Half-Size Seastead Prototype - Analysis & Estimations

1. Scaling & Geometry

Scaling down by half means lengths are 1/2, surface areas are 1/4, and volumes/weights are theoretically 1/8th. Keeping the target speed at 4-5 knots (ignoring Froude scaling) means the stabilizer foils will have twice the relative power-to-weight ratio, which is excellent for testing active stabilization in the sheltered waters off Anguilla.

Half-Size Dimensions:

Top Triangle Frame: Left/Right sides = 35 ft, Back side = 17.5 ft. Truss height = 3.5 ft (Note: 3.5 ft interior height is a crawl space; you may want to keep the truss at 4-5 ft for seating headroom, adding a bit of weight).
Legs/Floats (NACA 0030): Length = 9.5 ft, Chord = 5 ft, Width (Max Thickness) = 1.5 ft.
Stabilizers: Main wing = 5 ft span, 6 in chord. Elevator = 1 ft span, 3 in chord.
Dinghy: 7 ft RIB.

2. Buoyancy vs. Weight Analysis

                Critical Physics Check: A 50 kWh lithium battery bank weighs roughly 1,200 to 1,400 lbs. The total buoyancy of the half-size legs at 60% submersion is only about 1,680 lbs. A 50 kWh battery would sink the prototype before anyone stepped aboard! Therefore, we must size the battery to fit the boat, rather than picking an arbitrary capacity.
            

Buoyancy Calculation (Static)

The cross-sectional area of a NACA 0030 foil is approximately 20.5% of its bounding box (Chord × Thickness).

Area per leg = 0.205 × 5 ft × 1.5 ft = 1.537 sq ft
Volume per leg = 1.537 sq ft × 9.5 ft = 14.6 cu ft
Total Volume (3 legs) = 43.8 cu ft
Displacement at 50% submersion = 21.9 cu ft × 64 lbs/cu ft (saltwater) = 1,401 lbs
Displacement at 60% submersion = 26.3 cu ft × 64 lbs/cu ft = 1,682 lbs

Estimated Weight Breakdown

Using standard marine aluminum (1/8" / 3mm for legs, 1/16" / 1.5mm for truss framing) and realistic equipment:

Component	Estimated Weight (lbs)	Notes
3x Aluminum Legs (1/8" skin + internal bulkheads)	550	~313 sq ft of plate + framing
Truss Frame (Triangle, cross-bracing, bolt plates)	300	Aluminum extrusions + 1/16" cladding
2x Yamaha HARMO RIM Drives	130	65 lbs each (motor + bracket)
Battery Bank (15 kWh LFP)	350	Scaled down from 50kWh to fit weight limits
Solar Panels & Mounts (1.5 kW array)	80	Lightweight flexible or rigid panels
Stabilizer Actuators & Hardware	50	3x mini-foils + actuators + wiring
Netting, Seats, 7ft Dinghy, Misc Hardware	140	Trampoline net, lightweight seating
Total Empty Weight	1,600 lbs

Payload & Dynamic Lift

At 1,600 lbs empty weight, the seastead sits at roughly 57% submersion without anyone on board. With 3 passengers (~450 lbs), it sits at ~73% submersion.

Extra Buoyancy: You have about 80 lbs of static reserve before hitting 60% draft. However, when moving at 4-5 knots, the 3 stabilizer foils will act as lifting foils. They will dynamically carry several hundred pounds, reducing the static draft back down toward the 50% mark while underway. This makes it a very capable day-sailor for 2-4 people.

3. Range & Power

With the sleek NACA 0030 foil legs, drag will be remarkably low. Pushing a 1,600 lbs semi-submersible at 4-5 knots requires very little power.

Speed: 4.5 knots average
Estimated Continuous Draw: ~1.5 kW to 2.0 kW (frictional + wake drag on foils and truss)
Battery Capacity: 15 kWh (usable ~12.8 kWh at 85% DoC)
Run Time: 12.8 kWh / 1.75 kW = ~7.3 hours
Range: 7.3 hours × 4.5 knots = ~33 Nautical Miles

With the 1.5 kW solar array, on a sunny Caribbean day you will generate roughly 6 kWh, effectively extending your range by another 15 nautical miles or allowing slow station-keeping indefinitely.

4. Off-the-Shelf Truss Parts & Sourcing

Yes, there are excellent off-the-shelf aluminum truss systems that use bolt-together conical spigots or flanges. You do not need custom marine extrusions for the truss frame.

Stage/Lighting Truss: Companies like Global Truss, Applied Electronics, or Tomcat make modular 12-inch box trusses or triangular trusses out of 6082-T6 aluminum. They bolt together rapidly. While normally used for stages, they are frequently repurposed for architectural and marine frameworks. You would just need to spec heavy-duty swivel flanges for the triangle corners.
Marine Aluminum: Ensure the legs are 5052 or 5083 marine-grade alloy. The Chinese fabricator can weld the NACA foils and ship them, while the truss can be off-the-shelf bolt-on components packed into the same container.

The 2x Yamaha HARMO RIM drives are a perfect choice. Removing the outboard mounting bracket and rigidly bolting the rim unit to the trailing edge/sides of the NACA foils is highly feasible and keeps the hydrodynamic drag exceptionally low.

5. Estimated Build Cost

Assuming you and your boys provide the labor and you already have crane access, here is a realistic budget for a functional prototype:

Item	Estimated Cost (USD)
China Fabrication (3 NACA legs, bolt-together brackets, foil frames, packed in container)	$9,000
Off-the-shelf Truss System (flanges, cross-braces, hardware)	$4,000
2x Yamaha HARMO RIM Drives	$10,000
15 kWh LFP Battery Bank (cells + BMS + enclosures)	$3,500
Solar Array (1.5 kW) + MPPT Controllers	$1,200
3x Stabilizer Actuators + RC-style control electronics	$1,500
Shipping (40ft Container China to Anguilla)	$4,500
Anguilla Import Duties / Taxes (estimate ~15-20%)	$3,500
Misc (Netting, Seats, 7ft Dinghy, Wiring, Paint/Anodizing)	$2,800
Total Estimated Cost	$40,000

Note: The Yamaha HARMO drives and shipping/duties make up nearly half the budget. If budget is tight, basic 5kW electric outboards could substitute for the HARMO drives, saving ~$7,000, though you'd sacrifice the sleek rim-drive hydrodynamics.

Conclusion

A half-size prototype is an incredibly smart way to test the seastead concept. By shrinking the battery to 15 kWh to match the 1/8th scale buoyancy, you still achieve an excellent 30+ nautical mile range. The over-powered stabilizer foils will make active pitch/roll control highly responsive in the sheltered Anguilla waters, giving you invaluable software tuning time. Using modular stage-truss for the top frame will allow you to bolt the entire structure together on-site in a weekend, getting you out on the water faster.