This is a rough engineering-sketch analysis for a half-linear-scale prototype of the trimaran-style, foil-legged seastead, intended as a day sailor for calm sheltered water (e.g. the lee of Anguilla).
| Item | Full size | Half size (prototype) |
|---|---|---|
| Triangle side length | 70 ft | 35 ft |
| Triangle back width | 35 ft | 17.5 ft |
| Frame (floor-to-ceiling) | 7 ft | 3.5 ft (open frame / seating) |
| Leg length (NACA 0030) | 19 ft | 9.5 ft |
| Leg chord | 10 ft | 5 ft |
| Leg thickness (width) | 3 ft | 1.5 ft |
| Submerged leg length | 9.5 ft | ~4.75 ft |
| Stabilizer wingspan | 10 ft | 5 ft |
| Dinghy | 14 ft RIB | 7 ft inflatable |
Each leg is a NACA 0030 foil, chord 5 ft, thickness 1.5 ft, with
9.5 ft total length. For a NACA 00xx section the cross-sectional area
is approximately A ≈ 0.685 × chord × thickness.
If the waterline rises to, say, 65% of the leg length (still well below the deck), buoyancy grows to roughly 6,080 lb (~2,760 kg). That "reserve" is what the crew, batteries, and cargo are spending.
Scaling mass as 1/8 is a geometric ideal; in reality aluminum plate has minimum practical gauges (~3 mm / 0.12"), so small structures end up heavier than pure cube-law scaling suggests. Using reasonable marine-aluminum plate + extrusion estimates:
| Component | Estimate (lb) | Notes |
|---|---|---|
| 3 foil legs (Al skin + internal ribs) | 600 | ~24 ft³ displacement hulls, ~3 mm skin, ribbed. ~200 lb each. |
| Triangle truss frame (3 sides × 35 ft) | 700 | Off-the-shelf aluminum box truss ≈ 6–8 lb/ft. |
| Cross-bracing / node gussets / hardware | 150 | Bolted, no welding. |
| Solar frame + ~2 kW panels | 250 | Optional on prototype. |
| Trampoline net (center + rear decks) | 60 | Polyester catamaran netting ≈ 0.4 lb/ft². |
| Seats (6, light pedestal) | 120 | |
| 3 stabilizer "airplanes" + actuators | 120 | 40 lb each incl. servo. |
| 2 Yamaha HARMO RIM drives (motor units only) | 240 | ~50 kg each bare; cabling + controllers ~20 kg. |
| Wiring, controllers, helm, autopilot PC | 100 | |
| 50 kWh LiFePO4 battery | 900 | Modern packs ≈ 18 lb/kWh installed. |
| 7 ft inflatable dinghy + davits | 120 | |
| Misc (lights, nav, safety, fasteners) | 140 | |
| Empty structural + systems total | ~3,500 lb | ~1,590 kg |
Each stabilizer main wing: 5 ft span × 1 ft chord = 5 ft² area, three of them = 15 ft². At 5 knots (≈ 2.57 m/s), dynamic pressure in seawater:
q = ½ ρ V² = 0.5 × 1025 × 2.57² ≈ 3,390 Pa (~70.8 lb/ft²)
At a modest CL of 0.4 (safe, not stalling): Lift ≈ 15 × 70.8 × 0.4 ≈ 425 lb. At CL = 0.7 you could carry ~740 lb. So 300–700 lb of dynamic lift is realistic at cruise — enough to noticeably raise the hulls and reduce wetted drag, but not a true hydrofoil takeoff.
Three half-submerged NACA 0030 struts, each with wetted surface of roughly 2 × chord × submerged length × 1.03 ≈ 2 × 5 × 4.75 × 1.03 ≈ 49 ft² per leg, × 3 = ~147 ft² (~13.6 m²) total wetted area.
At 5 kt (2.57 m/s), Reynolds ≈ 4×10⁶ → Cf ≈ 0.0035. Friction drag ≈ Cf × q × S_wet ≈ 0.0035 × 3390 × 13.6 ≈ 161 N.
Add form factor (~1.15 for thick foil), thrusters, stabilizer foil drag, small wave/spray/interference drag, and ventilation losses → realistic total resistance ≈ 300–400 N at 5 kt.
| Speed | Battery draw | Endurance (50 kWh, 90% usable = 45 kWh) | Range |
|---|---|---|---|
| 4 kt | ~1.1 kW | ~41 h | ~160–170 nm |
| 5 kt | ~2.0 kW | ~22 h | ~110 nm |
| 6 kt | ~3.3 kW | ~13.5 h | ~80 nm |
With ~2 kW of deck solar in Caribbean sun averaging ~6 peak-sun hours, you gain ~10–12 kWh/day — roughly 5–10 additional nautical miles per day of "free" cruising at 4 kt, more if you run slow.
There are several marine- and architectural-grade aluminum truss lines that could serve as the triangle frame sides with only bolted gussets at the corners:
For saltwater service, specify 6082-T6 or 5083 members, stainless or A4 bolts, and isolate dissimilar metals. A stage-truss F44/F54 side at 35 ft will weigh ~100–140 lb, matching the budget above.
| Item | USD |
|---|---|
| 3 custom NACA-foil legs, fabricated in China, shipped flat-pack | $18,000 |
| Triangle truss (3 × 35 ft F44-class, + solar & interior framing) | $6,000 |
| Corner gussets, bolts, SS hardware | $2,000 |
| 3 stabilizer "airplanes" w/ actuators + IMU control | $4,000 |
| 2 Yamaha HARMO RIM drives (re-mounted) | $10,000 |
| Battery: 50 kWh LiFePO4 (marine) | $15,000 |
| Solar ~2 kW + MPPT + BMS + wiring | $4,000 |
| Helm, autopilot computer, sensors, lights, nav | $3,000 |
| Trampoline netting, seats, bimini | $2,500 |
| 7 ft inflatable dinghy + davit ropes | $2,000 |
| 40 ft container shipping China → Anguilla | $6,000 |
| Customs / duty / contingency (~15%) | $10,000 |
| Total, turnkey (self-assembled) | ~$82,000 |
Realistic range: $75k–$110k, depending on how much is truly off-the-shelf versus custom, and how far the HARMO mounts need to be re-engineered.