When a shipyard or marine OEM asks what gauge of aluminum to use for a boat hull, the honest answer is that thickness is an engineering decision, not a catalogue lookup. It depends on vessel length, service environment, structural role, and the rule set the vessel must satisfy - typically classification-society rules backed by material standards such as ASTM B928/B928M and EN 485-2. This guide gives buyers the working thickness ranges for the common marine alloys, the standards that govern them, and a specification checklist that can be pasted straight into an RFQ.
Key Takeaways
- Work in millimetres: marine drawings, class rules and fabricators all use mm. US "gauge" numbers are a legacy shorthand that differs between aluminum and steel - an 8-gauge aluminum sheet is 3.26 mm, while 8-gauge steel is about 4.2 mm.
- Typical hull thickness ranges: 3.2–6.4 mm for small craft under 20 ft; 6.4–9.5 mm for 20–50 ft vessels; 9.5–19 mm for large vessels; 25.4 mm and above for ice-class or heavy-impact structures.
- Workhorse alloys: 5083-H116/H321 and 5086-H116 for primary structure; 5052-H32 for light panels and small-craft topsides; 5456-H116/H321 for extreme impact service.
- Temper is as important as alloy: H116 and H321 are the marine tempers under ASTM B928/B928M and resist sensitization; an "O" temper changes seawater corrosion performance.
- Specify a full designation (alloy–temper–thickness), a tolerance class per EN 485-2 or ASTM B209, and a mill test certificate per EN 10204 type 3.1 in every RFQ.
Gauge numbers vs millimetres
North American practice still quotes aluminum sheet thickness in gauge numbers. The US aluminum gauge table runs from 10 gauge (0.1019 in / 2.59 mm) to 2 gauge (0.2576 in / 6.54 mm) across the range used for small craft:
| US gauge (aluminum) | Thickness, in | Thickness, mm | Typical use in boats |
|---|---|---|---|
| 10 | 0.1019 | 2.59 | Interior panels, small-boat topsides |
| 9 | 0.1144 | 2.91 | Non-structural partitions |
| 8 | 0.1285 | 3.26 | Small-craft hull plating (lower bound) |
| 7 | 0.1443 | 3.67 | Small-craft hull and stringers |
| 6 | 0.1620 | 4.11 | Small-craft hull, decks |
| 5 | 0.1819 | 4.62 | Medium-vessel deck plating (~0.188 in / 4.8 mm) |
| 4 | 0.2043 | 5.19 | Medium-vessel bottom plating |
| 3 | 0.2294 | 5.83 | Medium-vessel hull plating |
| 2 | 0.2576 | 6.54 | Medium-vessel primary structure (0.25 in / 6.4 mm) |
Note that an 0.125 in (3.2 mm) plate sits between gauges 9 and 8, and an 0.25 in (6.4 mm) plate sits between gauges 3 and 2. Because the same gauge number means different thicknesses for aluminum and steel, and because classification rules and fabrication drawings are dimensioned in millimetres, this article - and every serious RFQ - uses millimetres as the primary unit.
Governing standards and what each one controls
| Standard | Scope | Why it matters to a buyer |
|---|---|---|
| ASTM B209 | General specification for aluminum and aluminum-alloy sheet and plate | Baseline for alloy/temper designations, thickness tolerances and mechanical property limits on US-spec material. |
| ASTM B928/B928M | High-magnesium aluminum-alloy sheet and plate for marine service and similar environments | The marine-specific spec covering 5083, 5086 and 5456 in H116/H321; adds corrosion testing and tighter controls for seawater service. |
| ISO 209-1 / ISO 209-2 | Wrought aluminum: chemical composition (Part 1) and mechanical properties (Part 2) | International designation baseline for composition and properties. |
| EN 573-3 | Chemical composition and product designation of wrought aluminum alloys (e.g., EN AW-5083) | European designation system; the composition reference used in this article. |
| EN 485-2 | Aluminum sheet, strip and plate: mechanical properties and tolerances | Standard supply and tolerance basis for European orders. |
| GB/T 3880.1–.3 | Chinese national standard for wrought aluminum sheet and plate (general requirements, mechanical properties, tolerances) | Applicable standard for material supplied from mainland China; specify it in the RFQ for Chinese-origin plate. |
| Classification-society rules (ABS, DNV, Lloyd's Register) | Scantlings (required thickness), welding and testing for classed vessels | Final authority on minimum thickness for classed hulls; the material must also meet the standard those rules reference. |
Recommended thickness ranges by vessel type
The ranges below reflect common shipbuilding practice for welded aluminum craft and are a starting point for RFQ discussion. Final scantlings must come from approved structural calculations and the applicable classification-society rules.
Small craft, under 20 ft (6.1 m)
Hull plating is typically 0.125–0.25 in (3.2–6.4 mm) in 5052-H32/H34 or 5083-H116. The thinner 5052 grades suit cabin interiors and non-structural panels; hull bottoms and structural members go to the upper end of the range, usually in 5083-H116.
Medium vessels, 20–50 ft (6.1–15.2 m)
Primary hull structure is commonly 0.25–0.375 in (6.4–9.5 mm) in 5083-H116/H321 or 5086-H116 to carry hydrodynamic and impact loads. Decks are often held at 0.188 in (4.8 mm) to control topside weight while retaining strength.
Large vessels, over 50 ft (15.2 m)
Critical structure uses 0.375–0.75 in (9.5–19 mm) 5083-H116/H321 plate. Ice-breaking vessels and military craft specify 1 in (25.4 mm) and above, frequently in 5456-H116/H321 for maximum impact resistance.
| Vessel type | Typical thickness | Alloy / temper | Typical application |
|---|---|---|---|
| Small craft < 20 ft | 3.2–6.4 mm (0.125–0.25 in) | 5052-H32/H34, 5083-H116 | Hull, stringers, interior panels |
| Medium 20–50 ft | 6.4–9.5 mm (0.25–0.375 in); decks ~4.8 mm (0.188 in) | 5083-H116/H321, 5086-H116 | Primary hull, decks, frames |
| Large > 50 ft | 9.5–19 mm (0.375–0.75 in) | 5083-H116/H321 | Hull, bulkheads, main frames |
| Ice-class / military | 25.4 mm (1 in) and above | 5456-H116/H321 | Ice belt, impact zones |
Alloy selection: strength, weldability and seawater corrosion
All the marine alloys discussed here belong to the 5xxx (Al–Mg) series - non-heat-treatable, weldable and corrosion-resistant - plus one 6xxx option for members where welding is avoided.
5052 (2.2–2.8% Mg)
Good formability and corrosion resistance at moderate strength. Common for small-boat topsides, cabin interiors and splash-zone components. For primary hull structure in open seawater, most builders step up to 5083 or 5086.
5086 (3.5–4.5% Mg)
A cost-effective middle option between 5052 and 5083. Slightly lower strength than 5083, so it is often specified for plating where the 5083 margin is not required.
5083 (4.0–4.9% Mg)
The marine workhorse: high strength, good weldability with ER5183 or ER5356 filler (AWS A5.10), and the default choice for primary hull structure in welded aluminum boats.
5456 (4.7–5.5% Mg)
The highest-strength 5xxx plate, used where extreme impact resistance is required - ice class and heavy-duty structures.
6061-T6 (0.8–1.2% Mg, 0.4–0.8% Si)
Weldable, but the T6 temper is lost in the heat-affected zone, so it is typically specified for bolted or adhesive-bonded, above-waterline members where welding is avoided.
Temper matters as much as alloy. For seawater service, order 5xxx plate in H116 or H321 - the tempers covered by ASTM B928/B928M and designed for marine environments. Fully annealed (O temper) material is more prone to sensitization - precipitation of Mg₂Al₃ at grain boundaries during service - which can lead to intergranular corrosion in seawater.
| Alloy | Mg range (EN 573-3 / ISO 209-1) | Marine temper | Typical role |
|---|---|---|---|
| 5052 | 2.2–2.8% | H32 / H34 | Small-craft panels, splash zone |
| 5086 | 3.5–4.5% | H116 / H321 | Plating where 5083 margin is not needed |
| 5083 | 4.0–4.9% | H116 / H321 | Primary hull structure |
| 5456 | 4.7–5.5% | H116 / H321 | Ice class, high-impact structure |
| 6061 | 0.8–1.2% (Si 0.4–0.8%) | T6 | Bolted/bonded above-waterline members |
Engineering factors that set minimum thickness
Hydrostatic and dynamic loads
Minimum plate thickness is driven by hydrostatic pressure, slamming loads and grounding scenarios. As a common design practice, hull-bottom plating is specified roughly 20–30% thicker than side plating of the same alloy; this is a cross-check rule of thumb, not a standard value - approved scantling calculations govern.
Corrosion allowance
Builders commonly add 0.5–1 mm of thickness above the structural requirement for long seawater exposure, most importantly in splash zones and bilge areas where protective coatings wear.
Welding and fabrication
For 5083 and other 5xxx alloys, corrosion performance at welded joints is managed by temper selection (H116/H321), matching 5xxx filler wire (ER5183 or ER5356 per AWS A5.10), and controlling interpass temperature - commonly kept below about 65 °C to limit sensitization of the heat-affected zone. Post-weld heat treatment is neither required nor recommended for 5083: it is a non-heat-treatable alloy, and heating accelerates sensitization rather than restoring properties.
Tolerances and fit-up
Thickness tolerance classes per EN 485-2 or ASTM B209 affect welded assembly fit-up. Specify the tolerance class in the RFQ, and confirm flatness for wide plates used in hull panels.
RFQ checklist: how to specify marine plate to a supplier
To get comparable quotes and compliant material, every enquiry should include:
- Full designation - alloy + temper + thickness in mm, e.g. 5083-H116, 6.0 mm.
- Dimensions - width × length, quantity, and whether plates are cut to size (sawed or sheared edges) or full mill sheets.
- Tolerance class - per EN 485-2 or ASTM B209, plus a flatness requirement.
- Certification - mill test certificate per EN 10204 type 3.1 (chemical + mechanical test results); for 5083/5086/5456 destined for classed hulls, confirm ASTM B928 compliance and request the corrosion test records.
- Packaging and delivery - seaworthy crating, port of discharge and incoterm.
- Compliance - confirm the shipment is subject to no sanctions or export-control restrictions in the destination market.
Common buying mistakes: mixing the aluminum and steel gauge tables when converting drawings; ordering "5083 plate" without a temper (H116 vs O is a real performance difference); and accepting the lowest price without a mill test certificate - mis-certified plate is a liability in a classed hull.
Send Inquiry - specify with confidence
Send us your alloy, temper, thickness, dimensions, quantity and destination port, and we will confirm availability, tolerance class and delivery within 24 hours. A mill test certificate per EN 10204 type 3.1 is issued with every shipment.
