Each of the three foil legs carries a high-thrust submersible mixer (2.5 m propeller, ±7000 N / ±1574 lbf) oriented so it can push the leg up or pull it down. A flight-control-style computer commands the three thrusters in real time to counteract wave-induced heave, pitch and roll while the seastead is stationary and headed into the prevailing seas. An optional curved-track mechanism would allow each mixer to be winched through 90° so the same unit could also provide horizontal thrust.
where kh = 57.3 kN m−1 and η is local wave elevation.
With 21 kN of total thrust the system can null a uniform 0.37 m elevation change.
Real short-crested Caribbean waves produce differential elevations across the 13 m triangle, so the three thrusters also generate correcting pitch and roll moments. Maximum restoring moment is on the order of 80–90 kN·m—valuable because the water-plane second moment of the three slender foils is modest.
Typical Caribbean sea states (protected to open trade-wind)
Condition
Hs
Tp
Approx. peak η
Static force demand
Authority margin
Protected anchorage
0.3–0.6 m
3–5 s
0.25–0.45 m
14–26 kN
Comfortable excess
Typical trade-wind lee
0.8–1.2 m
5–7 s
0.6–0.9 m
34–52 kN
Partial (50–70 %)
Open Caribbean winter
1.5–2.0 m
6–8 s
1.1–1.5 m
63–86 kN
Saturated most of the time
Bottom line on authority: the thrusters are sized very well for the protected and moderate conditions the design already targets for helical mooring. In true open-sea winter trades they become saturated and act more as dampers than as full cancellers.
3. Expected Motion Reduction
A simple linear model (mass–damper–spring + wave force + control force) with realistic thruster bandwidth (>1 Hz) and a modest heave-plate damping ratio (ζ ≈ 0.2–0.3) yields the following practical expectations after closed-loop control:
Protected (Hs ≤ 0.6 m)
Heave residual: 2–6 cm RMS
Pitch / roll: < 0.5°
Vertical accel: < 0.03 g
Reduction vs. passive: 75–90 %
Near-land comfort
Moderate trades (Hs 0.8–1.2 m)
Heave residual: 8–18 cm RMS
Pitch / roll: 0.7–1.5°
Vertical accel: 0.04–0.08 g
Reduction vs. passive: 55–75 %
Very liveable
Open winter (Hs 1.5–2 m) – for reference
Heave residual: 25–45 cm RMS (thrusters saturated on the larger crests)
Still 30–50 % better than passive because the thrusters clip peaks and add damping
Noticeable motion; most people would want the helical tension legs down
Because the three actuators are independent, the controller can null the first-order wave frequencies at the three leg locations simultaneously (a simple discrete MIMO or even three SISO loops with cross-feed). Feed-forward from a short bow-mounted wave staff or from the existing motion sensors further improves performance. Residual motion is then dominated by:
higher-order wave components and diffraction that the hydrostatic model misses,
thruster dynamics and any thrust-allocation limits,
the intentional “softness” left in the loop so the thrusters do not fight every tiny ripple (saves power and wear).
4. Human Comfort Assessment
Motion-sickness incidence is driven mainly by vertical acceleration in the 0.1–0.5 Hz band (ISO 2631 / MSI indices).
Vertical accel (RMS)
Typical reaction (4–6 h exposure)
Achievable with active system
< 0.02 g
Essentially no one sick
Protected waters
0.02–0.05 g
Low single-digit % MSI
Moderate trades
0.05–0.1 g
Noticeable; 10–30 % MSI
Open winter (active)
> 0.15 g
Uncomfortable for most
Passive platform in same seas
In the design’s intended operating envelope (Caribbean, often protected or semi-protected) the active system should keep the platform inside the “almost nobody gets sick” regime. The aluminum-grating walkway and the already-soft foil buoyancy further help; the thrusters simply remove the residual that would otherwise remain.
5. Power & Energy Cost
Bollard-pull induced velocity for a 2.5 m propeller at 7000 N is ~0.8 m s−1. Ideal fluid power is ~5.8 kW; real electromechanical + propeller efficiency will bring each unit to roughly 12–18 kW at continuous full thrust. Average power while station-keeping in moderate seas is far lower—typically 15–40 % of peak because the thrusters reverse and because the controller only fights the instantaneous error.
Peak electrical load for three units ~40–50 kW. Given that ~25 % of displacement is already allocated to LiFePO4 batteries and each leg has its own inverter, the energy budget is realistic for hours-to-days of active stabilization between sunny periods. Solar on the 800+ ft² roof can recharge comfortably in the trade-wind sun.
6. Practicality of the 90° Curved-Track Dual-Mode Mount
Interesting on paper, questionable in practice.
Pros
One piece of hardware serves both transit propulsion and vertical stabilization.
Intermediate angles give a limited vectoring capability (counteract wind drift while still stabilizing).
Cons / risks
Marine mechanical complexity – underwater curved rails, wheels or skids, winch, latches. Biofouling, corrosion, galvanic issues and jamming are almost guaranteed without heroic maintenance.
Structural loads – 7 kN thrust at the end of a moment arm while the carriage is at an intermediate angle produces large side loads on the track.
Packaging – a 2.5 m propeller does not fit through the 7.7 ft container door or width; the props must be shipped detached and fitted at the assembly yard anyway. The track mechanism adds further bulky parts.
Single-point failure – a jammed track leaves you with neither good propulsion nor good stabilization on that leg.
Control & safety – changing mode requires the vessel to be stopped and the thruster unloaded; a failure during the transition is awkward.
Recommended alternatives
Dedicated vertical thrusters (simpler fixed mounts) + keep the six existing RIM drives for horizontal thrust and dynamic positioning. Cleanest reliability path.
If multi-use is mandatory, a robust 0–90° tilting azimuth pod on a single heavy hinge with a linear actuator is preferable to a long curved track; still complex but fewer fouling surfaces.
Active ballast (water pumps between leg compartments) can supply the low-frequency vertical force while the thrusters handle only the wave-frequency component—reducing required thruster size and duty cycle.
Once the helical tension legs are deployed, vertical motion is already tightly constrained; the mixers become optional “ride-quality” devices rather than primary station-keeping tools.
7. Overall Verdict
Active vertical thrusters of the proposed size are a strong match for the seastead’s intended Caribbean use case.
In protected and moderate conditions they can cut residual heave and attitude motion by roughly 60–85 %, bringing accelerations into the very-comfortable zone (<0.05 g).
The three-leg geometry is ideal for simultaneous heave/pitch/roll control.
Power demand is compatible with the planned large LiFePO4 bank and solar array.
The curved-track dual-mode mechanism adds more mechanical risk than operational value; fixed vertical thrusters (or a simpler tilt hinge) plus the existing RIM drives are the more practical route.
Combined with the already-soft foil buoyancy, heave plates, and optional tension-leg moorings, the platform can offer a ride quality far closer to a small apartment than to a conventional monohull or even most catamarans of similar displacement.
8. Suggested Next Engineering Steps
Linear and non-linear time-domain simulation (3-DOF vertical + simple thruster model) in JONSWAP spectra for Hs = 0.5, 1.0, 1.5 m.
Thruster selection: ducted vs. open, actual bollard-pull curves, reverse-thrust transient times, cavitation at shallow submergence.
Structural integration: local reinforcement at the bottom of each NACA leg for ±7 kN + shock from heave plates.
Control architecture: sensor suite (IMUs at each corner + optional wave staff), allocation algorithm, failure modes (loss of one thruster still leaves a controllable pair).
Prototype one leg with a smaller electric thruster in a wave flume or protected harbor to validate control authority and riding comfort before committing to 2.5 m units.