Seastead Leg Strength Analysis

Design Summary

Platform

Triangle Frame80 ft × 40 ft
Height (inside)7 ft
Living Space14 ft × 45 ft
StructureTruss + 4 ft safety railing

Legs / Buoyancy

Quantity3 (front, left, right)
Length19 ft vertical
Submerged9.5 ft (50%)
Cross SectionNACA foil, 10 ft chord × 3 ft width
MaterialMarine Aluminum, ½" thick
OrientationLeading edge forward

Simplified Top View
FRONT LEG ────────────────► (10ft chord)
▲
LEFT RIGHT
(40ft wide at back)

Each leg has 6 rim-drive thrusters (2 per leg) and integrated ladders on upper half.

Analysis Assumptions

Model: Each leg is treated as a vertical cantilever beam fixed at the triangular truss and free at the bottom.

Cross-section approximation: NACA foil approximated as a thin-walled rectangular tube (120" × 36" outer) with 0.5" wall thickness for calculation purposes.

Material: Marine-grade aluminum (similar to 5086-H116). Yield strength used: 35,000 psi (conservative for welded marine application).

Loading: Uniformly distributed lateral force along the entire 19 ft length of the leg (as requested).

Bending Axis: Weak axis (sideways force perpendicular to the 10 ft chord). This is the critical case for beam seas.

Key Results

Maximum uniform lateral load per leg

37,800 lb/ft

≈ 719,000 lbs total force per leg

(360 tons) before yield

Property	Value	Notes
Moment of Inertia (I_xx)	42,800 in⁴	Weak axis (sideways bending)
Section Modulus (S)	2,343 in³	I/c where c ≈ 18.25 in
Maximum allowable moment	6.83 × 10⁶ ft-lb	At yield (35 ksi)
Uniform load capacity (w)	37,800 lb per foot	Over 19 ft length
Total lateral force capacity	719,000 lbs (360 tons)	Before plastic deformation

Calculation Steps (simplified):

1. I ≈ 42,768 in⁴ (two long walls dominate: 2 × A × d²)
2. Max stress σ = M·c/I → M_max = σ·I/c = 6.83 million ft-lb
3. For cantilever with uniform load: M = (w·L²)/2
4. w = (2·M)/L² = 37,800 lb/ft (L = 19 ft)

Wave Height Estimate

Converting structural capacity into wave height is complex and depends on wave period, direction, platform mass, metacentric height, and dynamic response.

Estimated Threshold

18–26 ft

Significant wave height in beam seas that could approach critical stress in the legs.

Why this range?

Wave orbital velocities and accelerations create both drag and inertial forces on the 9.5 ft submerged section.
Platform roll induces significant lateral loading on the leeward legs.
The tripod geometry provides good stability, but small waterplane area means relatively low roll stiffness.
At ~20 ft waves (8–12 second period), lateral forces can easily exceed 300–500 tons per leg in extreme crests.

Important: This is a rough first-order estimate only. Real wave forces are dynamic, include slam loads, and depend heavily on the exact NACA profile, internal stiffening of the legs, and the rigidity of the triangular truss. The structure may fail at the attachment points or through buckling long before the leg material reaches yield stress.

Recommendation: Professional naval architecture analysis (including CFD, tank testing, and finite element modeling) is essential before construction.

Engineering Disclaimer

This is NOT certified engineering advice.

Calculations use simplified geometry and conservative material values. Real-world performance depends on:

Internal ring frames and longitudinal stringers (not modeled)
Weld quality and heat-affected zones
Exact NACA foil geometry and center of pressure
Dynamic amplification from wave slamming
Corrosion allowance over time

Consult a licensed marine structural engineer. The lives of everyone aboard depend on it.