Here's my full engineering estimate for the half-scale prototype. Quick highlights before the file:
- **Range on 50 kWh:** ~170 nm at 4 knots, ~130 nm at 5 knots (practical, calm water; theoretical up to ~250/165 nm). Solar adds roughly 10–14 nm/day in Caribbean sun.
- **Dry weight:** ~3,000 lb — which is almost exactly the buoyancy of the three legs at the 50% waterline (~3,000 lb). So like the full-size design, the payload lives in the top half of the legs: **~300 lb at 55% submergence, ~600 lb at 60%, ~900 lb at 65%**, plus dynamic lift from the stabilizer foils underway. Your instinct is right — dropping to 35 kWh buys back ~400 lb of payload.
- **Build cost:** roughly **$30k (lean) to $40k (full spec)** with you doing assembly, including ~$4,500 freight from China.
- Your stabilizer scaling logic checks out: ¼ area at the same speed against ⅛ the weight = 2× the lift-to-weight margin, which is exactly what a small boat needs.
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Half-Scale Seastead Prototype — Engineering & Cost Estimate
Day-sailor validation platform · ½ linear scale · marine aluminum bolt-together kit · Anguilla assembly
~170 nm
Range @ 4 kn (50 kWh, practical)
~130 nm
Range @ 5 kn (50 kWh, practical)
~3,000 lb
Estimated dry weight
300–900 lb
People + cargo (55–65% waterline)
$30–40k
Build cost, self-assembled
~830 lb/ft
Immersion per ft of waterline change
Key Assumptions (please sanity-check these first)
Leg geometry: each leg modeled as a vertical fin — 21.5 ft span (athwartships) × 14.5 ft tall × 8.5 ft chord (fore-aft), with the NACA 0035 section (~3.0 ft max thickness) seen in plan view. Half-submerged at the design waterline. This is the reading that matches your ½ × 14.5 ft draft, the ladder on the "top half out of the water," thrusters on the lower side faces, the conduit on the trailing edge, and the container packing (chord vertical, 8.5 ft < 8.9 ft interior height).
Scaling: ½ linear → ¼ area → ⅛ volume/displacement. Plate gauge does not scale (you correctly flagged this) — prototype uses ⅛″ (3.2 mm) 5083 plate vs ~¼″ full-size.
Batteries: LiFePO₄ prismatic (e.g., EVE LF280K class), ~90–110 Wh/kg at pack level → 50 kWh ≈ 1,000 lb.
Resistance: empirical estimate for a small-waterplane trimaran (3 fin legs + heave plates + foils), overall propulsion efficiency ~60% for rim-driven pods.
Prices: indicative 2024-era USD; China fab rates $4–8/kg finished aluminum; freight China→Anguilla for a 40 ft HC.
All figures are conceptual (±20–25%) — worth a naval architect's pass before cutting metal.
Scaled Geometry
Dimension
Full Size
Prototype (½)
Triangle side (frame / living area)
44.0 ft
22.0 ft
Wall / truss height
7.0 ft
3.5 ft
Leg span (athwartships)
21.5 ft
10.75 ft
Leg height (vertical)
14.5 ft
7.25 ft
Leg chord (fore-aft)
8.5 ft
4.25 ft
Leg max thickness (NACA 0035)
~3.0 ft
~1.49 ft
Draft at 50% submergence
7.25 ft
3.63 ft
Waterplane area (3 legs)
~52 sq ft
~13 sq ft
Buoyancy at 50% waterline (legs)
~24,100 lb
~3,010 lb
Immersion rate
~3,320 lb/ft
~830 lb/ft
The waterplane check agrees with your "1 ft = ~1/7 of buoyancy" figure: 3,320/27,500 ≈ 1/8.3 for the bare legs — heave plates and walkway posts nudge it toward your 1/7.
The tight spot: the legs displace only ~3,010 lb at the 50% waterline, and the dry boat is ~3,000 lb. Just like the full-size design, all useful payload comes from deliberately sitting deeper in the top half of the legs — which you said is acceptable. See next section.
Buoyancy & Payload for People + Cargo
Immersion rate ≈ 830 lb per foot of waterline change. Leg is 7.25 ft tall; 50% = 3.63 ft draft.
Condition
Draft
% of leg submerged
Total displacement
Payload (people + cargo)
Design waterline (empty)
3.63 ft
50%
3,010 lb
~0 lb
Light crew
3.99 ft
55%
3,310 lb
~300 lb
3 adults
4.36 ft
60%
3,610 lb
~600 lb
3 adults + dive gear + cooler
4.72 ft
65%
3,910 lb
~900 lb
55% — ~300 lb
300
60% — ~600 lb
600
65% — ~900 lb
900
Dynamic lift: with the stabilizer foils carrying part of the weight at 4–5 kn, effective payload grows by several hundred pounds underway — your "lifting foil day sailor" concept is sound (see notes below).
Emergency reserve: the sealed upper halves of the legs hold ~47 cu ft of air ≈ 3,000 lb of flood-up reserve before you'd approach the truss — a genuinely reassuring safety margin. Adding closed-cell foam blocks there (~30 lb) makes it effectively unsinkable.
Recommendation: consider launching with 35 kWh (~600 lb) instead of 50 kWh. That returns ~400 lb of payload (≈ 1,000 lb total at 60%), and cells can be added later — the racks are the expensive part to retrofit, not the cells.
Range on 50 kWh @ 4–5 Knots
Calm-water estimate: displacement ~1.5 t, wetted surface ~115 sq ft (legs + heave plates + foils), small-waterplane wave drag, rim-pod overall efficiency ~60%, plus 100 W hotel load. Usable energy: 45 kWh (90% DoD).
Speed
Resistance
Total power draw
Endurance
Theoretical range
Practical range*
4 kn
~180 N
~0.72 kW
~62 h
~250 nm
~170 nm
5 kn
~290 N
~1.34 kW
~34 h
~167 nm
~130 nm
*Practical = 65–70% allowance for chop, maneuvering, throttle-ups, and battery aging.
Solar assist: 1.6 kW × ~5.5 sun-hours ≈ 8–9 kWh/day → roughly +10–14 nm/day at 4 kn, or covers hotel load + dinghy charging at anchor.
Caveats: at ½ scale but the same absolute speed, Froude number doubles — relatively more wave drag than full size, especially in chop. Expect +10–20% power over the calm-water numbers on a breezy day. Also confirm the HARMO units sustain ~1–2 kW continuous each; if they're closer to 1 kW, treat 5 kn as a flat-water cruise and 4 kn as the all-conditions speed.
Even the pessimistic case (>100 nm at 4 kn) comfortably covers a full day out of Sandy Ground / Crocus Bay with big margins.
Build Cost (self-assembled, free labor, own crane)
Item
USD
3 legs — CNC-kitted, welded 5083, ~310 kg finished
316 SS fasteners, anodes, sealants, isolation paste
1,300
Bottom paint + coatings
500
Net, cushions, odds & ends
500
7 ft dinghy + small electric motor
1,200
Mini helical mooring screw set (optional)
900
Freight, 40 ft HC China → Anguilla
4,500
Rigging slings, consumables
400
Subtotal
34,600
Contingency (15%)
5,200
Realistic all-in
≈ $39,800
Lean variant ≈ $30–32k: 30 kWh (−$2,400), one HARMO + one budget pod (−$3,000), skip mooring screws (−$900), smaller solar array (−$400). Good strategy: prove the platform lean, upgrade later.
Off-the-Shelf Aluminum Truss / Extrusion Sources
T-slot framing (bolts together, no welding):80/20 Inc (US), item International, Misumi, MiniTec — 6061-T6 profiles up to 3″+. Excellent for seats, rails, electronics racks, solar mounts, and even secondary bracing. For primary load paths, prefer welded tube or T-slot with heavy gussets and through-bolts.
Marine plate:Alloys International, Clinton Aluminum for 5083 sheet/plate in small quantities.
Done-for-you sub-kits: the aluminum boat-building cluster around Qingdao/Weihai (via Alibaba) routinely welds hull sections and truss assemblies to drawing — ideal for the legs and triangle sections, shipped as a bolt-together kit with machined flanges.
Corrosion discipline: 316 fasteners isolated with Tef-Gel or Duralac, backing plates everywhere, bedded joints in polyurethane sealant. Bolted aluminum + stainless works well at sea if isolation is religious.
Design Notes & Confirmations
Your stabilizer scaling is correct: lift ∝ v²·A. Same speed, ¼ area, ⅛ weight → 2× the lift-to-weight ratio. Smaller foils do suffer a Reynolds-number drag penalty, but the 2× margin dwarfs it. Running them at a modest angle as lifting foils at 4–5 kn should visibly stiffen the ride and add effective payload. Budget ~10–15% extra power when deployed, and consider shallow fixed incidence rather than active control for v1.
Packing check: three prototype legs at 7.25 ft tall × 4.25 chord × 1.49 thick lie easily in a half-height footprint; the full-size packing scheme (chord vertical, 8.5 < 8.9 ft) works as you described, with the trailing-edge trim absorbing bracket/lug height.
Electrical: a 16S LiFePO₄ bank is 51.2 V nominal — a natural match for 48 V-class rim drives. For the prototype, one bank with independent fused feeds per thruster is fine; keep the triple-redundant per-leg architecture for the full-size vessel.
Keep the airtight compartments in the prototype legs exactly as planned — they're your reserve buoyancy and your damage-control boundary. Add foam if the weight budget allows.
Test plan fits Anguilla perfectly: the west-side lee gives you protected water for the first foiling/stabilizer trials; the ½-scale boat at 2× Froude will actually be a harder seakeeping test than the full-size ship — good, conservative ordering.
Next steps: (1) weigh-and-moment audit of the kit before launch, (2) tow test to validate the ~180/290 N resistance figures, (3) incrementally ballast to map the waterline-vs-payload curve, (4) then deploy stabilizers.
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A few things worth flagging honestly: the biggest uncertainty in the weight budget is the legs (plate gauge and frame spacing dominate), and the biggest uncertainty in range is the actual resistance of those three fin-legs plus heave plates — a cheap tow test behind the dinghy would nail both down before you commit to the kit order. If your intended leg orientation differs from my "vertical fin" reading, tell me and I'll rerun the numbers — the buoyancy and payload conclusions are quite sensitive to it.