Half-Size Seastead Prototype

35 ft × 17.5 ft • ~1/8th mass • Day Sailor / Test Platform

Anguilla Protected Waters Testbed

Executive Summary

42–58 nm

Range at 4–5 knots with 50 kWh

$58,000 – $74,000

Estimated total build cost

~5,400 lbs

Total displacement at 50% submersion

A half-scale prototype is not only feasible but highly practical. At 1/8th the mass but using the same 5–6 knot target speed, the stabilizers become relatively more powerful. The design works well as a fun day sailor in protected waters (west Anguilla reef area). Bolted aluminum construction from China + local final assembly makes this realistic.

Weight & Buoyancy Estimates

Component	Estimated Weight	Notes
3 × Legs/Floats (9.5 ft × 5 ft chord × 1.5 ft thick NACA 0030)	1,050 lbs	3/16" marine aluminum skin, internal bulkheads every 3 ft, ladder integrated on forward face. ~350 lbs each.
Triangle Truss + Roof Frame (35 ft sides, 17.5 ft base, ~3.5–4 ft height)	1,650 lbs	6061-T6 aluminum tubing (3–4" box & round), bolted nodes. Includes roof solar frame. Minimum gauge kept for durability.
Windows & Enclosure Skin	680 lbs	Polycarbonate or tempered glass panels in aluminum frames. Catamaran-style net forward.
50 kWh LiFePO4 Batteries	620 lbs	~12.4 lb/kWh (good marine-grade cells). Distributed low in triangle truss.
2 × Yamaha HARMO Rim Drives + Mounts	320 lbs	~160 lbs each including custom leg mounts.
3 × Stabilizers ("mini-airplanes")	180 lbs	Carbon fiber or aluminum wings + actuators. Very light.
Solar Array (~6–8 kW on roof)	420 lbs	Lightweight marine flexible panels + aluminum mounting extrusions.
Seats, Net, Misc (7ft RIB, lines, anchors, etc.)	480 lbs	Trampoline-style nets between legs, lightweight aluminum seating.
TOTAL ESTIMATED DRY WEIGHT	5,400 lbs	~2.45 metric tons

Buoyancy & Reserve

5,400 lbs

Displacement at 50% leg submersion

→

≈ 1,800–2,200 lbs reserve buoyancy

Enough for 8–10 adults + gear on a day sail.
Waterline can safely rise to ~60–65% of leg height.

The stabilizers can also provide 300–600 lbs of dynamic lift at 5 knots, effectively increasing payload and softening the ride.

Range with 50 kWh

At 5 knots

46 nm

~9.2 hours

at ~5.0 kW total average draw

Conservative

Aggressive

At 4 knots

62 nm

~15.5 hours

at ~2.8–3.2 kW total average draw

Rim-drive efficiency is excellent at low power. At these speeds the dominant drag is from the three foil legs. The 1.5 ft diameter Yamaha HARMO units are perfectly sized for this hull.

Assumptions: 88% overall propulsion efficiency, 15% hotel load (electronics, autopilot, small winches), modest wave drag in protected water. Real-world testing with the stabilizers deployed should beat these numbers.

Cost Estimate (USD)

Chinese fabrication (3 legs + triangle truss + stabilizer parts + solar frame)

$21,000 – $26,000

2 × Yamaha HARMO Rim Drives

$10,000

50 kWh LiFePO4 battery bank + BMS + chargers

$12,500 – $14,500

Solar panels (~7 kW), wiring, MPPTs

$4,800

Windows, netting, seats, 7 ft inflatable RIB, hardware

$5,500 – $7,000

Total Build Cost

$58k – $74k

Does not include shipping container from China (~$4–6k) or your time (assumed free). You already have a crane — this dramatically lowers final cost.

Off-the-Shelf Marine Aluminum Truss Options

01
6061-T6 Aluminum Box Tubing (3"×3"×1/4" and 4"×2")
Available from OnlineMetals, Metals Depot, or local suppliers in the Caribbean. Can be bolted with internal splice plates.
02
Truss-style modular aluminum scaffolding / stage rigging components
Several Chinese manufacturers sell marine-grade bolted truss systems (often used for floating docks). Look for “aluminum event truss” rated for outdoor use.
03
80/20-style heavy marine extrusions or “Bosch Rexroth” equivalent
Larger 4–6" profile versions exist for structural work. Good for the roof solar frame.

Recommendation: Have the Chinese supplier build the three legs and main corner nodes. Ship the rest as standard box tubing that you and your boys bolt together on site. No welding required.

This is a really fun and practical project.

The half-scale version gives you real-world data on stabilizer performance, software tuning, and seakeeping at 1/8th the cost and risk of the full seastead. Because you’re not Froude scaling speed, the foils and stabilizers are relatively over-powered — exactly what you want for a smaller boat in chop.

Ready to move to detailed engineering drawings and stabilizer CFD analysis next?

Analysis by Claude • March 2025 • Designed as a living web page for your project