Wing-Spar Seastead — Minimal Viable Product Estimate

39 ft wing-spar · 10 ft chord · 5 ft thick · 8× RIM-drive thrusters · deep suspended ballast · duplex stainless steel · Caribbean service

~60,000 lb

Displacement

~15,500 lb

Structure weight

~12 kW

Solar peak

~50 kWh/day

Avg Caribbean yield

~2.1 kW

Avg available watts

~3 mph

Est. cruise speed

~$200k

China build cost

1 · Displacement

The spar cross-section is modeled as an ellipse (chord 10 ft × thickness 5 ft = 39.3 ft²), extruded 39 ft, with tapered tips (≈88% mean section). Normal operation: 70% immersed.

Item	Value
Immersed length (70% × 39 ft)	27.3 ft
Mean immersed cross-section (with tip taper)	≈34.6 ft²
Immersed volume	≈945 ft³
Seawater density	64 lb/ft³
Design displacement	≈60,000 lb (27 t)

Freeboard: 11.7 ft of spar above waterline, plus the 20×20 ft porch above that.

2 · Structure Weight (Duplex Stainless) & Weight Budget

Duplex plate ≈ 490 lb/ft³. Shell assumed 3/16″ (7.7 lb/ft²); internals lighter gauge.

Component	Basis	Weight (lb)
Spar shell	≈945 ft² surface × 7.7 lb/ft²	7,300
Spar internals (5 floors, bulkheads, frames)	Floors + longitudinal framing	3,200
Ladders, hatches, fittings	External + internal ladders, hatch	500
Spar subtotal		11,000
Porch deck + framing	400 ft² @ ~5 lb/ft²	2,000
Railing (80 lin ft), supports, folding arms		2,000
Porch subtotal		4,000
Cable, fairings, ballast hardware	~100 ft heavy SS cable + terminations	500
Total structure		≈15,500 lb

Full weight budget (must equal displacement)

Category	Weight (lb)
Structure (above)	15,500
8× RIM-drive thrusters (@ ~200 lb ea.)	1,600
Battery bank + inverters + power electronics	6,000
Basic interior outfitting	3,000
People, water, provisions	2,500
Internal water ballast (trim tank, floor 1)	7,400
Deep suspended ballast	24,000
Total	60,000 ✔ floats at 70%

Key insight: structure + equipment alone (~28,500 lb) is far less than the 60,000 lb needed to sink the spar to 70%. The remaining ~31,500 lb must be ballast — this is actually good news: it becomes your stability reserve and is tunable.

3 · Solar, Energy & Batteries

Array	Area	Peak power
Porch-deck fixed array (minus hatch/access)	≈360 ft² @ 18 W/ft²	≈6.5 kW
Fold-out wings (8 ft × 20 ft, both sides, flat deployment)	320 ft² @ 17 W/ft²	≈5.5 kW
Total installed solar		≈12 kW

Energy figure	Value
Caribbean average sun-hours (annual)	≈5.5 h/day
Average daily yield (12 kW × 5.5 h × 0.80 derate)	≈50 kWh/day
Seasonal range	≈38–62 kWh/day
Average available watts (50 kWh ÷ 24 h)	≈2,080 W continuous

4-day battery bank

Item	Value
Usable energy (4 × 50 kWh)	200 kWh
Nominal LiFePO₄ capacity @ 90% DoD	≈220 kWh
Cells + pack @ ~120 Wh/kg	≈4,000 lb
Inverters, chargers, BMS, cabling, enclosures	≈1,000 lb
Total battery system weight	≈5,000 lb

All of this sits on floor 1 as planned — it doubles as fixed low ballast, which is exactly where you want it.

4 · Deep Ballast: How Much, Winch vs Fixed, How Long?

Recommended ballast

Deep suspended ballast: 20,000–25,000 lb (encapsulated scrap steel / concrete), on ~40–60 ft of cable below the keel. This is ~35–40% of displacement — enough to make the pendulum dominate the dynamics without making the system unmanageable.
Internal water ballast: 5,000–10,000 lb in floor-1 tanks for fine trim (compensating for provisions, people, solar panel positions).

Longer cable = longer pendulum = slower, gentler angular motion (period ≈ 2π√(L/g): 40 ft → ~7 s, 60 ft → ~8.6 s) and lower angular accelerations. But it also adds drag when underway and depth hazards. ~50 ft is a good MVP sweet spot.

Winch vs fixed cable

	Fixed cable + rotating fairings	Powered winch
Reliability	Excellent — few failure modes	Motor, brake, drum = failure points at sea
Vibration/drag	Good with fairings	Good
Flexibility	None — depth fixed	Raise for transit/shallows/maintenance
MVP verdict	Recommended	Add later; weld a strong hardpoint now

Two details you must not skip:
① Swivel: every turn you make twists the ballast cable. Fit a marine swivel at the ballast, or adopt a "turn left twice as often" discipline.
② Heave resonance: spar + deep ballast gives a heave period around 6.5–7 s — right in the Caribbean swell band. The ballast plate helps as a damper, but during detailed design, tune ballast mass/depth (or waterplane area) to push heave off resonance. This is the single biggest dynamic issue in the concept.

5 · Speed on 60% of Average Power

Propulsion budget: 60% × 2,080 W ≈ 1,250 W to thrusters.

Speed	Hull drag (wetted area ≈660 ft²)	Electric power needed*
2 mph	~150 N	~300 W
3 mph	~330 N	~900 W
4 mph	~570 N	~2,000 W

*assuming ~50% overall propulsive efficiency (RIM drives are good at low speed).

Estimated cruise: ~3 mph (2.5–3.5 mph) in calm conditions. Noon-time surplus can push 4 mph briefly.

Honest limitation: windage dominates. The porch + exposed spar present several hundred ft² of area; a 15-knot breeze exerts more force than your entire thrust budget. Treat this as a maneuverable drifting homestead, not a commuter: easy downwind, slow crosswind, essentially unable to beat upwind above ~10 knots. Plan routes with weather routing.

6 · Motion Control Effectiveness

Differential upper/lower thrust vs pitch 10–20% reduction

Your thrusters can generate perhaps 3,000–5,000 N·m of differential pitching moment. Wave pitching moments on a 39-ft body in 5-ft seas are orders of magnitude larger (~10⁵ N·m). So active pitch control works as a damping and trim layer — taking the edge off, tuning attitude — not as wave cancellation. The deep ballast does the heavy lifting passively: the ~50-ft pendulum has a ~8 s natural period, far slower than 5–8 s waves, so pitch amplitudes stay small naturally.

Turning with the waves vs roll 30–50% reduction

This is genuinely effective. Roll excitation peaks in beam seas; keeping seas on the bow or quarter cuts roll input dramatically, and the deep-ballast pendulum resists what remains. Combined effect vs a naive beam-on drift: expect roughly half the roll. Limitations: you need sea room, reliable swell-direction knowledge, and it helps least in confused/conflicting seas.

7 · Comfort & G-Loads by Level

Estimated peak accelerations (% of g) with active thruster assistance engaged. Heave affects all levels equally; rotational contribution grows with height above the effective pivot (deep ballast).

Sea state	Floor 1	Floor 2	Floor 3	Floor 4	Floor 5	Porch
3 ft waves	0.03 g	0.035 g	0.04 g	0.045 g	0.05 g	0.06 g
5 ft waves	0.05 g	0.06 g	0.07 g	0.08 g	0.09 g	0.12 g
8 ft waves	0.08 g	0.09 g	0.11 g	0.13 g	0.15 g	0.20 g

(For reference: a car braking hard ≈ 0.3–0.4 g; a mild amusement ride ≈ 0.2 g.)

Qualitative comfort

Location	3 ft seas	5 ft seas	8 ft seas
Porch	Pleasant, gentle sway; ideal living space	Lively but usable; deploy curtains, stow loose items	Unsafe — stay off; fold panels, secure everything
Floors 4–5	Very comfortable	Noticeable sway; fine for work/meals	Usable with care; sleep marginal
Floor 3 (near CG)	Excellent — lowest motion	Best working/sleeping floor, as you planned	Acceptable; sleep OK for most people
Floors 1–2	Excellent	Very steady; slight low-frequency surge	Steady but "elevator" heave felt; best storm refuge

Your instinct is correct: floor 2 (just above the battery floor) will feel the gentlest rotational motion, while floor 1 trades a bit of that for being closest to the "anchor point" of the pendulum. Both are good heavy-weather spaces.

8 · Fabrication Cost — China (basic interior)

Line item	Low	High
Engineering & design (amortized)	$10k	$20k
Duplex stainless material (~7,300 kg @ $6–8/kg)	$45k	$60k
Hull/spar fabrication labor (curved duplex welding, ~4,000 hr)	$35k	$50k
Porch fabrication & assembly	$8k	$12k
Passivation, QC, testing	$5k	$8k
8× RIM-drive thrusters	$16k	$28k
Solar 12 kW + mounting	$6k	$10k
Battery 220 kWh LiFePO₄ + BMS	$35k	$55k
Inverters, chargers, switchgear	$4k	$8k
Basic interior (galley, heads, bunks, flooring)	$6k	$10k
Ballast, cable, fairings, swivel	$3k	$5k
Final assembly, trials, container packing	$8k	$14k
Total ex-factory	$181k	$280k

Planning figure: ~$200k–$230k USD (ex-China, before shipping and import duties). Without the battery bank: roughly $140k–$170k.

9 · Verdict & Recommended Changes

Yes — this can work as an MVP, with correctly framed expectations: a stable, comfortable, solar-powered drifting/micro-maneuvering homestead for calm-to-moderate Caribbean conditions, not an ocean-crossing motor yacht. The core ideas (wing-spar form, deep pendulum ballast, no through-hulls, top-only entry, floor-by-floor motion zoning) are sound and mutually reinforcing.

What I would change or add

Tune out heave resonance (§4) — the highest-priority design task.
Swivel on the ballast cable so turning doesn't twist it.
Second emergency egress: single-ladder access is elegant but a fire or blocked hatch below decks is a serious scenario. Add a small emergency escape scuttle per compartment, even if manual.
Backup power: 4 days of batteries is good, but Caribbean cloud stretches and hurricane-season overcast happen. Add a small quiet genset (or accept strict rationing) — even 3–5 kW covers watermaking and refrigeration indefinitely.
Steel choice: duplex everywhere is premium. Consider duplex below the waterline and at splash zone, 316L or coated carbon steel above — saves real money with little durability loss.
Antifouling plan: a slow-moving hull fouls fast. Silicone foul-release coating or ultrasonic antifouling (fits your no-through-hull philosophy).
Thruster guards — entanglement (lines, divers, wildlife) protection.
Lightning protection — you are a grounded metal tower in the sea; bond the spar and add air terminals.
Water: rain-catchment off the porch + a 40–60 gal/day DC watermaker (~1.5–2 kWh/day) closes the loop nicely.
Moor or drift deliberately: decide the station-keeping story early — coastal mooring field, protected anchorage, or free-drift with weather routing.
Build a 1/4-scale model first (~$15–25k) to validate the ballast/pendulum behavior and heave tuning before committing to full scale.