Each leg is a NACA 0030 foil, chord 8.5 ft, with the last 0.5 ft of the trailing edge removed. The cross-sectional area of a NACA 0030 foil is approximately:
A ≈ 0.685 × t × c = 0.685 × (0.30 × 8.5) × 8.5 ≈ 14.85 ft²
Subtracting the small trailing-edge cutoff (a thin sliver ~0.05 ft²), call it ~14.8 ft² of waterplane area per leg.
Buoyancy of one additional foot of submersion:
F = 14.8 ft² × 1 ft × 64 lb/ft³ ≈ 947 lbs per foot
So each leg gains/loses roughly ~950 lbs of buoyancy per foot of vertical motion, or about 79 lbs per inch.
The stabilizer main wing: 10 ft span × 2 ft chord = 20 ft² area. Assume aspect ratio 5, NACA-style foil, operating at a modest angle of attack (≤ ~8°) to stay well below stall. Lift coefficient up to ~0.7 is realistic. Drag coefficient at that lift ≈ 0.04 (profile + induced).
Lift in seawater (ρ = 1.99 slug/ft³):
L = ½ ρ V² S C_L
With V in ft/s (1 knot = 1.688 ft/s) and full deflection C_L ≈ 0.7:
| Speed | V (ft/s) | Max Lift (lbs) | Drag at max lift (lbs) | Drag Power (W) |
|---|---|---|---|---|
| 4 kt | 6.75 | 635 | 36 | 330 |
| 5 kt | 8.44 | 990 | 57 | 650 |
| 6 kt | 10.13 | 1,430 | 82 | 1,120 |
| 7 kt | 11.82 | 1,945 | 111 | 1,780 |
| 8 kt | 13.50 | 2,540 | 145 | 2,660 |
Convert max lift into "inches of leg buoyancy" using 79 lb/inch:
| Speed | Lift / 79 lb·in⁻¹ | Inches removed from crest (or trough) | Total wave-height reduction |
|---|---|---|---|
| 4 kt | 8.0 in | ~8 in | ~16 in |
| 5 kt | 12.5 in | ~12 in | ~25 in |
| 6 kt | 18 in | ~18 in | ~36 in |
| 7 kt | 24.5 in | ~24 in | ~49 in |
| 8 kt | 32 in | ~32 in | ~64 in |
Batch of 20, fabricated in China:
| Component | Est. cost (USD each) |
|---|---|
| Main wing (10 ft × 2 ft, aluminum skin over ribs, foam-filled) | $700 |
| Fuselage / pivot housing (6 ft, aluminum tube + bearings) | $450 |
| Elevator (2 ft × 6 in) + linkage | $120 |
| Servo-tab actuator (marine, ~100 W, waterproof) | $350 |
| Position sensors, wiring, connectors | $120 |
| Control computer (own MCU + IMU, redundant) | $180 |
| Anodizing / paint / hardware | $150 |
| Assembly & test labor | $300 |
| Subtotal per stabilizer | ~$2,370 |
| Shipping + import + margin (×1.4) | ~$950 |
| Delivered cost | ~$3,300 each |
Three per seastead → roughly $10,000 for the full active stabilizer set.
At ~$10k for a complete trio, this is a highly attractive option. For comparison:
Critically, the stabilizers attack the dangerous case of resonant motion buildup, where a sequence of waves near the platform's natural period can amplify motion. Even one working stabilizer kills the Q of that resonance. I'd estimate ~80–90% take rate among customers — most seastead buyers will see this as a must-have safety and comfort feature for the price.
Deep-water wavelength:
L = (g/2π) T² ≈ 5.12 × T² ft = 5.12 × 144 ≈ 737 ft
The Caribbean is mostly deep water for this period, so ~737 ft (about 225 m) wavelength.
Seastead length (corner-to-opposite-side, equilateral triangle 44 ft on a side):
h = 44 × √3 / 2 ≈ 38.1 ft
The maximum slope of a sine wave of amplitude A = 6 ft and wavelength 737 ft is:
slope_max = 2π A / L = 2π × 6 / 737 ≈ 0.0512 (≈ 2.93°)
Across 38.1 ft of seastead at peak slope:
Δh = 0.0512 × 38.1 ≈ 1.95 ft ≈ 23 inches
So one end can be about ~2 feet higher than the other at the steepest part of a 12 ft / 12 s swell.
The seastead moves at, say, 5 knots forward; the swell at celerity c = gT/2π ≈ 61 ft/s ≈ 36 kt overtakes us. Encounter speed of water past the stabilizer wings is dominated by orbital motion + boat speed: a few knots. At ~5 kt boat speed, each stabilizer can produce roughly ±1,000 lbs vertical force.
To create a pitching moment: front stabilizer pushes down 1,000 lb, two rear ones lift 500 lb each. Moment arm front-to-back ≈ 25 ft → pitching moment ≈ 25,000 ft·lb.
Restoring moment per degree of pitch (from waterplane geometry): each leg's waterplane × moment arm² gives a pitch stiffness of roughly 6,000–8,000 ft·lb per degree. So 25,000 ft·lb ≈ 3–4 degrees of active pitch correction, which is roughly the entire wave slope of this swell. Excellent.
In a beam sea the roll moment arm uses the full triangle width (~44 ft) and only one stabilizer needs to push down while the other two lift in roll-coordinated fashion. The geometric advantage is roughly 1.5–2× better, and the roll-restoring stiffness of the triangular waterplane is also higher. Net: in a beam sea the active stabilizers can likely null out 80–100% of the roll induced by the 12-ft swell, an even bigger gain than in head sea.
At zero forward speed the wing has no flow to give it hydrodynamic balance, and the unbalanced 25%/75% chord about the pivot will cause it to flop with vertical heave. Solution: a dedicated brake/lock on the pivot shaft.
| Caliper + disk + pads (marine SS) | $140 |
| Solenoid release + spring pack | $60 |
| Sealed housing + wiring | $50 |
| Assembly | $40 |
| Delivered cost per stabilizer | ~$290 |
|---|
So adding a brake/lock to all 3 stabilizers is roughly $900 extra per seastead.
Two competing effects:
In typical 3-ft seas, vertical leg motion reduction from active stabilization saves perhaps 5–10% of total propulsion power (rough estimate; depends strongly on wave spectrum). Stabilizer drag, averaged over a wave cycle (not always at full lift), is roughly 40% of the peak figure in Table 2.
| Speed | Avg. stabilizer drag power per unit (W) | Total 3 units (W) | Estimated leg-motion savings (W) | Net extra power (W) |
|---|---|---|---|---|
| 4 kt | 130 | 400 | 150 | ~250 |
| 5 kt | 260 | 780 | 350 | ~430 |
| 6 kt | 450 | 1,340 | 700 | ~640 |
| 7 kt | 710 | 2,130 | 1,200 | ~930 |
| 8 kt | 1,060 | 3,190 | 1,900 | ~1,290 |
So the realistic penalty for running the stabilizers underway is on the order of 0.25–1.3 kW, far less than the naive "max-deflection drag" calculation would suggest, and well within what the solar roof can provide during daylight.