Friction Hitches
A Hitch is a type of knot that is tied around an object and if the object is removed, the knot comes untied. Hitches are used for many purposes but one of the main uses is as a rope grab. This allows a rope to be attached to another rope in a way that allows the position to be easily adjusted. There are many different types of hitches but there are two that we use most commonly.
Three Wrap Prusik Hitch
Prusik Hitch. The Prusik Hitch is named after Austrian mountaineer, Karl Prusik. He first made the knot popular in a 1931 manual showing how it can be used to ascend rope. The Prusik Hitch is a very easy knot to tie. It is also very easy to inspect to see if it was tied correctly. Because of this, it is a very good knot for use by beginners.
Valdotain Tresse (VT) Hitch
The VT was also first used in mountaineering. In the 1990s it started to become popular with arborists and tree climbers. Since then, it has started to be used by rescue teams and rope access professionals. The VT is a more advanced knot since it is more complicated to tie and use.
Hitch Differences
One of the main differences between the Prusik Hitch and the VT is the Prusik Hitch works in both directions. A Prusik Hitch can be pulled in any direction and it still holds securely. A VT will only hold when pulled in one direction. A Prusik Hitch also can not be moved when under load. If a weight is pulling on the Prusik Hitch, it will hold in position. A VT can be moved by pushing directly on the knot even with a heavy load attached. This makes the VT more useful under some situations such as tree climbing. When used with a pulley, a VT does not need a specalty load release strap in order for the load to be removed from the knot. This is very different than when a Prusik Hitch is used.
Both of these knots only work correctly when tied with the appropriate type of hitch cord. If the rope used to tie the Prusik Hitch or VT is the wrong type, it will not grip correctly. The diameter must also be matched to the main rope. If the hitch cord is too small diameter it will grip very tightly making it difficult to move. If the hitch is used as part of a climbing system this will cause extra energy to be used making the climber tire much quicker. If the hitch cord is too large diameter, it will not grip securely and will slip. If being used while climbing, this can cause the climber to fall. When tieing a VT, the length of the cord is also important since if it is too short, the knot can not be tied correctly.
Specialty hitch cord such as the CMC Prusik Cord which is softer than normal life safety rope. This allows the hitch cord to grip better. Aramid (Technora) prusik cords such as BlueWater VT Prusik are also available which are softer than normal rope but also much more durable. This is important since a major function of this type of knot is as a ‘safety valve’. If too high of a load is applied, the hitch will slip preventing damage to the life safety rope. When it slips, the heat created can cause the hitch cord to melt reducing its strength. When this happens the hitch cord is physically damaged and must be replaced. This is different than with a mechanical metal rope grab. When the load is too high with a mechanical rope grab there is a chance of it cutting into the life safety rope instead of slipping. The life safety rope must then be replaced which is not only much more expensive but also much more dangerous.
Another feature to look for in hitch cords is sewn connectors. This can be a sewn loop such as the Courant Ellipse or sewn eyes at the end of the cord such as on the Courant Phoenix or Bluewater VT. We have found hitch cord with sewn eyes to be more flexible in use allowing a wider number of knots to be tied than with a traditional loop. Using sewn connectors keeps the hitch cord less bulky and improves safety by removing a chance for human error in tieing the loop.
Choosing A Friction Hitch Cord
A few tips to help choose the best friction cord include:
- Choose a hitch cord a few millimeters smaller than the diameter of the rope it will be attached to.
- If a three-wrap prusik grips too tight, try only using two wraps.
- Materials with higher melting points like technora make good sheath material since it is less likely to melt in case of a slip.
- Avoid materials with a low melting point like UHMWPE (Dyneema) for use in friction hitches.
The only way to know the best size and type of hitch cord to use on a rope is by testing it. Each hitch cord must be tested to make sure it grips and holds as expected. This can only be done by experimenting to find the right balance. Because of this, it is difficult to make recommendations. Sea Air Thai has tested many different types of hitch cord on ropes make from different materials and of different diameters. Please contact us for more information.
