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Understanding of mechanical advantage systems
A river with a place to practice
Someone to set safety
Understanding how wraps occur
Spotting obstacles that can wrap a raft
Avoidance tactics
What to do when a wrap occurs
Techniques for unwrapping a raft
In this lesson, Trevor is discussing how to identify, avoid, and recover from wrapped boat scenarios. We will be focusing specifically on: Why wraps occur, where you are likely to wrap, how to avoid a wrap, what to do when a wrap occurs, and techniques to unwrap a raft.
Simply put wraps occur due to the force of hydraulic pressure overcoming the rigidity of the hull of a boat. Both inflatables and hard-shell craft can wrap, however we see it more often in inflatables due to the lower rigidity and more pliable material used in their construction. Loss of buoyancy especially due to a boat being filled with water is also a contributing factor to a wrap. Losing buoyancy can cause the boat to sink deeper into a pressure wave forming on the upstream side of an obstruction. This in turn causes the water to fill the boat and if that hydraulic pressure, then overcomes the rigidity of the boat it will wrap around the obstacle.
A classic wrap involves hitting an obstruction sideways and the bow and stern of the raft act like sails for the water to catch. After the river begins to wrap the boat, it will eventually settle into an equilibrium where the boat is pinned in place.
Bridges, trees, narrow rocks directly in the current, undercut obstacles, pour over drops all pose wrapping hazards. In most cases the likelihood of wrapping is a function of the velocity of the current pushing on the obstacle. The wider the obstacle the less likely a wrap is to occur due to the velocity of water slowing down and moving around the obstruction. Very thin obstructions like trees and bridge piers in the main current have a low surface area and high velocity of water passing over them.
There is also another variety of less common wrap which involves the boat being pinned to the river bottom. We call this a flat wrap. This can occur due to the sides of the boat pinning and water begins flushing into the boat from above reducing buoyancy and sinking the boat farther into the water column. This can also happen when the raft passes over a pour over drop into a plunge pool. If the force of the plunging water is sufficient enough it can force the raft down and onto the river bottom below a vertical drop.
Tactics for avoiding wraps in the first place starts from the second phase of your scouting techniques. The goal is to Identify and avoid wrap prone obstacles during the scout. If you are having trouble with this, please take a look at our scouting lessons. Specific tactics you can use to avoid a wrap include:
Point your bow at an obstacle and bounce off of it
High side before hitting the obstacle
Boof to get on top of the obstacle – (this is a bit of a desperation move though and can be un reliable resulting in further dangers like flips and swims.)
Safety in a wrap scenario is your top priority and you should always seek to prioritize people before equipment. You want to make sure you are getting people in the boat to a safer and more stable location out of the water to prevent them from being flushed downstream. Next, you will want to account for swimmers by looking downstream and communicating with swimmers or other boaters near you to see if they either have any swimmers who may have flushed down stream or if they can assist with picking them up. Establishing and setting up safety is the first step in the situational awareness required to recover from teh wrap.
The second step in your situational awareness in a wrap scenario is to observe the water. You need to understand where most of the water is going and how much hydraulic force is being applied to your boat. This will help you determine where the current vector is strongest and where you want to start peeling the raft off of the wrap.
Next you need to figure out how the boat is wrapped. is it a single rock with water pinning the raft in place? Is there another rock under the boat? Is it pinned on a log underwater, or is the bottom of the boat curled under a hidden undercut below the rock? These are all elements you need to understand to help you build your strategy on how you plan to remove that boat from it’s current state.
We want to start with some theories for how and when to apply different techniques to unwrapping a raft. First, try to peel the boat away from the obstacle by getting the boat to lift up and the water flushing under the raft to help gain lift. Typically the raft is resting in a state of equilibrium so an effective strategy is to Alter the shape of the raft to disturb the equilibrium. This may require a shore-based rescue including a mechanical advantage system. This is why you want to be confident with mechanical advantage systems before getting into a wrap scenario where you may not know what to do.
The next theory we want to look at is utilizing minimal equipment tactics. Start with the smallest amount of equipment and escalate from there. Start with hands on the raft attempting to move or slide the boat first then if that doesn’t work escalate to a flip line. From there you can move to multiple people on a throwbag stationed on the obstacle. Finally move to a mechanical advantage system like a 2:1 or vector pull inside the boat. By starting with an escalation of minimal equipment tactics you are not initially jumping into utilizing lots of ropes and hardware which and introduce additional elements of danger into the scenario for yourself, your team, and other bystanders. As you escalate more you may find that you need additional people in the mix as you work into bigger more complex mechanical advantage systems from shore.
It has been a widely practiced principal in the rafting world that deflating a tube can be useful, however deflating your boat generally is not a good idea. There are two main reasons for this rigidity is a key factor in removing the boat from the wrap. The boat is only wrapped because the hydraulic pressure is greater than the resistance from your boat’s rigidity. If you can somehow reduce the hydraulic pressure you can use the rigidity of your boat to spring your raft out of the wrap. The only thing maintaining rigidity in most rafts in the air pressure in the tubes thus letting air out of the boat removes rigidity from the raft. This has the compounding effect of reducing buoyancy which generally tends to make the boat wrap more readily because it allows the raft to sink deeper under the water.
