I do not see one simple physics reason that the concept is impossible. Your displacement estimate for the three NACA 0040 legs is roughly consistent with the stated 27,500 lb buoyancy at the desired waterline. However, I do see several areas that could become project-killers unless they are redesigned or professionally analyzed.
The biggest concerns are:
For a NACA 0040 section with chord 8.5 ft:
So the stated displacement at 50% immersion is plausible.
The waterplane area would be roughly:
That matches your statement that a 1 ft waterline change is roughly 1/7 of total buoyancy.
This means the basic displacement math is not the first thing I would worry about. The bigger risks are structural loads, motion behavior, maneuverability, storm survival, and realistic weight control.
The container can carry much more weight than the seastead can float at the desired waterline. The important number is not the container payload; it is the 27,500 lb floating displacement.
If 25% of displacement is batteries:
That leaves about:
for everything else:
This may be possible with disciplined lightweight construction, but it is not generous. A detailed weight spreadsheet with a 15% to 25% contingency is essential.
A dangerous failure mode is that the design slowly gets heavier during development. Every 3,800 lb of excess weight sinks the structure about 1 ft deeper.
A 45 ft high-cube container has approximately the dimensions you listed internally, but the door opening is smaller than the internal height. Typical high-cube door height is around 8 ft 5 in, sometimes slightly less depending on the container.
Your foil chord is 8.5 ft, or 8 ft 6 in. That may fit inside the container height but may not pass cleanly through the door if loaded upright. You might be able to rotate the parts during loading, but this needs to be verified with a real container drawing, not just nominal internal dimensions.
The 44.0 ft wall sections also leave very little end clearance in a 44.6 ft internal length container:
That is extremely tight once you include:
This is not necessarily impossible, but I would treat container fit as a high-risk item. Before doing detailed design, make a full container loading CAD model using the actual door opening, not just the internal dimensions.
Because the three waterplane areas are far apart near the triangle corners, the platform may have a large metacentric height. That helps static stability, but it can also make roll and pitch accelerations sharp.
A platform can be very stable yet uncomfortable.
The design is somewhat SWATH-like because the waterplane area is modest, but it is not a true deep SWATH. The waterplane stiffness is approximately:
Without enough added mass and damping, the natural heave period could land in an uncomfortable range. The heave plates may help, but they also add:
This design needs a seakeeping model before money is spent on full-scale construction. The claim of a “very soft ride” is plausible only after analysis or scale testing.
The three legs are large buoyant columns placed near the triangle corners. The living structure is a large 44 ft triangular frame. The loads between those systems will be high.
Important load cases include:
The bolted joints are especially important. A structure that is assembled from containerized pieces can work, but the joint design may become heavier and more complicated than expected.
For this concept, I would not proceed without finite element analysis and a naval architect or offshore structural engineer reviewing the corner connections.
The living floor appears to be roughly 7 ft above the waterline if the legs are half-submerged and attached near their tops. That sounds like good clearance, but it is not enough for open-ocean storm survival.
Even in the Caribbean, hurricanes, squalls, steep chop, and boat wakes can produce damaging events. Potential problems:
If this is intended only for protected anchorages and fair-weather moves, that is one design category. If it is intended to survive offshore storms unattended, that is a very different design category.
You should explicitly define the survival condition, for example:
The proposed six fixed RIM drives can provide forward and reverse thrust, and differential thrust can yaw the vessel. But fixed forward-facing thrusters do not give direct lateral force.
That means the seastead may be poor at:
Because the above-water structure has a lot of windage, crosswind control may be harder than expected. The foil legs may reduce forward drag, but they do not solve low-speed lateral control.
I would strongly consider at least some of the following:
For harbor maneuvering, fixed forward/reverse thrusters alone are likely inadequate.
The triangular roof area is approximately:
If covered efficiently with solar panels, peak solar power might be roughly:
But the daily average is much lower. In good Caribbean sun, you might get something like:
That is useful, but not large for propulsion of a 27,500 lb vessel with heave plates, appendages, thruster losses, windage, and real sea conditions.
Solar may be enough for:
But it may not be enough for reliable cruising against current, wind, or weather windows. You should do a propulsion-power estimate at 1, 2, 3, 4, and 5 knots, including:
Thrusters mounted about 2 ft above the bottom of each leg would be well below the waterline. That is good for avoiding ventilation, but it creates other issues:
RIM drives can be attractive, but in marine use they must be very robust against fouling and corrosion. Make sure the design allows a failed thruster to be replaced without hauling the entire seastead if possible.
Pulling the platform down by 3 ft increases displacement by approximately:
That means the mooring system must hold at least about 11,400 lb of extra vertical tension, before dynamic loads, waves, gusts, current, safety factors, and unequal line loading.
In practice, design loads could be much higher.
Other concerns:
A tension-leg system is not impossible, but it is an offshore-engineering problem, not a simple anchoring problem.
Multiple airtight compartments are good and necessary. However, the design should be checked for realistic damage cases:
With three widely spaced buoyant supports, losing buoyancy in one corner can create a large heel/trim moment. Even if the vessel does not sink, the living area may become unsafe or unusable.
The design should be able to answer:
Putting LiFePO4 batteries low in the legs helps center of gravity and stability. That is good. But battery compartments inside float/foil legs create safety and maintenance complications:
LiFePO4 is safer than many lithium chemistries, but a large battery bank in a sealed marine compartment still needs serious electrical and fire-safety design.
The idea of connecting two seasteads with a walkway while underway is one of the highest-risk parts of the concept.
Two separate floating bodies will have independent:
Even if both vessels are computer-controlled, wave-induced relative motion can be fast and powerful. Thrusters cannot cancel high-frequency wave motion reliably, especially with fixed forward-only thrusters.
Risks include:
If this feature is kept, the walkway should be treated like an offshore motion-compensated gangway problem. It needs articulation, handrails, fail-safe release, load monitoring, and strict operating limits. I would not rely on software coordination alone.
The 7 ft high triangular living wall creates a large above-water sail area. In strong wind, forces can become large quickly.
Approximate dynamic wind pressure:
A 44 ft by 7 ft side has about 308 ft² of projected area before accounting for shape factors. At storm wind speeds, loads on the structure, mooring, and stationkeeping system could be severe.
This affects:
Even if the design works physically, practical operation may require satisfying rules related to:
These are not physics problems, but they can stop the project if ignored.
The basic buoyancy concept is plausible. The NACA 0040 legs can provide roughly the displacement you estimated. So the design does not obviously fail at the first hydrostatic calculation.
However, the following are potential showstoppers:
If this were my project, I would next spend money on:
Those steps could reveal whether the concept is merely challenging or fundamentally uneconomic before you commit to expensive fabrication.