Seastead Engineering Report: Leg Side-Load & Wave Analysis

This report analyzes the structural integrity of the main buoyancy legs of your trimaran-style seastead design. The focus is specifically on the sideways force limits of the 1/2-inch marine aluminum NACA struts and the wave conditions that could induce those forces.

        Design Parameters Assumed:
        Leg Dimensions: 19 ft long (vertical), 10 ft chord (front-to-back), 3 ft max width.
Material: Marine-grade aluminum (e.g., 5083-H116), 1/2 inch (0.5") wall thickness.
Connection: Fixed at the top to the triangular truss structure, free at the bottom (Cantilever model).
Waterline: 50% submerged (9.5 ft underwater, 9.5 ft exposed).
Force Direction: Uniform broadside wave impact pushing against the 190 sq ft flat side of the leg.

    

1. How Much Sideways Force Can the Leg Handle?

To determine when the leg will break or permanently bend (yield), we must look at the leg as a giant cantilever beam jutting down from the main deck. Because the force hits the "broad side" of the wing, the beam resists this bending using its 3-foot width (the minor axis of the airfoil).

Assuming a hollow aluminum shell without accounting for internal structural ribs, the approximate Moment of Inertia (I) for a 10ft x 3ft ellipse with 1/2" walls is roughly 24,000 in&sup4;. Using a typical marine aluminum yield strength of 30,000 psi, we can calculate the breaking point:

Failure Mode	Force Required to Fail	Description
Global Bending (Snapping off)	≈ 350,000 lbs total	The total distributed force pushing against the side of the 19ft leg required to bend the main aluminum shell at the connection point to the truss deck.
Surface Pressure Loading	≈ 1,840 lbs / sq-ft (psf)	The pressure applied uniformly across the entire 190 sq ft broadside of the leg necessary to reach global bending limits.

CRITICAL ENGINEERING NOTE: Local Buckling ("Oil-Canning")
While the leg as a whole can handle ~350,000 lbs of force before breaking off the deck, a sheet of 1/2-inch aluminum stretched across a 10-foot chord will dent, buckle, and cave in (oil-canning) at much lower pressures if it is purely hollow. You absolutely must include internal horizontal ribs and vertical stringers inside the NACA foil. With proper internal bracing, the 1/2-inch skin is more than enough.

2. What Kind of Wave Height Creates That Force?

Ocean waves apply force in two ways: hydrostatic pressure (water piling up) and dynamic pressure (wave slap). For a small-waterplane-area seastead, rolling, non-breaking swells generally wash past the thin legs smoothly. The real danger is a broadside breaking wave.

Using coastal engineering standards (like the Minikin formula for wave slap on vertical structures), we can estimate the dynamic pressure of a breaking wave hitting a flat surface. A breaking wave can exert a brief but massive localized pressure.

Non-Breaking Waves: Even very tall steep swells (e.g., 20-30 ft) that are not breaking will not exert 1,840 psf. The water velocity is relatively low, and water will simply flow around the smooth NACA foil shapes.
Breaking Waves ("Rogue" or storm waves): A cresting, breaking wave slams into structures at high velocity. As a rule of thumb, wave slap pressure (in psf) can be approximated as $100 \times H$ (where $H$ is wave height in feet).

Wave Failure Threshold

To achieve the 1,840 psf needed to threaten the structural integrity of the leg, the seastead would need to take a direct, broadside hit from a breaking wave approximately 18 to 25 feet high.

A wave of this size breaking directly against the flat 19x10 side of the leg would generate immense hydrodynamic slap, likely causing the 1/2 inch aluminum skin to buckle inward, followed by the leg suffering severe structural deflection.

3. Design Recommendations Based on Analysis

Internal Bulkheads: Ensure the interior of the 19ft legs contain horizonal ribs every 2 to 3 feet, and vertical stringers supporting the broad sides of the NACA foil. This shifts the failure mode from localized denting to global bending, maximizing the strength of the 1/2" marine aluminum.
Active Heading Control: You have 6 RIM drive thrusters and 3 stabilizer airplanes. In storm conditions with breaking waves over 15 feet, the onboard system should automatically orient the seastead so the waves approach the leading edge (the blunt front) of the NACA foils, rather than broadside. The leading edge is structurally stronger and hydrodynamically slices through waves.
Damping Assessment: The trailing stabilizers (the "little airplanes") will be excellent for mitigating pitch and heave during normal transit. However, under sheer broadside wave impacts, those stabilizers (being highly delicate at 10ft spans with 1ft chords) will be subjected to immense cross-flow drag and must be built to hinge or feather to prevent them from snapping off.