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Seastead Trimaran 1/10 Scale Design Calculations
Seastead Trimaran Design & 1:10 Scale Test Parameters
📐 1:10 Froude Scale Dimensions
| Component / Measurement | Full Scale (ft) | Full Scale (in) | Model Scale (ft) | Model Scale (in) |
|---|
| Triangle Side Lengths (Front → Back) | 80.0 ft | 960 in | 8.0 ft | 96.0 in |
| Triangle Base (Back Width) | 40.0 ft | 480 in | 4.0 ft | 48.0 in |
| Leg Length (Vertical Span) | 19.0 ft | 228 in | 1.9 ft | 22.8 in |
| Leg Chord Length | 10.0 ft | 120 in | 1.0 ft | 12.0 in |
| Leg Max Thickness ("Width") | 3.0 ft | 36.0 in | 0.3 ft | 3.6 in |
| Draft / Waterline Submersion (50%) | 9.5 ft | 114 in | 0.95 ft | 11.4 in |
🧱 Foam Mix Requirements (Per Scale Leg)
Calculated using a typical NACA cross-section area coefficient of ~0.66. Final density target: 2.0 lb/ft³.
- Final Expanded Volume (1 leg): ~0.376 ft³ (≈ 45.1 cups)
- Total Mixed Liquid Volume Needed: ~1.8 cups
- Per Part (assuming 1:1 volumetric mix & 25:1 expansion ratio): 0.9 cups Part A + 0.9 cups Part B
Liquid Volume = (Final Leg Volume) ÷ (Foam Expansion Ratio)
Typical 2-lb rigid foams expand 20:1 to 30:1. Check your datasheet for exact "Yield" or "Expansion Ratio" and adjust accordingly.
💡 Pro Tip: When pouring vertical leg molds, mix slightly less (≈10%) to account for trapped air and prevent overflow. Vent the top of the mold.
⚖️ Target Displacement / Weight
To maintain the designed 50% leg submersion, the total mass (legs + frame + payload + ballast) must equal the buoyant force of the submerged volume. Calculated using Caribbean seawater density (~63.9 lb/ft³).
| Parameter | Full Scale | 1:10 Model |
|---|
| Submerged Volume (3 legs, 50% each) | ~564.3 ft³ | ~0.564 ft³ |
| Target Total Weight (in Caribbean Water) | ~36,090 lbs (16.4 tons) | ~36.1 lbs |
🚤 Towing Test Speed & Power Conversion Constant
For Froude number similarity, model speed scales with the square root of the linear scale ratio.
| Parameter | Value |
|---|
| Full Speed Target | 5.0 knots |
| Model Towing Speed | 1.58 knots |
| Model Towing Speed (Imperial) | 2.67 ft/s |
Thruster Power Conversion Constant
Multiply your scale model towing sensor reading (lbs) by the constant below to estimate the electrical watts required at the thrusters for full scale at 5 knots.
P_full (Watts) = R_model (lbs) × 11,440
Derivation (Froude Scaling):
Force scales as L³. Velocity scales as L⁰·⁵. Power scales as Force × Velocity = L³·⁵.
λ = 1/10 → Scale Factor = (1/0.1)³·⁵ × V_fs(ft/s) × 1.35582(W/ft-lb/s) ≈ 3,162 × 8.439 × 1.356 ≈ 11,440
📈 Natural Roll Period Scaling
Hydrodynamic periods scale with the square root of linear dimensions.
T_full = T_model × √10
Multiply Measured Model Roll Period By: 3.162
⚠️ Engineering Notes & Assumptions
- Buoyancy Estimate: Leg cross-sectional area assumes a typical NACA profile area coefficient of ~0.66 × chord × thickness. Exact area varies by series (e.g., NACA 0015 vs 4412). Adjust volume if your specific profile differs significantly.
- Froude vs. Reynolds Scaling: The thrust constant (×11,440) assumes pure Froude scaling. In reality, viscous drag (surface friction) does not scale perfectly at 1:10. Add a 15–25% margin for real-world thruster sizing, or apply ITTC friction correction if doing rigorous model testing.
- Target Weight: Includes ALL mass. If the bare frame + floats weighs more than ~36,090 lbs (full) or ~36.1 lbs (model), you will sit deeper than 50%. If lighter, you'll need to add ballast.
- Seawater Density: Caribbean tropical water (~80°F / 27°C) typically ranges 63.8–64.0 lb/ft³ due to temperature & salinity variations. Calculations use 63.9 lb/ft³.
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