Leg Specifications: 19 ft tall NACA foil (10 ft chord × 3 ft max thickness), ½-inch marine aluminum skin, 50% submerged (9.5 ft draft). Legs act as vertical cantilevers fixed to the overhead triangular truss.
Leg modeled as thin-walled elliptical tube (10 ft chord, 3 ft thickness, 0.5" wall)
Weak-axis bending (lateral to foil chord) — the critical direction for side waves
Leg is a pure cantilever: fixed at top (truss connection), free at bottom
Uniformly distributed lateral load along entire 19 ft length (as requested)
Factor of safety not applied in "break" calculation (real design should use 2.0+)
Stabilizer "airplane" wings add some damping but are ignored for ultimate strength
Maximum Distributed Load
19,950
lb per foot
Total Lateral Force: 379,000 lbs(~190 tons)
Maximum Moment at Base
3.60
million ft-lbs
This is the bending moment at the truss connection when the leg reaches yield stress.
Section Properties (weak axis)
Moment of Inertia (I)
25,920 in⁴
Distance to extreme fiber (c)
18 in
Section Modulus (I/c)
1,440 in³
Final Strength Result
The leg can withstand approximately 379,000 lbs (190 tons) of total sideways force distributed evenly along its 19 ft length before the aluminum yields.
⚠️ This is the theoretical yield point. A proper engineering design would incorporate a safety factor of at least 2.0, reducing the allowable load to ~95 tons per leg.
Estimated Wave Height to Reach Yield
Rough Estimate
18–25+ foot waves
Breaking or near-breaking beam seas with wavelengths comparable to the 40 ft platform width could generate lateral forces approaching this magnitude.
Why such large waves?
Platform displacement per leg ≈ 12,000 lbs (6 tons)
379,000 lbs lateral force ≈ 30× displacement per leg
Small waterplane area (SWATH-like) reduces coupling to small waves
Stabilizer wings and RIM thrusters provide additional damping
Most force comes from inertial loading and wave slamming, not steady drag
Important Caveat: Accurate wave force prediction requires computational fluid dynamics (CFD) or scale-model tank testing. The above is an engineering-order-of-magnitude estimate only. The seastead's triangular spacing (80 ft × 40 ft) and the damping from the three stabilizers will significantly reduce roll amplitude in real conditions.
Recommendations
Finite Element Analysis (FEA) of the actual NACA foil shape with welds and internal bulkheads is essential.
Consider internal stringers or bulkheads every 4–5 ft to prevent local buckling.
The ½-inch skin is quite robust; local denting from debris is more likely than global bending failure.
Add strain gauges on the first prototype at the top of each leg (highest stress area).
The "small airplane" stabilizers should be designed to also act as passive roll dampers.