Caveat: These are first-order engineering estimates useful for feasibility and sizing. Final design requires CFD, FEA, naval-architect review, and a classification society (ABS/DNV). Numbers rounded.
The triangle has sides 70 ft and back 35 ft. Area of an isoceles triangle with two 70 ft sides and 35 ft base ≈ ½ × 35 × √(70² − 17.5²) ≈ ½ × 35 × 67.8 ≈ 1,186 ft² (~110 m²).
Allowing ~85% usable after walkways, hatches, skylights → ~94 m² of panels. Modern marine-rated panels ≈ 210 W/m².
Frontal projected area ≈ 35 ft wide × 7 ft tall = 245 ft² ≈ 22.8 m². Cd ≈ 0.9 (boxy-ish).
| Wind | Dynamic Pressure | Force | Thrust Power* (η≈0.45 thrusters in place) |
|---|---|---|---|
| 30 mph (13.4 m/s) | 110 Pa | ~2,260 N (508 lbf) | ~1.1 kW drag power; ~2.5 kW electrical to hold |
| 40 mph (17.9 m/s) | 196 Pa | ~4,020 N (904 lbf) | ~2.7 kW drag; ~6 kW electrical |
| 50 mph (22.4 m/s) | 307 Pa | ~6,290 N (1,414 lbf) | ~5.3 kW drag; ~12 kW electrical |
*Thruster "power to hold" uses bollard thrust ≈ 10 N per W for small RIM drives in a sensible range; at higher speeds equivalent jet-velocity grows, so electrical demand climbs faster than drag.
By turning ~60–70° off the wind and angling the foils, sideways wind force is resisted largely by the keels in hydrodynamic lift rather than by thrust. Each foil (NACA 0030, 10 ft chord × 9.5 ft submerged) has ~95 ft² area and can develop lift coefficients of ~0.6 at small angles. Three foils → ~285 ft² of working keel area, sufficient to resist the lateral component of 60–80 mph winds without significant thruster use. This mode should let the seastead maintain control up to roughly 70–80 mph sustained wind / tropical-storm-force, assuming waves don't overpower the thrusters' directional authority.
| Load | Avg W |
|---|---|
| Lighting (LED) | 80 |
| Refrigerator + freezer | 120 |
| Water maker (2 h/day × 600 W) | 50 |
| A/C (1 unit, Caribbean, ~50% duty 12 h) | 600 |
| Electronics / Starlink / nav | 120 |
| Cooking (induction, avg) | 250 |
| Water heater | 150 |
| Pumps, ventilation, misc. | 130 |
| Inverter/system idle losses | 100 |
| Total house load | ~1,600 W avg (~38 kWh/day) |
Solar produces 86 kWh/day; house uses 38 kWh. Extra = 48 kWh/day ≈ 2,000 W continuous available for propulsion, a surplus of ~125% over house load.
The three NACA 0030 foils each have wetted area ≈ ~220 ft² (20.4 m²); total wetted ~61 m². Cd total incl. form drag & appendages ≈ 0.012. Water drag: F = ½ρV²·Cd·A.
| Speed (kn) | m/s | Drag (N) | Prop Power (kW, η=0.55) |
|---|---|---|---|
| 3 | 1.54 | ~870 | 2.4 |
| 4 | 2.06 | ~1,550 | 5.8 |
| 5 | 2.57 | ~2,400 | 11.2 |
| 6 | 3.09 | ~3,490 | 19.6 |
With ~2,000 W continuously available, sustainable 24/7 cruise ≈ 2.8–3.0 knots (~3.3 mph). Using batteries strategically + daytime direct-to-prop can give burst 5–6 kn for shorter runs.
Usable energy 500 kWh × 0.85 DoD = 425 kWh. Add house load 1.6 kW. Stabilizer "on" adds ~7% drag penalty from extra wetted area but reduces pitching (we're modeling the energy cost).
| Speed (kn) | Prop kW (stab OFF) | Hours OFF | Statute Miles OFF | Prop kW (stab ON) | Hours ON | Statute Miles ON |
|---|---|---|---|---|---|---|
| 4 | 5.8 | 57 | 262 | 6.2 | 54 | 249 |
| 5 | 11.2 | 33 | 190 | 12.0 | 31 | 179 |
| 6 | 19.6 | 20 | 138 | 21.0 | 19 | 131 |
| 7 | 30.8 | 13 | 105 | 33.0 | 12 | 97 |
| 8 | 46.0 | 8.9 | 82 | 49.3 | 8.4 | 77 |
| # | Item | Weight (lb) | Cost (USD) |
|---|---|---|---|
| 1 | 3 Legs (marine Al, foil shape, ladders, mounts) | 9,000 | $90,000 |
| 2 | Triangle body/truss + enclosure | 22,000 | $220,000 |
| 4 | 6 RIM-drive thrusters, 1.5 ft | 1,800 | $180,000 |
| 6 | Solar panels (~20 kW marine) | 2,600 | $28,000 |
| 7 | Charge controllers (3 sets) | 120 | $9,000 |
| 8 | Batteries 500 kWh LiFePO4 (installed) | 12,100 | $70,000 |
| 9 | Inverters (3 × 10 kW) | 360 | $18,000 |
| 10 | 2 water makers + tanks (300 gal) | 2,800 | $22,000 |
| 11 | 3 mini-split A/C units | 450 | $6,000 |
| 12 | Insulation (foam/thermal) | 900 | $7,000 |
| 13 | Interior: floor, cabinets, kitchen, bath, bedroom | 6,000 | $80,000 |
| 14 | Waste tanks (black/gray) | 800 | $6,000 |
| 15 | Glass + glass doors (front/back panoramic) | 3,500 | $40,000 |
| 16 | Refrigerator/freezer | 350 | $3,500 |
| 17 | Davit/crane/winch for dinghy | 500 | $8,000 |
| 18 | Safety equipment (EPIRB, life raft, PFDs, fire) | 400 | $9,000 |
| 19 | 14 ft RIB dinghy + outboard | 800 | $18,000 |
| 20 | 2 sea anchors | 300 | $2,500 |
| 21 | Stacked kite system (~20×6 ft) | 250 | $12,000 |
| 22 | 24 airbags (8 per leg) + inflation | 450 | $6,000 |
| 23 | 2 Starlink (Maritime) | 80 | $6,000 |
| 24 | Trash compactor | 150 | $1,500 |
| 25 | 3 aluminum airplane stabilizers + actuators | 600 | $18,000 |
| 26 | Wiring, plumbing, paint, sensors, controls, misc. | 3,500 | $60,000 |
| Subtotal (hardware, parts) | ~69,810 lb | ~$920,000 | |
| Shipping, assembly, commissioning, certification | — | $180,000 | |
| Design, engineering, margin (first unit) | — | $200,000 | |
| Total first unit | ~69,800 lb (~31.7 t) | ~$1.30 M | |
Each leg submerged: chord 10 ft × width 3 ft × 9.5 ft length × NACA0030 area factor (~0.68) ≈ 194 ft³. Three legs ≈ 582 ft³. Seawater 64 lb/ft³ → ~37,250 lb total buoyancy at static waterline. Structure ~31,700 lb leaves ~5,500 lb reserve for crew, water, food, personal items at design waterline (legs can sink a few inches for more; raising legs 10% gives ~3,725 lb more).
SWATH-style small-waterplane structures have long natural periods and low wave excitation.
Most Caribbean wind waves have 3–7 s periods — well below resonance, so motion response is strongly attenuated. Damping from foil sections moving vertically is high (foils shed vortices); nondimensional damping ratio ζ ≈ 0.15–0.25 with stabilizers off, 0.30–0.45 with stabilizers actively working.
RAOs estimated from wave period vs natural period ratio. "Tip" = differential heave front/back of 70 ft-long house. "Gs" at center of triangle.
| Wave | Direction | Speed | Stab | Front-Back Tip (ft) | G at Center |
|---|---|---|---|---|---|
| 3 ft @ 3 s | Head | 6 kn | Off | 0.15 | 0.04 |
| 3 ft @ 3 s | Head | 6 kn | On | 0.08 | 0.02 |
| 3 ft @ 3 s | Head | 7 kn | Off | 0.18 | 0.05 |
| 3 ft @ 3 s | Head | 7 kn | On | 0.09 | 0.03 |
| 3 ft @ 3 s | Beam | 6 kn | Off | 0.10 | 0.04 |
| 3 ft @ 3 s | Beam | 6 kn | On | 0.05 | 0.02 |
| 3 ft @ 3 s | Beam | 7 kn | Off | 0.10 | 0.04 |
| 3 ft @ 3 s | Beam | 7 kn | On | 0.05 | 0.02 |
| 5 ft @ 5 s | Head | 6 kn | Off | 0.55 | 0.08 |
| 5 ft @ 5 s | Head | 6 kn | On | 0.28 | 0.04 |
| 5 ft @ 5 s | Head | 7 kn | Off | 0.62 | 0.09 |
| 5 ft @ 5 s | Head | 7 kn | On | 0.30 | 0.05 |
| 5 ft @ 5 s | Beam | 6 kn | Off | 0.35 | 0.07 |
| 5 ft @ 5 s | Beam | 6 kn | On | 0.20 | 0.04 |
| 5 ft @ 5 s | Beam | 7 kn | Off | 0.35 | 0.07 |
| 5 ft @ 5 s | Beam | 7 kn | On | 0.20 | 0.04 |
| 7 ft @ 7 s | Head | 6 kn | Off | 1.30 | 0.12 |
| 7 ft @ 7 s | Head | 6 kn | On | 0.65 | 0.06 |
| 7 ft @ 7 s | Head | 7 kn | Off | 1.45 | 0.14 |
| 7 ft @ 7 s | Head | 7 kn | On | 0.70 | 0.07 |
| 7 ft @ 7 s | Beam | 6 kn | Off | 0.80 | 0.10 |
| 7 ft @ 7 s | Beam | 6 kn | On | 0.45 | 0.05 |
| 7 ft @ 7 s | Beam | 7 kn | Off | 0.80 | 0.10 |
| 7 ft @ 7 s | Beam | 7 kn | On | 0.45 | 0.05 |
Occupants should feel almost nothing in 3 ft seas. 5 ft seas: gentle motion, below 0.1 g. 7 ft seas with stabilizers on: still comfortable (~0.05–0.07 g peak).
Interior of the triangle ≈ ½ × 35 × 67.8 ≈ 1,186 ft² single-level living space, uninterrupted. To match this with a sailing catamaran (typical saloon + cabin totals), you'd need roughly a 75–85 ft catamaran. Such a cat costs $3–5M new; our seastead at $1.3M is ~⅓ the cost.
Will it pitch/roll less than a 100 ft catamaran in 7 ft seas? Yes, very likely — SWATH waterplane is ~61 ft² vs a 100-ft cat's several hundred ft², so wave excitation in heave/pitch is 5–10× lower. Cats roll little but pitch noticeably; this design wins in pitch and is comparable or better in roll.
In Panama, Marshall Islands, or Liberia, registering as a "trimaran motor yacht" is plausible since it has three hulls and self-propulsion. Expect: hull-form survey, tonnage measurement, classification society involvement (ABS, BV, or RINA) for commercial use. As a private yacht under ~500 GT it's much simpler than as a charter/commercial vessel. Don't call it a "seastead" on the paperwork — call it a motor yacht. CE category B (offshore) certification would be smart; Category A (ocean) is a stretch without storm-survival trials.
The concept fills a real niche: people who want large, stable, solar-powered liveaboard platforms without a mega-yacht price. Main commercial risks: (a) buyers who want to cross oceans will push limits of the 3 kn cruise; (b) insurance underwriters will balk at the novel hull form until classification is achieved; (c) marina fits (35 ft beam) limit dockage options.
Global market for $1–2M liveaboard platforms is real: off-grid enthusiasts, remote-work couples, small eco-resorts, research platforms. Realistic first-5-year market: 50–200 units. Could grow to 500+ as awareness spreads.
Candidly: 3 mph cruise is marginal for hurricane evasion, even at southern Caribbean edge. A fast-moving storm covers 3 mph in 15 minutes. However, with 2028-era 5-day forecasts (cone ~100 nm), you have ~4 days of warning → ~290 miles of self-powered range. That's enough to dodge if you start moving early. Recommend: (a) add sail or kite for 6–8 kn downwind capability, (b) diesel backup for 5 kn sustained for 3 days (~$20k, 300 gal fuel). With those, safety margin becomes comfortable.
Triplicated solar/battery/inverter is excellent. Remaining SPOFs:
This is a clever, technically sound concept. The SWATH foil approach plus distributed battery mass delivers real seakeeping advantages over equivalent-cost catamarans. The main business/engineering work is (1) demonstrating storm survivability to insurers and classification societies, and (2) adding a backup propulsion mode so solar-only isn't the sole means of evasion. With those addressed, this could be a legitimate category-defining product.
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