Seastead 1/6-Scale Model — Design & Test Plan

1. Displaced Water Mass (Full Scale)

Each leg is a cylinder, D = 3.9 ft (r = 1.95 ft), 24 ft long, with 16 ft immersed. Displacement depends only on submerged volume, so the 45° angle does not change it:

V_leg = π r² L_sub = π × (1.95)² × 16 = 191.1 ft³

V_total = 3 × 191.1 = 573.4 ft³ (≈ 16.24 m³)

36,700 lb

Seawater @ 64.0 lb/ft³
(≈ 16,650 kg ≈ 1,141 slugs)

35,780 lb

Freshwater @ 62.4 lb/ft³
(≈ 16,240 kg)

573.4 ft³

Total displaced volume
(3 legs)

Design implication: to float with exactly ⅔ of each leg immersed, the complete structure (frame + legs + cables + payload) must weigh ≈ 36,700 lb in seawater.

2. Froude Scaling Laws (λ = 1/6)

Quantity	Scale factor	Value
Length	λ	1/6 = 0.1667
Area	λ²	1/36
Volume, mass, displacement, force	λ³	1/216
Time, wave period	√λ	0.4082
Velocity, wave celerity	√λ	0.4082
Acceleration	λ⁰	1 (unchanged)
Wave steepness H/L	—	unchanged

3. 1/6-Scale Model Dimensions (inches)

Item	Full scale (in)	Model (in)
Leg diameter	46.8	7.8
Leg length	288	48
Draft (immersed length along leg)	192	32
Freeboard (along leg)	96	16
Frame height above waterline (vertical)	67.9	11.3
Leg tip depth below waterline (vertical)	135.8	22.6
Triangle frame side	720	120
Frame width, corner to corner	831.4	138.6
Footprint across leg tips	1,238.7	206.4

≈ 170 lb

Target total model weight
(36,698 ÷ 216 = 169.9 lb)

± 3 lb

Acceptable tolerance;
trim with ballast

Ballasting: build light, then add ballast low in each leg until the model floats level at 32 in of leg immersed. Weigh the whole assembly (legs + frame + cables + phone + mounts) on a bathroom scale before launch.

4. Cable Geometry (Top View)

The legs project 16.97 ft horizontally and 16.97 ft vertically (24 ft at 45°). Each leg tip sits 11.3 ft below the waterline, 51.6 ft from the frame center in plan.

5. Cable Lengths

Cable	Count	Straight-line, full scale	Scale length (in)	Recommended cut length
Leg tip → adjacent corner	6	77.1 ft	154.1 in	162 – 168 in (13.5 – 14 ft)
Loop: tip → tip → tip	1	268.2 ft	536.4 in	552 – 564 in (46 – 47 ft)

Cut lengths include ~5 % margin for thimbles, knots, shackles, and tension adjustment.
Use low-stretch line (thin Dacron/polyester or small Dyneema) so pretension stays consistent.
Add a small turnbuckle or lashing at each corner tie so you can tune symmetry — uneven pretension will tilt the model and corrupt your accelerometer data.
Pretension only until the frame stops lifting; the 45° legs convert cable tension into downward restoring load.

6. Wave Simulation Targets

Wave heights (scale directly by 1/6)

Target full-scale H_s	Model wave height
3 ft	6 in
5 ft	10 in
8 ft	16 in

Wave periods & wavelengths (deep water: L = 5.126 T², feet)

Full-scale period	Model period (×0.408)	Model wavelength	Deep-water depth needed (≥ L/2)
4.9 s	2.0 s	20.5 ft	≥ 10.3 ft
6.1 s	2.5 s	32.0 ft	≥ 16.0 ft
7.4 s	3.0 s	46.1 ft	≥ 23.1 ft
9.8 s	4.0 s	82.0 ft	≥ 41 ft (not feasible in bay)

Practical recommendation: work with model periods of 2.0 – 2.5 s. These correspond to 4.9 – 6.1 s full-scale seas — shorter than typical ocean swell, but the only way to keep waves behaving "deep-water" in a small bay.

Water depth & placement in Sandy Hill Bay

Absolute minimum to avoid depth-induced breaking: depth ≥ ~2 × wave height → ≥ 32 in for the 16-in waves. This alone is not enough for realism.
Target: the deepest water you can find, ideally ≥ 8 – 10 ft, combined with ≤ 2.5 s model periods.
Stay ≥ 1 – 2 wavelengths (roughly 25 – 60 ft) away from the beach, reef edges, and any shoal so waves haven't started shoaling.
Sandy Hill Bay faces south — it works best with a clean southerly swell and light wind. Trade-wind chop adds high-frequency noise your phone will happily record.
Scout depths beforehand with the Navionics/Boating app, a handheld fish finder, or a weighted line marked in feet.
By moving the model along the depth gradient you effectively step through sea states — just re-log the local depth for each run.

7. Free Android Apps for Acceleration Logging

App	Why it's useful	Export
phyphox (RWTH Aachen)	Best scientific option. Selectable sample rate, absolute/linear acceleration, live view from a desktop browser over Wi-Fi during tests.	CSV / Excel
Physics Toolbox Sensor Suite (Vieyra Software)	Simple g-force meter and multi-sensor recorder; very easy for field volunteers.	CSV via email/Drive
Sensor Logger	Logs accelerometer + gyroscope + GPS together in the background; good for tagging each run's location and depth spot.	CSV / ZIP
AndroSensor	Reliable all-sensor CSV logger (works best on older Android versions).	CSV

Field tips

Sample at 50 – 100 Hz — plenty for model motions of a few Hz.
Airplane mode ON (saves battery, removes radio noise); screen timeout OFF.
Mount the phone rigidly to the frame in a waterproof pouch — a flopping phone measures the pouch, not the platform.
Note the phone's axis orientation, and do a sharp tap test at the start of each file so you can align/sync traces later.
Transfer files by USB cable, or upload to Drive and pull them to the desktop.

8. Caveats & Reminders

Froude scaling is valid here: gravity/wave forces dominate; viscous and surface-tension effects are second-order at this size.
Reynolds-number mismatch means model drag coefficients are slightly higher than full scale — expect model responses to be mildly conservative (a bit more damped).
The stretchy line to the human handler is a position keeper, not a mooring: keep it slack enough that it never influences the motion records. Use a quick-release knot.
Log depth, wave period (count 20 crests with a stopwatch), wave height against a staff, wind, and time for every run — future-you will thank present-you.