This tip was written with the help of Bryan Hall, who is certified by the Society of Professional Rope Access Technicians (SPRAT) at their highest level. Connect with Bryan at www.rosecityropes.com
If you tie your cordelette “bunny ears” style, as we recommend in this Tip, you can use it to do this Crafty Rope Trick (CRT) - make a low-tech block and tackle system.
(History side note: The term “block” comes from the wooden blocks that were originally used on ships to raise heavy sails, and the “tackle” refers to the ropes/rigging running between the blocks.)
Climbers typically think that mechanical advantage systems only come into play in a rescue-type scenarios. But there are a handful of more mundane climbing situations where they can come in handy.
When might you want to do this?
In the example shown in the video below, you can transfer or share the load in a crevasse rescue off an sketchy initial gear placement to a stronger second placement.
Maybe you're on a big wall climb, you screwed up your rigging somehow, and you need to lift your haul bag a few inches to get it unclipped. (Note, if you use a docking cord to attach your haul bag to the anchor, you’ll never have this problem.)
Maybe you're in some other kind of rescue scenario, and you need to momentarily lift a load off of a carabiner an inch or two to unclip something.
You have a strand of rope with a weighted knot, and you need to unweight the rope so you can untie the knot. To do this, put a prusik loop on the rope above the knot, another prusik on the rope below the knot, and rig the block and tackle between the two prusiks. See images below.
In the crevasse rescue video below, the guide makes an initial quick anchor. He then adds another stronger picket, and uses the alpine block and tackle to share the load with the new stronger picket.
The block and tackle part starts at 2:20.
The video covers things pretty well, but below is a step-by-step with some photos.
A few things to note:
To make the math simple, for the example below let’s assume a load of 100 pounds.
Try to use an HMS pear shaped belay biner. These tend to be wide and flat on the bottom, which minimizes the friction of the cord rubbing against itself.
You could probably use a double length/120 cm dyneema runner to accomplish this also, but the bunny ears cordelette is overall going to be easier.
The real-life mechanical advantage, especially in improvised situations like this that are not using low friction pulleys, rigging plates, etc. will always be less than the theoretical due to friction. So, anytime you read “mechanical advantage”, insert “theoretical” in front of it.
Every loop you put through the LOAD carabiner increases your mechanical advantage by a factor of two. One loop gives you a 2 to 1, the second loop gives you a 4 to 1, the third loop gives you a 6 to 1.
Every loop that you put through the ANCHOR carabiner does nothing to increase your mechanical advantage. This only is redirecting the pull.
Every change of rope direction adds friction that robs you of pulling force. While it varies depending on carabiner size and rope diameter, pulling a rope over a carabiner results in roughly a 50% efficiency loss. This is easiest to notice when you compare picking up a 100 lb weight vs simply redirecting it through a carabiner. Much harder to lift it through a redirect right?
So . . . whenever you can do so, avoid doing a redirect. The red “Pull” text in the photos below shows the preferred upward direction of the pull to minimize friction.
In the photos below, there are two anchor carabiners. This is only done for visual clarity. In real life, it's fine to do all this from one anchor carabiner, which is the method is shown in the video above.
First, start with a cordelette tied “bunny ears” style, with a single 18 or 20 foot strand of 6-7 mm cord with two small overhand on a bight knots in either end.
Next, clip one of the bunny ears onto the anchor, and pass the other end of the cord through the carabiner at the load. If you pull up on the free end of the cord, your mechanical advantage is 2 to 1.
For every 2 feet you pull the rope, you lift your load 1 foot. You need to lift with 50 pounds (100 / 2) of force to move your 100 pound load.
If it helps you to visualize it, notice there are 2 strands of rope moving upwards from the load, hence 2:1.
2 strands going upward from the load: 2 to 1
Now, pass the free end of the cordelette through the anchor carabiner. Doing this only redirects the pull and does not add any mechanical advantage and unfortunately adds friction, something covered in more detail at this Tip.)
changes direction of pull, no mech. advantage gained, adds friction
Now, we repeat by passing the free end of the cordelette through the load carabiner. We now have 4 strands of rope moving upward from the load. This gives us a 4 to 1 mechanical advantage, requiring 25 pounds (100 / 4) of pull to lift our 100 pound load. We need to haul 4 feet of rope to move the load 1 foot.
4 strands going upwards from the load: 4 to 1
Let's try this one more time. Pass 1 more loop through the load carabiner. This gives us 6 strands heading up from the load, with a theoretical 6 to 1. We need to pull 6 feet of rope to move the load 1 foot, but the force required is only about 17 pounds (100 / 6). Again, in reality, it’ll be more due to friction.
(Gorilla tape is optional. =^)
6 strands going upwards from the load: 6 to 1
In the real world with an improvised system like this, you probably don’t want to start going past about a 6 to 1, because all of the increased friction really starts to diminish your mechanical advantage. (Besides, as you can see, in this case we’ve come to the end of our 18 foot cordelette. =^)
But, for the situations mentioned above, such as transferring the load in a crevasse rescue to a new anchor, or lifting up your haul bag off the anchor a few inches so you can unweight a carabiner, a 6 to 1 should be all the assistance that you need.
If you need to lift something seriously heavy, like a loaded haul bag or person, you may need to add a sort of extension or even a loop so you can stand in it and give it your full body weight.