Target: a triangular SWATH-like seastead that ships as kits inside one 45 ft High Cube container, assembles into a 44 ft equilateral living platform with three NACA 0040 foil legs, electric propulsion, solar/battery power, and a TLP-style screw mooring.
Your geometry, container-packing plan, propulsion architecture, and mooring concept are creative and internally consistent in many ways. I have verified the central buoyancy number and a few stability numbers — those check out. However, there is one make-or-break problem and several serious secondary issues you should resolve before cutting metal. If you only read one box, read the red one.
Your rated buoyancy at the design waterline is 27,500 lb (verified — see math below). My best estimate of the empty assembled weight of the structure (legs + living triangle + walkway + roof + solar + batteries + equipment) is roughly 35,000–55,000 lb with conventional aluminum construction. That means the seastead, fully assembled but empty of people, will likely be 7,000–27,000 lb heavier than the water can support. It will not float at the design waterline; it will submerge much deeper (and likely breach the deck or fail structurally) long before reaching equilibrium.
| Component | Lower bound (lb) | Upper bound (lb) | Assumptions |
|---|---|---|---|
| 3 foil legs (aluminum shell + internal frames + bulkheads for compartments) | 8,000 | 20,000 | 0.25"–0.5" plate; includes watertight bulkheads and the cable conduit on the trailing edge |
| Triangle frame, floor, ceiling, 7 ft walls (aluminum) | 8,000 | 15,000 | 838 ft² floor + 924 ft² walls + roof structure at 5–8 lb/ft² |
| Walkway grating + frames + diagonal braces + railings (118 ft of 3 ft walkway) | 3,500 | 6,000 | Aluminum grating and stringers |
| Solar panels + racking on 838 ft² roof | 3,000 | 5,000 | ~4 lb/ft² for panels, ~2–3 lb/ft² for racking |
| LiFePO₄ batteries (25 % of 27,500 lb displacement) | 6,875 | 6,875 | ~90 Wh/lb → ~619 kWh. This is huge — about 46 Tesla Powerwalls |
| 6 × 1.5 ft RIM drives, mounts, nozzles, cabling | 500 | 1,500 | Small units, but each still has a motor, duct, and mount |
| Helical mooring system (6 screws + 3 motor units + tension members) | 2,000 | 4,000 | Steel screws and gearmotors are heavy |
| Dinghy (14 ft RIB deflated + Yamaha HARMO) | 250 | 350 | |
| Heave plates (steel, bolted on) | 2,000 | 5,000 | "Several" per leg of unspecified size — at minimum a few hundred lb each |
| MEP, plumbing, interior fit-out, safety gear, water, provisions, people | 4,000 | 8,000 | |
| Total | ~38,000 | ~72,000 |
Even at the optimistic end, the structure is about 10,000 lb heavier than 27,500 lb of buoyancy. At the conservative end, it is about 45,000 lb heavier. The deck would be awash or underwater.
Until you do a real weight estimate from a chosen material and cross-section schedule, the design is not buildable as drawn.
The 27,500 lb buoyancy figure is right for three NACA 0040 foils of 8.5 ft chord and 14.5 ft span, half-submerged. The waterplane area is dominated by the three foils, and the foils present about 3.2 ft of thickness at the waterline (95 % of max thickness) × 14.5 ft of span = ~46 ft² each, or ~140 ft² total. That gives a heave stiffness of ρgA_wp ≈ 64 · 140 ≈ 9,000 lb/ft.
With the three foils at the corners of a 44 ft triangle (legs ~25.4 ft from center), the waterplane second moment is huge:
I_T (roll) ≈ 3 · [14.5·3.2³/12 + 46.4·25.4²] ≈ 90,000 ft⁴ BM_T = I_T / V_displaced = 90,000 / 430 ≈ 210 ft GM_T ≈ BM_T + KB − KG ≈ 200+ ftRoll and pitch natural periods will be on the order of 2 seconds. That is very stiff — the seastead will snap back to upright quickly, which is good for seakeeping in the harbor, but in beam seas it can deliver high accelerations to occupants. You will probably want some controlled compliance (softbilge tanks, or simply accept a stiffer ride).
3 legs × 14.5 ft end-to-end = 43.5 ft, fits 44.6 ft container length. 8.5 ft chord fits 8.9 ft container height with 0.4 ft to spare. 3 walls × 10 in = 30 in on the left leaves ~3 ft clear; the central section has room for the batteries, RIM drives, dinghy, etc. No dimensional contradiction here.
838 ft² of roof × 20 % efficient panels ≈ 16 kW peak. In the Caribbean with 5–6 equivalent sun-hours, that's 80–100 kWh/day — more than a low-speed seastead will ever use. The batteries are wildly oversized relative to the solar input, which feeds back into the weight problem above.
Six small (1.5 ft / ~450 mm) RIM drives are good for low-speed thrust and the kind of differential steering you describe, but the total bollard thrust of six units this size is typically only a few hundred pounds total. Against a 27,500 lb (and probably 35,000+ lb) displacement, you will be doing well to make 2–3 knots. Hull-speed calculations for a SWATH use the underwater hull length (14.5 ft), giving ~5 knots theoretical, but you will not approach that with this thruster package.
If 2–3 knots is acceptable (it probably is for a houseboat/seastead), fine. If you ever want to make any headway against current or in stronger trades, you need bigger thrusters (≥ 2.5–3 ft diameter) or a different propulsion approach. The good news is the 6-thruster, 3-redundant layout itself is sensible.
"Several bolt-on heave plates" is not a specification. Heave plates work by adding horizontal waterplane area below the natural waterline. They also add drag. For this hull, I would target something like 40–80 ft² of additional plate per leg, mounted near the bottom, with slots or holes if you want to limit drag. Otherwise the seastead will bob noticeably in 2–3 ft Caribbean chop — fine at mooring, but uncomfortable when underway. Specify plate count, area per plate, and attachment detail before building.
Pulling the structure down 3 ft to take up slack works in principle, but the pretension is set by the buoyancy change over 3 ft:
Δ buoyancy over 3 ft = 3 · 9,000 lb/ft ≈ 27,000 lb total Per leg / per tension member = ~9,000 lb Per screw (2 per corner) = ~4,500 lbThat pretension is high for typical small-boat helical screws (which are usually rated 1,000–5,000 lb each in sand, less in mud). The good news is that the screws are mounted in pairs and the holding capacity is roughly additive. You will want:
Two free-floating hulls, even with computer-coordinated thrust, will not