This document outlines a concept for a self‑propelled, semi‑submersible seastead that can be shipped in a standard 40‑ft container and assembled with bolts (no on‑site welding). It addresses the geometry, structural concept, weight estimate, and assembly method.
1. Overall Geometry
Hull: Circular aluminum pipe, 12 ft (3.66 m) diameter, 40 ft (12.2 m) long, with rounded end caps (≈5 ft each) → overall length ≈ 50 ft (15.2 m).
Legs / Floats: Four duplex‑stainless‑steel columns, each 24 ft long, extending from the hull corners at 45° (≈ 12 ft horizontal projection, 12 ft vertical). Half of each leg is submerged.
Cable Net: Two sets of tension cables connect the leg bottoms: (a) a rectangular “ground‑track” net (redundant) and (b) diagonal “cross‑brace” cables between adjacent legs.
Propulsion: Two 2.5 m (8.2 ft) low‑speed axial mixers (essentially large propellers) powered by solar panels, intended for 0.5–1 mph (0.22–0.45 m/s) cruise.
2. Structural Concept
2.1 Hull (Aluminum)
Material: 6061‑T6 aluminum (marine grade).
Side shell thickness: 6 mm (≈ ¼ in).
End‑cap thickness: 12 mm (≈ ½ in) – dished to reduce stress concentration.
Internal ring frames (stiffeners) every 2 m (≈ 6.5 ft) – extruded aluminum I‑beams welded (or bolted) to the shell.
A central full‑length box‑girder (or tube) running the length of the hull to carry torsional loads from the leg reactions.
2.2 Leg Attachments
Each leg attaches to a mounting plate (25 mm thick Duplex‑steel) welded to an internal reinforcement ring in the hull.
The plate has a set of high‑strength bolts (M20‑8.8 or ¾‑in Grade 8) passing through a flanged “saddle” on the leg. The saddle can be bolted to the plate, allowing disassembly.
To avoid field welding, the leg‑mounting plates are pre‑welded in the shop to the hull structure; all other connections are bolted.
2.3 Cable Net
Cables: 12 mm (½‑in) Dyneema® (UHMWPE) rope – extremely strong, light, and corrosion‑free.
Each leg bottom has a stainless‑steel thimble (eye) for cable termination.
Cable tension is adjusted with turnbuckles (or tensioning nuts) to keep the leg angle at 45° under all load cases.
At ~30 k lb (≈ 13.6 t) the structure is within the lift capacity of the two 2.5 m propellers for the intended low‑speed cruise (drag ≈ 200 lb at 1 mph).
4. Shipping & Assembly
4.1 Containerisation
The 40‑ft container internal dimensions: 39 ft 6 in L × 7 ft 7 in W × 7 ft 10 in H.
The hull is split into two 20‑ft half‑cylinders (each 12 ft diameter). Each half‑cylinder is shipped lying on its side; the overall width (12 ft) exceeds the container width, so the halves are shipped flat‑packed as sets of 10‑ft panels (see sketch below).
Panels are 10 ft long, 6 ft wide, with pre‑drilled bolt holes. Two panels form a full ring. All stiffeners, end caps, and the central torsion tube are shipped as separate pieces.
4.2 Assembly Sequence (no welding)
Prepare foundation: Place a flat steel frame (or concrete slab) on the dock to serve as a jig.
Assemble hull rings: Bolt the 10‑ft panels together using flange plates (M12 bolts, 50 mm long). Use gasket tape (e.g., silicone) to seal.
Install stiffeners & torsion tube: Bolt the extruded I‑beams to the interior of each ring, then attach the central box‑girder with high‑strength bolts.
Mount leg plates: Pre‑welded mounting plates on the hull are already in place; attach the leg saddles with bolts (M20‑8.8). Use a torque wrench to achieve the design preload.
Install cables: Thread the Dyneema ropes through the thimbles, tension with turnbuckles, and lock with nyloc nuts.
Add equipment: Lift the propulsion units, solar arrays, batteries, and interior fit‑out into the hull through the end caps (which can be removed temporarily).
Check alignment & pressure test: Pressurise the hull to 10 psi (≈ 0.7 bar) with air, monitor for leaks, and verify leg angles.
5. Can It Be Fully Bolted? (Welding vs. Bolting)
Hull shell & stiffeners: Fully bolt‑together using extruded flanges. No on‑site welding is required.
Leg mounting plates: These must be welded to the hull in a shop environment (they are high‑load attachments). After that, the legs attach to the plates with bolts, so field welding is avoided.
End caps: Can be bolted to the hull using a flange joint; a small amount of sealant (marine silicone) provides leak tightness.
Conclusion: The entire structure can be assembled with bolts; only the leg‑mount plates need shop welding. This satisfies the “no on‑site welding” goal.
6. Performance Notes
Drag: Frontal area ≈ π·(6 ft)² ≈ 113 ft² (10.5 m²). With a drag coefficient ≈ 0.8, the drag at 1 mph is ≈ 200 lbf (≈ 0.9 kN). Two 2.5 m axial mixers each delivering ~150 lbf of thrust at 60 rpm are more than adequate for continuous low‑speed cruising.
Stability: The leg geometry (45° spread) combined with the cable net provides a wide base and resists overturning moments. The central torsion tube greatly reduces hull twist when one pair of legs is lifted (e.g., by wave action).
Redundancy: The rectangular cable net ensures that even if a single cable fails, the remaining three sides hold the leg positions, preventing catastrophic loss of leg alignment.
7. Next Steps & Recommendations
Perform a detailed finite‑element analysis (FEA) of the hull‑leg‑cable system to verify stresses, especially under asymmetric loading (e.g., one side lifted by a wave).
Select specific bolt grades and torque settings for each joint (e.g., M20‑8.8 at 450 Nm for leg mounts).
Prototype a single 10‑ft hull panel and test its buckling capacity under compressive load.
Validate the thrust calculation with model‑scale tests or CFD for the chosen propeller‑mixer design.
Check local regulations for floating structures (stability, safety, wastewater, etc.) before construction.
Key Take‑aways:
The proposed round‑aluminum hull can be broken into 10‑ft panels, shipped in a 40‑ft container, and assembled with bolts only (except for the leg‑mount plates).
Estimated total weight ≈ 30 k lb, with about 8 k lb of payload.
Torsional rigidity is provided by an internal box‑girder; the cable net gives redundancy.
Propulsion via two 2.5 m low‑speed axial mixers is sufficient for the intended 0.5–1 mph cruise speed.
Safety Note: This concept is preliminary. Detailed engineering, including structural analysis, safety factors, and compliance with marine regulations, must be carried out before construction. All welding should be performed by certified welders in a certified shop.
Figure: Conceptual side‑view (not to scale). Grey cylinder = aluminum hull; red lines = stainless‑steel legs; blue rectangle = cable net; yellow circles = low‑speed mixers.