Engineering review of modular, low-speed, tensegrity seastead design
A structure's displacement dictates how much weight it can carry (structural weight + payload) to float at the designed waterline.
You have proposed two excellent marine-grade options. Because you are factoring in differing thicknesses, the physics of the two choices yield interesting results.
| Factor | Option 1: Duplex St. Steel (2205) (1/4" side, 1/2" ends) |
Option 2: Marine Aluminum (e.g., 5083) (1/2" side, 1" ends) |
|---|---|---|
| Weight per Leg | ~4,200 lbs (Heavier) | ~2,850 lbs (Lighter) |
| Total Leg Weight | ~12,600 lbs | ~8,550 lbs |
| Material Cost | High ($$$). Raw DSS 2205 is expensive, and specialized welding requires inert gas purging. | Moderate ($$). Even at double thickness, raw aluminum is cheaper and easier to cut/weld. |
| Life Expectancy | 50 - 100+ years. Incredible pitting and corrosion resistance. Virtually maintenance-free if isolated from other metals. | 20 - 40 years. Highly susceptible to galvanic corrosion. Requires strict maintenance of sacrificial anodes and anti-fouling coatings. |
| Payload Impact | Eats up ~27% of your total buoyancy budget. | Eats up ~18% of your total buoyancy budget (leaving ~4,000 lbs more for batteries/living space). |
The living space is a 3-sided pyramid on a 60 ft equilateral triangle base, peaking at 25 ft high.
Because the walls slope inward, the floor area shrinks rapidly as you go up. Calculations below represent only the square footage where the ceiling height is 7 feet or higher.
To have 7ft headroom, we measure the area of the triangle at a height of 7 ft.
Usable Area: ~808 sq. ft.
Note: The actual floor footprint is ~1,558 sq ft, leaving lots of edge space (<7ft) for beds, sitting areas, and storage.
Measuring the cross-section at a towering height of 15 ft to guarantee 7ft clearance on this floor.
Usable Area: ~250 sq. ft.
Measuring the cross-section at 23 ft.
Usable Area: ~10 sq. ft.
Best utilized as an observation deck or ventilation/solar hardware hub.
The "banana blade" submersible mixers (2500 mm, 3000W, 2090N thrust) are highly efficient at moving massive amounts of water slowly. Differential thrusting using 2 active units provides ~4180N of thrust total, which is excellent for a slow-moving, high-drag structure.
Design A: 3 x cylinders, 3.9ft diameter, 20ft submerged.
Design B: 3 x (10ft cylinder + 球 Ball on bottom). Let's calculate the ball size.
Calculated using hydrodynamic drag equations (V = √(2 × Force) / (ρ × Cd × Area)), factoring in the 45-degree angle of the legs which creates an elliptical frontal area.
| Power Input / Thrust | Design A (Straight Columns) Drag Coef ~1.1 |
Design B (Column + Ball) Drag Coef ~0.8 |
|---|---|---|
| ~3000W (~2100 Newtons total) | 1.09 MPH (0.49 m/s) | 1.34 MPH (0.60 m/s) |
| ~4000W / 2 Mixers (~2800 N total) | 1.25 MPH (0.56 m/s) | 1.55 MPH (0.69 m/s) |
Conclusion: Both designs easily achieve your 0.5 - 1.0 MPH goal. Design B is roughly 23% faster due to the aerodynamic/hydrodynamic efficiency of a sphere compared to an acutely angled cylinder.
Design B is strictly superior for occupant comfort. Both models have the exact same small waterplane area (since the 3.9ft columns pierce the water surface). However, the 6.1ft spheres in Design B add massive "entrained water mass" (added mass) at the bottom. This acts like a giant dampener, drastically lowering the natural heave frequency. Short, choppy waves will wash past the thin columns without moving the structure vertically.
Design A places the leg bottom at ~14.1 ft deep (20ft string length at 45 degrees). Design B places the bottom at ~10 ft deep (10ft string length at 45 deg + 3ft ball radius). Design B gives you a shallower draft, which is safer for reef passages and shallow anchorages.
While Design B is hydrodynamically superior, it is structurally more expensive to build.
Your structural approach using a rigid top frame, flexible jointed legs, and tensioned cables is highly resilient.