⚓ 1:10.5 Scale Seastead Model – Froude Scaling Summary

All full-scale dimensions are converted to model inches using the linear scale factor 1 : 10.5. Conversion: model dimension (in) = full‑scale ft × 12 / 10.5 = full‑scale ft × 1.142857.

📐 Main Dimensions (Model in Inches)

Component	Full Scale	Model (inches)	Notes
Triangle Frame
Left side (port)	70 ft	80.0 in
Right side (starboard)	70 ft	80.0 in
Back side (base)	35 ft	40.0 in	Front point opposite this side
Truss height (floor–ceiling)	7 ft	8.0 in	Living area inside
Legs / Foils (3×)			NACA 0030 section
Leg length (vertical span)	19 ft	21.71 in	50% submerged = 9.5 ft full scale
Submerged length	9.5 ft	10.86 in	From bottom to waterline
Out‑of‑water length (incl. ladder)	9.5 ft	10.86 in	Ladder on front, top half
Chord (fore‑aft)	10 ft	11.43 in
Max thickness (width)	3 ft	3.43 in	30% of chord
RIM Drive Thrusters (2 per leg)
Thruster diameter	1.5 ft	1.71 in	Flat sides forward/back
Height from bottom of leg	3 ft	3.43 in
Depth below waterline	6.5 ft	7.43 in	(9.5 ft – 3 ft)
Dinghy (RIB)			Sideways behind back centre
Length	14 ft	16.0 in	Electric HARMO outboard
After Deck			Extends beyond back edge
Deck width (each side)	5 ft	5.71 in	Left & right of dinghy
Stabilizer “Little Airplane” (3×)			One near back of each leg
Wingspan (main wing)	12 ft	13.71 in
Main wing chord	1.5 ft	1.71 in
Fuselage length	6 ft	6.86 in
Elevator wingspan	2 ft	2.29 in
Elevator chord	6 in (0.5 ft)	0.57 in	Actuator adjusts elevator angle
Notch into wing (25% chord)	~0.375 ft	~0.43 in	Balance on pivot

📌 Note: Dimensions are rounded to two decimals. The 1:10.5 scale gives a very convenient 70 ft → 80.0 in, 35 ft → 40.0 in, 14 ft → 16.0 in, etc.

⚖️ Target Model Weight (Displacement)

Froude scaling for weight: mass (weight) scales with (length)³ → model weight = full‑scale displacement ÷ 10.5³.
First, we estimate full‑scale displacement from the submerged leg volume (the main buoyancy providers).

Full‑scale buoyancy estimate

Leg cross‑section area (NACA 0030, chord 10 ft, max thickness 3 ft):
Approximate area ≈ 0.75 × chord × thickness = 0.75 × 10 ft × 3 ft = 22.5 ft².
(The NACA 0030 is somewhat fuller than a pure ellipse; 0.75 coefficient is conservative.)
Submerged volume per leg = area × draft (9.5 ft) = 22.5 ft² × 9.5 ft ≈ 213.75 ft³.
Total submerged volume (3 legs) ≈ 3 × 213.75 ft³ = 641.25 ft³.
Displacement in salt water (64 lb/ft³): 641.25 ft³ × 64 lb/ft³ ≈ 41,040 lbs (≈ 20.5 short tons).
Additional topside structure weight (triangle, solar, equipment) is carried by the same buoyancy, so total displacement ~ 41,000 lbs.

Model target weight = 41,000 lbs ÷ 1157.625 ≈ 35.4 lbs.

⚡ Practical note: This is the design waterline displacement for the model. If you build lighter, you must add ballast to reach this weight. If heavier, the model will float deeper than the intended 50% leg immersion. Verify trim and draft during setup.

🌊 Wave Heights for Simulation

Froude scaling for wave heights is linear (same as length). Model wave height = full‑scale wave × 1 ⁄ 10.5.

Full‑scale wave (ft)	Model wave height (in)	Suitable test location
3 ft	3.43 in	Small chop inside the bay
5 ft	5.71 in	Moderate open‑water swells
8 ft	9.14 in	Larger waves near the mouth or exposed area

Use the measured wave pole to find areas in Sandy Hill Bay that deliver these heights. The test tether (stretchy line) will allow gentle restraint while preserving natural motions.

📱 Android Apps for Motion Recording

Several free apps can log accelerometer, gyroscope (pitch/roll/yaw), and derived quantities. Recommended:

Phyphox (RWTH Aachen) – Excellent physics toolkit. Record acceleration (without g), gyroscope, GPS, and more. Exports CSV/Excel directly. Can stream data to a laptop. Best choice for scientific analysis.
Physics Toolbox Sensor Suite – Simultaneous recording of acceleration, g‑force, gyroscope, magnetometer. Easy to use, also exports data.
Sensor Kinetics (or similar) – Good for real‑time visual observation, but export options may be limited.

Mount the phone securely on the model, aligned with the seastead axes. Use “linear acceleration” (without gravity) to measure pure motion. Combine with video for validation.

🍽️ Acceleration Thresholds for Sliding Plates

In full‑scale, a plate on a table begins to slide when the horizontal acceleration exceeds a > μ_s·g, where μ_s is the static friction coefficient (typically 0.3–0.5 for ceramic on wood/plastic).

Because Froude scaling keeps acceleration invariant (g is the same), the same g‑level in the model corresponds to the same sliding threshold in the real seastead. So if the model experiences lateral accelerations of 0.3g or higher, plates would likely slide in the full‑scale version.

Recommended metrics to monitor from the phone data:

Peak lateral acceleration (x‑ or y‑axis) – compare with friction‑coefficient × g.
Root‑mean‑square (RMS) horizontal acceleration – gives a measure of sustained motion “roughness”.
Cumulative exceedance – time fraction above, e.g., 0.2g or 0.3g.
Jerk (rate of change of acceleration) – high jerk can cause items to topple even at lower sustained g’s. If the phone can calculate jerk (Phyphox can differentiate), values above 5–10 m/s³ (~0.5–1.0 g/s) in full scale are noticeable; the model will give the same numerical values.

As a qualitative check, the glass with rocks and water will visually show sloshing. In the slowed‑down video, compare the water surface slope with a protractor overlay – slopes > 15–20° would likely spill drinks full scale.

🧸 Doll Size for Scale Reference

Human height ≈ 5 ft 8 in (68 inches). Divide by 10.5 → 6.48 inches. Use dolls or figures approximately 6.5 inches tall (16.5 cm) to give a sense of scale. For 6‑inch action figures the error is acceptable.

🌊 Water Depth for “Deep Water” Waves

Deep‑water condition: water depth d > L/2, where L is wavelength. For the highest target wave (8 ft full‑scale) we estimate the expected wavelength during the test. Typical wind‑driven 8‑ft waves have periods T ≈ 5–7 s, giving deep‑water wavelength L ≈ 130–250 ft (full scale). Model wavelengths are 12–24 ft (3.7–7.3 m).

To safely stay in deep‑water regime for all tests, the water depth at the model location should be at least 12 ft (3.6 m). This avoids wave shoaling and steepening that could introduce scale effects. Position the model where Sandy Hill Bay reaches that depth, preferably with a clean fetch.

📷 Camera & Measurement Strategy

Shore camera (tripod, fixed): High zoom, calibrated reference marks on the model (e.g. vertical tape at bow and stern). Record at 60 fps or higher; play back at normal speed ÷ √10.5 ≈ 3.24× slowdown to simulate full‑scale time. Use Tracker (free video analysis software) to extract heave, pitch, roll from marker motion.
On‑board GoPro: Records first‑person view, also logs accelerometer data. Slow down by the same factor for human‑like perception. The acceleration values are real‑time correct already, so no scale factor needed.
Wave pole: Pound into sand or moor with a weight; mark every inch. Use still frames to read wave height.
Optional: Place a small IMU logger (e.g., MSR, X‑IMU) if higher accuracy needed; otherwise the phone is sufficient.
Additional ideas:
- Attach brightly coloured ping‑pong balls at known heights as tracking markers.
- Use a waterproof Bluetooth accelerometer/Gyro for easier integration (e.g., WitMotion).
- Record weather conditions, especially wind speed (uses same Froude scale for velocity: model wind = full‑scale / √10.5).
- Measure mooring line tension with a small scale if you want data on wave drift forces.

With this setup you can quantitatively compare model motions to comfort criteria (e.g., ISO 2631 for seakeeping) and predict full‑scale behaviour reliably.

All calculations follow standard Froude similarity: length ratio λ, velocity ratio λ^½, time ratio λ^½, acceleration ratio 1, force/weight ratio λ³ (constant density).