A lifting bag raises a load by injecting air until it overcomes the load's submerged weight. The problem is that the air expands as it rises, so the buoyancy grows on its own and the ascent accelerates: that's why the basic rule is never to stand under the load, and to control the ascent from the surface whenever possible.
The lifting bag is one of those tools that seems simple until someone tells you how the accidents happened. It's a sack of tough fabric that you fill with air to lift whatever's hooked to it. Nothing more. And yet it's behind a fair share of the serious scares in commercial diving, because its behaviour isn't intuitive.
The reason is that bags aren't a controlled system: they're a system that feeds back on itself. Understanding that difference is what separates someone who uses them with judgement from someone who's just been lucky for a few years.
How it actually works
To lift something off the seabed you need buoyancy greater than its submerged weight. Submerged weight is the item's real weight minus the buoyancy of the water it displaces, and it's always less than its weight in air. A steel block loses around an eighth of its weight underwater; a hollow item can lose much more. Sizing the lift on catalogue weight instead of submerged weight is the first sizing mistake.
Up to this point, everything is static and manageable. The problem starts the moment the load lifts clear of the bottom.
The expansion that changes everything
The air inside the bag is at ambient pressure. As it rises, that pressure drops and the air takes up more volume. More volume means more water displaced, which means more buoyancy. And more buoyancy means the load rises faster, which makes the pressure drop faster still, which makes the air expand even more.
The effect is dramatic exactly where most people work: near the surface. Between twenty metres and ten metres, absolute pressure drops from roughly three atmospheres to two, so a given volume grows by about half. Between ten metres and the surface it doubles again. A load that started rising slowly can reach the surface at speed, break clear of the water and then drop back down suddenly as the bag empties into the air, falling uncontrolled onto whatever's underneath.
That cycle — shoots up, breaks the surface, empties, sinks — is the classic sequence, and it's the reason nobody and nothing can be under the load's vertical line in the work area.
Types of bag
Open-bottom bags
These are the most common in construction work and light salvage. They have an open skirt underneath, so excess air escapes on its own if the bag tilts or is overfilled. That gives them a degree of self-regulation, but it also makes them vulnerable: if the bag rolls over during the ascent, it loses its air all at once and the load drops.
Closed bags with a vent valve
These are sealed sacks with a relief valve that releases excess pressure during the ascent. They're the right choice when the load needs to rise in a controlled way, because the valve limits the acceleration. They need more care in setup and maintenance, but they're far more predictable.
Parachute-type bags and pontoons
For larger loads, high-volume formats or rigid pontoon-type units are used, mainly in salvage and refloating, where the goal isn't to lift a single item but to restore buoyancy to an entire hull. It's a field with its own engineering.
The rules that aren't negotiable
Nobody under the load, ever. Not during filling, not during the ascent, not even once it looks stable. The diver positions themselves clear of the vertical line and stays there.
Fill slowly and check. Filling is done in stages, checking between each one how the load responds and whether the rigging is behaving as expected. Filling fast is the most efficient way to lose control from the very first second.
Control from the surface whenever possible. A control line from a vessel or from shore lets you govern the ascent and recover the load without relying on the diver's reaction. It's the single improvement that most reduces the risk of this operation.
The rigging matters as much as the bag. Slings and shackles with the right working load, correct angles, protection against sharp edges, and confirming the load is genuinely free and not buried or snagged. Everything covered in the article on rigging for divers applies here even more, because a rising load pulls with more energy than it looks like it should.
Keep the umbilical under control. An umbilical that runs between the load and the seabed, or that loops around the sling, turns a minor incident into a serious one. Before filling starts, you need to know its route and tell the surface team.
Continuous communication. The surface team must know at all times what phase the operation is in, and the diver must call out every change before making it, not after.
When not to use bags
There are situations where the right answer is a different method. If a crane or gantry is available with access to the load, mechanical lifting is preferable because it keeps control throughout the whole lift. If the load is buried or stuck fast, it has to be freed first, because an oversized bag trying to tear something loose is a loaded spring. And if the load's weight or geometry isn't known with reasonable certainty, there's no sizing it properly: estimate first.
Nor is it a tool for working alone. Even with the diver clear of the vertical line, the operation needs the surface team attentive and able to respond.
How to size a lift
Sizing a lift properly is the part done at the table, calmly, before anyone gets in the water. It has four steps, and none of them can be skipped.
The first is estimating the item's real weight. If there's no documentation, it's calculated from its geometry and material, with a generous margin, because underestimating means a bag that can't lift it and several dives wasted.
The second is converting it to submerged weight, subtracting the buoyancy of the displaced volume. This is where people coming from surface work go wrong most often, in both directions: oversizing for solid items, or — much worse — underestimating how little a hollow item weighs underwater, which can shoot up with very little air.
The third is adding what the trade calls suction force: if the item has been sitting on mud for a while, you also have to overcome the sediment's grip, which can be considerable and, above all, unpredictable. Free it before trying to lift, using a water jet or digging under the object, not by forcing it with buoyancy.
And the fourth is deciding the configuration: one large bag or several smaller ones spread out, and how high above the load. Several bags share the effort and give stability, but multiply the failure points and complicate simultaneous filling. A single bag is simpler and more prone to rolling over. The decision depends on the item's geometry, and it's best made in advance, not improvised on the seabed.
The sequence in the water
With the sizing done, the operation follows an order that's worth keeping the same every time.
Inspect the load and confirm it's free. Rig the hook-up and check it in its final position. Position the bag, unfurled and untwisted, with its filling line accessible. Announce to the surface that you're about to start and confirm the control line is taken up. Fill in stages, with the diver clear of the vertical line and calling out each step. When the load starts to move, stop adding air and watch: a load that's already rising doesn't need more buoyancy, it needs less. And finally, follow the ascent from the surface until the load is recovered.
The point where control is lost is almost always the same: the instant the load lifts clear and the diver, still holding the air line, adds a bit more "just to be sure". That bit more is what turns a governable ascent into an object arriving at the surface at full speed.
The idea worth taking away
Lifting bags are useful, cheap, and work well when their rules are followed. But they're not a crane: they're a system that gains energy as it rises and that tends to run out of control by design, not by bad luck.
Accepting that changes how you plan. Instead of asking how much buoyancy is needed, the right question is what happens if this load rises twice as fast as expected, and where everyone will be at that moment. The answer to that second question is what prevents accidents, and it's one of the skills the good commercial diving schools drill hardest in the practical part.
Frequently asked questions
Why does a load rising on a bag accelerate?
Because the volume of trapped air increases as pressure drops during the ascent. More volume means more buoyancy, and more buoyancy means more speed, which in turn speeds up the expansion. Without a vent valve to release the excess, the process feeds back on itself until the load shoots to the surface.
What is a load's submerged weight?
It's what the load actually weighs in the water, i.e. its weight minus the buoyancy of the volume of water it displaces. It's always less than its weight in air, and it's the figure to use for sizing the lift, not the catalogue weight.
What's the most common mistake?
Working underneath or beside the load during the ascent. If the bag rolls over, comes unhooked, or the sling gives way, the load falls without warning. The second most common cause is dumping too much air at once trying to correct an ascent that's already got out of control.
Is a crane better than bags?
When a crane is available and there's access, almost always yes, because the load stays under mechanical control throughout. Bags are used when there's no lifting equipment, to move loads along the seabed, or as an aid to lighten an object before hooking it up.
What maintenance do bags need?
Inspection of the fabric, seams, attachment points, and fill and vent valves, plus a record of those checks. Like any lifting accessory, they need traceability and must be taken out of service as soon as damage appears in a load-bearing element.
Published 5 September 2026 · By Arthur More, professional saturation diver
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Photo: kmardahl · CC BY-SA via Openverse