Bagging and Lifting: Moving Heavy Things Underwater Safely

By Gus Marchetti, Salvage master. Reviewed by Amanda McCallister, editor.

Marine Salvage & Recovery | Hull & Bottom Cleaning October 17, 2010, Port of Galveston. I was 22 years old and thought I knew what I was doing. A 40-foot container full of copper wire had gone overboard during a storm the night before. It sat in 38 feet of water, half-sunk into the mud. The client was on the phone every hour, asking when we'd have it out. I remember the water was green and cold, with a five-knot current that made working feel like hand-to-hand combat. We had a 5-ton lift bag and a lot of confidence. What we didn't have was a plan for the suction effect of the mud. That container had settled into the bottom like a sleeping animal. The bag lifted, the shackles held, but the container didn't budge. Then the bag popped. It shot to the surface like a cork and took one of my divers' masks with it. That day taught me more than any manual. Bagging and lifting is a craft. It's about understanding the water, the load, and the laws of physics you can't negotiate with. Here's what I've learned in the 12 years since, in the only order that matters: safety first, then technique.

Start with the load, not the bag

You can't pick the right bag until you know what you're lifting. That means weight, shape, volume, and how it's sitting on the bottom. Steel weighing a ton on land only weighs a few hundred pounds in the water when you account for its displacement. But if it's buried in silt, the suction can add ten times that. I once found a concrete anchor block that looked like a 2-ton candy drop. It was actually a 9-ton block with the bottom buried in mud. Our 5-ton bag couldn't move it. We needed a bigger bag and a water jet to break the suction.

Choose your bag by depth

Lift bags are rated for surface lift capacity. Underwater, that rating changes. Air compresses as you descend, so you need more air to fill the same bag at depth. At 33 feet of seawater, the pressure is twice the surface pressure, so you'll need twice the air volume to get the same lift. For a lift at 70 feet, you're looking at three times the air. Choose a bag with a capacity at least 50 percent above your calculated load, and never rely on a bag that's near its rating. I always carry one size bigger than I think I need.

Rig it like your life depends on it

Your bag is just a balloon with a job to do. It's only as good as the rigging that holds it. Use a wide nylon sling, not a single line, to spread the load across the object. Always use stainless steel shackles—saltwater is hungry for carbon steel. And put a long pendant line on the bag, something you can grab from the surface if the bag starts to behave badly. That line has saved my crew more than once.

The lift is a slow dance

Inflation is where things go wrong. Never fill a bag fully at depth. As the bag rises, the air expands, and the lift increases dramatically. You want to lift with a partially filled bag and let it expand on the way up. On that Galveston container job, we learned the hard way. Since then, I use a valve to bleed air off as the bag rises. And I never let a bag ascend faster than about three feet per second. Anything faster is out of control.

Getting it aboard

The lift is only half the job. When the bag breaks the surface, you need to handle it. A wave can push your load sideways and swing it like a pendulum. Use a tag line from the bag to a deck cleat. If the bag is open-bottom, let the air escape slowly as you attach a crane hook. If it's closed, use the dump valve. Bring the load over the side, then snap a crane cable on. Once the crane takes the weight, you can release the bag. That's the safe, boring way. Boring is good.

Frequently Asked Questions

What’s the most common mistake in underwater bagging? Over-inflating the bag at depth. Divers forget that air expands as the bag rises, so a bag that's 80 percent full at 30 feet becomes 100 percent at the surface and keeps going. Always start with less air and bleed off as you ascend. Can I use a lift bag to break a stuck anchor loose? No. Lift bags are for lifting free or mostly-free objects. If you try to use them to pull something out of the mud, the bag will take all the load and then release like a slingshot when the object finally breaks free. That’s how bags and body parts get lost. Use a direct pull or a rock hammer. How do I know what size bag to buy? Start with the object’s weight on land, subtract its buoyancy (the weight of the water it displaces), and add 50 percent for the unknown. That’s your minimum bag rating. For something in mud, double the margin.