Underwater Cutting: Exothermic, Oxy-Arc and Mechanical
By Ray Whitfield, ADCI-certified commercial dive supervisor. Reviewed by Amanda McCallister, editor.
Underwater Inspection | Commercial Diving Contractors Alabama I was working on a trash rack at the bottom of a hydroelectric intake one day, and the bolts had rusted so bad you couldn't tell where the steel ended and the debris began. I tried an oxy-arc, but every time the arc struck, silt would swallow the electrode. So I reached for a Broco Ultrathermic rod, lit it off, and watched that bar chew through a 1-1/2-inch plate like it was butter. That's the moment you realize: underwater cutting isn't about theory, it's about what still works when the water's dirty, the current's pushing, and you're upside down in a harness. Over 28 years on dams, bridges, and power plant intakes, I've used every method you'll read about below, and I've seen them fail in spectacular ways. This isn't a classroom rundown. It's the practical breakdown of the three main underwater cutting methods—exothermic, oxy-arc, and mechanical—and how you actually choose between them on the job.
Exothermic Cutting – The Burning Bar
Exothermic cutting is the brute-force option. You're using a steel rod packed with a mixture that burns at an outrageous temperature—call it 10,000°F, give or take, once the oxygen hits it. The commercial version everyone knows is Broco's Ultrathermic rod, and it'll cut steel, stainless, cast iron, and even concrete. I've used it to cut old mooring chains, steel sheet piling, and once, to clear a concrete plug out of a water line. You need a special torch and an oxygen supply at around 80 to 100 psi, and you'll burn through a rod in a couple of minutes, so don't cheap out on rods. The beauty of the burning bar is that it doesn't need a perfect electrical ground or a clean surface. It just goes to work, even through rust, barnacles, or a coating of silt. The downside is the cut is rough. It's not precision work. You'll get a kerf that's a half-inch wide or more, and you'll throw a lot of molten metal around. That can be bad news if there's anything nearby you don't want to melt—hydraulic hoses, rubber seals, plastic-coated cables. And you're creating hydrogen gas, so in a confined space you need forced ventilation or you're asking for a boom. If the job calls for the bar, you know it. Thick, grungy steel, no place to set up a hydraulic saw, and you're at 100 feet with limited bottom time. Burning bars are the old reliable. But they're consumable-hungry, and after a few dives your wrists feel it from holding that rod against a vibrating plate. It's a tool, not a toy.
Oxy-Arc Cutting
Oxy-arc is what most divers think of when you say 'underwater cutting' because it's the method that uses an electric arc to heat the steel while a stream of oxygen blows it away. The electrode is a hollow steel tube with a flux coating on the inside, and you run DC current along it, strike an arc on the work, then hit the oxygen trigger. The arc does the melting, and the oxygen does the cutting. It's cleaner and faster than a burning bar on ordinary plate steel, and you can get a respectable edge that you can weld back to later. The classic rig is an Arcair Sea-Cut torch with a 400-amp welding machine. You need a heavy-duty lead to the diver, and the electrode consumption is a cost you have to track. You'll set your oxygen pressure lower than you would for a burning bar—maybe 40 to 50 psi—and you'll have to reset the electrode every time you use it, because the flux covers the end and you need to expose the center to start the arc. It takes practice to get the angle and the drag right. I've seen rookies bounce the arc for a full minute before they get the hole started. Oxy-arc is my go-to for making shaped cuts, like trimming a plate for a patch or removing a flange from a pipe. The kerf is narrower, and you can cut in a straight line if your hand's steady. But the arc needs clean contact with the metal, so if the work's covered in thick marine growth, you'll spend the first few minutes gouging it clean. And with the right electrode—like a stainless-cutting rod—you can cut stainless and even some non-ferrous metals. But watch the ventilation, because the flux gives off fumes that you don't want to breathe through your reg if you're in a tight space.
Mechanical Cutting
Mechanical cutting is the quiet, slow, and sometimes the only safe route. Hydraulic power is the workhorse: hand-held hydraulic shears, grinders with diamond blades, and those chain saws they use for wood and concrete. I've also used a diamond wire saw on a massive intake pipe that had to come out without any risk of sparks. That job took a full shift, but it was clean and nobody had to worry about hydrogen or flying hot metal. Mechanical methods don't require an electrical arc or a burning rod, so there's no fire hazard, and that makes a lot of safety people sleep easier. The downside is performance in tight quarters. A hydraulic saw is bulky and you need a power unit on the surface with hoses running down—extra drag, and you have to keep the hoses clear of the cut. Diamond wire gets stuck in a deep kerf if you're not careful, and it doesn't like rebar that's not straight. But for a controlled, precise cut on heavy steel or concrete, mechanical is often the better bet. I've cut steel piles with a diamond wire setup on a salvage job, and the cut was so smooth you could set a level on it, which you never get with a torch. Mechanical is also the fallback when environmental regulations limit hot work. On a few river jobs, the state permits said no in-water hot work during fish migration, so we used a hydraulic chain saw to notch a section of old timber crib. It was the difference between doing the job and tying up the bridge for another season. If you've got the time and the deck space for the power unit, mechanical's the honest, boring workhorse.
Choosing the Right Method
You don't pick a method by what's cool in the catalog. You look at the base metal, the thickness, the depth, and the surroundings. Exothermic handles the heavy, dirty steel and even concrete, but it's abrasive and rough. Oxy-arc gives you speed and control on plate steel and stainless, but it needs a clean surface and a good electrical setup. Mechanical is your heat-free, precise option, but it's slow and bulky. Give me a clean 2-inch plate at 30 feet and I'll take oxy-arc every time. Give me a 6-inch cast-iron intake grate clogged with sand and I'm grabbing a burning bar. Also think about the diver. Exothermic is hard on the diver's body—the bar gets hot, bubbles rise, and you're fighting the rod's buoyancy. Oxy-arc is easier on the wrist, but you're managing an electrical circuit in the water, which means strict check of the equipment's insulation. Mechanical can be the lowest stress, but you're wrestling a heavy saw and tending a hydraulic line all day. Talk to the dive team. A good supervisor listens when the diver says 'I can't see the cut' or 'the current's pushing me off.' It matters. And never forget the permit. Some inland inspections require a hot work plan even for an oxy-arc, and a few facilities won't allow any open flame or electric arc inside a water tank. In those cases, the mechanical route is your only option. So you call it: the method is a job decision, not a habit.
Frequently Asked Questions
What's the difference between exothermic and oxy-arc cutting? Exothermic cutting uses a self-oxidizing steel rod that burns at around 10,000°F with oxygen, and it doesn't need a clean metal surface—it'll eat through rust and concrete. Oxy-arc uses an electric arc to melt the steel and a jet of oxygen to blow it away, giving a cleaner cut but requiring a solid electrical contact with the work. Can you use a plasma cutter underwater? Yes, but it's not the usual tool for a working diver. Plasma torches need special long leads and gas shielding, and they're mostly used in robotic or hyperbaric systems. Most commercial divers rely on oxy-arc or exothermic bars. What safety precautions matter most when cutting underwater? You've got to worry about hydrogen gas building up, so in confined spaces you need ventilation. Also, with oxy-arc, you're running a live electrical circuit in the water—check the torch and lead insulation carefully. And always follow hot work permit rules; they're not paperwork, they're your life. Can mechanical saws cut concrete underwater? Yes, diamond-blade saws and hydraulic chain saws fire cutting concrete, rebar, and timber. They're slower than burning bars but produce no sparks or gas, which makes them the only acceptable option in many low-risk environments like water tanks or during fish spawning closures.