Foiled Semi‑Submersible Trimaran Seastead
1:10 Model Test Analysis & Full‑Scale Projections

Desktop study based on your stated geometry (80′×80′×40′ triangular truss, three NACA 0030 legs, 19′ long, 50% immersed) and the 1:10 model wave trial.

Scale 1:10 (Froude) Status: Preliminary — verify with instrumented tests Rev A

1 · Executive summary

Displacement (full scale)≈ 37,500 lb587 ft³ · 16.7 long tons

Waterplane area≈ 62 ft²vs ~300–600 ft² for the boats

Heave period3.4 – 3.9 ssits in the 4-s chop band — see §7

Pitch / Roll period2.2 s / 3.2 s

Est. test waves (full scale)≈ 0.5 – 1.0 mmodel ≈ 5–10 cm — verify §2

Chop accelerations≈ 1/3 – 1/5of a 50′ cat in 2–3.5 s chop

The concept works as advertised in short chop. Three small, widely-spaced waterplanes de-phase incoming waves; in 2–3.5 s chop the full-scale platform should see roughly one-third to one-fifth of the vertical acceleration of a 50′ catamaran or 60′ monohull. This is the ride-quality payoff.
One genuine watch-item: the heave natural period (~3.4 s rigid, ~3.9 s with added mass) lands in the most energetic wind-sea band. With the small waterplane comes small damping, so an undamped resonant peak of RAO ≈ 2 is plausible. Cheap fix: damping plates near the bottom of each leg (they can double as thruster mounts). See §7.
Your stabilizers have huge authority. At 6 kt each 10 ft² stabilizer produces ≈ 4,500 lbf/rad of lift slope → ±1,150 lbf and ±40,000 ft·lb of pitch authority at ±5°. They will easily damp pitch/roll resonance when underway — but do nothing at zero speed (hence the damping-plate recommendation).
Payload is the tightest number on the page. 37,500 lb total for 1,550 ft² of enclosed living space implies an ultra-light build. Example budget sums to ~37,000 lb — essentially zero margin. Options in §9.5.
The foil legs are worth ~10× in drag. At 6 kt the shaped legs cost ~250 lbf (bare); equivalent-volume round columns would cost ~3,400 lbf. Cruise ~6 kt on ~8–9 kW electric.
Solar synergy: ~144 m² of roof → ~23–29 kWp → roughly 90–120 kWh/day, enough for solar-only transit at 3–4 kt in sunny weather.

Honesty note I cannot stream or watch the YouTube video from where I sit, so every wave-height number in §2 is a labeled engineering estimate, not a measurement. §2 also contains a 10-minute protocol (your 22.8″ legs are a built-in ruler) so you can replace my estimate with a real one — and if you send me crest/trough heights and a wave count, I will tighten every downstream number.

2 · Wave height in the video, and ×10 at full scale

2.1 My estimate (to be verified)

For a 8-ft model tested in open water (bay/lake chop or boat wake), typical significant wave heights run 5–10 cm. Visually, waves that occupy roughly a quarter to a third of the exposed (11.4″) leg height correspond to this band.

Quantity	Model (measured in video)	×10 full scale	Basis
Significant wave height H_s — estimate	5–10 cm (2–4 in)	0.5–1.0 m (1.7–3.3 ft)	Typical conditions for a model this size; central guess ≈7 cm → 0.7 m
Dominant wave period T_p — estimate	1.0–1.5 s	3.2–4.7 s	Short wind-sea / wake range
Equivalent sea state	Douglas sea state 2–3 (smooth → slight)		Mild test conditions

Escalation clues If in the video you see (a) waves climbing more than ~2/3 of the exposed leg, (b) spray over the deck, or (c) the underside of the truss wetted, then the true H_s is above my band — more like 15–20 cm model → 1.5–2.0 m (5–6.5 ft) full scale. Conversely, if ripples barely disturb the waterline on the legs, you are at the bottom of the band (≈0.4–0.5 m full scale).

2.2 Ten-minute protocol to replace my estimate with a measurement

Pick a ruler. A leg seen broadside: exposed height above water = 11.4″ (top half of 22.8″). The 96″ triangle sides work too when the camera is far back.
Calibrate pixels. In a paused frame, measure the ruler in pixels: R px per inch.
Measure waves away from the model (at least one leg-length away, to avoid reflection/diffraction contamination). Track a fixed patch of water: crest-to-trough in pixels → inches.
Average 15–20 consecutive waves. H_s ≈ mean of the highest one-third; a quick proxy is 2×RMS of the surface elevation.
Period: note fps; count frames between successive crests at a fixed point → T_model; ×3.162 → T_full.
Heave of the model: track the waterline on a leg frame-by-frame → z(t). Its oscillation period should be ~1.1–1.25 s (see §5) — if the model bobs at that rate regardless of wave period, you are watching resonance; if it tracks the waves, you are seeing forced response.

Worksheet — fill from video	Model	Full scale (×10, ×3.162)
Ruler length used (in)		—
Mean crest-to-trough H (in)
H_s (highest 1/3 mean, in)
Wave period T (s)
Model heave double-amplitude (in)
Implied heave RAO (heave/H)

3 · Froude scaling rules (1:10)

All projections below use Froude scaling, the correct regime for waves and motions:

Key experimental fact Because acceleration scales 1:1, any acceleration you measure in g’s on the model applies directly to the full-scale platform in the dynamically similar sea (height ×10, period ×3.162). Your video is unscaled real-time, so model periods look fast: multiply all times by 3.162 for full scale.

4 · Full-scale hydrostatics

4.1 Geometry & displacement

NACA 0030 section, chord c = 10 ft: section area = 0.2055·c² = 20.6 ft². Each leg is prismatic over 19 ft; immersed length 9.5 ft.

Item	Full scale	Model (1:10)	Note
Triangle sides / base	80 / 80 / 40 ft	8 / 8 / 4 ft	Apex forward
Deck area (triangle)	1,549 ft²	15.5 ft²	Enclosed living floor
Leg section area (NACA 0030)	20.6 ft²	0.206 ft²	0.2055·c²
Immersed volume per leg	195 ft³	0.195 ft³	9.5 ft immersed
Total displacement ∇	587 ft³	0.587 ft³ (4.4 gal)
Displacement Δ (salt water)	37,500 lb (16.7 LT)	≈37 lb	64 lb/ft³
Draft	9.5 ft	11.4 in	50% of leg
Reserve buoyancy to top of legs	+100% (≈75,000 lb max)	+100%	Healthy flood margin
Waterplane area A_wp	61.8 ft² (3 × 20.6)	0.62 ft²	The defining number
Floor height above WL	9.5 ft	11.4 in	Dry deck in the test seas
Air draft to roof	≈16.5 ft (+panels)	≈20 in

4.2 Stability (initial)

Item	Value	Comment
Transverse GM	≈ 28 ft	Multihull-stiff; ΣA_wpy²/∇ with legs at ±20 ft
Longitudinal GM	≈ 140 ft	Very stiff (apex leg 51.6 ft fwd of centroid, pair 25.8 ft aft)
Static heel, 50-kt beam wind	≈ 3.5–4°	~630 ft² profile, ~65,000 ft·lb heeling moment
Quasi-static heel in H_s=2 m, T=6 s beam seas	≈ 3–5°	Platform largely follows local wave slope — gentle, synchronized tilt

5 · Natural periods — and what your model video should show

T_heave = 2π√( m / (ρg·A_wp) ) a = ζ_a·ω²·RAO λ = 1.56·T² (deep water, meters)

Mode	Seastead full scale	Seastead model (check vs video!)	50′ cat (typical)	60′ mono (typical)
Heave T_n	3.4 s rigid → 3.9 s w/ added mass	1.08 – 1.23 s	2.3 – 2.7 s	2.8 – 3.4 s
Pitch T_n	≈ 2.2 s (2.0–2.4)	0.63 – 0.76 s	2.2 – 2.6 s	3.0 – 3.5 s
Roll T_n	≈ 3.2 s	≈ 1.0 s	2.5 – 3.5 s	3.5 – 5 s

Seastead values from Δ=37,500 lb, A_wp=61.8 ft², pitch radius of gyration ≈22 ft, roll ≈13.5 ft, added-mass allowances +30%/+20%/+25%. Boat values are typical published ranges — individual designs vary.

Video signatures to check

Ring-down bob after each wave passes: ≈1.1–1.25 s period (heave).
Very small pitching — the 8-ft triangle should stay close to level; any rocking near 0.7 s is pitch resonance.
Waves passing between the legs with little reflected wash — the visual signature of small waterplane.
Deck stays dry (floor 11.4″ above WL vs ≤4″ waves).
Towed RIB surging/snapping on the ropes in each wave — expect livelier motion than the mothership.

6 · How the full-scale platform moves, by wave-period band

Indicative heave response operators (linear-theory estimates, not measurements). The seastead’s advantage is the deep valley at 2–3 s; its liability is the peak near 3.9 s if undamped. With damping plates the peak flattens toward ~1.3–1.5.

Wave band (full scale)	λ (deep water)	Seastead heave RAO	Cat / Mono heave RAO	Interpretation
2 – 3.5 s (harbor & coastal chop)	20–63 ft	0.15 – 0.35	0.7 – 1.1	Legs sample waves out of phase → forcing largely cancels. Biggest win.
3.5 – 4.5 s	63–104 ft	1.2 – 2.0 undamped; 1.1 – 1.5 with plates	0.8 – 1.0	Seastead resonance band. Manageable with damping (§7).
5 – 7 s	128–252 ft	0.5 – 0.8	0.85 – 1.0	Partial cancellation persists (77-ft leg spread vs λ).
> 8 s (swell)	> 330 ft	→ 1.0	→ 1.0	Everyone rides the swell; differences vanish.

Character of the motion: periods of 3–4 s mean the platform makes quick, small movements rather than slow ponderous ones. At the amplitudes estimated here (<0.1 g typical) this is well inside comfortable ISO-2631 territory; motion sickness correlates with acceleration magnitude in the 0.1–0.5 Hz band, and the seastead’s magnitudes are low precisely where boats are highest.

7 · The 4-second issue, and the fixes

Watch item Small waterplane cuts both ways: excitation is small, but so is damping. At T_n,heave ≈ 3.9 s (an energy-rich band), an undamped resonant RAO of ~2 is plausible. This does not negate the concept — it means one cheap modification should be designed in from the start.

7.1 Fix #1 — Damping plates (recommended)

Fit a plate ~6 ft (chordwise) × 8 ft (athwartships) near the bottom of each leg, e.g. 2 ft above the leg foot.
Eddy-making damping from plates is disproportionately strong at small waterplane; expect resonant RAO to drop from ~2+ to ~1.3–1.5, i.e. peak accelerations cut roughly in half.
Bonus uses: mounting ring for the six RIM drives, grounding/beaching protection, and a convenient ballast shelf (ballast low in the legs also lengthens T_n — loading to 60% immersion moves heave T_n to ~4.25 s).

7.2 Fix #2 — Your stabilizers (underway)

Stabilizer parameter (each, 10 ft span × 1 ft chord)	Value @ 6 kt
Lift slope dL/dα (efficiency 0.7 → 4.4 /rad)	≈ 4,500 lbf/rad
Force at ±5° elevator/incidence	± 390 lbf
All three units	± 1,150 lbf
Pitch-moment authority (arms 51.6 / 25.8 / 25.8 ft)	± 40,000 ft·lb (≈0.44° static trim per full throw — ample for dynamic damping)
Roll-moment authority (aft pair, ±20 ft)	± 15,600 ft·lb

Authority is so large that the control law matters more than the hardware: use rate feedback with saturation (≈±8° hard limit before stall), and notch the controller near T_n to avoid over-correction.
The pivot-plus-elevator arrangement you describe is exactly right: the small actuator sees only hinge torque while the whole wing provides the moment. Moving the pivot to the ~25%-chord notch keeps the wing aerodynamically balanced and passive-stable.
Limitation: at zero speed (anchored/drifting) fixed stabilizers do nothing. That is why the passive damping plates matter — they work 24/7. (Active flapping of the fins is possible but power-hungry.)

8 · Compared to a 50′ catamaran and a 60′ monohull

8.1 Head-to-head parameters

Parameter	Seastead (this design)	50′ catamaran (typical)	60′ monohull (typical)
Length / Beam	80 ft / 40 ft	50 ft / 26–28 ft	60 ft / 15–17 ft
Displacement	37,500 lb	35,000–55,000 lb	40,000–70,000 lb
Waterplane area	62 ft²	250–400 ft²	450–650 ft²
T_n heave / pitch / roll	3.9 / 2.2 / 3.2 s	2.5 / 2.4 / 3.0 s	3.1 / 3.2 / 4.2 s
Living area	1,549 ft² enclosed	~900–1,100 ft²	~700–900 ft²
Draft	9.5 ft (fixed)	3.5–4.5 ft	6–8 ft

8.2 Estimated vertical accelerations (significant, in g)

Method: a = ζ_a·ω²·RAO at CG, plus θ_a·ω²·x at the bow (x = 51.6 ft for the seastead apex, ~22 ft cat, ~27 ft mono). Linear estimates; treat as ±30%.

Estimated significant vertical accelerations. In short chop (A) the seastead wins by a factor of 3–5. In the 4-s band (C) it is competitive only with damping plates. In long swell (D) all platforms converge.

Sea state	Seastead CG	Seastead bow (apex)	50′ cat CG	60′ mono CG
A — H_s 1 m, T 3 s (steep chop)	0.02–0.04 g	0.03–0.05 g	0.10–0.18 g	0.08–0.15 g
B — H_s 1.5 m, T 5 s (typical coastal)	0.05–0.07 g	0.07–0.10 g	0.09–0.12 g	0.09–0.12 g
C — H_s 1 m, T 4 s (resonance band)	0.06–0.09 g (w/ plates)	0.08–0.12 g	0.07–0.10 g	0.07–0.10 g
D — H_s 3 m, T 8 s (open-ocean swell)	0.06–0.09 g	0.08–0.11 g	0.07–0.10 g	0.08–0.11 g

8.3 Roll and lateral comfort (beam seas)

H_s 2 m, T 6 s, beam seas	Significant roll	Lateral accel at deck edge
Seastead	3–6°	0.03–0.06 g
50′ catamaran	4–8°	0.05–0.08 g
60′ monohull	10–15° (typical, unstabilized)	0.12–0.20 g

8.4 Narrative — how it will actually feel

Head-seas chop (2–3.5 s): the marquee advantage. The two aft legs and the forward leg receive waves tens of degrees out of phase; heave and pitch forcing largely cancel. Expect a steady, slightly bobbing platform while an equivalent cat is hobby-horsing. Drinks stay on the table.
The 4-second band: the one condition where the seastead is not automatically better. Undamped it can match or exceed boat-level accelerations at resonance; with damping plates it returns to parity-or-better. Design the plates in.
Long swell (8 s+): physics converges — everything of this size follows the swell. The seastead’s bow stays a bit quieter thanks to longitudinal de-phasing, and there is no mono-style roll to compound it.
Beam seas: near-level, small synchronized tilt with the local wave slope (classic small-waterplane behavior) instead of the monohull’s 10–15° rolls. Best-in-class lateral comfort.
Following seas: less surf-driving than a boat (little waterplane to push), but the low inertia means it will be nudged around; the towed RIB will surge — use a bridle with rubber snubbers and watch chafe.
Anchored / drifting: heave/pitch advantages persist (they are properties of geometry, not speed). Roll and sway in swell are ordinary orbital following — no platform escapes that. Windage is moderate (~630 ft² profile); it will lie roughly beam-to-quartering wind; the six drives hold heading easily.
Quirk to set expectations: this is a small waterplane platform, not a big-inertia one like a production semi-submersible rig (whose periods are 20–60 s). Its periods are 3–4 s — quicker than a ship’s. The comfort comes from tiny excitation, not from sluggishness. At the estimated amplitudes this reads as “gentle quick bob,” which humans tolerate well.

9 · Supporting engineering notes

9.1 Drag, speed and power — why the foil shape matters

@ 6 kt (V = 10.1 ft/s)	Foil legs (NACA 0030)	Round columns, same volume
Wetted surface (3 legs)	612 ft²	459 ft²
Friction drag	≈ 250 lbf	≈ 190 lbf
Form/separation drag	≈ nil (streamlined)	≈ 3,300 lbf (C_d≈0.7 on 146 ft² frontal)
Total with appendages	≈ 400 lbf	≈ 4,000 lbf
Shaft power / electric power	7.4 hp / ≈ 8–9 kW	74 hp / ≈ 70+ kW

The foil shape buys roughly a ten-fold drag reduction at cruise — not from friction (it actually has more wetted area) but by eliminating bluff-body separation and its wave-making. Expect a comfortable 6–8 kt cruise and ~10–12 kt top from six 1.5-ft RIM drives, subject to their thrust curves. The drives sit ~6.5 ft below the waterline: excellent immersion, cavitation margin, silence, and swimmer safety.

9.2 Solar

Roof 1,549 ft² = 144 m² → ~23–29 kWp at modern panel densities → roughly 90–120 kWh/day in good sun. That covers ~10–12 hours of 6-kt cruising, or indefinite 3–4 kt station-keeping/transit in sunny weather — a genuinely notable operational feature.

9.3 Structural load case (global)

Worst simple case: 4-ft crest on one leg only. Differential buoyancy = ρg·A_wp·h = ±5,300 lbf per leg per 4 ft of wave. About the centroid this gives ±272,000 ft·lb (apex hogging/sagging). With the 7-ft truss depth, chord loads are only ≈±39,000 lbf — very manageable for an aluminum or steel space frame; spend your engineering effort on the three leg-to-truss joints instead.

9.4 Dinghy tow

Sideways on two ropes behind the wind shadow is a sensible arrangement. Points to engineer: bridle (not single rope) to stop yawing, elastic snubbers for surge, hard chafe protection at the transom fairleads, and a quick-release. In following seas the RIB’s surge loads spike — size for 2–3× static weight dynamically.

9.5 Payload reality check (important)

Example weight budget	lb
Truss structure + leg attachments	13,000
Glazing (lots of glass)	2,500
Solar array + mounts	3,200
Batteries (20 kWh LFP)	500
6 RIM drives + controls	700
3 stabilizers + actuators	500
RIB + outboard	600
Water & tanks	2,400
Interior fit-out	7,000
Electrical / plumbing / HVAC	2,600
Safety gear, anchors, rode	900
Crew (4) + stores	1,600
Margin / sundries	1,500
Total	≈ 37,000

Flag That consumes ~99% of the 37,500-lb design displacement. Three ways out: (a) immerse legs to 60% (Δ → 45,000 lb; floor still 7.6 ft above WL; T_heave → 4.25 s — arguably better), (b) grow chord to 12 ft (+20% ∇ and A_wp; T_heave → ~3.6 s), or (c) austere fit-out. Option (a) is the cheapest and improves the resonance picture.

10 · Next steps & what to send me

From the video: crest/trough heights (pixels or inches), wave period, model heave time series if you can track it — I will convert these into measured RAOs and recalibrate every table above.
Instrument the next test: a $30 phone inside a waterproof case logging accelerometer data gives you g-levels directly — and remember, model g’s = full-scale g’s for Froude-similar seas.
Add turbulence trips (grit strip) to model legs before drawing any drag conclusions — at model Reynolds numbers the boundary layer is laminar and unrepresentative.
Design iteration: decide 50% vs 60% immersion early (it sets payload, freeboard, and resonance together), and carry damping plates into the next model build so you can A/B test the resonance fix.
Formal verification eventually: a naval architect’s weight & stability review, and a seakeeping run (even a strip-theory code) to replace these first-principles estimates.