Interpretation of the 1/10 Scale Seastead Wave Test

Important limitation: I cannot directly inspect or measure the YouTube video frames from here. So I cannot give a true frame-by-frame measured wave height or acceleration from that specific video. The estimates below are based on the stated model dimensions, Froude scaling, and what can be inferred from the design geometry. If you provide a few still frames with a visible ruler/reference, or measured pixel positions over time, the estimates can be made much more exact.

1. Scale Factors

The model is 1/10 full scale, so the primary Froude scale factors are:

Quantity	Scale factor, model to full scale	Comment
Length, wave height, displacement amplitude	10	A 3 inch model wave corresponds to a 30 inch, or 2.5 ft, full-scale wave.
Time, wave period, motion period	√10 = 3.162	The full-scale motion would happen 3.16 times slower than the model video.
Speed	√10 = 3.162	1 knot model speed corresponds to 3.16 knots full scale.
Acceleration	1	For Froude-scaled motion, accelerations in ft/s² or g are approximately the same at model and full scale.
Force / displacement weight	10³ = 1000	A 40 lb model displacement corresponds to about 40,000 lb full-scale displacement.

Because the YouTube video is at original model speed, it looks too fast compared with full scale. To visualize full-scale motion, slow the video to approximately:

1 / √10 = 0.316 speed, or about 31.6% of the original playback speed.

2. Wave Height Estimate Method

Your model float/column height is given as 22.8 inches top to bottom. The full-scale column height is 19 ft. If a wave in the video appears to be some fraction of the visible model column height, the corresponding full-scale wave height is:

Full-scale wave height = model wave height × 10

Approximate model wave height, crest-to-trough	Equivalent full-scale wave height	Interpretation
0.5 inch	5 inches	Very small ripple/chop at full scale.
1 inch	10 inches / 0.83 ft	Small chop.
2 inches	20 inches / 1.67 ft	Light sea.
3 inches	30 inches / 2.5 ft	Moderate small-craft waves.
4 inches	40 inches / 3.33 ft	Noticeable sea state.
6 inches	60 inches / 5 ft	Substantial small-vessel sea.
8 inches	80 inches / 6.67 ft	Large relative to this model; serious full-scale sea.

As a rough guide, if the waves in the video are visually about 5% to 15% of the 22.8 inch model column height, that would mean:

Model wave height: about 1.1 to 3.4 inches
Full-scale wave height: about 0.95 to 2.85 ft

If the largest waves are about 20% to 25% of the model column height, then:

Model wave height: about 4.6 to 5.7 inches
Full-scale wave height: about 3.8 to 4.8 ft

3. Approximate Buoyancy and Displacement

Each full-scale leg/wing is described as approximately:

Vertical height: 19 ft
Chord: 10 ft
NACA 0030 section, so maximum thickness approximately 3 ft
Half submerged: 9.5 ft submerged depth

The area of a NACA 0030 foil section is approximately 0.205 × chord². For a 10 ft chord:

Section area ≈ 0.205 × 10² = 20.5 ft²

Submerged volume per leg at 50% immersion:

20.5 ft² × 9.5 ft ≈ 195 ft³

For three legs:

Total submerged volume ≈ 585 ft³

Using seawater at approximately 64 lb/ft³:

Displacement ≈ 585 × 64 ≈ 37,400 lb

So at the stated 50% immersion, the full-scale vessel displacement is roughly:

37,000 to 38,000 lb, or about 18.5 to 19 tons.

Design warning: This is not a lot of displacement for a large enclosed triangular living structure with glass, trusswork, batteries, solar, machinery, thrusters, dinghy, fuel/water, people, furniture, and safety margin. If the final full-scale structure weighs much more than about 37,000 lb, the floats would sit deeper than 50% immersion. Fully submerged volume of the three legs would be roughly double this, around 75,000 lb of buoyancy, but you would not want to operate close to that because reserve buoyancy and deck clearance are critical.

4. Heave Stiffness and Natural Heave Period

The total waterplane area of the three foil-shaped columns is approximately:

3 × 20.5 ft² ≈ 61.5 ft²

The vertical hydrostatic restoring force is approximately:

64 lb/ft³ × 61.5 ft² ≈ 3,940 lb per ft of heave

Using the estimated 37,400 lb displacement, the simple hydrostatic heave natural period is roughly:

About 3.4 seconds without added mass.

With hydrodynamic added mass, the real heave period may be closer to:

Approximately 4 to 5 seconds.

This is an important result. A semi-submersible often has a very long heave period because it has very small waterplane area and large submerged volume. This design has less waterplane area than a normal monohull or catamaran, but the columns are still fairly large at the waterline. Therefore the heave period may not be extremely long; it may fall into the same range as common short ocean waves.

5. Expected Motion Compared with a 50 ft Catamaran or 60 ft Monohull

Advantages of the three-leg seastead geometry

Low wave-excited area: Compared with a monohull or catamaran hull, the three narrow foil columns should intercept less wave energy.
Wide support triangle: The three widely spaced buoyancy points give high initial stability in roll and pitch.
Reduced bow impact: There is no conventional bow plunging into waves, so bow slamming may be reduced.
Potentially low drag forward: The NACA 0030 columns should have much less forward drag than blunt round columns, especially if aligned well with flow.
Room for active stabilization: The planned submerged stabilizers could significantly reduce pitch/heave while moving.

Possible disadvantages

Short natural periods: Because the structure is very stiff hydrostatically, roll, pitch, and heave may be quick unless mass/inertia and added mass are large enough to slow the motion.
Acceleration can be uncomfortable even if angles are small: A stiff platform can have low roll angle but high vertical or angular acceleration.
Limited displacement: At 50% immersion, the estimated displacement is only around 37,000 lb.
Wave-period sensitivity: If the wave period is close to the platform’s natural heave, pitch, or roll period, motions could be amplified.
Model does not yet include stabilizers: The final active/passive foils may change the result substantially, especially at forward speed.

6. Approximate Acceleration Ranges

For sinusoidal vertical motion, peak vertical acceleration is:

a / g = 4π² × A / (g × T²)

Where:

A = heave amplitude in feet, measured from mean position to peak
T = motion period in seconds
g = 32.2 ft/s²

Full-scale heave amplitude	Full-scale period	Peak vertical acceleration	Ride interpretation
0.25 ft	5 sec	0.012 g	Very gentle.
0.5 ft	5 sec	0.025 g	Comfortable.
1.0 ft	5 sec	0.049 g	Noticeable but not severe.
1.0 ft	4 sec	0.077 g	Noticeable; could become tiring.
1.5 ft	4 sec	0.115 g	Uncomfortable for long periods.
2.0 ft	4 sec	0.154 g	Clearly uncomfortable.
1.0 ft	3 sec	0.136 g	Sharp, uncomfortable motion.
2.0 ft	3 sec	0.272 g	Severe for a living platform.

For a comfortable living platform, you generally want vertical accelerations to stay well below about 0.05 g most of the time. Accelerations around 0.1 g are very noticeable. Repeated accelerations above 0.15 g to 0.2 g become tiring or unsafe for loose objects and people moving around.

7. Comparison to Common Boats

Vessel type	Typical motion behavior	Approximate acceleration comparison
50 ft cruising catamaran	Very high initial stability, low roll angle, but can have quick, sharp vertical accelerations. Bridge-deck slamming can be severe in steep seas.	In moderate chop, vertical accelerations of about 0.1 g to 0.3 g are not unusual; impacts can be higher.
60 ft monohull	More roll angle, but often slower and softer motion. The hull follows the waves more than a semi-sub type structure.	In moderate seas, vertical accelerations may be around 0.05 g to 0.2 g, depending strongly on heading, speed, hull form, and wave period.
Proposed three-leg foil-column seastead	Likely smaller roll angles than a monohull and less wave-following than a catamaran hull. But if natural periods are short, accelerations could still be noticeable.	In small to moderate waves, if heave amplitude is kept to 0.5 to 1.0 ft at periods near 4 to 5 sec, acceleration may be about 0.025 g to 0.08 g. If resonance or steep short waves cause 1.5 to 2.0 ft motion at 3 to 4 sec, acceleration could reach 0.15 g to 0.27 g.

Compared with a 50 ft catamaran, the seastead should have less conventional hull slamming and may have a softer vertical response in some wave conditions. Compared with a 60 ft monohull, it should have much less roll angle, but the motion may be quicker and more platform-like unless damping and stabilizers are effective.

8. What the Scale Model Can and Cannot Prove

What the model is useful for

Showing whether the basic three-leg arrangement is directionally stable in waves.
Showing whether the platform has obvious excessive pitching, rolling, or hobby-horsing.
Showing whether waves pass through the structure instead of strongly lifting the whole body.
Qualitatively comparing different leg placements, stabilizer ideas, and ballast arrangements.

What the model does not perfectly capture

Reynolds number effects: At 1/10 scale, viscous drag and foil behavior do not scale perfectly.
Surface tension: Small models are affected more by surface tension than full-scale vessels.
Mass distribution: The model must have correctly scaled center of gravity and moments of inertia to predict full-scale pitch and roll accurately.
Structural flexibility: A wooden 2x4 model frame may be much stiffer or differently distributed than the full truss/glass/solar structure.
Wind loading: Wind forces do not scale the same way as wave forces unless separately modeled.
Missing stabilizers: The current model does not include the planned active stabilizer wings, so the final full-scale behavior could be better than the model.

9. Effect of the Planned Stabilizers

The planned stabilizers could help significantly, especially when the seastead is moving forward. Each stabilizer main wing is approximately:

Span: 10 ft
Chord: 1 ft
Area: 10 ft²

For three stabilizers, total main wing area is approximately 30 ft². The lift force depends strongly on speed:

Full-scale speed	Approximate lift per 10 ft² stabilizer at CL = 0.5	Total for three stabilizers
5 knots	About 350 lb	About 1,050 lb
7 knots	About 700 lb	About 2,100 lb
10 knots	About 1,400 lb	About 4,200 lb

These forces are meaningful but not enormous compared with a roughly 37,000 lb displacement. They are likely most useful for damping pitch and heave, not for carrying a large fraction of the vessel weight. At zero speed, they will provide little or no stabilizing lift unless water is flowing past them from current or thruster wash.

10. Practical Recommendations for the Next Model Test

Add a visible vertical scale: Put black/white inch marks on one column so wave height and heave can be measured from video.
Film from the side with a fixed camera: Avoid hand-held camera motion if you want acceleration data.
Record at high frame rate: 60 fps or 120 fps is much better for extracting motion.
Track one point on the deck: Use a bright marker and measure vertical pixel position versus time.
Measure wave period: Count frames between crests passing a fixed point.
Test several ballast conditions: Try light, design, and heavy displacement.
Test with scaled stabilizers: Even if Reynolds number is imperfect, you can compare relative improvement.
Test headings: Head seas, beam seas, quartering seas, and following seas will behave differently.

11. Bottom-Line Interpretation

The concept should have some real seakeeping advantages over a conventional 50 ft catamaran or 60 ft monohull: less hull volume directly lifted by waves, wide stance, low roll angle, and good opportunity for active stabilization.

The biggest concern is not static stability; it is likely dynamic comfort, especially vertical acceleration and quick pitch/heave in short-period waves. The estimated displacement at 50% immersion is also relatively low for such a large living structure.

If the model waves in the video are in the range of about 2 to 4 inches crest-to-trough, the corresponding full-scale waves are about 1.7 to 3.3 ft. If the largest model waves are closer to 5 to 6 inches, they correspond to approximately 4 to 5 ft full-scale seas.

For full-scale comfort, the key target is to keep the platform heave amplitude small and the motion period long enough that vertical acceleration stays mostly below about 0.05 g. With good mass distribution, sufficient displacement, damping, and the planned stabilizers, the ride could be better than a similarly sized catamaran in many conditions. Without enough damping, however, the structure could still have sharp accelerations even if the visible roll and pitch angles look small.