One of the consistent problem areas for many rescue crews is not just the construction of mechanical-advantage systems, but their safe and efficient operation. If speed and simplicity are the objective, then rescue crews must develop systematic approaches to haul systems.
To get started, we have to cover some basic concepts about mechanical advantage. The overall goal is to train all of your personnel to count units of tension and understand mechanical-advantage physics.
However, crews are made up of diverse individuals with varied abilities to understand and implement these concepts. Here are the basics that everyone should understand first.
Mechanical-advantage ratios represent two things: the force required to move the load and the relationship between length of rope pulled by rescuers and the resulting distance traveled by the load or victim.
For example, a 3-to-1 mechanical-advantage ratio requires force that is one-third the load to move it. This means a rescue crew would have to pull with 100 pounds of force to move a 300-pound load.
A general rule is that the average rescuer will pull approximately 100 pounds of force. Additionally, for every three feet the rescuer pulls, the load will only rise one foot.
Based on this example, we can extrapolate some basic information to establish some operating parameters.
No more than three rescuers should be pulling on the haul system at one time. Use four-man haul teams if possible — have one rescuer in transition while three rescuers haul like a conveyor. Stagger the rescuers evenly through the first half of the haul lane.
As the lead rescuer approaches the end of the haul lane, the rescuer in staging or transition announces "on rope" and the lead hauler announces "off rope." The lead rescuer then lets go of the rope and returns back to the starting point.
Pulling rope hand over hand results in a bumpy ride for the victim and applies very small but unnecessary spikes in the forces to the overall system. If the haul team cannot generate enough force to efficiently move the load, then mechanical advantage should be upgraded.
I am a strong advocate of limiting the haul team to four personnel. If we simply add more horses, we decrease the ability to detect malfunctions or system overloading.
Adding more rescuers is one of the most common upgrades applied to mechanical-advantage systems because it is an easy answer and often times crews are not proficient at constructing mechanical-advantage systems other than the bread and butter 3-to-1 Z drag.
A 3-to-1 mechanical advantage is the general starting point when using the load-design model of 300 pounds for a single-person load (or two-person load with a victim and rescuer weighing less than 300 pounds combined) and applying the above dynamics of a collective haul team generating 300 pounds of force.
Use this as a guideline. In the field, mechanical advantage is always theoretical. It can be equated loosely to friction-loss calculations with fire hose.
As with appliances, when we add pulleys, edges and inefficient rope angles we lose efficiency in the hauling system, thus requiring greater force to move the load than what the mechanical advantage may represent. If the load cannot be effectively moved or if it exceeds this starting point, upgrade the mechanical advantage.
To upgrade, use the smallest manageable mechanical advantage to maintain an appropriate ratio between ease and speed of rescue. Although a 9-to-1 mechanical advantage is very easy to operate, a load that requires nine feet of pull to achieve one foot of rise is not very fast or efficient.
With guidelines established, we need to create a solution to the mechanical upgrade challenge. We need a solution that is fast, simple and maintains a high success rate based on the performance capabilities of your organization.
This is accomplished through the development of a standard progression of mechanical-advantage upgrades. Additionally, there are some equipment variables that will increase the speed and ease with which you perform these upgrades.
Use compound and complex systems with progress capture devices to allow system resets. These systems maximize rope use and length, reduce edge contact and friction, and provide greater versatility than simple systems such as a 4-to-1 block-and-tackle hauling system.
Apply the progressions in succession as needed. With that being said, experience is the greatest teacher and well-versed crews may bypass a progression to get to a higher mechanical advantage based on the difficulty required to move the load.
I discourage crews from doing this until they have that level of experience; the goal is to err on the side of fast and efficient movement of the victim, not ease for the rescuers. The victim must always drive the rescue.
Mechanical Advantage progressions:
- 3-to-1 MA: Start with a compound 3-to-1 Z drag. This system involves a single line going to the victim with a change-of-direction pulley and progress-capture device as the primary hauling pulley. Apply a Prusik and a moving pulley to the primary line. Using a double pulley in this position will facilitate quicker upgrades later in the progression sequence.
- 5-to-1 MA: If the 3-to-1 system is not appropriate due to haul lane length or significant heights, convert it into a compound 5-to-1 by placing an additional change-of-direction pulley at the anchor and attaching an additional moving pulley and carabiner to the existing Prusik on the primary rope. If a double pulley was already used for the 3-to-1, simply rig the rope into the open side of the pulley.
- 6-to-1 MA: This system is the most common progression to bypass because it affords minimal mechanical advantage upgrade and presents some management challenges. To convert the 5-to-1 to a 6-to-1, convert the secondary change-of-direction pulley at the anchor into a midline knot and unrig the secondary moving pulley portion of the Z drag. Convert that portion to an additional Z drag by rigging a Prusik and moving pulley onto the backside of the initial moving pulley. This system results in a "dead" section of rope. The innermost portion of rope coming out of the midline knot carries zero units of tension and increases in length as the hauling system progresses. Each reset results in more limp rope lying on the deck that must be managed so that it does not become a trip hazard. Resetting this system also requires the limp section of rope to be managed by pulling tension back towards the anchor.
- 7-to-1 Complex MA: This system should only be applied for short hauls or extremely long haul lanes and would be used for a 5-to-1 upgrade when a 6-to-1 upgrade has been bypassed. It is an extremely fast upgrade to build but collapses from both ends and requires more resets. To turn the 5-to-1 into this system, simply take the portion of the rope that the haul team is pulling on and rig it into a moving pulley with a Prusik and attach it to the rope on the backside of the primary hauling pulley.
- 9-to-1 MA: Converting the 6-to-1 to a 9-to-1 simply requires the midline knot to be replaced with a change-of-direction pulley. This results in stacked Zs, or a 3-to-1 pulling on a 3-to-1.
Insure that your systems are engineered to provide changeover operations or the ability to convert from a haul to a lower. This progression as well as the changeover component can be accomplished using a traditional RPH or a specialized device.
Using the CMC MPD as the primary hauling hub eliminates the need for a rigging plate as well as a Prusik or additional progress capture device. This greatly enhances both the speed and safety of both resets and progression upgrades. The CMC integrated swivel pulleys with detent button side plates also greatly enhance quick rigging transitions.
When this progression is implemented as a standard approach, it allows rescue crews to consistently drill on hauling operations with a more comprehensive approach.
They are not just developing proficiency in building and operating hauling systems. They are developing consensus and understanding about what type of systems will be used as well as why and when.
This allows operational commanders or lower and haul team leaders to communicate basic information to the team and trust that the appropriate system will be built and upgraded as needed without 10-minute conversations on the scene.
Additionally, these progressions can be taught to the organization's nonrescue members. If support personnel can be used to apply these progressions and operate them, the technically savvy personnel are now free to perform oversight and more technically intense phases of the operation that require their talents.