Question 1 — Can you build strong bolted trusses from aluminum?
Yes. Bolted aluminum trusses are thoroughly precedented (aluminum bridges, offshore helideck structures, antenna towers, stage-rigging trusses, ship superstructures). Aluminum is softer than steel, so the connections — not the beams — are where care is required: larger gussets and bearing areas, more/larger fasteners, friction-type (slip-critical) joints where possible, galvanic isolation of stainless hardware, and explicit fatigue design. Duplex stainless is not required for feasibility; it is a legitimate premium option that buys near-immunity to corrosion and very robust connections, at roughly 2–4× the installed material cost and a significant top-weight penalty.
Question 2 — Can the living-area skin be bolted rather than welded?
Achievable, but not with a single seal. Long-term watertightness at sea comes from redundancy: a bolted substrate + a continuous waterproof barrier + a bolted, replaceable impact rainscreen, with every joint built on the same principle proven by ship hatch covers (compressed gasket + sealant + drainage path). The single best trick available to you: a thermoplastic membrane (PVC/TPO) hot-air-welded on site gives you a continuous, shipyard-quality waterproof layer without needing aluminum welders, since membrane welding is cheap, portable, and semi-skilled labor friendly.
Aluminum (~1/3 the density, ~1/3 the stiffness, ~1/2 the yield strength of duplex) behaves differently from steel in bolted joints in four ways:
| Issue | What happens | Standard mitigation |
|---|---|---|
| Bearing / hole elongation | Soft aluminum crushes locally at bolt holes; holes ovalize under cyclic load. | Larger-diameter bolts, more bolts per joint, thicker gusset plates, generous edge/end distances (per Eurocode 9 or the Aluminum Design Manual). |
| Fatigue | Aluminum has no endurance limit — a seastead sees millions of wave cycles, so this must be designed explicitly. | Use slip-critical (friction-grip) joints: load transfers by friction between faying surfaces, never cycling through the hole. Bolted friction joints in aluminum fatigue far better than welded aluminum details (which suffer ~35–45% strength loss in the heat-affected zone). Bolting is actually the good news for aluminum fatigue. |
| Galvanic corrosion | Aluminum is strongly anodic to stainless steel. Bare stainless bolts in wet aluminum joints corrode the aluminum. | Isolate every stainless fastener: coated bolts (e.g., PTFE/Dacromet-finished A4), nylon or EPDM sleeves and washers, and seal the faying surfaces so no seawater enters the crevice. Never leave dissimilar-metal contact in a wet crevice. |
| Thread galling & preload relaxation | Aluminum threads gall; aluminum creeps slightly under sustained preload, relaxing friction joints over time. | Use steel-core or stainless bolts (not tapped aluminum threads in primary joints), anti-seize, calibrated torquing, and a re-torque schedule (typically at 1 month, 1 year, then per inspection). Disc-spring (Belleville) washers on critical joints hold preload. |
| Factor | Marine aluminum (5083-H116 plate / 6082-T6 extrusion) | Duplex 2205 (or super-duplex 2507) |
|---|---|---|
| Yield strength | ~215–260 MPa | ~450 MPa (2205); ~550 MPa (2507) |
| Density | 2.70 g/cm³ | 7.8 g/cm³ |
| Stiffness (E) | 69 GPa | 200 GPa |
| Corrosion in splash zone | Very good if 5083 + intact coating/anodizing; pits if crevices are wet and unsealed | Essentially immune in marine atmosphere and splash zones (PREN ≈ 34 for 2205, ≥40 for 2507) |
| Bolted-joint robustness | Good with proper detailing (Section 2.1) | Excellent — high bearing strength, conventional steel-style joint design, stainless fasteners need no isolation |
| Field welding (Caribbean yard) | Routine, widely available skill; distortion control needed | Requires disciplined procedures (heat-input window, nitrogen backing gas) to preserve corrosion properties; fewer qualified welders |
| Indicative installed cost index* | 1.4–1.9 | 2.5–4.0 |
| Service life, low maintenance | 40–60 yr | 60+ yr, near zero maintenance |
| Top-weight effect on stability | Low — helps center of gravity | High — a duplex truss at the same geometry weighs ~2.5–3× the aluminum one, raising CG and increasing required buoyancy |
| Fire behavior | Loses strength above ~200 °C; melts at 660 °C | Retains strength to ~500 °C+ |
*Relative indices only — nickel, molybdenum, and chrome markets swing duplex pricing hard; get current quotes from Chinese mills (which produce large volumes of 2205 plate, pipe, and sections for desalination and chemical plants).
| Option | Description | Assessment |
|---|---|---|
| AAll aluminum | Legs/floats and truss in 5083/6082; bolted truss, welded or bolted legs. | Cheapest durable system; lightest topweight; one material, no galvanic issues. Requires disciplined joint detailing and periodic inspection of coatings/joints. |
| BAll duplex | Everything in 2205; bolted truss with A4/A5 or duplex fasteners. | Your proposed option. Simplest corrosion story, most robust connections, longest life, minimal maintenance — a defensible "pay once" philosophy for a landless asset. Watch: cost volatility, top-weight/stability penalty, and stricter welding QA. |
| CHybrid (recommended study case) | Duplex (or coated carbon steel + anodes) for wetted legs/floats; aluminum for the above-water truss; electrically isolated flange kits at the interface. | Puts the expensive corrosion-proof metal only where seawater lives, keeps topweight low, cuts cost materially. The isolation flange is a known, catalog-able detail (EPDM/nylon isolation sets). Adds one interface to manage. |
| DValue option | Hot-dip galvanized / thermally-sprayed carbon steel legs with sacrificial anodes; aluminum truss above. | Lowest capex, proven shipyard practice, but commits you to a coating-maintenance program forever. Worth pricing before dismissing. |
Mixing metals rule: any aluminum-to-duplex/steel junction must be a designed isolation joint (sealed faying faces, isolation sleeve/bushings and washers on every fastener), and any aluminum kept near the waterline should sit inside the protection zone of sacrificial anodes. An unisolated stainless bolt dripping seawater onto an aluminum beam will eat the beam, not the bolt.
Bottom line, Q1: Strong bolted aluminum trusses are entirely feasible and precedented — the effort goes into connection engineering, not into abandoning aluminum. All-duplex is a coherent premium strategy (especially paired with duplex legs) if capex is acceptable and the stability analysis tolerates the extra topweight. The strongest value case is usually hybrid: corrosion-proof metal in the water, aluminum in the air, isolated interfaces.
This is not a roofing problem. Breaking-wave impacts generate short-duration pressures on the order of p ≈ C·½ρv². With crest velocities of 8–12 m/s and slamming coefficients of 1–3, expect localized peak pressures of roughly 30–220 kPa (milliseconds) plus quasi-static green-water heads on exposed faces. Consequences for design:
The proof this works: cargo-ship hatch covers — enormous bolted/cleated steel panels with compressed rubber gaskets — remain watertight across the North Atlantic for decades. The recipe scales down to your wall panels:
Done this way, bolted skins are reliable long-term — but they are a system, not a hardware-store detail. Every deviation (a lone screw through a panel, an unsealed penetration) becomes a leak path.
Get continuity without shipyard aluminum welding by separating the three jobs:
| Layer | Function | Construction |
|---|---|---|
| 1. Substrate | Structure & attachment | Bolted aluminum or FRP sandwich panels on the truss — no waterproof duty, so ordinary bolted joints are fine. |
| 2. Primary waterproof barrier | The actual seal | Thermoplastic membrane (PVC or TPO), hot-air welded on site into a continuous bag over the substrate. Portable welders cost a few hundred dollars, the process is learnable in days, seams are peel-tested on the spot. Alternative: liquid-applied polyurea/polyurethane with reinforcing fleece — seamless but more workmanship-sensitive. |
| 3. Rainscreen / armor | Impact, UV, aesthetics | Bolted aluminum or gelcoated-FRP panels on standoff clips, individually replaceable, with a drained & ventilated cavity behind. Takes the wave slam so layers 1–2 never do. |
This arrangement is standard high-performance building science (pressure-equalized rainscreens) upgraded for impact. Its virtues for your business model: the waterproof layer is continuous, inspectable, and renewable — a crew can re-weld a patch or replace a membrane section in a day, versus cutting open a welded skin.
Bottom line, Q2: You do not have to choose between "all bolted" and "all welded." The strongest configuration is a bolted substrate + site-welded thermoplastic membrane as the continuous barrier + bolted replaceable impact panels outside, with welded aluminum reserved for the slam-exposed lower band if desired. Purely bolted watertight skins are feasible using hatch-cover joint principles, but demand tighter QA than most builders sustain; purely welded skins are maximally reliable but tie you to aluminum welders at the yard for every future modification.