Concept Study

Tri-Foil SWATH Seastead — Performance & Cost Estimates

40-ft triangular aluminum frame · 3 NACA foil legs · 12×24 ft living area · active "airplane" stabilizers · solar-electric propulsion

Quick Answers (TL;DR)

≈ 680 ft²

Total solar area (roof + 2 fold-down wings)

≈ 13 kW

Installed solar (DC nameplate)

≈ 12,000 lb

Estimated structure weight

≈ 13,000 lb

Reserve buoyancy after structure

≈ 880 lb

Extra buoyancy per leg per ft of immersion

1.5 / 2.9 / 4.9 kW

Propulsion power @ 4 / 5 / 6 kn

~3 days

Caribbean sun to refill battery bank

$270k / $210k

Est. cost — 1 unit / batch of 20 (China)

1 · Geometry Check

Equilateral triangle, 40 ft per side → height = 40 × √3/2 ≈ 34.6 ft. A 12-ft-wide rectangle pushed as far forward as possible touches the two angled sides at:

y_max = 34.64 × (1 − 6/20) = 24.25 ft → Living area ≈ 12 ft × 24.2 ft ≈ 290 ft²

Living area: ~12 × 24 ft (≈ 290 ft² floor), 8 ft ceiling, window band fore/aft + sides.
Legs: NACA-section vertical foils, 10 ft chord (fore-aft) × 2 ft thick × 19 ft tall, 9.5 ft immersed.
Waterplane area: only 2 ft × 10 ft = 20 ft² per leg → 60 ft² total. This tiny waterplane is what gives SWATH-style stability.

2 · Solar Array

Surface	Dimensions	Area
Roof of living area	12 × 24.2 ft	291 ft²
Left fold-down wing (up & level)	8 × 24.2 ft	194 ft²
Right fold-down wing (up & level)	8 × 24.2 ft	194 ft²
Total collector area		≈ 680 ft² (63 m²)

Installed wattage: premium marine panels deliver ~200–210 W/m² → 63 m² × 205 ≈ 12.9 kW ≈ 13 kW installed. Realistic Caribbean yield after heat/wiring/MPPT losses: ~55–60 kWh/day.

3 · Structure Weight & Reserve Buoyancy

Component	Basis	Weight (lb)
Triangle frame + deck support grid	~180 ft of 8×8×¼" box beam + joints	2,500
Living-area shell (walls, roof, floor, glazing)	⅛" plate, ~580 ft² walls + 290 ft² roof + floor	3,000
3 foil legs (sealed, framed)	~420 ft² skin each + internal framing	3,600
Railing frame + catamaran netting	120 ft perimeter rail + net	2,000
Ladders, davit, hardware, misc.		900
Estimated bare structure		≈ 12,000 lb

Buoyancy budget

NACA section area ≈ 0.685 × chord × thickness = 0.685 × 10 × 2 = 13.7 ft²
Immersed volume/leg = 13.7 × 9.5 = 130 ft³ → 3 legs = 390 ft³
Buoyancy @ 64 lb/ft³ (seawater) = 390 × 64 ≈ 25,000 lb

Reserve buoyancy after structure ≈ 25,000 − 12,000 = 13,000 lb for batteries (4,000), thrusters (~900), stabilizers (~285), RIB + motor (~1,600), interior/systems/people/water/gear.
Each additional inch of immersion adds ≈ 219 lb of support (2,630 lb per full foot, all 3 legs).

4 · Active "Airplane" Stabilizers

4.1 Buoyancy per foot of immersion

ΔB per leg per ft = 13.7 ft² × 64 lb/ft³ ≈ 880 lb (all 3 legs: ≈ 2,630 lb per ft)

4.2 Does cutting 1 ft off crest + 1 ft off trough halve a 4-ft wave?

Yes. Felt height = wave height − (crest cut + trough cut) = 4 − 1 − 1 = ~2 ft. And because the SWATH base already passes only a fraction of the wave through, the residual motion can be reduced even further (Section 8).

4.3 Foil sizing to generate ±880 lb at 5 knots

Dynamic pressure q = ½ρV² = ½ × 1.99 × (8.47 ft/s)² ≈ 71 psf
Required: L = q × CL × S → S = 880 / (71 × CL)
CL ≈ 0.7 (flapped section, mean operating) → S ≈ 17.6 ft²

Recommended stabilizer wing: ~16–18 ft² — e.g., 8 ft span × 2.2 ft chord, flapped, mounted on the thin trailing-edge notch with the small tail/actuator setting angle of attack. At 4 kn it has ~0.8× the authority; at 6 kn ~1.4× (practically capped ~1.2).

4.4 Weight & cost (batch of 20, China)

Item	Weight (lb)	Cost (USD, ea @ qty 20)
Main wing (hollow 5083 aluminum)	40	$500
Tail surfaces + pivot bracket/notch fitting	40	$450
Marine linear actuator + feedback	10	$150
Anodizing, seals, wiring, QC, shipping share	5	$300
Per stabilizer "airplane"	≈ 95 lb	≈ $1,100 (range $900–1,400)

2 · Propulsion Power (legs + stabilizer drag)

Wetted surface ≈ 630 ft² (three 9.5-ft-immersed foils). Friction + form + small wave-making, driven through rim thrusters at ~65% overall efficiency. Power fits P ≈ 0.0234 × V³ kW very well.

Speed	Hull resistance	Propulsion power (electric)	+ Active stabilizers (avg, in waves)	Total with stabilizers
4 kn	~146 lb	1.5 kW	+0.3 kW	1.8 kW
5 kn	~218 lb	2.9 kW	+0.6 kW	3.5 kW
6 kn	~311 lb	4.9 kW	+0.9 kW	5.8 kW

Stabilizer induced drag: generating ±880 lb with an 8-ft-span wing costs ~60–65 lb of drag at peak — but it only works hard part of the time, hence the modest averages shown.

3 · Battery Endurance — 4,000 lb LiFePO₄

4,000 lb ≈ 1,814 kg × ~100 Wh/kg pack-level ≈ 175 kWh nominal / ~158 kWh usable (90% DoD).

Propulsion only

Speed	Draw	Hours	Nautical miles
4 kn	1.5 kW	~105 h	~420 nm
5 kn	2.9 kW	~55 h	~272 nm
6 kn	4.9 kW	~32 h	~193 nm

With stabilizers active

Speed	Draw	Hours	Nautical miles
4 kn	1.8 kW	~88 h	~350 nm
5 kn	3.5 kW	~45 h	~226 nm
6 kn	5.8 kW	~27 h	~163 nm

4 · Stabilizer Wave-Cutting Authority vs Speed

Speed	Cut per crest/trough	Peak-to-trough removed	Example: 4-ft wave feels like…
4 kn	~0.8 ft	~1.6 ft	~2.4 ft (before SWATH benefit)
5 kn	~1.0 ft	~2.0 ft	~2.0 ft (before SWATH benefit)
6 kn	~1.2 ft	~2.4 ft	~1.6 ft (before SWATH benefit)

5 · Recharging from the Sun (Caribbean)

13 kW × ~5.5 peak-sun-hours × ~0.80 system derate ≈ 57 kWh/day
Refill 175 kWh ÷ 57 kWh/day ≈ 3.1 days

≈ 3 average Caribbean days of sun to fully recharge the bank (2–2.5 in strong clear conditions, 4+ in unsettled weather).

6 · Actual Motion Estimates (SWATH base + active stabilizers)

Natural heave period ≈ 3.1 s (stiffness = 64 lb/ft³ × 60 ft² waterplane ≈ 3,840 lb/ft against ~30,000 lb total mass). Passive SWATH already transmits only a fraction of the wave; the stabilizers then attack that residual. Values below are estimated felt vertical motion (peak-to-trough) in typical trade-wind seas — final numbers deserve VPP/seakeeping simulation or tank testing.

Sea state	Passive (no stabilizers)	Active @ 4 kn	Active @ 5 kn	Active @ 6 kn
3-ft waves	~1.7 ft	~1.0 ft	~0.7 ft	~0.5 ft
4-ft waves	~2.2 ft	~1.5 ft	~1.0 ft	~0.8 ft
5-ft waves	~2.8 ft	~2.1 ft	~1.5 ft	~1.2 ft

Why stabilizers don't need to fight the full wave: cancelling the passive residual of a 4-ft/5-s wave requires F = |k − mω²| × amplitude ≈ 2,400 lb/ft × ~1.1 ft ≈ 2,600 lb — almost exactly the ±2,640 lb the three stabilizers can deliver. The concept closes nicely.

7 · 24/7 Ocean Crossing on Solar (1 kW hotel load)

Available for propulsion = 57 kWh/day − (1 kW × 24 h) = 33 kWh/day → avg 1.37 kW
Solve 0.0234·V³ = 1.37 → V ≈ 3.9 kn (no stabilizers)
With stabilizers (+~0.4 kW avg): 0.97 kW → V ≈ 3.5 kn

Sustainable 24/7 speed: ≈ 3.9 knots with stabilizers off, ≈ 3.5 knots with them on. The stabilizers buy a lot of comfort for ~0.4 kn — and the battery bank buffers cloudy stretches. Trade-off is yours to make day by day.

8 · Full Seastead Cost Estimates (built in China)

RIB tender and outboard not included (~$15k extra if desired).

Line item	1 unit (USD)	Batch of 20 (USD each)
Engineering, tooling, amortized design	25,000	8,000
Marine aluminum structure (12,000 lb fabricated)	120,000	100,000
Glazing / windows	12,000	10,000
Solar 13 kW + MPPT + mounting	10,000	8,500
LiFePO₄ 175 kWh battery bank	22,000	19,000
6 × rim-drive thrusters	15,000	13,000
3 × active stabilizer "airplanes"	4,500	3,600
Davit / crane	4,000	3,500
Electrical, plumbing, controls	14,000	12,000
Interior fit-out (basic)	18,000	15,000
Paint/anodize, assembly labor, QA	15,000	12,000
Shipping & delivery	8,000	6,000
Total per vessel	≈ $267,500	≈ $210,600

Call it ~$270k for one, ~$210k each at a batch of 20 (roughly 20% savings from shared tooling, bulk aluminum/cells, and learning-curve labor).

Assumptions & Caveats

Seawater 64 lb/ft³; NACA section area coefficient 0.685; ITTC friction line; form factor ~1.4; thruster train ~65% efficient.
LiFePO₄ pack energy ~100 Wh/kg; 90% usable depth of discharge.
Solar yield assumes flat-mounted panels, ~5.5 peak sun hours, 0.80 derate — typical of the Caribbean basin.
Motion figures are engineering estimates for trade-wind wind-sea/swell mixes; a proper seakeeping run (or tank test) should precede final stabilizer sizing.
Costs are order-of-magnitude 2020s China quotes for marine-grade (5083/6061) work, excluding duties and destination fees.