Seastead MVP — “Tri-Foil” One-Container Platform

Equilateral triangle · 44 ft sides · 3 vertical NACA-0035 fins · ~15 kW solar · ~300 kWh battery

Preliminary concept package — all figures ±15% pending naval-architecture review.

Headline Numbers

838 ft²

Gross indoor floor (≈800 ft² usable), 7 ft ceilings

14.7 kW

Solar DC on roof (≈60 kWh/day Caribbean avg)

300 kWh

LiFePO₄, 6,000 lb (25% of displacement), 3×100 kWh redundant

24,000 lb

Design displacement @ 7.25 ft draft (27,500 lb @ 8.3 ft)

13,400 lb

Structural kit — fits one 45′ HC container

~$125k

Structural kit, qty 10, EXW China (±15%)

Parameter	Value	Notes
Solar array (roof)	14.7 kW DC	771 ft² usable of 838 ft² roof; ~205 W/m² marine panels; 55–65 kWh/day typical Caribbean, 85+ kWh peak day
Battery	~300 kWh @ 48 V	6,000 lb LiFePO₄ = 25% of displacement; three independent 100 kWh banks, one per fin
Inverters / chargers	3 × 8 kW	One per fin → triple-redundant power; each thruster pair fed only by its own fin
Thrusters	6 × 4 kW rim-drive, Ø18″	Fixed, fore-aft; differential steering; pivot-in-place mode; 24 kW installed
Indoor area	838 ft² gross	≈800 ft² usable after beam footprints & core; plus ~400 ft² exterior walkway
Displacement (design)	24,000 lb	@ 7.25 ft draft (50% of fin). Option: 27,500 lb @ 8.3 ft draft (57%)
Light ship	~17,000 lb	Structure 13,400 + propulsion/electrical 1,600 + dinghy/davit 800 + outfit 1,200
Payload to design WL	~7,000 lb	Batteries 6,000 + crew/stores ~1,000. Up to ~10,500 lb if operated at 27,500 lb WL
Reserve buoyancy	~48,000 lb	Fins fully submerged — 2× design displacement. Waves washing the deck are a non-event
Speed	5–6 kt cruise / ~8 kt max	Cruise draw 2–4 kW; solar-sustained ~4.5 kt around the clock in good sun
Range (batteries only)	~450–500 nm @ 5 kt	Effectively unlimited at 4–4.5 kt under Caribbean sun
Waterplane sensitivity	~1/5 per ft	1 ft rise/swell ≈ 4,900 lb force (your 1/7 target is reachable with tapered fin tips, see §7)

1 · Geometry & Layout

An equilateral triangle, 44.0 ft per side, walls 7 ft tall, enclosed as the living area. Each side is a single 44-ft framed wall panel (this is the trick that makes one-container shipping work: three straight panels, stood upright along the container’s left wall). Corners join with bolted splice sleeves and gussets. Two doors sit on the aft wall, 2 ft in from each side; plexiglass windows throughout. Connecting the midpoints of the walls at floor and ceiling level are six 22-ft box beams forming an inner triangle — every remaining floor/ceiling span is ≤ 6.4 ft, filled by small bolted panels.

Plan view (schematic, not to scale). All three fins parallel, leading edges forward. Fins sit under the vertices, centers ~1.5 ft inboard of each point. Aft-center gap in the walkway is where the dinghy hangs.

Walkway: 3 ft wide aluminum grating bolted around both sides and the bow, 1 ft above the wall base, with diagonal braces landing on the fin “knees” (see §2) — the grating lets waves pass through.
Railing: 42″ stanchions, double rail + toe board, in ≤10 ft bolted sections.
Roof: entirely solar; structural walk strips for cleaning access; plexiglass skylight optional over the core.
Interior: 838 ft² open floor; suggested core (bath, galley, electrical closet) ~120 ft², leaving ~700 ft² cabin.

2 · The Fins (“Legs”) and Draft

Each fin is a vertical, prismatic NACA 0035 section: 8.5 ft chord (streamwise), 2.98 ft max thickness, 14.5 ft deep below the floor structure. The trailing edge is truncated to a ⅝″ blunt edge (your “cut the last 0.5 ft” — keeps it weldable and shippable; buoyancy loss <½%). Half of each fin is submerged: draft 7.25 ft, hull underside 7.25 ft clear of the water. The leg assembly totals 21.5 ft: 14.5 ft below the floor plus a 7-ft “knee” that rises alongside the wall — doubling as the walkway corner post and the top handhold of the boarding ladder.

Ladders are built into the top (exposed) half of each fin’s leading face — 7.25 ft climb from the water.
No through-hulls. Cables drop from the roof through a conduit welded to each fin’s trailing edge. Each fin has 4–5 airtight compartments; battery hatch sits in the dry, exposed top section.
Thrusters: six rim drives (one each side of each fin) on short struts near the fin tips, ~2 ft above the bottom ⇒ ~5.3 ft below the waterline: quiet, protected from surface turbulence and weed.
Heave plates: three bolt-on plates per fin (lower half), ~7 × 3.5 ft each, isolated fasteners, sacrificial-anode protection. They roughly halve heave amplitude at resonance and add ~60% added mass.

Profile (schematic). Deck ≈ 14.3 ft above waterline; walkway ≈ 15.3 ft.

3 · One-Container Packing (45′ High Cube)

Internal envelope used: 44.6 × 7.7 × 8.9 ft, 62,000 lb limit. Kit weighs ~13,400 lb — weight is a non-issue; volume is the game, and it closes with ~1,240 ft³ to spare.

Container loading plan (schematic, viewed from above; door end at left).

Lane	Width used	Contents
Right	3.1 ft	Nested fin pair (29 ft of length) + fin #3 (14.5 ft) + ~15 ft of loose gear: heave plates, 6 crated thrusters, 6 mooring screws + drive units, davit poles, deflated 14′ RIB, HARMO outboard
Left	3.0 ft	Three 44-ft wall panels upright (~7″ each); grating bundles and heave plates flat on top (1.8 ft headroom)
Center	1.6 ft	Six 22-ft inner beams (two layers of three), floor/ceiling infill panels stored on edge “file-style,” railing sections, diagonal struts, splice hardware

Note on batteries: ship the 6,000 lb of LiFePO₄ as a separate Class-9 hazmat shipment (or buy cells in-country). Even with thrusters, mooring gear, and the dinghy co-shipped, the container sits near 23,000 lb — far under the 62,000 lb limit.

4 · Systems

Power

Solar: 14.7 kW DC across the roof, wired as three independent strings (one per fin’s MPPT charge controller).
Storage: 3 × 100 kWh LiFePO₄ banks, one per fin, low and central — ballast, redundancy, and short cable runs to that fin’s thruster pair.
Conversion: 3 × 8 kW inverter/chargers; any one can run the whole hotel load; any two can run all six thrusters at cruise.
Daily budget: hotel + refrigeration + watermaker ≈ 6–10 kWh; surplus ~50 kWh/day goes to propulsion or bank top-up.

Propulsion & control

6 fixed rim drives, fore/aft only; differential thrust steers; reverse-one-side/forward-other pivots the boat in its own length for harbors.
Two computers (primary + hot spare) share thruster command; when two seasteads are coupled, both controllers run a light DP loop to calm the connecting walkway.
Estimated resistance: ~2.5 kW at 5 kt, ~8–10 kW at 8 kt (small-waterplane hulls make surprisingly little wave drag).

Mooring (tension-leg mode)

At each vertex: a pair of helical screw anchors with a motorized drive unit between them; 3 ft pull-down ⇒ ~4,700 lb pre-tension per corner, fins never go slack.
Caribbean-only deployment as you specified: micro-tides, protected anchorages, sand/clay seabeds.
Deploy/retrieve by 2 people in a few hours; screws stow in the right-lane crate space.

Dinghy & interface

14 ft RIB (shipped deflated) + Yamaha HARMO electric outboard on two aft davit supports with rope leads — fully in the hull’s wind shadow underway.
Aft-center coupling sockets (port/starboard of the dinghy) accept the inter-seastead gangway; alignment cones + hard pins, rated for 6 people.

5 · Performance Estimates

Condition	Speed	Power	Endurance
Economic (solar-sustained, good sun)	4.5 kt	~2.0 kW	Unlimited in Caribbean conditions
Cruise	5–6 kt	2.5–4 kW	~450–500 nm on batteries alone
Max sustained	~8 kt	8–10 kW	~25 h (full bank)
Harbor maneuver	0–3 kt	<2 kW	Pivot in place via differential thrust
Parked (tension legs)	0	0.3–0.8 kW hotel	Indefinite; solar covers 6× hotel load

Seakeeping: the 76 ft² total waterplane gives a heave natural period of ~2.5 s (≈3.1 s with heave-plate added mass), heavily damped — the platform “ignores” short chop and climbs long swells, exactly the behavior you described. Roll stiffness is enormous (fins 24 ft off centerline, batteries 5–9 ft below the waterline); expect near-table-flat at rest and gentle coordinated sway underway. Ultimate stability: with the deep-ballasted CG, the platform rights itself from any heel that keeps the leeward fin’s compartments sealed — and there is 2× displacement in reserve buoyancy before the deck structure is even threatened.

6 · Weight Budget (Structural Kit as Shipped)

Item	Weight (lb)	Construction
Wall panels ×3	2,600	5083 plate 3 mm + 6061 extrusion framing, ~7″ structural depth, ~10″ finished
Floor + ceiling infill	3,300	2.5 mm plate on extrusion joists, spans ≤ 6.4 ft, all bolted
Inner triangle beams ×6	1,180	8×8×¼″ 6061-T6 box, 22 ft
Fins ×3 (incl. knees, bulkheads, hatches, conduit)	2,000	2.5 mm skins, 4–5 airtight compartments each, truncated TE
Walkway grating + framing + railing + braces	2,450	Bar grating ~2.5 lb/ft², bolted sections
Heave plates ×9 + fasteners	800	¼″ plate, flanged edges, isolated bolts
Doors ×2, plexiglass windows + frames	550	Marine plexi, gasketed
Misc: splices, brackets, fasteners, anodes, conduit	550	316 SS fasteners, isolation bushings
Total structural kit	13,430	Fits 45′ HC with ~1,240 ft³ and ~48,000 lb of margin

In-service weight ledger (at 24,000 lb design WL)

Group	lb
Structural kit (above)	13,430
Thrusters, solar, inverters, wiring, controls	1,600
Dinghy, HARMO, davit	800
Outfit (galley, bath, furniture, watermaker, safety)	1,200
Batteries (25% of displacement)	6,000
Crew + stores + personal cargo	970
Design displacement	24,000

If you prefer to operate at your quoted 27,500 lb: trim ballast/cargo +3,500 lb and the draft deepens to 8.3 ft (57% of fin). Structure is designed for either waterline.

7 · Reconciling Your Numbers (and How to Hit Them Exactly)

“27,500 lb rated buoyancy at desired waterline.” With a prismatic 0035 fin at exactly 50% submergence the displacement computes to 24,000 lb; your 27,500 lb corresponds to 57% submergence (8.3 ft draft). Both are offered above; the structure is designed to the higher mark.
“1 ft of water-level change ≈ 1/7 of buoyancy.” Prismatic fins give ≈ 1/5. Tapering the bottom 4 ft of each fin to ~65% chord (a cheap robotic-cut variation) brings the ratio to ≈1/6–1/7 at the cost of ~8% of buoyancy — recommended if ride softness outranks payload.
“21.5 ft legs.” Interpreted as 14.5 ft below the floor + 7 ft knee alongside the wall (doubles as walkway post and ladder top). If you truly meant 21.5 ft below the floor, displacement at 50% becomes 35,500 lb and the deck rises to 17.8 ft — shippable (2 × 21.5 = 43 ft) but windier, heavier, and slower; not recommended for the MVP.
Truncated trailing edge. Modeled as a blunt ⅝″ edge at full 8.5 ft chord; buoyancy penalty <½%.

Why 44 ft is the sweet spot (and what “bigger” costs)

The one-container magic lives in the width budget: fins 3.1 ft + walls 3.0 ft = 6.1 ft of the 7.7 ft interior, leaving a 1.6 ft center lane for everything else. Growing the triangle requires more/longer wall panels that eat that lane, and structure weight grows faster than solar area. The 44-ft size lands where ~15 kW of solar covers hotel load plus 4.5–5 kt of perpetual motion — a genuinely fuel-free vessel. For completeness:

Variant	Side	Indoor ft²	Solar	Displacement	Kit wt	Kit cost (qty 10)
MVP (recommended)	44 ft	838	14.7 kW	24,000 lb	13,400 lb	$125k
Stretched (6 wall panels, fins 16.5 ft)	52 ft	1,171	22 kW	27,300 lb	~17,500 lb	~$170k
True 40′ HC fallback	39 ft	658	11.5 kW	21,500 lb	~11,800 lb	~$112k

8 · Cost Estimate — Structural Kits, Qty 10, Chinese Robotic Yard (EXW)

Line item	USD / unit	Basis
Aluminum material (5083 plate, 6061 extrusions, +15% waste)	28,000	~15,500 lb bought weight @ ~$1.8/lb China mill pricing
CNC cutting, forming, machining	18,000	Plasma/router nests, press-brake fin shells
Welding (robotic seams + manual finish)	30,000	Jig-built panels; fin shells seam-welded robotically
Fasteners, fittings, anodes, conduit (316 SS)	8,000
Doors, hatches, plexiglass glazing	7,000
Surface prep + primer (extrusions anodized)	6,000
Jigs & fixtures (amortized over 10)	6,000
QA, marking, build book	3,000
Packing & crating for container	4,000
Yard margin (12%)	13,200
Total per unit (EXW yard)	≈ $123,000	Planning figure $125k; realistic range $105k–$142k

Add ocean freight (~$4–6k/container to the Caribbean), duties, and site assembly.
A one-off (no jig amortization, no quantity pricing) would run ~$180–220k — the 10-unit batch saves ~40%.
Site assembly: 2–4 workers, a small mobile crane (heaviest piece ≈ 900 lb wall panel), ~3–4 weeks per unit including systems.

9 · Build & Assembly Notes

Yard laser-cuts everything from a single 3D model; panels jig-welded flat, fins seam-welded in rotators.
Ship flat-pack; in-country: set the three knees on leveling skids, hang fins, raise and splice walls, drop in the six beams, bolt floor/ceiling infill, hang walkway and railing, bond heave plates and thrusters.
Pressure-test every fin compartment at the yard; repeat annually (hatch-accessible).
Galvanic discipline: isolated heave-plate bolts, sacrificial anodes on each fin, antifoul below WL.
Fatigue watch-items: knee-to-floor joints and walkway brace roots — design with generous radii and specify fitted bolts at these nodes.
Flag/insurance survey early; the compartmentation and 2× reserve buoyancy story surveys well.

Recommended next steps: (1) naval architect pass on knee loads and bolted-joint fatigue; (2) CFD or towing-tank confirmation of the 5 kt / 2.5 kW figure; (3) full-scale mock-up of one corner joint (wall splice + knee + walkway brace) before committing to the 10-unit batch; (4) CAD-level container packing study to convert the schematic lanes above into a certified packing drawing.