A good general principle in setting up mechanical advantage (MA) systems: use as little as you need to get the job done. Are you able to move your load with a 3:1? Great, use that, and don't make it a 6:1!

Or, to put it another way, just because you CAN add more MA doesn't mean you SHOULD add more MA.

Every time you increase mechanical advantage:

• You add extra gear, changes of direction, introduce more friction, and create more chances for ropes to twist, rub, cross etc. that reduces the overall efficiency

• You increase the force on the anchor

A fundamental principle of economics (and many other aspects of life, including climbing) is the law of diminishing returns. In econ-speak, means that adding additional factors of production eventually results in smaller increases in output. Say that it takes one builder one year to build a house. So, if you have 365 builders, can you build a house in one day? Of course not, because after a certain point, the extra production (builders) result in lower output (less work getting done because they are tripping over each other).

MA systems work in somewhat the same way. Past a certain point, it doesn’t make sense to keep adding components to the system, because you're actually decreasing efficiency.

Here's an example, with a common self rescue techniques taught in many books and classes.

You’re belaying your second with a Grigri off the anchor. They get to the hard part, and need a short lift / assistance to get past a hard move or two.

No prob! You set up a theoretical 3:1 as shown on the left. Simply add a rope grab on the loaded rope going down to them, clip a carabiner onto the rope grab, and pull up. Voilà, a nifty 3:1! Start pulling and up they go, right?

Well, not quite! Let's have a closer look at the top photo.

Because you’re using a fairly inefficient carabiner (about 45% efficient) and a really inefficient Grigri (about 28% efficient) as your progress capture, at every change of direction where the rope goes through these devices, they are going to rob you of some of your pulling force. This means your real world efficiency is somewhere around 1.6 to 1, not 3 to 1.

Depending on your strength and the rope running over any edges or gear and adding additional friction, etc. might mean it's still impossible to move your partner!

No problem, let's make it a 5:1!

“Wait”, you say, “I know what to do! I have another rope grab. I’ll put that on the backside of the rope coming from the Grigri, and clip my pull strand to that. Now I have a 5:1 pulley system! With that I should surely be able to lift my lard-ass partner. Besides, now I get to lift DOWN with my body weight instead of UP with my arms. That must be easier, right?”

Well . . . Let's have a look at the photo on the right.

Turns out, rigging a theoretical 5:1 like this gives you something pretty much close to a 1:1 in real life! That red carabiner on the rope grab introduces so much additional friction at the start of the system, that the rest can't really overcome it. You probably be better off staying with that 3:1.

You have also created what's called a “complex” pulley system. In a complex system, you have two pulleys moving toward each other when you pull. This can be a bother when you’re at a small stance, and requires that you reset the system more often.

There are some good ways to set up a 5:1 or a 6:1 at a small stance that require minimal gear and offer reasonable efficiency. Unfortunately this is not one of them.

So, if you build a lame hauling system like this and it's not working . . . you need to yell down to your partner to get out a couple of their own friction hitches and start prusiking up. =^)

Takeaways:

• There can be a giant difference between theoretical MA and a real world MA.

• Adding more MA is not always better.

• Use pulleys when possible to increase your efficiency.

• If you have a limited pulleys, use them closest to your hand that’s doing the pulling.