Froude Scaling Basics: To make a scale model that behaves physically like the full-size version in water (wave interactions, wake, and buoyancy), we use Froude scaling.
If the geometric scale factor is λ (Lambda):
- Lengths scale by: λ
- Areas scale by: λ²
- Volumes and Weights scale by: λ³
- Time (wave periods, motion) scales by: √λ
You want to use 5-inch diameter cylinders for the model legs instead of the full-scale 4-foot (48-inch) diameter legs.
This means your model is exactly a 1:9.6 scale model.
Applying the scale factor of 9.6 to all linear dimensions:
| Component | Full Scale Dimension | Model Dimension (λ = 9.6) |
|---|---|---|
| Body Length | 720.0" (60 ft) | 75.0" |
| Body Width | 168.0" (14 ft) | 17.5" |
| Body Height | 96.0" (8 ft) | 10.0" |
| Leg Length | 420.0" (35 ft) | 43.75" |
| Leg Diameter | 48.0" (4 ft) | 5.0" |
| Legs Underwater (60%) | 252.0" (21 ft) | 26.25" |
For a floating vessel to sit at its designed waterline (legs 60% submerged), the total weight of the structure must exactly equal the weight of the water displaced by the submerged portion of the legs.
* Assumptions: Full scale operates in Seawater (64.0 lbs/ft³), Model operates in a Freshwater pool (62.4 lbs/ft³).
| Metric | Full Scale | Model Scale (λ³ = 884.736) |
|---|---|---|
| Volume of 1 Leg | 759,996 in³ (439.8 ft³) | 859.0 in³ |
| Total Volume (4 Legs) | 3,039,984 in³ (1,759.3 ft³) | 3,436.0 in³ |
| Submerged Volume (60%) | 1,823,990 in³ (1,055.6 ft³) | 2,061.6 in³ |
| Target Total Weight (Body + Legs + Cables + Payload) |
67,556.5 lbs | 74.5 lbs |
To test your model successfully, the entire fully assembled model (body, cylinders, cables, and any added ballast) must weigh exactly 74.5 lbs to sink the legs to the 60% mark.
You asked what dimensions it would represent at full scale if you used two standard 55-gallon plastic drums, connected end-to-end, as the "body" of the model.
A standard tight-head 55-gallon drum is roughly 23.5 inches in diameter and 35 inches tall (long).
Scaling these back up to Full Scale (multiplying by 9.6):
| Body Dimension | Model Dimension (2x Drums) | Represented Full Scale Dimension |
|---|---|---|
| Length | 70.0" | 672.0" (56.0 ft) |
| Diameter (Width/Height) | 23.5" | 225.6" (18.8 ft) |
| Internal Volume | 110.0 Gallons | 97,321 Gallons (approx 13,010 ft³) |
Conclusion on Drum usage: Using two 55-gallon drums gives you a full-scale length of 56 feet, which is very close to your original 60 foot design. However, because drums are cylinders, you are changing your living quarters from a flat 14'x8' rectangular box to a giant 18.8-foot diameter tube. This vastly increases the internal volume of the body, which is perfectly fine for a seastead, but structurally quite different from the original prompt.
Because you are building a tensegrity structure, pay close attention to your assembly angles: