What is Mechanical Advantage?

In the river world and in rescue scenarios boaters talk a lot about Mechanical Advantage with the venerable z-rig being the standard that we’ve been taught since the olden days when the first boaters rode dinosaurs to run their shuttles. These days there is a lot of debate on the best systems and equipment to use out there. The debate is going to rage for eternity in boating and for good reason. Standards improve as we develop best practices each year. There are a few things we want to examine for newer boaters though before we wade into those choppy waters. A lot of beginners ask us what Mechanical Advantage is and how it works, so buckle up because we’re gonna nerd out on physics!

How does Mechanical advantage work?

Mechanical advantage (often called MA) is a term used to describe the relationship between how much force you’re pulling with (the effort force) applied to a machine and how heavy your wrapped boat with all your crap in the river is (the load force) that the system is moving. MA is a measure of how much easier a system makes a task. There are a few ways to calculate this: you can attach a scale like a dynameter to the system and calculate the mechanical advantage by dividing the load force by the effort force. The higher the mechanical advantage, the less effort required to move your stuff. Another way is to measure how much line you pull in and compare that with how far the load moves and that will give you your ratio. Sometimes people count the number of lines supporting the system, but this can be inaccurate when you start getting into pulley redirects.

In a basic 2:1 system there are effectively two lines acting on the load and thus the weight (load force) of your stuff is divided equally between the two lines. For example, you have a 2:1 system built and your stuff weighs 100 lbs. to hold your stuff still against the force of gravity you need to pull on it with 50lbs. Any additional force you put into the system will begin to raise your stuff in the air.

Pulleys and how you use them…

In boating we aren’t typically vertically extracting things with pulleys, but from time to time we will use this when pulling a boat up a steep bank. Typically, a boat is pinned or wrapped and we need to pull that boat off the place it is pinned on. The river exerts a massive force on the boat, much greater than the force of gravity, and we have to work against that. Remember 1 cubic foot of water is 7.5 gallons or roughly 60 lbs. If you look at a wrapped boat and calculate the interior space in cubic feet of your boat you can pretty quickly calculate how much water is sitting in your boat…oh don’t forget to multiply that by the speed of the water. So, you gotta be a massive beefcake to move that, or use mechanical advantage.

If you don’t understand how a pully works in the system, here’s the basic rundown. In basic terms the pully splits the line into 2 separate lines that help to support the load. One line attaches to a static point like a rock or a tree and the other line you will be pulling on. The reason this works is because the pulley (in the 2:1 system described above) forces the tension in the system to be split between the two sections of line that the pulley is creating. In other words, the tension in the line is causing the anchor side of the line to hold half of the force and you pulling on the other side takes the other half.

Pulleys can also be an important safety tool when you use them to redirect the angle you are pulling in. Pulleys in a mechanical advantage system work best when the lines are parallel. As you increase the angle you decrease the mechanical advantage the pulley is offering. Why this is important is sometimes on the river you can’t pull lines parallel because of a cliff wall so you may need to pull parallel with the current. Another scenario you may want to use a pully redirect in is if you are concerned that the amount of force you are putting in the system will rip some metal hardware apart and send it flying directly towards those operating the system. To avoid this, you can place a pulley near the initial anchor point to turn the direction you are pulling by 90 degrees. It won’t add mechanical advantage, but it will help get you out of the way of flying metal shards. If you want to learn more about pulleys you can check out our pulley article here.

Your mechanical advantage ratios are a lie.

Stretchy rope kills Mechanical Advantage and friction also reduces the mechanical advantage numbers. The reality of how the physics works is that the more stretch in a system and the more friction you have on the system, the worse the mechanical advantage is. It is impossible to get the MA numbers to reach the ideal numbers that the system says it is. 3:1 is never what the sticker says. I’ve tested this with static rope from my throw bag and some pretty standard pulleys which resulted in a mechanical advantage ratio of 2.02:1 on a 3:1 z-rig system. So, yeah the 3:1 is not accurate, but largely that has to do with the amount of stretch in the rope.

Unlike climbing, in boating we don’t often use dynamic rope because there is no need for stretchy rope so we generally have static rope at our disposal. This is a huge advantage because most throw bags should offer us a higher mechanical advantage ratio than we would get if we were climbers using dynamic rope. This is the reason pin kits from manufacturers are stocked with static line. Static line does stretch though so it is good to understand your rope and how much elongation your rope has.

The loss of mechanical advantage due to elongation and friction is a good reason that you should have several tools in your toolkit for building mechanical advantage systems. One day you’re going to wrap or pin and it is going to suck. Training with ropes is so important for when this happens so you can react appropriately. There are a ton of ways to accomplish that mission from 2:1, z-rig, 5:1, to vector pulls, and combination systems to get it done. I’m not trying to debate the merits of any one system in this article, only that you keep training, practicing, and trying different options so you’re ready when it does go down. If you need some more insight on how to build these systems check out the intermediate rafting series: