Triangle Seastead — 1/6-Scale Model Test Analysis

Froude-scaled hydrostatics, motion & acceleration analysis • Comparison vs. 50′ catamaran and 60′ monohull • Falsifiable predictions for the loaded re-test

⚠️ Read First: What I Could and Couldn't Do

I cannot open or watch YouTube videos (or any video), so I could not literally measure wave heights or platform motion from your footage. Everything below is computed from the dimensions you provided (10-ft triangle, 2x8 frame, three 8-in × 4-ft legs, 1/3 submergence) using standard naval-architecture scaling and seakeeping physics. Wherever the video matters, I give scenario ranges plus a simple recipe so you can extract exact numbers from your clip. Treat my "video estimates" as hypotheses your next test can confirm or kill. 🙂

1. Froude Scaling Laws Used (λ = 6)

Your video was slowed by √6 ≈ 2.449, which is correct for Froude scaling. All results below use:

Quantity	Scale factor	Meaning
Length, wave height	× 6	1 ft model → 6 ft full scale
Area	× 36	—
Volume, weight, force	× 216	1 lb model → 216 lb full scale
Time, wave period	× √6 ≈ 2.449	1 s model → 2.45 s full scale
Speed	× √6 ≈ 2.449	—
Acceleration	× 1	g-levels measured on the model ARE the full-scale g-levels

The single most useful fact from model testing: because acceleration scales as L/T² = 6/(√6)² = 1, any accelerometer reading in g’s taken on your model transfers directly to full scale for corresponding seas. Tape a phone accelerometer to the deck next test — the g’s you record are the g’s a full-size unit would feel.

2. Derived Physical Properties (computed from your description)

Property	Model (as tested)	Model (loaded, planned)	Full scale (as tested)	Full scale (loaded)
Triangle side	10 ft	10 ft	60 ft	60 ft
Legs	8 in dia × 4 ft (×3)		48 in dia × 24 ft (×3)
Draft	16 in	32 in	8 ft	16 ft
Freeboard	32 in	16 in	16 ft	8 ft
Displacement volume	1.40 ft³	2.79 ft³	302 ft³	603 ft³
Total weight	≈ 87–89 lb	≈ 175–179 lb	≈ 19,000–19,300 lb	≈ 38,000–38,600 lb
Waterplane area (3 legs)	1.05 ft²		37.7 ft²
Heave natural period T_n*	1.3–1.8 s	2.2–2.6 s	3.1–4.4 s	4.4–6.3 s
Rocking (pitch/roll) period*	≈ 1.1 s	≈ 1.7 s	≈ 2.5–2.7 s	≈ 4.1–4.4 s

*Range reflects estimated added mass of 0.5–1.0 × displaced mass for heaving surface-piercing columns. T_n = 2π√(m+a)/(ρgA_wp). Full-scale periods = model × 2.449.

Ballast required to reach 2/3 draft: ≈ 90 lb total on the model (≈ 30 lb per corner), equivalent to ≈ 19,300 lb ≈ 8.8 tonnes full scale.

3. Estimating the Wave Heights in Your Video

Since I can't view the footage, here is the estimate framework plus my best guess:

How to measure it yourself (2 minutes of work)

The 8-in leg diameter and the 7.25-in wide face of a 2x8 are built-in rulers in every frame.
Count how many leg-diameters tall the crest-to-trough excursion is at a leg.
Because the video is already time-scaled, any period you time in the slowed video IS the full-scale period — no conversion needed.

Conversion table (heights only — multiply by 6)

Model wave height	2 in	3 in	4 in	5 in	6 in	8 in
Full-scale height	1.0 ft	1.5 ft	2.0 ft	2.5 ft	3.0 ft	4.0 ft

My best guess for your clip: sheltered-water test setups like yours typically produce 3–5 inch model waves → 18–30 inches (1.5–2.5 ft) full scale, with slowed-video periods of roughly 3–4.5 seconds. If your waves looked like they reached about halfway up the submerged 16 inches of leg, that's ~4 in model = 2 ft full scale.

4. Motion Analysis — What the Video Likely Shows

The critical number is the heave natural period. As tested, the full-scale equivalent is T_n ≈ 3.1–4.4 s — unfortunately sitting right inside the energy band of typical bay/harbor chop (2.5–4.5 s). That predicts one of three regimes; check your video against them:

Regime	Appearance in slowed video	Interpretation
A — Tracking	Platform bobs in sync with waves, similar amplitude	Wave period above T_n; RAO ≈ 1. Benign.
B — Resonant ringing	Platform heaves larger than the waves; keeps oscillating after a wave group passes	Wave period ≈ T_n; amplification factor ~1.5–3 because slender columns have low damping. Likely present in your clip.
C — Detuned	Platform nearly still while waves pass beneath	Short chop below T_n; SWATH-like behavior. The design goal.
Rocking	One leg dips as the opposite side rises, ~2.5 s full-scale rhythm	Pitch/roll mode about the stiff triangle; excited when the wavefront hits legs sequentially.

With only 87–89 lb of displacement and 16-in draft, the model is very light for its waterplane, so I expect your footage shows a mix of B and A: lively bobbing near the wave period with some continued ringing — energetic but not violent.

5. Accelerations — Numbers and Comparison

Vertical acceleration a ≈ (2π/T)² × RAO × (H/2) [ft/s²; divide by 32.17 for g]

Applying this to plausible full-scale seas (stationary platform, RAO = response/wave-amplitude ratio):

Full-scale sea state	RAO = 0.5 (detuned)	RAO = 1 (tracking)	RAO = 2 (moderate resonance)	RAO = 3 (strong resonance)
1 ft @ 3 s (light chop)	0.03 g	0.07 g	0.14 g	0.20 g
2 ft @ 3.5 s	0.05 g	0.10 g	0.20 g	0.30 g
3 ft @ 4.5 s	0.05 g	0.09 g	0.18 g	0.27 g
4 ft @ 8 s (swell)	0.02 g	0.04 g	0.08 g	0.12 g

Comfort reference (ISO 2631-style): <0.05 g easy living • 0.05–0.10 g fatiguing over hours • 0.10–0.20 g unpleasant • >0.20 g work becomes difficult.

Head-to-head (moderate chop: 2–3 ft @ 3.5–4.5 s)

Metric	Seastead as tested (19 klb)	Seastead loaded (39 klb)	50′ catamaran (22–30 klb)	60′ monohull (40–70 klb)
Dominant vertical period	3.1–4.4 s ⚠️ in chop band	4.4–6.3 s ✓ out of band	Tracks waves (~2–2.5 s heave)	3–5 s
Vertical accel, center	0.10–0.25 g	0.04–0.12 g	0.08–0.15 g	0.08–0.15 g
Vertical accel, extremities	uniform (symmetric platform)		0.15–0.35 g at bows	0.12–0.30 g at bow/stern
Roll	essentially none (tiny rocking mode)		< 2–3°	8–20° @ 7–9 s
Lateral accel from roll	≈ 0		< 0.02 g	0.05–0.15 g at deck edge
Heading sensitivity	omnidirectional		moderate	significant

Narrative verdict

As tested, the seastead is lighter than both yachts and tuned into the chop band, so its center-point vertical accelerations are probably comparable to or somewhat worse than the 50′ cat or 60′ mono in the same seas — but still only ~0.1–0.25 g, far from dangerous.
Loaded to 2/3 draft, it should beat both boats at the center point and crush them on lateral motion (no roll). The yachts' worst numbers are always at the bow/stern; the triangle has no bad seat.
In long swell (>6 s) everything is comfortable; the seastead is excellent there.
Caveat: yachts are usually compared underway; head-seas encounter shifts their periods. Zero-speed comparison shown above favors the loaded seastead.

6. 🎯 The Prediction: Double the Weight, Same Waterplane

Falsifiable predictions for your next test

Draft: 32 in ± 1.5 in at each leg (verify all three equal — checks trim/symmetry).
Ballast needed: ≈ 90 lb total (≈ 30 lb per corner).
Bob period (slowed video): grows from ~3.1–4.4 s to ≈ 5.4–6.3 s (best single guess ~5.8 s). Ratio must be ≈ √2 = 1.41 — this is pure physics, near-guaranteed.
Accelerations in the SAME wave field: drop to ≈ ½ of before (acceptable range 0.4–0.65×). If your current peaks are ~0.15–0.30 g, expect ~0.05–0.12 g.
Character change: resonant ringing at chop frequencies largely disappears; motion becomes slower, gentler, slightly larger in amplitude but at much lower frequency — and since a = ω²z, the lower ω wins. Even at its new resonance the g-levels are lower.
Rocking mode: slows from ~2.5 s to ~4.2 s full scale (longer still if ballast goes on deck).
Trade-off to watch: freeboard halves (16 ft → 8 ft full scale) and reserve buoyancy per leg drops — green-water risk begins around 7–8 ft seas full scale.

Why this must happen (physics)

Excitation force is set by waterplane area (Froude-Krylov + diffraction ∝ A_wp) — unchanged. Acceleration a = F/m with m doubled → a halves.
T_n ∝ √(m/A_wp) → grows by exactly √2 ≈ 1.41, moving out of the 2.5–4.5 s chop-energy band.
Deeper draft helps twice: wave orbital motion decays as e^−kd, so submerged excitation weakens further.
Cost: relative damping ζ = c/(2√(km)) decreases, so the resonant peak sharpens slightly — another reason to eventually add damping plates at the leg bottoms.

7. Recommendations for the Next Test

Put the ballast LOW — water bags or sand tubes lashed around each leg at the waterline, not on deck. Deck ballast raises KG and cuts GM (from ~23 ft toward ~10 ft full scale), softening the boat and lengthening the rocking period into the wave band.
Instrument it: phone accelerometer taped at deck center (g’s = full-scale g’s), a scale pole with 1-ft bands behind the model, and ideally an onboard camera for horizon reference.
Measure T_n directly: press the deck down ~2 in, release, and time the free oscillations in the slowed video. Compare to predicted 2.2–2.6 s model (5.4–6.3 s full). Count decay per cycle to get damping.
Static heel check: GM = w·d /(W·tanθ). E.g., 20 lb hung at one corner (d = 5.77 ft) should heel the loaded model only ~3°.
Log wind speed and keep the model orientation identical between runs; at 88→176 lb the model is light enough that windage on the tall exposed legs can bias results.
If resonance still shows after loading, bolt damping plates (12-in squares) at the bottom of each leg — cheap fix, big effect near T_n.

8. Assumptions & Limitations

Wave heights/periods attributed to your video are scenario estimates, not measurements — I could not view the footage.
Added mass (0.5–1.0 × displaced mass) and damping are engineering estimates for slender surface-piercing columns, not panel-code computations.
Linear theory assumed; steep chop adds harmonics that can bump accelerations ~10–20%.
Catamaran/monohull figures are representative values for typical vessels of those sizes, not specific designs.
Mooring-line snap loads and wind can contaminate light-model data; keep the tether slack.

Prepared from first principles + standard seakeeping practice. All model→full conversions use λ = 6 Froude scaling. Good luck with the loaded run — the √2 period check alone will validate the whole framework.