🛟 Wing-Spar Seastead — Minimal Viable Product Estimates

39 ft wing-profile spar · 10 ft chord × 5 ft thick · 70% submerged · 8 RIM-drive thrusters · 5 interior levels · 20×20 porch with 30×30 solar canopy · First-order desktop engineering estimates

Headline Numbers

~52,000 lb displacement (23.6 t)
~9,000 lb marine aluminum
~$40k China fab cost (alu only)
~12.9 kW solar · ~52 kWh/day
~5,000 lb 4-day battery bank
~2.5–3 mph cruise at 60% power

Verdict Snapshot

Mass budget closes?	YES — ~28% ballast margin
Stability (GM)	YES — ~4–6 ft, positive
Container shipping	FITS — tight diagonal, ~19° rotation
Motion comfort	GOOD — ≤0.08 g typical
Viable MVP?	YES — with listed changes

Schematic side elevation (not to scale). Wing cross-section: 10 ft chord × 5 ft thickness.

1 · Displacement Estimate

The submerged body is a 39-ft vertical wing with a ~10 × 5 ft foil cross-section, 70% immersed.

Cross-section area (symmetric foil, area coeff ≈ 0.65): 0.65 × 10 × 5 = 32.5 ft²
Immersed length: 39 × 0.70 = 27.3 ft
Tip-taper allowance: −8%
Volume ≈ 32.5 × 27.3 × 0.92 ≈ 817 ft³
Seawater 64 lb/ft³ → Δ = 817 × 64 ≈ 52,000 lb (23.6 metric tons)

Design displacement: ~50,000–55,000 lb. Freeboard: 11.7 ft of spar + porch above the waterline, so even 8-ft seas stay well below the porch deck.

2 · Marine Aluminum — Weight & China Fabrication Cost

Weight breakdown (5083 alloy)

Item	Basis	Weight (lb)
Spar shell plating	~900 ft² outer surface, mixed 3/16″–1/4″ (avg 2.9 lb/ft²)	2,600
Spar internal structure	Frames @16″, 5 decks, bulkheads, hatches, mounts	2,800
Spar subtotal (one piece)		~5,400
Porch deck + framing	400 ft² plate/beams	1,300
Railing	~80 lin ft @ 6 lb/ft	500
Solar canopy structure	900 ft² @ ~1.4 lb/ft²	1,260
Stairs/ladder, misc.		340
Porch kit subtotal (bolt-together)		~3,400
TOTAL ALUMINUM		~8,800–9,500 lb (≈4.1 t)

Fabrication cost in China

Component	Material	Labor/Fab	Est. Cost
Spar (one-piece weldment, jigged)	$6k	$22–30k	$28–36k
Porch kit (flat-pack, CNC + tack welds)	$5k	$5–8k	$10–13k
Total (ex-works, before shipping)			~$38–49k

Assumes ~$2.8–3.2/kg for 5083 plate and $5–9/kg fabricated welded-aluminum rates. Budget ~$45k ± 25%. The one-piece 39-ft spar needs a large jig and certified weld procedures — that's the cost driver. Container fit check: rotating the wing ~19° off the container axis gives a bounding box of ≈7.8 × 7.8 ft vs. interior 7.7 × 7.8 ft — it fits, but with only inches of clearance; specify cradles and confirm with your actual foil section.

3 · Solar, Daily Energy, Battery Bank, Average Watts

Quantity	Derivation	Value
Solar array	900 ft² canopy ÷ ~22 ft²/panel ≈ 30 × 430 W panels (allow walkway gaps)	~12.9 kWp
Average daily energy (Caribbean)	12.9 kW × 5.5 peak-sun-hrs × 0.80 system derate	~52 kWh/day
4-day battery bank (usable)	52 × 4 = 208 kWh usable → ~230 kWh installed LiFePO₄ (90% DoD)	230 kWh
Battery bank weight	LFP @ ~100 Wh/kg pack-level = 2,300 kg, + inverters/BMS/cabling	~5,000–5,200 lb
Average available power	52,000 Wh ÷ 24 h	~2,150 W continuous

Lead-acid would be ~13,000 lb for the same bank — reject it; LiFePO₄ is mandatory for this design. Seasonal swing: roughly 45 kWh/day in cloudy winter weeks to 60+ in summer.

4 · Does the Mass Budget Close? Is It Stable?

Load table (must equal displacement)

Item	Weight (lb)	Height above keel (ft)	Moment
Aluminum structure	9,000	16	144,000
8 RIM thrusters	1,400	9	12,600
Batteries + inverters	5,100	4	20,400
Solar panels	1,200	40	48,000
Systems (plumbing, HVAC, wiring, galley)	3,500	12	42,000
Outfit & furnishings	2,500	15	37,500
Crew, provisions, water	1,500	15	22,500
Fixed ballast (keel scrap/concrete)	12,000	2	24,000
Water ballast (adjustable trim)	16,000	6	96,000
TOTAL	52,200	KG =	8.6 ft

KB ≈ 0.53 × 27.3 draft = 14.5 ft → B sits 5.9 ft ABOVE G ✔
Waterplane I (roll, about chord axis) ≈ 55 ft⁴ → BM ≈ 0.07 ft
Waterplane I (pitch) ≈ 220 ft⁴ → BM ≈ 0.27 ft
GM(roll) ≈ 6.0 ft · GM(pitch) ≈ 6.2 ft — strongly positive
Natural roll period ≈ 2.0–2.5 s (well away from 6–9 s swell → small roll angles)
Natural heave period ≈ 5.5 s ⚠ (near Caribbean swell band — see §7)

IT WORKS OUT Aluminum is only ~17% of displacement and batteries ~10%, leaving a healthy ~28% for ballast — exactly what a spar needs. Your instinct to put heavy stuff low is correct and sufficient: with the ballast split shown, the vessel is stably upright with a large range of stability, helped by 11.7 ft of freeboard. The 5-ft effective beam means stability comes entirely from the deep CG, so ballast security (bolted, flooded compartments drained only by pump) is a critical safety item.

5 · Speed at 60% of Average Power

Propulsion budget: 2,150 W × 60% ≈ 1,290 W
Drag model: skin friction (S≈61 m², Cf≈0.003) + form drag (frontal area 12.7 m², Cd≈0.045)
D ≈ 405·v² (N) → hull power ≈ 405·v³; ÷ 0.60 propulsive efficiency = input power
1,290 W → v ≈ 1.2 m/s

Cruise ≈ 2.3–2.7 knots ≈ 2.5–3 mph. At 100% of average power: ~3–3.5 mph. Short battery surges might touch 4 mph. Honest framing: this is a weather-routing, station-keeping, slow-repositioning vessel, not a commuter. The upside: at 2.5 kt the wing is extremely efficient per mile, and the same thrusters give you dynamic-positioning authority.

6 · Effectiveness of Pitch & Roll Control

Pitch via upper/lower differential thrust — MODERATE

Differential of ~150–200 lbf between upper and lower thruster pairs on a ~12–15 ft arm yields ~2,000–3,500 ft-lb of couple.
Expect 20–40% pitch reduction in seas ≤5 ft; in 8-ft seas it takes the edge off but cannot fight the wave couple directly.
Caveats: upper units may ventilate in wave troughs (keep them ≥4–5 ft submerged or ducted), differential thrust adds drag, and it eats into propulsion power.
Recommendation: size thrusters 5–8 kW each so control runs at low duty with battery surge headroom.

Turning to combat roll — HIGHLY EFFECTIVE

Keeping the narrow (5 ft) dimension aligned with the wave crests — i.e., bow into the seas — cuts roll excitation by 60–80%. This is your single best motion tool.
Trade-off: you convert roll into pitch, which is why the pitch-control items above matter.
The wing itself is a giant keel: lateral resistance is excellent, so heading holds well once established, and it resists leeway far better than a round spar.
Note: adrift with no power, the body will likely weathervane downwind (following seas). An autopilot holding bow-to-sea needs dedicated thrust margin in windy conditions.

7 · Comfort & Estimated G-Levels by Deck and Sea State

Assumes active heading control + pitch damping working (roll ~5° max, pitch ~5.5° max in 8-ft seas). Without active control, beam-sea roll could be 2–3× worse. Heave is the dominant sensation at every level (natural period ~5.5 s sits near the Caribbean swell band, but the large wetted surface damps it hard).

Location	3-ft seas (T≈6 s)	5-ft seas (T≈7 s)	8-ft seas (T≈8 s)	Feel
L1 — bottom (bunks/heavy weather)	0.04 g	0.05 g	0.06 g	Sleep through almost anything
L2	0.04 g	0.05 g	0.06 g	Sweet spot — office/workshop
L3	0.04 g	0.05 g	0.06 g	Nearly identical to L2
L4	0.04 g	0.05 g	0.06–0.07 g	Slight sway felt
L5 — top of spar	0.04 g	0.05 g	0.06–0.07 g	Best views, most motion
Porch deck	0.04–0.05 g	0.07 g	0.08 g (peaks 0.12)	Exhilarating in 8 ft — hold the rail

3 ft: Coffee stays in the cup on every level. Genuinely comfortable everywhere.
5 ft: Gentle rhythmic bobbing; fine indoors, handrails appreciated on the porch.
8 ft: Noticeable heave (±2.5–3 ft) everywhere; secure loose items, cook with gimbaled precautions, sleep fine on L1–L3. Porch is "sporting." Still far gentler than a monohull of this size.

Your floor-by-floor instinct is validated: accelerations barely differ between levels (rotation radii are small), so choose levels for view vs. quiet, not for G-load — except the porch, which always gets the lever-arm multiplier.

8 · MVP Verdict & Recommended Changes

YES — THIS CAN WORK AS AN MVP The concept is sound: the mass budget closes with generous ballast margin, stability is strongly positive, motions are benign, the power system is self-consistent, and the container-shippable one-piece spar is a genuine manufacturing advantage. It is best positioned as a moored/anchored or slow-drifting homestead, not a transport vessel.

What I'd change or add

Living space reality-check: 5 levels × ~30 ft² = ~150 ft² total interior. Consider 3 roomier levels + a gear level, or embrace "sleeping capsule" minimalism explicitly in marketing.
Detune heave resonance: the ~5.5 s heave period sits near Caribbean swell. Add damping plates / bilge keels near the waterline region, or slightly flare the section near the waterline to add waterplane area.
Storm plan (non-negotiable for the Caribbean): hurricane-grade mooring (submerged plate anchor + nylon snubber), collapsible or storm-rated solar canopy (it's big windage up high), sealed hatches, and pre-storm ballast procedure.
Thruster redundancy & depth: wire as 4 independent quadrant pairs (any 2 can steer); verify upper units stay submerged in troughs or duct them.
Galvanic isolation: RIM-drive rims are often stainless/carbon — isolate from the aluminum hull and add sacrificial anodes; slow speeds mean fouling matters, so specify copper-coat or similar.
Rainwater catchment: the 30×30 canopy is a free 400+ gal/month collector in the Caribbean.
Electrical redundancy: two independent battery strings and inverters; a small DC emergency bus.
Safety kit: AIS, radar reflector, EPIRB, nav lights, and foam reserve buoyancy in the foil tips.

Rough all-in hardware budget (context)

System	Est. USD
Aluminum spar + porch (China fab)	$45,000
8× RIM-drive thrusters + drives	$30,000–50,000
230 kWh LiFePO₄ + inverters	$45,000–65,000
Solar panels, MPPT, mounting	$12,000–16,000
Ballast, plumbing, safety, electronics, outfit	$20,000–30,000
Total hardware (ex-shipping, ex-labor-on-site)	~$155,000–205,000