Aluminium is the default for a small welded hull because it is light, it does not need paint to survive salt water, and it can be cut and welded in a small shop. The catch is that aluminium alloys get their strength two completely different ways, and only one of them survives the arc.
Two families, two jobs
5xxx (aluminium-magnesium) is plate. 5083, 5086, 5052 get their strength from magnesium in solution and from being rolled — the H tempers. They are the most corrosion-resistant aluminium alloys in sea water, they are not heat-treatable, and a weld does not undo their basic strength: the heat-affected zone softens back towards the annealed condition, but annealed 5083 is still a useful structural material. That is the whole reason 5xxx is the hull alloy.
6xxx (aluminium-magnesium-silicon) is extrusion. 6061 and 6082 are heat-treated to T6 and extrude into any section you can draw — angles, tees, bulb flats, machined fittings. They are stronger than 5xxx as parent metal and much weaker after welding, because the heat dissolves the precipitates the T6 temper is made of. A 6061-T6 weld is nearer the annealed strength than the T6 strength unless it is re-solution-treated and aged, which nobody does to a hull.
So the usual division: 5xxx plate for the shell, decks and bulkheads; 6xxx extrusions for stiffeners, rails and machined parts, ideally bolted or with welds placed where the stress is low.
What the weld costs you
Design against the welded allowable anywhere within the heat-affected zone, which is a band of the order of 25 mm either side of the weld — and on a stiffened panel that is most of the structure. Take the parent and welded allowables from your own specification: the classification rules and EN 1999 (Eurocode 9) both tabulate them per alloy and temper, and they differ enough between sources that a number from memory is worth nothing.
The good news is in the arithmetic. Plate bending stress goes as 1/t², so the thickness penalty is only the square root of the strength penalty: lose 30% of the allowable and you need 20% more plate, not 30%. The bad news is that plate covers the whole boat, so that 20% is 20% of the largest single weight in the structure.
Try it — what a weld costs in plate
The two allowables above are placeholders so the sliders do something. Put your own specification’s numbers in — they are the whole answer.
Three traps
Sensitisation. A 5xxx alloy with more than about 3% magnesium — 5083 and 5086 — will, if it is held warm for long enough, precipitate a magnesium phase along its grain boundaries that corrodes preferentially. The result is exfoliation or stress-corrosion cracking years later. The marine standards draw the line at sustained service around 65 °C and specify stabilised tempers (H116, H321) with an exfoliation test for hull plate. Keep 5083 out of engine spaces, exhaust runs and anything that bakes in the sun under insulation, and buy the stabilised temper for the shell.
Filler metal. Match the family: 5183 or 5356 for 5xxx, 5356 or 4043 for 6xxx. Do not put a silicon filler such as 4043 into a high-magnesium 5xxx alloy — the magnesium and silicon form a brittle phase in the weld. Your welding procedure specification decides this, not the shop floor.
Everything else you bolt to it. Aluminium is anodically active: put it in sea water next to stainless steel or bronze and it is the aluminium that dissolves. The area ratio is what decides how fast — a small stainless fastener in a large aluminium plate is survivable, the reverse eats the fitting — and the fix is isolation, bedding compound and cathodic protection. See galvanic corrosion for the series and the area rule.
You need a welded bracket at 240 MPa parent strength. 6061-T6 extrusion or 5086-H116 plate?
The bracket is only as strong as its weld, and 6061-T6 gives up a great deal more of its strength there than 5086-H116 does. 6xxx earns its place where you need a shape you cannot roll, or where the joint is bolted — not where the load runs through the weld. The strength on the mill certificate is the parent strength, and the parent metal is not what fails.
Working practice
Aluminium moves when you weld it: roughly twice the thermal expansion of steel and a third of the stiffness, so distortion is the fabrication risk, not strength. Keep welds short, balanced about the neutral axis and away from corners; expect to fair a thin panel; leave the plate a little oversize until the stiffeners are on. Cut with a saw or waterjet rather than a grinder wheel loaded with steel, keep steel tooling and steel dust off the plate, and treat a shop that welds both metals as a contamination risk.
Anodising is decorative and thin; it is not a substitute for cathodic protection. Bare 5xxx above the waterline is perfectly happy to be left bare. Below it, paint and anodes are doing the work, and the fasteners are where the trouble starts.