Half-Scale Seastead Prototype Analysis

Design Overview

This analysis evaluates a half-scale prototype of a foiling trimaran-style seastead designed for sheltered water testing around Anguilla. The prototype maintains the same NACA foil-shaped legs, triangular living platform, and active stabilizers as the full-scale concept but at reduced dimensions suitable for day sailing.

Key Scaling: Half linear dimensions = 1/8th volume & mass (if geometric scaling held perfectly). In practice, material thickness limits and structural needs mean weight savings may be less, but the goal is a lightweight, assemblable prototype.

Half-Scale Dimensions

Component	Full-Scale	Half-Scale Prototype
Triangle Side Lengths	70 ft	35 ft
Triangle Base Width	35 ft	17.5 ft
Truss Height (Floor to Ceiling)	7 ft	3.5 ft (open frame, not enclosed)
Leg/Wing Length	19 ft	9.5 ft
Leg Chord (NACA 0030)	10 ft	5 ft
Leg Width (Thickness)	3 ft	1.5 ft
Stabilizer Wing Span	10 ft	5 ft
Dinghy	14 ft RIB	7 ft inflatable

Weight & Buoyancy Estimate

Assumptions: Marine aluminum construction (5083/H116), bolt-together truss, minimal accommodations. Skin thickness likely ~3mm (0.12") minimum for durability, not scaling to 1/8th of full-scale thickness.

Component	Estimated Weight (lbs)	Notes
3 Foil Legs (Aluminum, hollow)	1,200	Each ~400 lbs, NACA 0030 shape, 5ft chord, 1.5ft width, 9.5ft long
Triangle Truss Frame (35' sides)	900	Aluminum truss members, 3.5 ft tall, bolted connections
Solar Frame & Panels (Light coverage)	300	~2kW solar, minimal framing
2x Yamaha HARMO Rim Drives	200	~100 lbs each mounted
50 kWh Batteries (LiFePO4)	1,100	~22 lbs/kWh
3 Stabilizers (small, active)	150	Aluminum/SS, actuator, pivot
Netting, Seats, Misc Hardware	250	Marine mesh, simple seats, bolts, wiring
Total Empty Weight	4,100 lbs	~1,860 kg
People & Gear (4 adults + gear)	800	200 lbs/person
7 ft Inflatable Dinghy	100	Lightweight
Total Loaded Weight	5,000 lbs	~2,270 kg

Buoyancy Analysis

Each half-scale leg (foil) displacement approximation:

Leg volume ≈ length × chord × width × shape factor
≈ 9.5 ft × 5 ft × 1.5 ft × 0.3 (NACA 0030 approx. volume factor) = 21.4 ft³ per leg
Total for 3 legs = 64.2 ft³
Seawater displacement: 64.2 ft³ × 64 lb/ft³ = 4,109 lbs buoyancy.

This matches the estimated empty weight almost exactly. With the waterline at 50% submergence (leg half in water), the legs alone provide ~2,055 lbs of buoyancy. Additional buoyancy comes from:

Potential slight increase in operational waterline (e.g., 60% submerged adds ~820 lbs)
Stabilizer foils providing dynamic lift at speed (significant)
Possible additional buoyancy in truss if sealed (not assumed here)

Conclusion: The design, as estimated, is buoyancy-limited for static conditions. To carry 4 people + gear (~800 lbs), either the waterline needs to be slightly higher (legs ~60-65% submerged), or the stabilizers must provide dynamic lift at speed. This is feasible for a day sailor where precise static trim is less critical, and foiling assistance is expected when moving.

Range Estimate with 50 kWh Battery

Assumed speed: 4-5 knots (same as full-scale plan). Power consumption estimated for 2x Yamaha HARMO rim drives (likely ~5-10 kW total at cruise, depending on efficiency, hull drag, and foiling assistance).

Conservative power use: 8 kW continuous at 4.5 knots
Range = Battery Capacity / Power × Speed
= 50 kWh / 8 kW × 4.5 knots
= 6.25 hours × 4.5 knots
= 28 nautical miles

Optimistic power use (with foiling lift): 5 kW continuous
Range = 50 kWh / 5 kW × 4.5 knots = 10 hours × 4.5 knots = 45 nautical miles

This range is ample for sheltered water testing and day trips around Anguilla's protected west side. Recharge via solar (assuming 2kW peak, ~6kWh/day average in good sun) can extend operations without shore charging.

Cost Estimate

Assumptions: Parts fabricated in China, shipped in one 40ft container, assembled in Anguilla with owner labor. Major costs:

Item	Estimated Cost (USD)
Custom Aluminum Legs & Truss Parts (CNC cut/welded)	$18,000
2x Yamaha HARMO Rim Drives	$10,000
50 kWh LiFePO4 Marine Batteries	$12,000
3x Active Stabilizers (foils, actuators, controls)	$6,000
Solar Panels, Charge Controllers, Wiring	$3,000
Control System (computer, sensors, joystick)	$4,000
Netting, Seats, Hardware, Fasteners	$2,000
7 ft Inflatable Dinghy	$800
Shipping, Customs, Misc.	$4,200
Total Estimated Cost	$60,000

Note: This is a rough estimate. Actual costs can vary ±30% based on fabrication details, supplier relationships, and shipping logistics.

Off-the-Shelf Marine Aluminum Truss Options

For bolted construction, consider these systems:

Bosch Rexroth Aluminum Framing: Modular t-slot profiles (metric 45mm, 90mm series). Not marine-grade but can be used if anodized or painted. Widely available.
80/20 Inc. Similar t-slot extruded aluminum profiles (imperial 1.5" series). Can specify 6063-T6 alloy, anodized.
Marine-specific welded truss nodes from companies like Marine Aluminum (New Zealand) or Aluma yacht – might sell weld-free bolt-together kits for catamaran beams.
Custom CNC-cut plates with pre-drilled holes to bolt standard aluminum rectangular tubes (e.g., 3"x6"×0.25" wall 6061-T6) – likely the most cost-effective for one-off.

Recommendation: Design using standard aluminum rectangular tubing (readily available globally) with custom CNC-cut gusset plates and bolted connections. This avoids custom extrusion costs and simplifies sourcing.

Prototype Viability & Recommendations

Strengths:

Half-scale allows affordable testing of hydrodynamics, stabilizers, and control software.
Sheltered waters of Anguilla provide ideal testing environment.
Bolt-together design enables shipping and assembly without specialized welding.
Stabilizers have relatively more power (2× per weight) to handle smaller-scale wave disturbances.

Challenges & Considerations:

Buoyancy margin is tight – consider slightly enlarging leg volume or accepting higher operational waterline.
Weight control is critical – use minimum practical skin thickness (3mm) and avoid over-building.
Ensure all aluminum is marine-grade (5083/H116 or 6061-T6 with proper isolation from steel fasteners).
Test stabilizer control software thoroughly on dry land before water trials.

Overall: The half-scale prototype is a feasible, exciting project. With an estimated cost of ~$60,000 and careful weight management, it can serve as a functional day sailor and invaluable testbed for the full-scale seastead. The range of ~30-45 nautical miles at 4-5 knots is sufficient for extended testing in Anguilla's protected waters.