter grey and white for the longest. These are shown in Figure 20. Multiples of a yard are recommended.
Figure 20. Recommended colours for various lengths of webbing.
By no means are all these lengths required. My three webbings are red, yellow, and blue.
Two ends of webbing may be connected using a water knot. A water knot is simply an overhand knot with the second end of webbing run in the reverse direction.
First, start with two ends of webbing (either the same to make a runner or two different pieces of webbing to connect them), as shown in Figure 6.1.
Figure 6.1 Two ends of webbing.
Fold one webbing over and back, as shown in Figure 6.2.
Figure 6.2 Fold one back.
Rotate it under, as shown in Figure 6.3.
Figure 6.3 Rotate the webbing underneith.
Slip the webbing through the created loop, as shown in Figure 6.4. The result should have the shape of a loose pentagon.
Figure 6.4 Pass it through the loop.
Insert the second piece of webbing in the same loop but in the opposite direction, as shown in Figure 6.5.
Figure 6.5 Pass the other webbing back through the loop.
Follow the first piece of webbing around, as shown in Figure 6.6.
Figure 6.6 Follow the first webbing with the second.
And slip the second working end of webbing back through the loop, following the fixed end of the first.
Figure 6.7 Slip the second webbing through the loop.
Tighten the water knot by pulling all four ends in pairs. The front side is shown in Figure 6.8 while the reverse is shown in Figure 6.9.
Figure 6.8 Tight the webbing.
Figure 6.9 Opposite side.
Suppose we have created two runners (a circular piece of webbing), perhaps by using a water know, and we wish to connect two runners. Ideally, one would use a carabiner to connect two runners, as shown in Figure 7.1.
Figure 7.1 Connecting two runners with a carabiner.
Without a carabiner, you can connect two runners using a girth hitch. To create such a connection, first run one runner (easier if you use the smaller runner) through the other, as shown in Figure 7.2.
Figure 7.2 Pass the shorter runner through the loop of the larger runner.
Create a girth hitch by passing the length of the shorter runner through the loop, as shown in Figure 7.3.
Figure 7.3 Pass the second runner through the loop.
Pulling tight creates the knot shown in Figures 7.4 and 7.5.
Figure 7.4 Pull tight.
Figure 7.5 Pulling tighter creates this knot.
Capsize the loop to create the connection shown in Figure 7.6.
Figure 7.6 Capsize the knot.
The following are photographs of me in a rappel at Rattlesnake Point. Note the sweat - it was 31oC that day. You may also see the pen in my pocket. C'est la vie.
Figure 8.1. Getting into position.
Figure 8.2. First steps down.
Figure 8.3. A small break.
Figure 8.4. Coming up to an overhang.
Figure 8.5. Finished.
Copyright (c) 2007 by Douglas Wilhelm Harder. All rights reserved.
Douglas Wilhelm Harder
Department of Electrical and Computer Engineering
University of Waterloo
200 University Avenue West
Waterloo, Ontario, Canada N2L 3G1
Phone: +1 519 888 4567 extension 37023
Fax: +1 519 746 3077
Contact Me | Department of Electrical and Computer Engineering