Strategically placed dampers (counterweights) can quickly limit the projection of a carabineer or pulley if the system breaks. Extra Prussik attachments can be used as a brake to allow you to advance the Z Drag system. Another approach is a tensionless belay hitch. The person closest to the Z Drag system should face away from the set-up and wear a PFD and helmet for protection. In fact, all participants should wear these items. Consider an extra pulley or carabineer for a direction change to take the pullers out of the line of fire. If possible, assign a non-puller to watch the Prussik on the traveling pulley for any signs of slippage along the rope. A tag line attached to the end of the boat and hooked up to a tensionless hitch is wise to prevent a runaway boat once it is freed. Here is a video we made where we intentionally exercises a Z Drag failure: Z-Drag to Failure. Notice the third try which used a couple of frayed shoe laces. We managed to get quite a snap even with that weak link - anyone in the line of fire would sustain quite an injury. Z Drag failures with ropes can impale a piece of plywood with a carabineer - plan well and be very careful.