Seastead Model Test Analysis & Full-Scale Motion Prediction

Subject: 1/10th Scale Seastead Model (8 ft × 8 ft × 4 ft triangle; 22.8" NACA-0030 legs)
Analysis Date: Based on geometric description & Froude scaling laws
Scale Factor λ: 10

Limitation Note: This analysis is generated from your design description and the principles of Froude scaling. I cannot directly playback or view YouTube content. The wave-height estimates and observed-motion commentary below are therefore derived from typical 1:10 scale wave-tank test appearances for craft of this geometry, sized relative to the model dimensions you provided (e.g., the 11.4 in draft and 7 ft deck height). If you can provide timed measurements or still frames with a reference ruler, the estimates can be refined.

1. Wave Height Estimation & Scaling

Because the video is not slowed by the Froude time-scaling factor, the motions you see are playing at real-world model speed. In the model tank (or test pond), waves that look significant relative to an 11.4-inch draft and a 22.8-inch leg are usually in the range of:

Estimated model wave height: 2 to 4 inches (5–10 cm)
With occasional larger crests that may reach 5–6 inches (12–15 cm)

Under strict geometric (Froude) scaling, linear dimensions scale by λ = 10. Therefore:

Parameter	Model Scale	Full-Scale Equivalent
Wave Height (moderate)	2–4 in (5–10 cm)	20–40 in (0.5–1.0 m)
Wave Height (occasional peaks)	5–6 in (12–15 cm)	50–60 in (1.25–1.5 m)
Leg Draft	11.4 in	9.5 ft
Freeboard (to top of leg)	11.4 in	9.5 ft
Deck Height (roof)	0.7 ft (8.4 in)	7 ft

Time Scaling Reminder: Time scales by √λ ≈ 3.16. A 1-second wave period in the model is dynamically equivalent to a 3.16-second period at full scale for the same steepness. Typical tank wave periods of 1.0–1.8 s therefore correspond to 3.2–5.7 s full-scale periods—representative of chop and short coastal swell rather than long ocean swells (>8 s).

2. How the Full-Scale Craft Will Move (Seakeeping Analysis)

Your design is essentially a small-waterplane-area triple-hull (SWATH-like) semi-submersible riding on three streamlined, widely separated columns. The governing physics strongly favor motion reduction compared to conventional hulls.

2.1 Natural Periods & Stiffness

Because the waterplane area is small (~60–65 ft² total for three NACA-0030 columns) and the buoyancy is distributed at the three vertices of an 80/80/40-ft triangle, the restoring coefficients behave differently than a conventional hull:

Transverse Roll Stiffness: Very high. The 40-ft beam between aft legs plus the forward leg gives a transverse metacentric height (GM_T) likely exceeding 25 ft. This creates a stiff, stable platform with minimal roll.
Longitudinal Pitch Stiffness: Also very high. The ~77 ft longitudinal spread between the forward apex and the aft base gives a large longitudinal moment of inertia of the waterplane. Pitch restoring is strong.
Heave Stiffness: Low. The total waterplane area is a small fraction of the displacement. Heave restoring is therefore soft, leading to a relatively long heave period compared to the vessel's weight.

Estimated full-scale natural periods:

Motion	Estimated Natural Period	Typical Encounter Period in 0.5–1.0 m Seas
Heave	5.5 – 7.5 s	4 – 6 s
Pitch	6 – 9 s	4 – 7 s
Roll	7 – 10 s	4 – 8 s

Resonance Watch: Because the full-scale heave period is estimated to be fairly close to the wave periods of short 0.5–1.0 m seas, the uncontrolled vessel could experience modest heave amplification in those conditions. However, because the wave-exciting force on a slender NACA column is extremely small compared to a conventional hull, the Response Amplitude Operator (RAO) remains low even near resonance.

2.2 Expected Full-Scale Motions in Scaled Seas (0.5–1.0 m)

Heave amplitude: 0.3 – 0.8 ft peak-to-peak at the center of gravity. The short columns cut through the waves without pushing a large waterline up and down.
Pitch amplitude: ±1.5° to ±3° peak. The long lever arm (77 ft) and high pitch stiffness keep the living-space deck exceptionally level.
Roll amplitude: ±1° to ±2.5° peak. The three widely spaced legs act like an stable tripod; the design has natural roll decoupling.

3. Acceleration Estimates & Comparison

Under Froude scaling, acceleration is dimensionally preserved. A model acceleration of X g in a correctly scaled seaway is exactly the acceleration the prototype would feel in the full-scale seaway. Because your video is at real speed, you are seeing the physically correct acceleration amplitudes—only the time axis is compressed by 3.16×.

Based on the SWATH-like geometry and typical responses for small waterplane platforms, the full-scale accelerations in a 0.5–1.0 m sea state are estimated as follows:

Vessel / Location	Vertical Heave Acceleration (RMS)	Pitch-Roll Related Acceleration at Perimeter (RMS)	Peak Acceleration (approx)
Your Seastead (CG)	0.02 – 0.05 g	0.03 – 0.06 g	0.08 – 0.15 g
Your Seastead (Apex / rear decks)	0.03 – 0.06 g	0.04 – 0.08 g	0.10 – 0.18 g
50-ft Performance Catamaran	0.05 – 0.12 g	0.10 – 0.20 g	0.20 – 0.40 g
60-ft Displacement Monohull	0.08 – 0.20 g	0.15 – 0.35 g	0.30 – 0.60 g

Interpretation: At the center of gravity, your seastead is expected to produce roughly one-third to one-half the heave acceleration of a comparable catamaran, and roughly one-quarter that of a 60-ft monohull. At the forward apex (80 ft from the aft edge), the low pitch angles keep extremity accelerations far below what a monohull bow or catamaran bridge experiences in the same sea state.

3.1 Why the Acceleration is Lower

Small Exciting Force: The NACA-0030 legs present minimal cross-section to passing waves. A monohull or catamaran hull pushes a massive column of water up and down; your slender legs do not.
Separated Buoyancy: Distributing buoyancy at three far-separated points averages out the local wave slope. When one leg encounters a crest, another may be in a trough, creating natural self-compensation.
High Structural Damping: The truss structure and enclosed volume have high inertia relative to the exciting force. The platform changes velocity slowly.
Draft / Wave-Orbit Avoidance: At a 9.5-ft draft, the legs extend below the most energetic orbital motion of typical 0.5–1.0 m waves, reducing dynamic pressure variations on the buoyancy volume.

4. Analysis of the Missing Stabilizers

The model in the video does not include the tail-stabilizer airplanes described in the full-scale design. Their absence is significant:

What You Are Likely Seeing in the Video

Free-decay oscillations: Without the stabilizers, the model lacks active pitch damping. Any pitch or heave disturbance will ring at the natural period with slow decay.
No active trim control: If the model has any weight-offset (likely in a wooden truss), it may sit with a slight static pitch or roll bias that the full-scale active system would trim out in seconds.
Side-force yawing: In oblique waves, the model may exhibit slow yaw “weathervaning” because there is no aft fin damping to stabilize the heading.

Expected Full-Scale Improvement with Stabilizers

Each 10-ft-span hydrofoil tail is essentially a moveable automated flap on a wing. At forward speeds as low as 2–4 knots, these can generate substantial lift forces to create trimming moments:

Condition	Without Stabilizers	With Active Stabilizers
Peak Pitch Angle (0.5 m seas)	±2.5° – 4°	±1° – 1.5°
Pitch Damping Ratio	Low (~0.05–0.08)	Moderate-High (~0.15–0.25)
CG Heave Acceleration	0.03 – 0.06 g	0.02 – 0.04 g
Station-Keeping in Beam Seas	Passive drift	Actively reduced lateral rocking

Key Insight: Because the elevator actuator on each stabilizer changes the incidence of the main wing without needing to move the entire surface against full hydrodynamic load, the power required is tiny—ideal for a solar-electric seastead. Combined with the 6 RIM-drive thrusters for dynamic positioning, the seastead should achieve “steady platform” behavior in most operational conditions.

5. Qualitative Comparison Summary

vs. 50-ft Catamaran

Motion: The seastead will have significantly less pitch, roll, and heave. A catamaran bridges waves with buoyant hulls; your craft averages waves with separated, low-area struts.
Slamming: Catamarans suffer cross-deck slamming and hull-slap in moderate seas. Your 9.5-ft draft and 7-ft high living truss place the wet-deck well above the 0.5–1.0 m wave crests, eliminating slam.
Speed: A performance catamaran is faster. However, your design is not optimized for planing but for minimal drag while transiting; the NACA legs are superior to cylindrical SWATH struts in that role.
Space: Your 80-ft equilateral-ish triangle gives a massive 1,550+ ft² deck with no hulls interfering in the living space; a 50-ft catamaran hulls constrain the floor plan.

vs. 60-ft Monohull

Motion: The monohull will roll substantially in beam seas and pitch heavily in head seas. Your tripod-column geometry separates the roll/pitch stiffness from the vessel weight, yielding a far more stable living platform—closer to a dock than a boat.
Seasickness: RMS accelerations on the order of 0.02–0.05 g are below the human comfort threshold for most occupants (0.05–0.10 g is where sensitivity begins for many people). A monohull in the same 0.5–1.0 m seas often exceeds that threshold.
Survivability: The monohull has deep hull form stability; your design relies on the three-column waterplane. In extreme breaking seas, a SWATH can be vulnerable if a crest hits the platform underside. Your 9.5-ft clearance mitigates this up to roughly 2–3 m significant wave heights.

6. Recommendations for Model Testing & Validation

To replace the estimated values above with measured data, the next model test campaign should include:

Instrument the Model: Add a small IMU (accelerometer + gyro) at the CG and at the forward apex. Record at 50 Hz.
Wave Probes: Place a capacitive wave probe next to the model to measure actual H_m during the run.
Time-Scale the Video: Play back the footage at 0.316× speed (√λ slowdown in post-production) so stakeholders can subconsciously “feel” the correct full-scale sluggishness of the motions.
Add Stabilizers: Print three scaled RC-airplane tails and attach them to the aft end of the foam legs. Even manual RC control in the tank will demonstrate the damping improvement.
Weight Budget Check: If the full-scale displacement is intended to be higher than ~17–20 tons (the geometric displacement of three half-submerged 19-ft NACA foils), consider lengthening the legs, increasing chord, or adding lower bulbous floats to preserve the 50%-submergence target.

7. Bottom Line

Based on the 1:10 model geometry and SWATH physics:

The waves in the video likely represent full-scale seas of 0.5 to 1.0 m (20–40 in), with occasional larger crests.
The full-scale seastead should exhibit very low accelerations—roughly 0.02–0.05 g RMS at the CG in those conditions.
It will ride more comfortably than a 50-ft catamaran and dramatically more comfortably than a 60-ft monohull, primarily due to the small waterplane area and widely separated buoyancy.
The video likely understates the final performance because the active stabilizers (the “little airplanes”) are missing. With them, pitch and heave should drop by 30–60% when the vessel is under way.

The design concept is hydrodynamically sound for a “soft ride” living platform, provided the weight budget is controlled to maintain the intended 50% leg submergence and the solar/thruster package can manage station-keeping loads.

Seastead Scale-Model Test Analysis