SRT Mechanical Advantage & Z-Drag 2 — Questions and Answers
Question 1: In a 3:1 Z-drag system, if the load weighs 300 lbs, approximately how much force must the hauling team apply (ignoring friction)?
- 300 lbs
- 150 lbs
- 100 lbs (Correct answer)
- 200 lbs
Correct answer: 100 lbs
A 3:1 mechanical advantage divides the load force by 3, so 300 lbs ÷ 3 = 100 lbs of required input force.
Question 2: What is the primary function of the progress-capture device (PCD) in a Z-drag system?
- Increase the mechanical advantage
- Prevent the load from slipping back while haulers reposition (Correct answer)
- Reduce rope friction on edges
- Serve as the anchor attachment point
Correct answer: Prevent the load from slipping back while haulers reposition
The PCD (typically a prussik or ratchet) holds the load in place so haulers can reset their grip without the load sliding back.
Question 3: When rigging a Z-drag, the rescue rope from the anchor traveling to the load and back creates what characteristic shape?
- S-curve
- Z-shape (Correct answer)
- V-shape
- Loop
Correct answer: Z-shape
The rope path traces a Z (or N) shape from anchor to traveling pulley to load attachment, which gives the system its name.
Question 4: Which knot or hitch is most commonly used as a progress-capture device on the haul line in a Z-drag?
- Bowline
- Munter hitch
- Prussik hitch (Correct answer)
- Cleat hitch
Correct answer: Prussik hitch
A prussik hitch grips the haul line under load and slides freely when unweighted, making it ideal as a PCD.
Question 5: A rescuer is building a Z-drag and only has one pulley. Where should the single pulley be placed for maximum efficiency?
- At the anchor
- On the load attachment
- On the traveling (moving) block position (Correct answer)
- On the progress-capture device
Correct answer: On the traveling (moving) block position
Placing the pulley at the traveling (load-side) block reduces the most friction, since that sheave turns the most under a changing angle.
Question 6: What effect does rope friction at pulley sheaves have on the actual mechanical advantage (AMA) of a Z-drag?
- It increases the AMA above the theoretical value
- It reduces the AMA below the theoretical value (Correct answer)
- It has no effect on the AMA
- It doubles the required input force
Correct answer: It reduces the AMA below the theoretical value
Friction losses at each sheave reduce the actual mechanical advantage below the theoretical 3:1, requiring more input force than the calculation predicts.
Question 7: In swiftwater rescue, why is it critical to ensure the Z-drag anchor is downstream of the load when hauling a victim from a hydraulic?
- To maximize rope length available
- To allow the haul vector to pull the victim toward the bank rather than deeper into the feature (Correct answer)
- Downstream anchors always provide better mechanical advantage
- To keep rescuers out of the water
Correct answer: To allow the haul vector to pull the victim toward the bank rather than deeper into the feature
An anchor positioned to pull the victim at an angle toward shore rather than straight upstream can break them free from the hydraulic and direct them to safety.
SRT Mechanical Advantage & Z-Drag 2
In a 3:1 Z-drag system, if the load weighs 300 lbs, approximately how much force must the hauling team apply (ignoring friction)?