| Item | Assumption |
|---|---|
| Seawater density | ~64 lb/ft³ (≈1025 kg/m³) |
| Leg cylinders (buoyancy) | 4 cylinders, diameter 3.9 ft, submerged length (your stated) 12 ft each |
| Leg total length | 24 ft each, oriented ~45° down/out |
| Wind drag coefficients | Blunt-ish body: Cd ≈ 1.0–1.2 |
| Solar panel power density | ~180–220 W/m² installed nameplate (panels only), then derate for temperature/wiring/shading |
| LiFePO₄ pack energy density | ~120–160 Wh/kg (pack-level, incl. structure/BMS/cables). I use 140 Wh/kg typical. |
| Thruster claim you provided | ~2090 N thrust at 3 kW per mixer (≈697 N/kW). (This is high but plausible for “bollard pull” style mixing; verify in seawater.) |
Cross-sectional area per leg:
A = π (d/2)² = π (1.95 ft)² ≈ 11.94 ft²
Submerged volume per leg:
V_leg = A · L = 11.94 · 12 ≈ 143.3 ft³
Total submerged volume (4 legs):
V_total ≈ 573.3 ft³
Displacement (saltwater):
W_disp ≈ 573.3 · 64 ≈ 36,700 lb (≈16.6 metric tons)
Buoyancy per foot of submerged cylinder (one leg):
11.94 ft² · 64 ≈ 764 lb/ft.
All 4 legs together: ~3056 lb/ft (per additional submerged foot across all legs).
| Total target weight | Required total volume | Required submerged length per leg |
|---|---|---|
| 40,000 lb | 625 ft³ | ~13.1 ft |
| 50,000 lb | 781 ft³ | ~16.4 ft |
| 60,000 lb | 938 ft³ | ~19.6 ft |
So you have a practical “knob” to turn: increase submerged length (by geometry/trim) or increase leg length (if you keep the same diameter).
Surface area per leg (24 ft long cylinder + two ends), rough:
circumference πd ≈ 12.25 ft; times 24 ft → ~294 ft²2 · πr² ≈ 2 · 11.94 = 23.9 ft²~318 ft² (≈29.5 m²)| Option | Thickness assumption (your spec) | Approx mass per leg | Approx mass (4 legs) | Net buoyancy impact |
|---|---|---|---|---|
| Duplex 2205 |
Sides: 1/4" (6.35 mm) Ends: 1/2" (12.7 mm) |
~3,450 lb | ~13,800 lb | Heavier legs reduce payload by ~4,200 lb vs Al option (see below). |
| Marine Al (e.g., 5083/5086) |
Sides: 1/2" (12.7 mm) Ends: 1" (25.4 mm) |
~2,400 lb | ~9,600 lb | Lighter. More payload capacity for same displacement. |
These weights are “plate-only” style estimates; real fabricated weights can move ±15–30% depending on stiffeners, weld prep, internal frames, hatches, pads, etc.
| Dimension | Duplex 2205 legs | Marine aluminum legs |
|---|---|---|
| Raw material cost | Higher ($/kg) and harder fabrication (welding procedures matter a lot). | Moderate. Plate is cheaper; welding is common, but still needs marine-qualified procedures. |
| Fabrication cost | High: duplex requires good control of heat input, pickling/passivation, crevice-avoidant details. QA/QC needs to be strong. | Moderate-to-high: thick Al plate, long welds, distortion control, and NDT. Needs anode system and good isolation from dissimilar metals. |
| Corrosion risk in seawater | Very good if you avoid crevices/stagnant seawater and properly finish/passivate. Susceptible to crevice corrosion in bad details. | Good in open seawater, but pitting/crevice in stagnant zones + strong galvanic sensitivity. Must design anodes, coatings, and isolation carefully. |
| Fatigue | Duplex is strong; fatigue still governed by weld detail categories and stress ranges. | Al fatigue can be limiting; require generous radii, avoid hard points, and keep stress ranges low. |
| Expected service life | ~25–40+ years plausible with excellent details + maintenance + cleaning. | ~15–30 years plausible; depends strongly on anode maintenance, coating, and avoiding dissimilar-metal mistakes. |
| Maintenance burden | Lower corrosion maintenance, but still needs inspection for crevice + fatigue cracks. | Higher: anodes, paint systems (if used), and stricter isolation discipline. |
πr² = 11.94 ft² = 1718 in².
At 10 psi, axial load on each end ≈ 17,000 lbf.
That drives end-cap thickness, weld size, and (most importantly) catastrophic energy release risk if a hatch/fastener fails.
If the goal is leak detection + keeping water out, you can often get most of the benefit with:
If you do pressurize at all, treat the leg as a pressure vessel: relief valves, burst analysis, safe maintenance procedures, and “no single hatch failure can explosively decompress”.
Yes—if a leg unloads in a wave trough and a cable goes slack, re-tension can create shock loading. Design approaches to reduce this:
| Component | Routine checks | Detailed inspection | Typical replacement |
|---|---|---|---|
| Jacketed Dyneema | Monthly: chafe, jacket cuts, UV damage, termination slippage, creep/tension drift. | Every 6–12 months: remove/inspect terminations; measure diameter loss at wear points. | ~3–7 years depending on UV/chafe/load spectrum (earlier if any core damage is suspected). |
| Stainless/duplex wire/rod | Monthly: broken wires, rust staining at sockets, movement/fretting. | Annually: NDT at high-stress terminations, check for crevice corrosion under covers. | ~7–15 years depending on design detail and environment. |
You described roof + 3 sides, with swing-out side arrays ~6 ft each side. A reasonable first-pass is:
40×16 = 640 ft² (≈59.5 m²)2 × (40×6) = 480 ft² (≈44.6 m²)16×9 = 144 ft² (≈13.4 m²)1,264 ft² (≈117.5 m²)At ~200 W/m² panel nameplate: ~23.5 kWp possible. After practical packing gaps, hatches, curvature, shading, and “marine mounting reality”:
Caribbean “peak sun hours” might be ~5–6, but you have mixed angles and some shading. A conservative “delivered DC into charge controllers” estimate:
If you want to store 2 days of average production/usage:
~180 kWh usable180,000 / 140 ≈ 1,285 kg ≈ 2,830 lb90/24 ≈ 3.75 kW average continuous.180/24 ≈ 7.5 kW average for one day (if you were willing to drain it).
You asked: “turn into the wind; drag is the end of a 20 ft diameter cylinder.”
End area: A = π(10 ft)² ≈ 314 ft² (≈29.2 m²).
Using Cd ≈ 1.1:
| Wind speed | m/s | Estimated wind force | Equivalent lbf | Power to match force (using 697 N/kW from your thruster claim) |
|---|---|---|---|---|
| 30 mph | 13.4 | ~3,500 N | ~800 lbf | ~5 kW |
| 40 mph | 17.9 | ~6,300 N | ~1,400 lbf | ~9 kW |
| 50 mph | 22.4 | ~9,900 N | ~2,200 lbf | ~14 kW |
A reasonable efficient “hotel load” estimate (varies wildly with air conditioning behavior):
| Load | Avg W (typical) | Notes |
|---|---|---|
| Starlink (2 units) | 150–250 W | Depends on model, heat mode, usage |
| Fridge + freezer | 80–200 W | Efficient DC units lower |
| Lighting + electronics | 50–200 W | All LED assumed |
| Watermaker(s) | 0–800 W avg | Often run midday; instantaneous could be 1–3 kW each |
| Pumps/controls/ventilation | 50–150 W | |
| Cooking | 0–1,500 W avg | If electric; propane greatly reduces electrical load |
| Air conditioning | 500–2,500 W avg | Dominant load; best to use inverter mini-splits + insulation + shading |
| Total “hotel” average | ~1.5–4.5 kW avg | ≈36–108 kWh/day |
If solar is ~90 kWh/day and hotel loads are ~36–60 kWh/day (typical if you run 1–2 mini-splits part time), then “extra” energy is roughly:
Pure Euler buckling of a 24 ft long, 3.9 ft diameter metal tube is generally not what will fail first. More likely issues:
A rough crossflow drag estimate (submerged portion only) shows that even a few knots of sideways flow does not usually create enormous bending stress in a tube of this diameter; the dynamic wave/current load spectrum and fatigue details matter more than “instant buckling”.
If you pick aluminum legs and an aluminum body, you still need to manage galvanics with thrusters, fasteners, rails, solar frames, etc. If you pick duplex legs, you still must avoid crevices and stagnant zones.
I provide two totals:
| # | Item | Weight (lb) – Option A (Al focus) | Cost USD – first unit | Notes |
|---|---|---|---|---|
| 1 | Legs (4) | ~9,600 | $80k–$140k | Al plate is thick; cost depends heavily on welding + end-cap complexity. |
| 2 | Body shell + internal frame | ~8,000–14,000 | $120k–$220k | Corrugated panels + beams + hard points + coatings + assembly fixtures. |
| 3 | Tensegrity cables + terminations | ~300–900 | $10k–$35k | Dyneema cheaper/lighter; stainless rods/cables pricier. |
| 4 | Motors + controllers | ~400–900 | $25k–$60k | 4 thrusters + controllers + spare is wise. Verify seawater rating, seals, reverse capability. |
| 5 | Propellers | incl. | incl. | Usually part of thruster. If custom props, add $5k–$20k. |
| 6 | Solar panels + mounts | ~1,500–3,500 | $20k–$60k | Panels are cheap; marine mounts, hinges, wiring, lightning protection cost real money. |
| 7 | Solar charge controllers (4) | ~80–200 | $4k–$12k | Multiple MPPTs due to multiple angles/strings is correct. |
| 8 | LiFePO₄ batteries (target 180 kWh usable) | ~3,200–4,000 | $45k–$110k | Large swing by supplier, marine enclosure, fire mitigation, certifications. |
| 9 | Inverters (4) | ~150–350 | $6k–$20k | Consider split-phase needs, surge, redundancy. |
| 10 | 2 watermakers + storage tanks | ~400–1,200 (+water) | $15k–$40k | Stored water weight dominates: 200 gal = ~1,670 lb; 500 gal = ~4,170 lb. |
| 11 | Air conditioning (4 mini-splits) | ~250–600 | $8k–$25k | Plus ducting/condensate management/corrosion protection. |
| 12 | Insulation | ~800–2,500 | $8k–$25k | Closed-cell foam panels/spray + thermal breaks are important. |
| 13 | Interior (flooring/cabinets/baths/furniture) | ~3,000–10,000 | $40k–$200k | Big swing. “Yacht finish” explodes cost/weight. |
| 14 | Waste tanks | ~200–1,200 (+contents) | $2k–$10k | Consider composting to reduce blackwater tank size. |
| 15 | Glass + glass doors (ends) | ~500–2,000 | $10k–$60k | Laminated/tempered, storm shutters recommended. |
| 16 | Refrigerator | ~150–300 | $1k–$4k | |
| 17 | Biofouling weight gain (1st year) | ~800–2,000 | $2k–$15k | Depends on coatings + cleaning interval. Not “initial weight” but affects displacement/power over time. |
| 18 | Safety equipment | ~150–600 | $5k–$25k | EPIRB, AIS, liferaft, PLBs, firefighting, medical, flares, MOB gear. |
| 19 | Dinghy + outboard | ~250–600 | $6k–$20k | |
| 20 | 2 sea anchors | ~40–150 | $1k–$6k | Plus strong attachment points + chafe gear. |
| 21 | Kite propulsion system | ~50–300 | $2k–$20k | Engineering/control is nontrivial; treat as experimental at first. |
| 22 | Air bags inside legs (32) | ~200–600 | $3k–$15k | Ensure materials survive marine heat, abrasion, mold, and long-term aging. |
| 23 | 2× Starlink (hardware only) | ~20–60 | $1k–$2k | Service plan not included. |
| 24 | Trash compactor | ~80–200 | $500–$2k | |
| 25 | Davit/crane/winch (x2) | ~300–1,200 | $5k–$30k | Marine-rated, stainless hardware costs add up. |
| 26 | Everything else to finish | ~1,500–6,000 | $25k–$120k | Plumbing, wiring, switchgear, lightning protection, coatings, anodes, sensors, pumps, spares. |
| Scenario | All-up build weight (excluding stored water & consumables) | Cost – first unit | Cost each if ordering 20 (rough learning curve) |
|---|---|---|---|
| Option A: Aluminum-focused | ~28,000–40,000 lb | $450k–$900k | $350k–$700k |
| Option B: Duplex legs (heavier) + higher QA | ~32,000–46,000 lb | $600k–$1.2M | $480k–$950k |
If the front buoyancy points and back buoyancy points are separated by roughly:
A simplistic “worst-case” when a wave crest is under the front and a trough under the back: pitch angle ≈ arctan(H / 57). The vertical difference between the ends of the 40 ft body would be ~tan(angle)·40.
| Wave height H | Approx pitch angle | Approx end-to-end height difference along 40 ft body |
|---|---|---|
| 3 ft | ~3° | ~2.1 ft |
| 5 ft | ~5° | ~3.5 ft |
| 7 ft | ~7° | ~4.9 ft |
With four legs angled out, your effective waterline beam could be on the order of ~30+ ft, which usually implies large initial stability. Pure “wind-only capsize” is unlikely until very high winds, but damage cases matter:
A crude estimate suggests capsize in intact condition might require something like 80–120+ mph beam winds, but this is not a design basis. The real design basis should be:
Bad cases to worry about:
Typical drift rates with a properly sized sea anchor are often on the order of ~1–3% of true wind speed (varies a lot by design). Example: 40 kt wind → 0.4–1.2 kt drift (≈10–30 nautical miles per day). Storm duration in Caribbean winter gales can be ~1–3 days; some systems linger longer.
Forecasting: You usually get days of warning for organized systems, but squall lines can be fast. Your low speed means you must design to ride out a meaningful subset of storms, not “run away”.
40×16 = 640 ft² gross. A comparable “inside” feel is often in the 60–80 ft catamaran class depending on layout.Rental payback: at $1,000/day gross, that’s ~$7,000/week gross. If build cost is $600k:
600,000 / 7,000 ≈ 86 weeks (before maintenance, crew, insurance, downtime, marketing, permitting).| Summary item | Estimate |
|---|---|
| 1) Estimated total cost (first unit) and cost each if ordered 20 |
First unit: $450k–$1.2M (depends mainly on metal choice, QA/QC, interior finish, and “marine-grade” requirements). Order of 20: $350k–$950k each (learning curve + tooling + standardized interior). |
| 2) Average solar produced; average solar used (no propulsion); average power left for propulsion |
Average solar produced: ~70–110 kWh/day (planning ~90 kWh/day). Hotel loads (no propulsion): ~36–60 kWh/day typical if AC is managed (can be 100+ kWh/day if AC runs hard). Left for propulsion (average): ~30–50 kWh/day typical → ~1.25–2.1 kW average continuous equivalent (higher in bursts using batteries). |
| 3) Lbs extra buoyancy for customers and their personal stuff |
With your stated displacement at 12 ft submerged each leg: ~36,700 lb total displacement. A plausible “light build” might come in around 28,000–34,000 lb before guests/consumables. That implies payload margin roughly ~2,000–8,000 lb depending on final build weight and how much water/stores you carry. If you want consistently larger payload, plan on more submerged length (e.g., 14–18 ft each leg) or longer legs. |