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Seastead Concept – Displacement, Materials, Space, and Low-Speed Propulsion Estimates
Estimates for 3-Leg Tensegrity Seastead Concept
Important note on uncertainty: The numbers below are “first-pass” naval-architecture estimates to help compare options.
Real results can differ significantly due to: drag of the above-water structure, wave conditions, thruster/prop efficiency at forward speed,
biofouling, appendages, leg angle vs direction of travel, and structural details (stiffeners/bulkheads).
Assumptions Used (so you can edit later)
- Leg outer diameter D = 3.9 ft (1.189 m). Radius r = 1.95 ft (0.594 m).
- Leg overall length = 30 ft; “2/3 in the water” interpreted as 20 ft submerged length of the cylindrical portion (as you stated).
- Ends are treated as hemispherical “dished ends” for mass estimate (close enough for comparison; real heads can be ellipsoidal/torispherical).
- Material densities: Duplex stainless ≈ 7800 kg/m³; marine aluminum (5083/5086 class) ≈ 2700 kg/m³.
- Salt water density for buoyancy: 1025 kg/m³ (≈64 lb/ft³).
- Propulsion speed estimate uses a simple drag model
D = 0.5 * ρ * (CdA) * v², with power to drag P = D*v.
- Overall electric-to-effective-propulsive efficiency assumed η = 0.50 (very uncertain; sewage mixers used as thrusters may be lower).
1) Total Displacement (from your stated submerged cylinder volume)
Per leg (submerged cylinder only)
- Submerged length: L = 20 ft
- Volume:
V = π r² L = π × (1.95 ft)² × 20 ft = 238.8 ft³
All 3 legs
- Total displaced volume: 716.4 ft³ = 20.29 m³
- Buoyant force (salt water): 20.29 m³ × 1025 kg/m³ = 20,800 kg ≈ 20.8 metric tons
- Equivalent weight support: 716.4 ft³ × 64 lb/ft³ = 45,850 lb
Note: This is just the displacement implied by “3 cylinders submerged 20 ft each.” If the legs are angled, and/or if end-caps/balls are submerged,
the real displaced volume at the waterline for a given draft can differ.
2) Leg Material Choice: Duplex Stainless (2205) vs Marine Aluminum
Shell thicknesses you proposed
- Duplex stainless: side shell 1/4" (6.35 mm), ends 1/2" (12.7 mm)
- Marine aluminum: side shell 1/2" (12.7 mm), ends 1" (25.4 mm)
Approximate leg mass (shell + 2 hemispherical ends; no internal stiffeners)
| Item |
Duplex 2205 (1/4" sides, 1/2" ends) |
Marine Al (1/2" sides, 1" ends) |
| Side area (30 ft cylinder) |
~34.15 m² |
~34.15 m² |
| Side shell volume |
34.15 m² × 0.00635 m = 0.217 m³ |
34.15 m² × 0.0127 m = 0.433 m³ |
| Side shell mass |
0.217 m³ × 7800 = 1690 kg |
0.433 m³ × 2700 = 1170 kg |
| Ends area (2 hemispheres) |
~4.436 m² |
| Ends shell volume |
4.436 m² × 0.0127 m = 0.056 m³ |
4.436 m² × 0.0254 m = 0.113 m³ |
| Ends shell mass |
0.056 m³ × 7800 = 440 kg |
0.113 m³ × 2700 = 305 kg |
| Total per leg (estimate) |
~2130 kg (≈ 4700 lb) |
~1475 kg (≈ 3250 lb) |
| Total for 3 legs |
~6400 kg (≈ 14,100 lb) |
~4425 kg (≈ 9,760 lb) |
Real legs typically also include: bulkheads, ring frames, local reinforcement at cable/prop mounts, manholes, anodes, and coating systems.
Those additions can be material-significant.
Cost and life expectancy (qualitative + rough order-of-magnitude)
| Factor |
Duplex Stainless (2205) |
Marine Aluminum (5083/5086-class) |
| Relative weight (for your thicknesses) |
Heavier overall in this design (ends + density). Helps lower CG but consumes displacement. |
Lighter overall; leaves more payload for structure, stores, batteries, water, etc. |
| Material cost (plate) |
Typically high. Rough ballpark: $12–$20/kg (varies strongly by market and thickness). |
Typically moderate. Rough ballpark: $5–$10/kg (varies by alloy, thickness, market). |
| Fabrication cost drivers |
Harder cutting/forming; welding procedures and consumables costlier; distortion control; passivation/pickling sometimes needed. |
Welding large thick plate needs skill; heat input/distortion; but generally common in marine yards. Avoid copper/steel contamination. |
| Corrosion behavior in seawater |
Very good general corrosion resistance; watch crevice corrosion, MIC, and galvanic coupling (esp. with aluminum parts). |
Good when properly designed; vulnerable to pitting/crevice corrosion and galvanic corrosion. Needs careful isolation from stainless/bronze. |
| Coatings / cathodic protection |
Often still uses coatings + CP in real marine service to control crevices and stray current risks. |
Commonly uses coatings + sacrificial anodes; design must avoid trapped seawater crevices and dissimilar metals. |
| Life expectancy (practical) |
Potentially 20–40+ years in seawater with good crevice management and CP; but failures can be severe if crevices/stray current are ignored. |
Potentially 15–30+ years with good detailing, anodes, coating upkeep, and galvanic isolation; can suffer faster local attack if poorly detailed. |
| Repairability at sea / remote |
Weld repair is doable but harder (procedures, consumables). Some repairs may need better tooling. |
Aluminum weld repair is common, but needs cleanliness and correct process. Easier to “patch” in many yards. |
Rule of thumb cost multiplier: For one-off marine structures, “fully fabricated + QA + coatings + fittings” can easily be
2× to 5× the raw plate cost, depending on complexity and labor rates.
3) Usable Living Space (7 ft headroom or more)
Base is an equilateral triangle, side 60 ft. Pyramid height 25 ft.
Three floors at approximately z = 0–8 ft, 8–16 ft, 16–25 ft.
- Base area
A0 = (sqrt(3)/4) * s² = 0.4330127 × 60² = 1559 ft²
- For a pyramid, cross-sections scale as
(1 - z/H)²
Floor plate areas (regardless of headroom)
| Floor (at height z) |
Cross-section scale |
Approx floor area |
| Floor 1 (z = 0 ft) |
(1 - 0/25)² = 1.000 |
1559 ft² |
| Floor 2 (z = 8 ft) |
(1 - 8/25)² = 0.4624 |
~720 ft² |
| Floor 3 (z = 16 ft) |
(1 - 16/25)² = 0.1296 |
~202 ft² |
| Total floor plate area |
|
~2481 ft² |
Area with ≥ 7 ft headroom (approximate)
For each floor at elevation zf, the region with headroom ≥ 7 ft is similar to the base cross-section at height zf + 7.
Usable area ≈ A0 * (1 - (zf+7)/H)² (if zf+7 ≤ H).
| Floor |
zf (ft) |
Scale factor (1 - (zf+7)/25)² |
Usable area ≥ 7 ft headroom |
| Floor 1 |
0 |
(1 - 7/25)² = 0.5184 |
~808 ft² |
| Floor 2 |
8 |
(1 - 15/25)² = 0.16 |
~249 ft² |
| Floor 3 |
16 |
(1 - 23/25)² = 0.0064 |
~10 ft² |
| Total ≥ 7 ft headroom |
|
|
~1067 ft² |
This ignores interior walls, stairs, structural members, and any “flat ceiling” areas you might introduce.
If you add dormers/vertical walls, you can greatly increase high-headroom area on upper floors.
4) “20 ft Column + Ball” Option
Ball diameter to replace 10 ft of the 3.9 ft cylinder (equal volume)
- Volume of 10 ft cylinder section:
V = π r² L = π × (1.95 ft)² × 10 ft = 119.4 ft³
- Set equal to sphere volume
(4/3)πR³:
R³ = (3 V)/(4π) = (3 × 119.4)/(4π) = 28.51 ft³
- Sphere radius: R ≈ 3.06 ft
- Sphere diameter ≈ 6.12 ft (≈ 1.87 m)
5) Low-Speed Propulsion: Estimated Speed for 3–4 kW
Thruster caution: “2090 N thrust at 3000 W” for sewage mixers is usually a static mixing thrust in a very different flow regime.
Forward-speed thrust can be substantially lower, and seals/bearings may not be designed for continuous open-ocean duty.
Drag model used (comparison-level only)
- Use an “equivalent” CdA for the submerged legs. (Does not include the above-water structure, cables, platform edges, etc.)
- Legs are at ~45° to horizontal, so projected area is reduced versus a fully side-on cylinder; actual depends on travel direction relative to the triangle.
- Assumed:
- Cylinder-like portions: Cd ≈ 1.0 (bluff body)
- Sphere: Cd ≈ 0.47
Equivalent CdA (submerged parts only; approximate)
| Design |
Submerged geometry assumption |
Estimated total CdA (all 3 legs) |
| A) 30 ft column legs |
Each leg has ~20 ft submerged cylindrical length contributing drag |
~17.7 m² |
| B) 20 ft column + ball |
Assume the bottom 10 ft of the previously-submerged column is replaced by the sphere |
~11.6 m² |
Speed estimates (assuming η = 0.50 overall efficiency)
Effective power to overcome drag: P_effective = η * P_electric.
Solve P = 0.5 ρ CdA v³ for v.
| Total electric to propulsion |
Design A speed (30 ft columns) |
Design B speed (20 ft + ball) |
| 3,000 W (effective ≈ 1,500 W) |
~1.23 mph (0.55 m/s) |
~1.41 mph (0.63 m/s) |
| 4,000 W (effective ≈ 2,000 W) |
~1.36 mph (0.61 m/s) |
~1.55 mph (0.69 m/s) |
If you actually run four 3 kW units (12 kW total electric)
| Total electric to propulsion |
Design A speed |
Design B speed |
| 12,000 W (effective ≈ 6,000 W) |
~1.95 mph (0.87 m/s) |
~2.25 mph (1.00 m/s) |
| 16,000 W (effective ≈ 8,000 W) |
~2.15 mph (0.96 m/s) |
~2.46 mph (1.10 m/s) |
In real ocean conditions, wind/waves can add large resistive forces; your “achievable” average speed might be lower,
but your target of ~0.5–1 mph looks plausible at modest power if the platform is not excessively windage-heavy and if biofouling is controlled.
6) Rough Cost Comparison for the Legs (very approximate)
Raw material mass basis (from Section 2)
- Duplex: ~6.4 t total for 3 legs (shells only)
- Aluminum: ~4.4 t total for 3 legs (shells only)
Very rough cost ranges
| Cost item |
Duplex 2205 |
Marine Aluminum |
| Raw plate material (order of magnitude) |
6,400 kg × ($12–$20/kg) = $77k–$128k |
4,400 kg × ($5–$10/kg) = $22k–$44k |
| Fabricated leg assemblies (typical one-off multiplier) |
~$150k–$500k for all 3 legs (depends heavily on labor rates, QA/NDT, head forming, fittings) |
~$80k–$300k for all 3 legs (depends heavily on welding, distortion control, QA) |
| Effect of “ball” option on cost |
Likely small-to-moderate change in shell mass (sphere surface area is similar to the removed 10 ft cylinder area).
However, forming/fabricating a sphere may increase labor vs simple cylindrical shell + standard dished heads.
|
7) Other Helpful Observations (Design A vs Design B)
- Drag: The ball option likely reduces drag somewhat at low speed (as estimated), but the benefit depends on how much of the original submerged
cylinder length is actually replaced in the submerged region and on travel direction vs leg orientation.
- Draft: Replacing 10 ft of column with a ~6.1 ft diameter sphere likely shortens the deepest extent of the leg (reduced draft), but the exact draft
depends on how the leg is attached and the waterline position under load.
- Heave / motion comfort: A small waterplane area (your 3 narrow legs) can reduce wave-force coupling, but it can also produce
low hydrostatic restoring in heave/pitch/roll unless damping is strong. The sphere can increase added mass/damping at the lower end, which may help.
- Structural loads: The cable tensions and leg bending moments in waves can be large. The bottom “ball” also becomes a high-leverage attachment point
for cables and can see big slamming/impact loads from steep waves and debris.
- Propulsors: Differential thrust can work well for yaw control given wide spacing. But consider:
(a) debris/entanglement protection,
(b) maintainability (seal changes),
(c) corrosion and galvanic isolation,
(d) actual propulsive efficiency at 0.5–2 mph.
If you want, I can refine this with 5 clarifying inputs
- Is the 3–4 kW you asked about the total propulsion power, or per thruster?
- What is the intended direction of travel relative to the triangular planform (one corner forward, or one side forward)?
- Are the legs sealed buoyancy tanks with air, or flooded + foam, or something else?
- Do you expect the legs to be smooth (painted/antifouled), or bare metal?
- Any estimate of above-water projected area (for wind drag), and the mass of the pyramid + solar + stores?
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