Seastead Leg Structural & Wave Load Analysis

1. Core Assumptions

This analysis treats each leg as a cantilever beam fixed at the triangular frame and free at the bottom. The load is assumed to be evenly distributed laterally along the full 19 ft length for calculation simplicity. Real wave loads are concentrated near the waterline and are highly dynamic.

Parameter	Value / Assumption
Leg Length (L)	19 ft (228 in)
Submerged Depth	9.5 ft (50% of length)
Material	Marine Aluminum 5083-H321 (σ_yield ≈ 28 ksi)
Wall Thickness	0.50 in
Foil Geometry	NACA, 10 ft chord × 3 ft max thickness
Section Modulus (S)	Estimated 120–1,800 in³ (varies by internal stiffening)
Design Factor of Safety	2.0 (standard for marine fatigue & dynamic loads)

⚠️ Actual section modulus depends heavily on internal bulkheads, rib spacing, and cutouts for thrusters/ladders. CAD/FEA is required for final values.

2. Structural Capacity (Evenly Distributed Side Load)

For a cantilever with uniformly distributed lateral load w (lb/ft):

M_max = wL² / 2 | σ = M / S | F_total = w × L
Combining: F_yield = (2 × σ_yield × S) / L

Conservative Baseline

Equivalent 18" OD Tube

S ≈ 117 in³
F_yield ≈ 28,700 lbs (14.4 tons)
Design Limit (FS=2) ≈ 7.2 tons

Realistic Foil Estimate

Hollow NACA Shell w/ Stiffeners

S ≈ 1,400 in³ (approx)
F_yield ≈ 540,000 lbs (270 tons)
Design Limit (FS=2) ≈ 135 tons

📐 How to adjust: Run a quick CAD export for your exact internal structure. Insert your Section Modulus (S) into the formula above to get your precise capacity.

3. Wave Height ↔ Side Force Correlation

Wave loading is dynamic and depends on wave period, breaking behavior, and submergence depth. Using Morison's Equation with a drag coefficient C_d ≈ 1.5 and impulsive slamming factor:

F_wave ≈ ½·ρ·C_d·A·v² · (Slamming Factor)
Where: ρ = 1.94 slugs/ft³, A = projected area (95 ft²), v ≈ πH/T (particle velocity)

Wave Height (H)	Period (T)	Est. Max Side Force / Leg	vs Conservative Limit
10 ft	8 s	~12,000–18,000 lbs	Safe
15 ft	9 s	~22,000–32,000 lbs	Approaching Yield
20 ft	10 s	~35,000–50,000 lbs	Exceeds Conservative Limit
25+ ft	11+ s	45,000+ lbs	High Risk

📉 Real forces are not evenly distributed. ~70% of wave load concentrates in the upper 5 ft of the submerged section, creating higher bending stress at the frame attachment. Breaking waves add impulsive loads 2–4x higher than non-breaking.

4. Critical Engineering Considerations

Stress Concentrations: Cutouts for thrusters, ladders, and the frame-to-leg welds will reduce effective capacity by 20–40%. Reinforcement plates and radiused cutouts are mandatory.
Fatigue: Marine aluminum is sensitive to cyclic loading. A 1/2" wall with proper weld planning is acceptable, but fatigue life requires S-N curve analysis.
Buckling vs Yielding: Thin-walled 0.5" aluminum under combined bending + compression may buckle before reaching σ_yield. Internal ring frames/bulkheads at 3–4 ft spacing are strongly recommended.
Dynamic Amplification: If wave encounter frequency matches the natural frequency of the leg/structure, resonance can multiply loads by 2–3x.

5. Recommended Next Steps

Model the exact internal structure in CAD and extract Section Modulus (S) about the lateral bending axis.
Run a Linear Static & Buckling FEA with hydrostatic pressure distribution and point loads at thruster mounts.
Perform a Wave Load Spectrum Analysis (using API RP 2A or DNV-RP-C205 guidelines) for your intended operating region.
Design internal stiffeners (rings + bulkheads) to prevent local buckling and distribute weld stresses.
Consider composite or steel transition joints at the frame connection if aluminum fatigue life is marginal.

🔍 This analysis provides a preliminary engineering baseline. Final structural certification should be reviewed by a licensed naval architect or marine structural engineer.