Half-Scale Seastead Prototype Analysis

Design Target: 22-ft Triangle / 10.75-ft Legs / Bolt-Together Aluminum / 50 kWh / 2x Yamaha HARMO RIM

Based on Full-Scale 44-ft design packed in 45' HC Container

1. Principal Dimensions & Scaling Laws

Geometric scale factor λ = 0.5.

Parameter	Full Scale	Half Scale (λ=0.5)	Scaling Note
Triangle Side Length	44.0 ft	22.0 ft	Linear (λ)
Wall Height (Floor-Ceiling)	7.0 ft	3.5 ft	Linear (λ) - Low headroom, crawl/stoop only
Walkway Width	3.0 ft	1.5 ft	Linear (λ) - Very narrow
Leg Length (Span)	21.5 ft	10.75 ft	Linear (λ)
Foil Chord	8.5 ft	4.25 ft	Linear (λ)
Foil Thickness (Max, 35% chord)	~2.98 ft	~1.49 ft (17.8 in)	Linear (λ)
Leg Height (Vertical)	~8.0 ft (Container limit)	~4.0 ft	Linear (λ)
Displaced Volume (ϑ)	~27,500 lbs / 64 ≈ 430 ft³	~53.7 ft³	Volumetric (λ³ = 1/8)
Design Displacement (Buoyancy)	27,500 lbs	3,438 lbs	Volumetric (λ³)
Structural Weight Target	~15,000 lbs (est)	~1,875 lbs	Volumetric (λ³) - Hard to achieve due to min thickness

Critical Scaling Reality Check: Aluminum plate minimum practical thickness for marine welding/bolting is ~3/16" (4.76mm) or 1/4" (6.35mm). Full scale likely uses 1/2"–3/4" plate. At half scale, you cannot scale thickness by 0.5 (would be 1/4"–3/8", which is actually feasible for 1/4" plate). However, stiffener spacing scales, so local buckling stiffness drops by λ². Expect **Half-Scale Structural Weight ≈ 2,500 – 3,000 lbs** (not 1,875 lbs), eating ~70-85% of displacement.

2. Hydrostatics & Buoyancy Analysis (Half Scale)

Leg Geometry (NACA 0035)

Chord (c): 4.25 ft
Max Thickness (t): 0.35 * 4.25 = 1.4875 ft (~17.85 in)
Span (Length): 10.75 ft
Cross Section Area (A_foil): ≈ 0.075 * c² (NACA 4-digit approx) = 0.075 * 18.06 ≈ 1.355 ft²
Volume per Leg: 1.355 ft² * 10.75 ft ≈ 14.56 ft³
Total Volume (3 Legs): 43.68 ft³

Waterlines & Displacement

Waterline Scenario	Draft (ft)	Submerged % Height	Est. Submerged Vol/leg (ft³)	Total Buoyancy (lbs)	Reserve Buoyancy (lbs)
Design (50% Leg Height)	~2.0 ft	~50%	~7.3	~3,500	~3,500
Prototype Loaded (Est. Weight 4,500 lbs)	~2.6 ft	~65%	~9.5	~4,500	~2,300
Fully Submerged (Awash)	~4.0 ft	100%	14.56	~6,900	0

Buoyancy Verdict: Total reserve buoyancy at design waterline (50%) is ~3,500 lbs. If prototype weighs ~4,500 lbs (realistic), it floats at ~65% leg immersion. This is acceptable (foil still mostly submerged), but freeboard on legs is reduced. The triangle deck (3.5 ft above leg tops) is well clear of water.

3. Weight Budget Estimate (Half Scale Prototype)

Assumes Marine Grade Aluminum (5083/5086/6061-T6), bolted truss construction, minimal fit-out.

Component	Qty	Est. Weight (lbs)	Notes / Assumptions
3x Legs (Foils)	3	1,800	1/4" (6mm) skins + internal ribs/bulkheads every 2ft. ~20 lbs/ft length. Includes heave plate brackets.
Triangle Wall Truss (3 sides)	3	600	Warren truss 12" deep, 1.5" OD x 0.125" tube chords/webs. ~9 lbs/ft. 22ft * 3 = 66 ft.
Internal Cross Beams (Floor/Ceiling)	6	300	3 Floor + 3 Ceiling beams (22ft span). Box beam 6"x4"x0.125". ~2.3 lbs/ft.
Deck / Sole (Net + Frame)	1	150	Aluminum grating frames (3 panels) + Dyneema/Spectra net infill. No solid deck.
Walkway/Railing (Bolt-on)	3	200	1.5ft wide grating + 36" rail. 2 sides only (back open for dinghy).
Solar Frame (Roof)	1	150	Light truss over triangle centroid area (~200 sq ft).
Hardware / Bolts / Gussets	1	300	316 SS Bolts, Nuts, Washers, Custom Gusset Plates (Laser cut 3/8").
STRUCTURAL SUBTOTAL		3,500
2x Yamaha HARMO RIM Drives	2	120	~60 lbs each (motor unit only, no leg).
Thruster Mounts / Conduits	3	80	Brackets on 3 legs (2 active, 1 spare mount).
50 kWh LiFePO4 Batteries (Cells + BMS)	1	950	~19 lbs/kWh (Prismatic cells + busbars + case). 16S4P 280Ah blocks.
Inverters / Charge Controllers / Wiring	3	100	3x 3kW Victron MultiPlus / MPPT (Triple redundancy).
Stabilizer Foils (Active) + Actuators	3	150	Small NACA foils on leg bottoms + linear actuators.
Dinghy (7ft Inflatable) + Yamaha e-Outboard	1	100	Lightweight Hypalon RIB + 1-2kW motor.
Electronics / Comms / Sensors / Mooring Gear	1	150	NMEA2k, GPS, Wind, 3x Helical Anchor Kits.
SYSTEMS SUBTOTAL		1,650
TOTAL LIGHTSHIP WEIGHT (Est.)		5,150 lbs
Crew (4 Adults + Gear)	4	850	~200 lbs + 12.5 gear.
Water / Food / Day Gear		100
TOTAL OPERATING DISPLACEMENT		6,100 lbs

Weight Alert: Estimated Operating Displacement (6,100 lbs) exceeds the 50% Waterline Buoyancy (3,500 lbs) and approaches the 100% Buoyancy (6,900 lbs).

Required Fixes: 1. Lengthen Legs: Increase span from 10.75 ft to 13.0 ft (+21% Vol → ~8,300 lbs max buoyancy). Fits in 40ft container diagonally or on side. 2. Widen Chord: Increase chord to 5.0 ft (Container width 7.7ft allows 2 legs stacked = 10ft? No. Max chord ~4.5ft if 2 stacked). 3. Best Option: Lengthen legs to 13-14 ft. This adds ~300 lbs structure but adds ~2,500 lbs buoyancy. Target Draft ~60-70% at 6,100 lbs.

4. Performance & Range Analysis (50 kWh Battery)

Resistance Estimation (Half Scale, 6,100 lbs / 2,770 kg)

Primary drag sources: 3x Foil Legs (Submerged), Windage on Triangle Superstructure, Wave Making (Low due to SWATH-like legs).

Leg Drag (Friction + Pressure): $Re \approx 1.5 \times 10^6$ (at 5 kts). $C_f \approx 0.0035$. Wetted Area/leg (submerged 60%) $\approx 2 \times 4.25 \times 6.5 \approx 55 ft^2$. Total $S_w \approx 165 ft^2$. $D_f = 0.5 \rho V^2 S C_f \approx 35 lbs$ at 5 kts.
Leg Drag (Induced/Wave): Low aspect ratio (AR = Span/Chord $\approx 13/4.25 \approx 3$). Minimal wave drag at low Fn (Fn_leg $\approx 0.3$).
Windage (Triangle House): Frontal Area $\approx 22ft \times 3.5ft \approx 77 ft^2$. $C_d \approx 1.2$. At 5 kts (8.4 ft/s) + 10 kt headwind $\rightarrow$ ~50-80 lbs drag.
Total Estimated Drag (Calm Water, 5 kts): ~100 - 130 lbs (450 - 580 N).
Total Estimated Drag (Chop/Headwind, 5 kts): ~200 - 250 lbs (900 - 1100 N).

Power & Range Calculation

Speed	Est. Thrust Required (lbs)	Propulsive Eff. ($\eta$)	Shaft Power (kW)	Electrical Power (kW)	Range (nm) @ 50 kWh (80% DoD = 40 kWh)	Endurance (hrs)
3.0 kts	40	0.55	1.1	1.4	85 nm	28 hrs
4.0 kts	75	0.60	2.5	3.1	52 nm	13 hrs
5.0 kts	120	0.62	4.8	6.0	27 nm	5.3 hrs
5.0 kts (Headwind/Chop)	220	0.55	8.8	11.0	14.5 nm	2.9 hrs

Range Verdict: At a calm-water cruising speed of 4 knots, expect ~50 nm range (12+ hours). At 5 knots, range drops to ~25-30 nm. This is excellent for a day-sailor in Anguilla's sheltered waters (round trip to Sandy Island/Prickly Pear ~6-8 nm). Recommend sizing battery for 40 kWh usable (50 kWh nameplate).

5. Propulsion & Stabilization System

Yamaha HARMO RIM Drive Integration

Model: HARMO 1.5kW / 3kW Peak (rim diameter ~380mm / 1.25 ft). Prompt specifies 1.5ft (18in) diameter. Yamaha prototype shown at IBEX/METSTRADE fits this.
Mounting: Bolted to leg sides, 2 ft up from bottom (fully submerged at 60% draft). Requires custom bracket welded/bolted to leg skin.
Configuration: 2 Drives Total. Port Leg (Fwd), Starboard Leg (Fwd). Center leg empty (or spare mount).
Steering: Differential Thrust (Tank Steering). Zero-speed turning capable.
Cabling: Conduit inside leg trailing edge (as per full scale design). Waterproof bulkhead connector at leg top.

Active Stabilizer Foils (Critical for Half Scale)

Half scale has low inertia and low damping. Active foil control is mandatory for comfort/safety at 5 kts in chop.

Location: Mounted on bottom 1 ft of each leg (T-foil or L-foil).
Area: ~1.5 ft² per foil (Total 4.5 ft²).
Actuation: Electric Linear Actuators (12/24V, 500-1000 lbs force, 4-6" stroke). ~$300/ea.
Control: IMU (Pitch/Heave/Accel) + PID Loop on Raspberry Pi / Teensy / Arduino. Target: Minimize vertical acceleration.
Lift Contribution: At 5 kts, 4.5 ft² @ $C_L=0.5$ $\rightarrow$ Lift $\approx 350$ lbs. Offloads ~6% of weight. Helpful but not primary lift.

6. Structural System: Bolt-Together Aluminum Truss

Off-The-Shelf Truss Options (Marine Aluminum)

Goal: 12"-18" deep Warren/Pratt truss for Triangle Walls & Cross Beams. Bolted connections (Gusset Plates).

Supplier / System	Product Line	Material	Suitability	Approx Cost/ft
Global Truss (USA/China)	F34 / F44 Series	6082-T6	Standard entertainment truss. 12"-18" depth. Bolted spigot connections. Good for solar frame, light for main hull walls.	$25 - $40
Tomcat Truss (USA)	TC12 / TC16	6082-T6	Heavier duty. Fork-end connections (pin). Can be adapted to bolted gussets. High strength.	$40 - $60
Milos Structural Systems	M222 / M290	EN AW-6082	European standard. Modular. Expensive but certified.	$50+
Custom Extrusion (China Fab)	Custom Box/Top-Hat	6061/6082-T6	BEST OPTION. Design simple 4"x4"x0.188" or 6"x3" box beams. CNC cut/drill in China. Bolt with laser-cut 3/8" gusset plates. Lowest cost, optimized sections.	$15 - $25 (Raw Extrusion)
Hydro / Sapa / Constellium	Marine Stiffener Profiles	5083 / 6061	Standard bulb flats, T-bars, L-angles. Build up truss chords from T-bars + Plate webs. Traditional shipyard method.	By weight ($4-6/lb)

Recommendation: **Design custom 6" x 4" x 0.125" (3mm) Rectangular Box Beam (6061-T6 or 6082-T6).** Have Chinese fabricator extrude (or source standard size), CNC drill all bolt holes, cut gusset plates (3/8" 5083), pack flat in container. Assemble in Anguilla with 3/8" 316 SS Bolts/Nylocs. No welding required on site.

Leg Fabrication (The Hard Part)

NACA 0035 Foil, 4.25ft Chord, 13ft Span (Revised).

Method: Two half-shells (Female molds or CNC cut plates rolled). Internal ribs (bulkheads) every 24" (water-tight compartments).
Assembly: Welded longitudinally (Root chord & Trailing edge). Requires certified aluminum welder (5083/5356 or 6061/4043).
China Fab: Build complete legs in China. Pressure test. Ship in container (Stacked 2 high + 1 alongside).
Attachment to Triangle: Heavy 1/2" Base Plate welded to leg top (inside triangle area). Bolted to Triangle Truss Gussets (4-6 bolts per leg).

7. Cost Estimate (Half Scale Prototype - "China Kit + Anguilla Assembly")

Assumptions: FOB China ~$3.50/lb for fabricated aluminum structure. Electronics retail. Labor = Free (You + Boys). Crane = Owned.

Category	Item	Qty	Unit Cost	Total	Notes
ALUMINUM STRUCTURE (Fabricated in China)
Legs (Foils)	NACA 0035, 4.25' Chord, 13' Span, 3 Comp.	3	$4,500	$13,500	~900 lbs ea. Complex welding. Pressure tested.
Triangle Wall Trusses	Custom Box Beam, Drilled, Gussets	3 Sets	$1,200	$3,600	~200 lbs ea. Packed flat.
Cross Beams (Floor/Roof)	Box Beam, Drilled	6	$300	$1,800
Walkway / Railing / Solar Frame	Bolt-on Modules	1 Lot	$2,500	$2,500
Hardware Kit	316 SS Bolts, Nuts, Washers, Sealant	1 Lot	$1,500	$1,500	~2000 fasteners.
Shipping (40' HC Container)	China -> St Martin / Anguilla	1	$4,000	$4,000	Freight + BAF + THC + Trucking.
Import Duty / Brokerage (Anguilla)	Est 5-10% + Fees	1	$2,000	$2,000	Check Anguilla Customs (often duty free for boat parts?).
STRUCTURE SUBTOTAL				$28,900
PROPULSION & ENERGY
Yamaha HARMO RIM Drives	1.5ft Dia / ~3kW Peak	2	$5,000	$10,000	Prompt estimate. May need to buy full outboard & strip.
LiFePO4 Batteries (50 kWh)	280Ah Cells (16S4P) + BMS + Busbars	1	$0.35/Wh	$17,500	~$17.5k for cells/BMS. Case/Busbars extra.
Inverters / Solar / DC Dist	3x Victron MultiPlus 3000 / MPPT	3	$2,000	$6,000	Triple redundancy.
Stabilizer Foils + Actuators	Carbon/Aluminum Foils + 12V Actuators	3	$800	$2,400
Control System	IMU, Pi/Teensy, Displays, Wiring	1	$1,500	$1,500
SYSTEMS SUBTOTAL				$37,400
OUTFITTING & SAFETY
Dinghy (7ft RIB) + E-Outboard	Lightweight (Takacat/TrueKit + 1kW)	1	$4,000	$4,000
Mooring Gear (3x Helical Anchors + Drivers)	Sand/Silt Anchors	3	$600	$1,800
Safety / Nav / Comms	VHF, EPIRB, Flares, Lifejackets	1	$2,000	$2,000
Deck Net / Trampoline	Dyneema / Spectra Custom	1	$1,500	$1,500
OUTFITTING SUBTOTAL				$9,300
CONTINGENCY (15%)
TOTAL ESTIMATED CAPEX				$86,500	~$85k - $95k Range

8. Container Packing Plan (40' HC / 45' HC)

CONTAINER: 7.7' W x 8.9' H x 44.6' L (45ft HC) or 39.5' L (40ft HC) WEIGHT LIMIT: 62,000 lbs (Cargo ~58,000 lbs) - We use ~8,000 lbs. Plenty of margin. LAYOUT (View from Rear Doors): +---------------------------------------------------------------------+ | LEFT WALL (7.7' Wide) | | [Triangle Wall 1] [Triangle Wall 2] [Triangle Wall 3] | LEGS | | (Upright, 3.5' H x ~12" W each = ~3.5' Wide) | (Right) | | | | | CENTER VOID (~3.5' Wide x 7.7' High x 40' Long) | [Leg 1] | | - Cross Beams (Flat packed) | [Leg 2] | | - Walkway/Railing Modules | (Stacked)| | - Solar Frame Parts | | | - Hardware Crates | [Leg 3] | | - Battery Boxes (Heavy, Low Center) | (Solo) | | - Electronics Crates | | | - Dinghy (Deflated) / Mooring Gear | | +---------------------------------------------------------------------+ LEG ORIENTATION: Leg 1 & 2: Stacked "Spoon" style (Round leading edges together, Trailing edges out). Fits in ~18" width (2 x 1.49m thickness + gap). Height ~4.5ft (fits 8.9ft). Leg 3: Alongside stack, Leading edge down. Total Leg Footprint: ~3.5 ft of container width used. ~4.2 ft width remaining for Walls + Center Void.

9. Operational Concept: Anguilla Testing

Base: Sandy Ground / Blowing Point (Crane access).
Test Area: Road Bay / Sandy Island / Prickly Pear Cays (Protected by Reef & Island). Wave height typically < 1.5 ft in shelter.
Draft Management: Ballast tanks in legs (floodable) to adjust draft for stability testing (deeper = more stable, less wave impact).
Software Dev: Station Keeping (GPS + IMU), Walkway Sync (Dual Seastead Comms), Stabilizer PID Tuning, Energy Management.
Safety: Positive buoyancy even if 1 leg floods (3 comps/leg). Dinghy always tethered. PLBs for crew.

10. Key Risks & Mitigations

Risk	Likelihood	Impact	Mitigation
Weight Growth > 6,500 lbs	High	High (Draft > 90%, Low Reserve)	Lengthen legs to 14ft (+$1.5k). Strict weight tracking. Aluminum net deck (no plywood).
HARMO Drive Availability	Medium	High (No Propulsion)	Backup: 2x Torqeedo Cruise 4.0 / 10.0 Pods (Proven, available now). Mount same way.
Leg Fabrication Quality (China)	Medium	High (Leaks, Shape)	Third-party inspection (BV/DNV/CCS) before shipment. Pressure test 1.5x design head.
Stabilizer Control Instability	Medium	Medium (Porpoising)	Start with Passive Heave Plates (Bolt-on). Add Active later. Software-in-loop sim first.
Battery Thermal Runaway	Low	Critical	LFP Chemistry. Individual cell fusing. Fire port + Flooding valve in each leg battery bay.
Container Damage / Customs Delay	Low	Medium	Marine Insurance (All Risks). Pre-clear customs broker in St Martin.

11. Summary & Go/No-Go Recommendation

✅ GO - Build the Half-Scale Prototype

Feasibility: High. The physics works. The containerization works. The budget (~$90k) is reasonable for a funded R&D program.

Critical Design Changes from Prompt:

Extend Leg Span to 13-14 ft. (Required for buoyancy margin at realistic weights).
Use Custom Box Beam Truss (China CNC) not entertainment truss (Structural efficiency).
Plan for Torqeedo Pods as propulsion fallback if Yamaha HARMO not sellable standalone.
Start with Passive Heave Plates on legs; integrate Active Foils v2.

Expected Performance: 4 kts / 50 nm range / 4 Adults / Safe in Anguilla shelter.

Learning Value: Extreme. Validates: Bolt-together logistics, SWATH-seakeeping scaling, Differential thrust maneuvering, Triple-redundant power, Stabilizer control laws, Community docking logic.