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Aluminum boat repair is almost entirely thin sheet and patch work, not heavy plate. Most jon boats, bass boats, and welded aluminum hulls use 0.080″ to 0.125″ (14 to 11 gauge) for hull sides and bottoms, with 0.160″ to 0.190″ (around 3/16″) for transoms, stringers, and keel reinforcements. That matters because you don’t need a 60-amp industrial machine to cut it, but you do need a clean, fast cut that doesn’t warp the panel or leave heavy oxide dross that will contaminate your TIG or MIG weld later. A plasma cutter is the right tool for this, but only if you size it and set it up correctly.

Why aluminum cuts differently than steel

On paper, aluminum melts at a lower temperature than steel, but it conducts heat about five times faster and is covered in a tough aluminum oxide layer that melts at over 3,600°F. A plasma cutter has to power through that oxide and keep the cut moving before the heat spreads and warps the sheet. That is why aluminum needs roughly 30-40% more amperage than the same thickness of mild steel.

For boat repair, the practical translation is this: a machine rated to sever 1/2″ steel will only sever about 1/4″ to 5/16″ aluminum cleanly. If you are cutting 1/8″ aluminum hull patch, you want at least 25-30 amps of clean output at the torch, not just on the spec sheet. Underpowered machines will cut, but slowly, with a wide bevel, heavy bottom dross, and a heat-affected zone that makes the edge gummy and hard to weld without extensive grinding.

How much power you actually need

Forget max sever ratings. Look at rated clean cut at 10-12 inches per minute. For the thicknesses you see on aluminum boats:

0.080″ – 0.090″ hull skin: 20-30A is enough for a fast, low-distortion cut.

0.125″ (1/8″) hull bottom and patches: 30-40A gives the best balance of speed and edge quality.

0.160″ – 0.190″ (5/32″ – 3/16″) transom, braces, and keel: 40-50A to get a square edge without dragging.

A 30-amp 120V machine will handle 90% of patch jobs on a 14-16 foot aluminum boat. The trade-off is speed. It will cut 1/8″ aluminum at about 15-20 IPM. A 50-amp machine on 240V will do the same cut at 35-45 IPM, which means less heat input and noticeably less warping on long cuts. If you only have 120V in your shop or at the dock, a 30A machine is fine. If you have 240V available, the extra speed of a 45-50A machine is worth buying.

Duty cycle matters more than people think for aluminum. Cutting at 30A on a 30A-rated machine puts you at 100% load. Many budget machines are rated 35-40% duty cycle at max amps, so at 30A you get 3-4 minutes of cutting before a 6-minute cool down. That is enough for patches, but if you are cutting out a full floor section or a 4-foot side panel, you will hit thermal shutdown. A 50A machine cutting that same 1/8″ material at 35A is only at 70% load and will run a 60% duty cycle, so you can keep moving.

Pilot arc is non-negotiable for boats

Do not buy a scratch-start or high-frequency touch-start machine for aluminum boat work. Hulls are painted, oxidized, anodized, and often have rivets and undercoating. A non-pilot-arc torch has to touch the metal to start, and it will misfire constantly on dirty or painted aluminum.

A pilot arc plasma cutter creates the arc inside the torch and blows it out to the workpiece, so it starts without contact and can re-establish the arc over rust, paint, and expanded diamond plate. It also lets you cut expanded metal and start in the middle of a panel to remove a damaged section without edge-starting. For boat repairs where you are cutting out a cracked section in the middle of a hull sheet, that is essential. High-frequency start without pilot arc will also interfere with nearby fish finders and electronics if you are working on the boat rigged.

Look specifically for “blowback” pilot arc, not HF pilot. Blowback is more reliable on low-cost inverters and does not require a high-frequency board that fails in damp conditions.

Air supply and consumables will make or break the cut

The plasma cutter itself is only half the system. Aluminum is far less forgiving of wet or low-pressure air than steel. Moisture causes double-arcing, premature tip failure, and a black, porous cut edge loaded with hydrogen that will cause weld porosity.

You need 3.5 to 5 CFM at 75-90 PSI delivered at the back of the machine, not just at the compressor tank. Most direct-drive pancake compressors cannot sustain that. A 20-gallon or larger oiled compressor rated for 5+ CFM @ 90 PSI is the minimum. Set the regulator at the cutter to 65-75 PSI while air is flowing, not static. Too much pressure blows the molten aluminum away before it can be fully severed, causing bevel and dross. Too little pressure lets molten aluminum stick to the tip.

Use a dedicated inline desiccant filter or motorguard filter within 10 feet of the cutter, even if your compressor has a water separator. In humid air, drain the tank before every session. For aluminum, switch to a smaller orifice tip than you use for steel at the same amperage. For example, a 0.8mm / 30A tip at 28A on 0.125″ aluminum will give a narrower kerf (about 1.5mm) and less top rounding than a 1.0mm / 50A tip run turned down.

Consumable life on aluminum is 30-50% shorter than on steel because the reflective molten puddle erodes the hafnium insert faster. If you see the arc wandering, the kerf widening, or you need to drag slower to get through, change the electrode and nozzle. Running worn consumables on aluminum guarantees oxide contamination.

What to buy for this job

For most DIY boat owners doing patch, transom cap, and stringer repairs, a dual voltage plasma cutter in the 45-50 amp range with blowback pilot arc is the sweet spot. It will run on 120V at reduced output (around 30A) for light hull work, and on 240V gives you full power for 3/16″ work without having to max the machine. You don’t need CNC or 4T modes.

If your budget is tight and you only repair 0.080″ to 0.125″ hulls, a 30-amp 120V inverter will do the job cleanly if you accept slower travel and more filing. Do not go cheaper by skipping the air dryer or buying a non-pilot unit; you will spend the savings on tips and re-work.

Avoid 60A+ machines unless you regularly fabricate 1/4″ aluminum brackets, pods, or thick transom plates. On thin hull skin they are harder to control. At 60A, even a brief hesitation will blow a 1/4″ wide divot in 0.080″ material and warp the edge beyond straightening.

Whatever you buy, check three specs that cheap listings hide: 1) rated clean cut on aluminum, not steel, 2) air requirement in CFM, and 3) included torch consumables availability. If tips and electrodes are proprietary and not sold in 10-packs, skip it.

Technique for weld-ready edges on aluminum

Aluminum does not leave like steel. Set amperage 5-10% higher than you think you need and move faster. Dragging slowly puts heat into the sheet and leaves heavy, crusty dross that is actually re-solidified aluminum mixed with oxide. That dross is extremely hard and will not brush off; you have to file or grind it, which smears oxide into the weld zone.

Hold a 1/16″ to 1/8″ standoff, or use a drag tip with a standoff guide if you have one. Keep the torch perpendicular. Even 10 degrees of tilt creates a 10-15 degree bevel that shows up as a poor fit-up on butt patches. For long straight cuts on hull sides, clamp a straightedge 1/8″ off the cut line to account for the kerf and to act as a heat sink.

Cut with the good side down. Most dross forms on the bottom side, so the top stays cleaner for marking. After cutting, remove dross with a dedicated stainless wire brush or a clean file, then wipe with acetone immediately before welding. Do not use a flap disc that has touched steel; it embeds iron particles that cause corrosion later.

When a plasma cutter is the wrong choice

Plasma is poor for very small holes (under 1/2″ diameter) in thin aluminum because the arc blows out the backside. Use a step drill or hole saw instead. It is also poor for cutting within 1/4″ of a weld you want to keep, because the arc can undercut the adjacent weld toe. For cutting out riveted sections with sealant underneath, the sealant will flare and smoke; ventilate well and expect to clean the edge twice.

If you only have one repair – a single 6-inch crack – a jigsaw with a fine metal blade and cutting wax will give you a usable edge for half the total setup cost. Plasma pays off when you have multiple patches, need to cut curves, or are replacing transom skin where speed and low distortion matter.