This guide introduces the core naval architecture principles needed to evaluate the seastead design described in the brief. The design is a semi-submersible, trimaran-inspired platform with three NACA 0035 foil legs, a 44-ft equilateral triangle deck, and a 7-ft tall living area. It is engineered to pack entirely inside a single High Cube 45-ft container for global shipping. Below, we break down how the seastead achieves stability, minimizes drag, and ensures a comfortable ride.
Every floating vessel has a natural period at which it wants to roll. This is governed by the vessel's
metacentric height (GM) and its radius of gyration (k):
T = 2π · k / √(g · GM)
A longer roll period is generally more comfortable for occupants, as it avoids the quick, jerky motions that cause seasickness.
The goal is to make the natural period longer than the dominant wave periods (typically 5–15 seconds).
The 44-ft wide triangle gives a large radius of gyration (k), which lengthens the roll period. The SWATH-like legs (small waterline area) provide a high GM, but the heave plates and low battery mass in the legs add damping. The design targets a "soft ride" by keeping the roll period well above 10 seconds. The 1/7th buoyancy change per foot of immersion ensures the platform is not too stiff.
A Small Waterline Area Twin Hull (SWATH) vessel concentrates buoyancy deep underwater, connected to the deck by narrow struts. Because waves act on the waterplane area, a smaller waterplane means the vessel is less affected by passing waves. This is the same principle used by semi-submersible oil platforms.
The three NACA 0035 legs are exactly this: 50% submerged (14.5 ft underwater), with a narrow 8.5-ft chord at the waterline. The designer notes that a 1-ft wave only changes buoyancy by ~1/7th of the total, meaning the platform rides through waves rather than over them. This is a classic SWATH advantage, but with a trimaran layout for better stability.
Drag is the force resisting motion through water. It has three main components:
form drag (shape), skin friction (surface area), and
wave-making drag (energy lost creating waves). The drag equation is:
FD = 0.5 · ρ · V² · A · Cd
Minimizing drag is critical for efficient electric propulsion.
The NACA 0035 foil shape is inherently low-drag. The blunt leading edge faces forward, reducing form drag compared to a cylinder. The RIM drives are flush-mounted on the legs, eliminating shaft and strut drag. The conduit for wires is welded to the trailing edge to avoid disturbing the flow. The heave plates are thin and aligned with the flow, so they add minimal horizontal drag.
Wind acts on the above-water profile of the vessel. The same drag equation applies, but with air density. Large, flat surfaces create high wind loads, which can cause leeward drift and increase mooring loads. Perforated or rounded surfaces reduce the effective wind drag coefficient.
The 7-ft tall living area presents a significant wind profile, but the triangular shape helps deflect wind. The walkway grating allows wind to pass through, reducing overall load. The dinghy is shielded behind the living area, preventing it from acting as a sail. Solar panels are flat, minimizing added windage. The design calculates wind load for mooring and anchoring scenarios.
Active stabilizers use sensors and actuators to counteract motion in real-time. Common examples include active fins, gyroscopic stabilizers, and differential thruster control. They are especially useful at low speeds or when stationary, where passive stability is less effective.
The seastead has 6 RIM drive thrusters (one on each side of each leg). These are used for differential thrust to control yaw and roll. The control system can rapidly adjust thrust to cancel out wave-induced motion. When two seasteads connect, both computers coordinate 12 thrusters to keep the walkway stable. This is a sophisticated active stabilization system.
A semi-submersible has most of its buoyant volume (pontoons/legs) deep below the water surface, connected to the deck by slender columns. This decouples the deck from wave action, because waves only affect the small columns near the waterline. It is the gold standard for offshore stability.
The three 21.5-ft NACA 0035 legs are the pontoons. The top half of each leg is out of the water, acting as the column. The lower half provides buoyancy. The design is not an extreme SWATH, as the waterline area is still significant (1/7th buoyancy change per foot), but it is a true semi-submersible layout. The multiple airtight compartments in each leg ensure damage stability.
The coefficient of drag (Cd) quantifies how much a shape's geometry contributes to fluid resistance. A sphere has Cd ≈ 0.5, a cylinder Cd ≈ 1.0, and a streamlined foil Cd ≈ 0.05–0.1. The shape's fineness ratio, leading edge radius, and trailing edge all affect the Cd.
The NACA 0035 is a symmetric foil with a thickness ratio of 35%. It is specifically chosen for a low Cd while still providing enough volume for buoyancy and battery storage. The trailing edge is cut short by 0.5 ft to fit inside the container height—this slightly increases the base drag (Cd penalty), but it is a necessary trade-off for shipping. The heave plates are flat (high Cd for vertical motion, good for damping) but thin for low horizontal Cd.
Key geometric relationships from the design brief, visualized.