🚢 Seastead Trimaran — Leg Profile Optimizer
Compares NACA 0040 / 0030 / 0025 leg profiles for stability, speed, wave response & cost
Fixed Design Parameters (all profiles): Leg Length = 39 ft |
Chord = 10 ft | 3 Legs total | Draft = 50% of leg length |
Leg width (thickness) varies per NACA # to maintain constant volume
NACA 0030 Baseline: Max Width = 3.00 ft (30% × 10 ft chord) |
Volume per leg = 835 ft³ | Submerged volume = 417 ft³ per leg
Approach: Width adjusted so volume is identical for all three profiles.
Thicker foils (0040) are narrower; thinner foils (0025) are wider.
📊 Comparison Results
| Metric |
NACA 0040
(Thick, Narrow) |
NACA 0030
(Baseline) |
NACA 0025
(Thin, Wide) |
📝 Key Findings & Notes
- Volume is constant across all three profiles (~835 ft³ per leg). Thicker foils (0040) are narrower to compensate, giving less waterplane area.
- Natural heave period depends on waterplane area and total mass (structure + added mass). Smaller waterplane = shorter natural period = potentially moves away from wave period.
- Near resonance (Tnatural ≈ Twave), response is huge without damping. The stabilizer is absolutely critical in this regime.
- Below resonance (Tnatural > Twave), response is naturally moderate. The stabilizer still helps but is less critical.
- Above resonance (Tnatural < Twave), the seastead is too stiff to follow the wave — response is low even without stabilizer.
- Speed is estimated from thrust = drag on submerged foils (skin friction + form drag, Cd ≈ 0.012). Higher power → higher speed → more stabilizer force.
- Stabilizer effectiveness scales with V² — doubling speed quadrifies the stabilizer's damping force.
- Added mass coefficient = 1.0 for submerged elliptical foils (conservative). Actual value depends on proximity to surface and foil shape.
- Weight & cost assume solid marine aluminum construction. Actual design may use hollow sections, reducing weight by 40–60%.
- Try changing wave period to see how dramatically near-resonance behavior changes. At Twave = 7s, NACA 0030 moves away from resonance and performs much better.
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## What This Tool Does & Key Physics
### The Core Tradeoff You're Exploring
The three NACA profiles, all with **identical volume (~835 ft³ per leg)**, create a fascinating design space:
| Profile | Width | Waterplane Area | Natural Period | Character |
|---------|-------|-----------------|----------------|-----------|
| **NACA 0040** (thick) | 2.45 ft | ~25 ft² | ~5.0 s | Short period, near/at resonance at 5s wave |
| **NACA 0030** (baseline) | 3.00 ft | ~31 ft² | ~8.8 s | Below resonance, moderate response |
| **NACA 0025** (thin) | 3.60 ft | ~37 ft² | ~11.4 s | Well below resonance, small RAO |
### Why This Matters
**Resonance is the dominant effect.** When the natural heave period equals the wave period, the response amplification without damping is:
$$RAO = \frac{1}{2\zeta} \approx \frac{1}{2 \times 0.015} \approx 33\times$$
For a 2.5 ft wave amplitude, that's **~80 ft of heave** — completely unlivable. The stabilizer (adding ~0.15–0.18 effective damping ratio) brings this down to 3–7 ft.
**NACA 0040** has the smallest waterplane area → shortest natural period → potentially moves **above** resonance where response is naturally small. But at exactly 5s wave period, it's right at resonance — making the stabilizer life-or-death critical.
**NACA 0025** has the largest waterplane area → longest natural period → sits well **below** resonance where the RAO is naturally ≤1.0. Less need for stabilizer, but larger legs mean more drag and cost.
### The Stabilizer Effectiveness
The stabilizer force scales as **V²** — so doubling speed quadruples effectiveness. At the relatively modest speeds achievable (4.5–5.5 knots with 10 kW), the stabilizer provides 0.1–0.4 ft of equivalent heave reduction. This is meaningful when the baseline heave is 1–7 ft, but the stabilizer's real value is **preventing resonance catastrophe**, not just trimming small motions.
**Try changing the wave period to 7–8 seconds** in the tool — you'll see how dramatically the response changes as we move away from resonance.
### Suggestions for Your Design
1. **NACA 0030 at 10 kW is the sweet spot** — it balances waterplane area, speed, and avoids being right at resonance
2. **The stabilizer is non-negotiable** — without it, any profile near resonance will have catastrophic heave
3. **Consider increasing power to 20–30 kW** — speed scales as power^(1/3), but stabilizer force scales as V², so more power has a compounding benefit on stability
4. **Hollow aluminum construction** would reduce weight by 40–60%, improving the power-to-weight ratio and speed