Seastead Froude Scale Model Design
Scale Ratio Determination
Your available 5-inch diameter cylinders dictate the scale ratio using Froude scaling laws:
Calculation:
Full-scale leg diameter = 4 feet = 48 inches
Model leg diameter = 5 inches
Scale Ratio (λ) = 5/48 ≈ 0.1042 or 1:9.6
Froude Scaling Factors
- Lengths: Multiply by 5/48 (≈0.1042)
- Areas: Multiply by (5/48)² ≈ 0.0109
- Volumes/Weights: Multiply by (5/48)³ ≈ 0.00113
- Velocities: Multiply by √(5/48) ≈ 0.323
- Time: Multiply by √(5/48) ≈ 0.323
Model Dimensions Table
| Component |
Full Scale |
Model Dimensions |
Notes |
| Body Length |
60' (720") |
75.0" (6'-3") |
~6.25 feet long |
| Body Width |
14' (168") |
17.5" |
1'-5.5" wide |
| Body Height |
8' (96") |
10.0" |
10 inches tall |
| Leg Diameter |
4' (48") |
5.0" |
Your available material |
| Leg Length |
35' (420") |
43.75" |
3'-7.75" (43¾") |
| Leg Submergence |
21' (60%) |
26.25" |
Waterline at 60% of leg length |
Cable Length Calculations
Assuming legs angle at 45° in both front and side views (resulting in true angle of 54.7° from vertical):
- Leg Tip Spread (each direction): 43.75" × sin(45°) × sin(45°) ≈ 21.9" horizontal offset from attachment point
- Front Triangle Cable (between front leg tips):
If legs attach at front corners: ~68" (5.7 feet)
If legs attach at center point: ~51" (4.25 feet)
- Diagonal Cables (leg bottom to opposite body center): ~106" (8.8 feet) each
Weight & Buoyancy Calculations
Model Weight Target
To achieve 60% submergence of the legs in fresh water (62.4 lbs/ft³):
Submerged volume per leg = π × (2.5")² × 26.25" ≈ 515 in³ = 0.298 ft³
Buoyancy per leg = 0.298 × 62.4 ≈ 18.6 lbs
Total Model Weight Required: 4 × 18.6 ≈ 74-75 lbs
This 75 lbs includes:
- The body structure
- The four leg structures
- Cables and fittings
Full-Scale Weight Equivalent
Scaled back up to full size:
74.4 lbs ÷ (5/48)³ = 74.4 ÷ 0.00113 ≈ 65,800 lbs (33 tons)
This is the approximate displacement/weight of your full-scale seastead design.
55-Gallon Barrel Body Analysis
If you use two standard 55-gallon plastic drums (each ~22.5" diameter × 33.5" tall) connected end-to-end:
| Parameter |
Barrel Model Size |
Represents Full Scale |
Design Spec |
Deviation |
| Length |
67" (2×33.5") |
53.6 feet |
60 feet |
-11% (close) |
| Width |
22.5" (diameter) |
18.0 feet |
14 feet |
+29% (too wide) |
| Height |
22.5" (diameter) |
18.0 feet |
8 feet |
+125% (much too tall) |
Recommendation: The barrels are usable for length but problematic for width and height.
Options:
- Cut the barrels: Slice vertically to reduce width to ~17.5", flatten or cut down height to 10"
- Use as-is: Accept that your model represents a 54'×18'×18' structure rather than 60'×14'×8'
- Alternative: Build a custom body from foam, wood, or fiberglass to exact 75"×17.5"×10" specs
Construction Guidelines
Materials
- Legs: 5" diameter cylinders (PVC pipe, heavy cardboard tubes, or foam cylinders wrapped in fiberglass)
- Body: Wood frame with fiberglass or thin plywood skin (target weight: ~40-50 lbs for body alone)
- Cables: 50-100 lb test monofilament fishing line or braided Spectra/Dyneema
Ballasting the Legs
Each 5"×43.75" leg must displace exactly 18.6 lbs of water when 60% submerged to maintain equilibrium.
- If legs are hollow sealed PVC: Add lead shot or steel ballast inside until each leg weighs ~18.6 lbs minus its share of body weight
- Alternatively, make legs from solid foam (density ~37 lbs/ft³) which naturally floats at 60% submerged
Assembly Sequence
- Build the 75"×17.5"×10" body frame
- Attach leg mounting points at 45° angles (compound angle of 54.7° from vertical)
- Install the four 43.75" legs with temporary flotation
- Connect front triangle cable and back triangle cable
- Install four diagonal cables from leg bottoms to opposite body centers
- Adjust ballast until waterline is exactly 26.25" up each leg (60% of 43.75")