Seastead 1/6 Scale Model Design Guide

1. Froude Scaling Rules Overview

Froude scaling maintains the ratio of inertial to gravitational forces, which is critical for wave-structure interaction studies. The key scaling relationships are:

Parameter	Scaling Factor	For λ = 6
Length	λ	÷ 6
Area	λ²	÷ 36
Volume	λ³	÷ 216
Mass/Weight	λ³	÷ 216
Time	√λ	÷ 2.449
Velocity	√λ	÷ 2.449
Acceleration	1	Same
Force	λ³	÷ 216

2. Scale Model Dimensions

Living Area

Dimension	Full Scale (ft)	Model Scale (ft)	Model Scale (inches)
Length	40	6.67	80 inches
Width	16	2.67	32 inches

Support Columns

Dimension	Full Scale (ft)	Model Scale (ft)	Model Scale (inches)
Column Diameter/Width	4	0.67	8 inches
Column Length	24	4	48 inches
Submerged Length	12	2	24 inches

Column Angle: The 45-degree angle remains the same in the scale model (angles don't scale).

Target Model Weight

Full Scale Weight: 36,000 lbs
Scale Factor for Mass: λ³ = 6³ = 216
Model Weight: 36,000 ÷ 216 = 166.7 lbs

Construction Tip: Aim for approximately 165-170 lbs total model weight. This should include the living platform, all four columns, and any onboard instrumentation. Weigh the completed model before testing and adjust ballast if needed.

3. Wave Height Scaling

Wave heights scale linearly with the scale factor:

Full Scale Wave Height (ft)	Model Wave Height (ft)	Model Wave Height (inches)
3 feet	0.5	6 inches
5 feet	0.83	10 inches
8 feet	1.33	16 inches

Wave Measurement: Your wooden pole with marks is a good approach. Consider marking it in 2-inch increments for easy reading from shore. Paint alternating colored bands for visibility.

4. Cable Tension Measurement

Surgical Tubing Approach

Surgical tubing (latex rubber tubing) typically has the following characteristics:

Property	Typical Range
Stretch Capacity	400-600% of original length
Useful Tension Range (1/4" OD tubing)	2-15 lbs
Useful Tension Range (3/8" OD tubing)	5-30 lbs
Useful Tension Range (1/2" OD tubing)	10-50 lbs

Full-Scale Force Equivalents

Force scales as λ³ = 216

Model Tension → Full Scale Tension:
5 lbs → 1,080 lbs
10 lbs → 2,160 lbs
25 lbs → 5,400 lbs
50 lbs → 10,800 lbs

Assessment: The surgical tubing range (roughly 5-50 lbs in the model) corresponds to approximately 1,000-11,000 lbs in full scale. This seems reasonable for initial cable tensions during moderate wave conditions. For a 36,000 lb structure, peak cable loads during storms could be higher, but this range should capture normal operating conditions well.

Digital Scale Alternatives

Amazon Search Recommendations:

Unfortunately, dedicated "rope tension data loggers" are specialized industrial equipment typically costing $500+ and not commonly found on Amazon. However, here are practical alternatives:

Search terms to try:

"Digital hanging scale" (most common, $15-40)
"Crane scale with data logging"
"Fish scale digital 100lb"
"Luggage scale digital"
"Force gauge digital"
"Dynamometer handheld"

Recommended Products:

Etekcity Digital Hanging Scale (~$15) - Simple, water-resistant, 110 lb capacity. No logging but easy to read.
Klau Digital Crane Scale (~$40-80) - Higher capacity, some models have data hold features.
Voltcraft VC-231 or similar force gauges (~$50-100) - More precise, some have data output.

DIY Data Logging Solution:

Use a simple digital hanging scale
Point a waterproof camera (GoPro) at the display
Review footage frame-by-frame later
This gives you temporal correlation with your other videos

Waterproofing Note: Most affordable digital scales are NOT waterproof. Consider:

Housing the scale in a clear waterproof container (Pelican case with clear lid)
Mounting scales above the waterline with extension cables/rods
Using the surgical tubing method for submerged measurements and scales for above-water lines

5. Android Apps for Motion Recording

Recommended Apps

App	Features	Best For	Cost
Phyphox	Accelerometer, gyroscope, magnetometer; data export to CSV; high sample rates	Detailed data logging and analysis	Free
Physics Toolbox Suite	Multiple sensors; real-time graphing; CSV export; G-force meter	Real-time visualization	Free (Pro ~$3)
Sensor Kinetics	All device sensors; data logging; good UI	Comprehensive sensor access	Free (Pro ~$5)
Accelerometer Analyzer	FFT analysis; vibration monitoring	Frequency analysis of motion	Free
AndroSensor	All sensors; background logging; timestamps	Long-duration recording	Free

My Recommendation

Primary Choice: Phyphox

Developed by RWTH Aachen University - scientifically validated
Excellent accelerometer and gyroscope recording
Can record pitch, roll, and inclination directly
Exports to CSV, Excel-compatible formats
Can be controlled remotely via WiFi (start/stop from shore)
High sample rates (up to 400 Hz on most phones)

Secondary Choice: Physics Toolbox Suite

More intuitive real-time display
Good for quick visual checks

Video + Acceleration Data Combined

Regarding Phyphox and Video: Unfortunately, Phyphox cannot record video with embedded sensor data. Most Android apps cannot do both simultaneously due to system limitations.

Solutions for Combined Video + Acceleration:

Option	Description	Pros/Cons
Option 1: Two Phones	One phone records video, another runs Phyphox. Start both with a visible/audible sync signal (clap, flash)	Simple, reliable, requires syncing in post
Option 2: Sensor Logger + Video	App "Sensor Logger" can record sensors in background while another app records video	Single phone, but may affect performance
Option 3: Post-Processing Overlay	Record separately, use video editing software (DaVinci Resolve - free) to overlay data graphs on video	Best visual result, more work
Option 4: HUD Apps	Apps like "GPS Speedometer" or "Digi HUD" can overlay some data on video	Limited sensor data, but real-time overlay

Recommended Approach:

Use Phyphox on a phone mounted on the model for detailed sensor data
Use a GoPro or separate phone for FPV video on the model
Record a sync event (sharp tap on the model) at the start
Combine data in post-processing using timestamps
Your shore-based camera provides the primary visual record

6. Acceleration Scaling and Comfort Metrics

Key Froude Scaling Insight

Acceleration scales 1:1 in Froude scaling

This means: Whatever acceleration you measure on your 1/6 scale model is the SAME acceleration that would occur at full scale under equivalent (scaled) wave conditions.

This is extremely useful! Your model accelerometer readings directly represent full-scale accelerations without any conversion needed.

Sliding Objects Threshold

Objects begin to slide when horizontal acceleration overcomes static friction:

Sliding begins when: a_horizontal > μ_s × g

where μ_s = coefficient of static friction
g = 9.81 m/s² = 32.2 ft/s²

Surface Combination	μ_s (typical)	Sliding Acceleration Threshold
Ceramic plate on wood table	0.3 - 0.5	0.3 - 0.5 g (2.9 - 4.9 m/s²)
Glass on wood	0.2 - 0.4	0.2 - 0.4 g (2.0 - 3.9 m/s²)
Rubber mat on wood	0.6 - 0.8	0.6 - 0.8 g (5.9 - 7.8 m/s²)
Paper on wood	0.3 - 0.4	0.3 - 0.4 g

Practical Test: When your model accelerometer shows peaks above 0.3 g (about 3 m/s²), unrestrained plates and glasses would likely start sliding on a wooden table in the full-scale seastead.

Human Comfort Acceleration Thresholds

Acceleration Level	Human Perception / Effect
< 0.02 g	Imperceptible to most people
0.02 - 0.05 g	Perceptible, comfortable for most
0.05 - 0.1 g	Noticeable motion, some discomfort possible
0.1 - 0.2 g	Significant motion, difficult to work, seasickness likely for sensitive individuals
0.2 - 0.4 g	Difficult to walk unaided, widespread discomfort
> 0.4 g	Need to hold on, risk of injury, objects sliding

Angular Motion (Roll/Pitch) Metrics

Besides linear acceleration, angular motion causes discomfort:

Metric	Comfortable Range	Uncomfortable
Roll/Pitch Angle	< 4°	> 8°
Roll/Pitch Rate	< 2°/second	> 4°/second
Roll/Pitch Acceleration	< 1°/s²	> 3°/s²

Note: Angular measurements from Phyphox on the model are also directly applicable to full scale, as angles don't scale. However, angular rates and accelerations scale differently:

Angular velocity scales as 1/√λ (model values ÷ 2.449 = full scale)
Angular acceleration scales as 1/λ (model values ÷ 6 = full scale)

7. Water Glass Indicator - Excellent Idea!

Your water glass idea is excellent for visualization!

A glass with water (and rocks for ballast to prevent tipping over) provides an intuitive visual indicator of motion that will:

Show the direction and magnitude of acceleration
Be easily visible in video footage
Provide immediate qualitative assessment
Connect with viewers who can imagine living aboard

Suggested Setup

Use a clear cylindrical glass (consistent diameter makes analysis easier)
Fill about 2/3 full with colored water (food coloring helps visibility)
Add smooth rocks or marbles to lower center of gravity
Mark the glass at the water line for reference
Consider adding floating markers (small cork pieces) to track water surface tilt

Interpretation

Water Surface Tilt	Corresponding Acceleration
5°	~0.09 g (tan 5°)
10°	~0.18 g
15°	~0.27 g
20°	~0.36 g

Water surface tilt angle θ = arctan(a_horizontal / g)
Or: a_horizontal = g × tan(θ)

8. Additional Measurement Methods to Consider

Highly Recommended Additions

Method	What It Measures	Implementation
Draft Marks on Columns	Heave motion, waterline changes	Paint visible scale marks at 1" intervals on columns near waterline
Inclinometer App	Real-time pitch/roll angles	Phone-based (Phyphox or Clinometer app)
Reference Grid Background	Absolute position/motion	Large gridded board on shore behind model
GPS Logger	Drift, position changes	Phone GPS logging app (though accuracy may be limited)
String Potentiometer	Mooring line extension	DIY with marked line and video

Additional Suggestions

Bubble Level Visualization: Mount a large circular bubble level on the deck, visible to cameras. Provides instant roll/pitch indication.
Pendulum Indicator: Hang a small weight on a string from a fixed point. The deflection angle shows horizontal acceleration. Mark a protractor scale behind it.
Freeboard Measurement: Mark the expected waterline and measure how much freeboard changes during wave encounters.
Wave Period Timing: Use your marked pole to count waves passing per minute. Wave period scales as √λ (full-scale period = model period × 2.449).
Wind Measurement: A small anemometer on shore helps correlate conditions. Wind speed scales as √λ.
Spray/Splash Documentation: Note when waves begin splashing onto the deck - this is scale-independent information about freeboard adequacy.

Time Scaling for Video Playback

Time Scale Factor = √6 = 2.449

To show "real-time" full-scale motion:
Slow down your model video to 40.8% speed
(or multiply playback time by 2.449)

Frame Rate Recommendation: If you shoot at 60 fps and play back at 24 fps, you get 2.5× slow motion - almost perfect for Froude scaling at 1:6 scale! This is a happy coincidence worth exploiting.

9. Test Protocol Checklist

Pre-Test

☐ Verify model weight (target: 166.7 lbs)
☐ Check all cable connections
☐ Confirm draft marks visible
☐ Waterproof phones in cases
☐ Start Phyphox recording with timestamp
☐ Position wave measurement pole
☐ Set up shore camera with zoom
☐ Record "zero state" (calm water baseline)

During Test

☐ Record sync event (clap/tap) on all devices
☐ Call out wave heights periodically for audio record
☐ Note any cable tension observations
☐ Monitor for any structural issues
☐ Document approximate wave period
☐ Note wind conditions

Post-Test

☐ Save all sensor data files
☐ Back up all video files
☐ Note any damage or unexpected behavior
☐ Record actual wave height range encountered
☐ Document water temperature (affects density slightly)

10. Quick Reference Card

1/6 Scale Model Key Dimensions
Living Area	80" × 32"
Column Width	8"
Column Length	48" (24" submerged)
Target Weight	166.7 lbs

Wave Heights to Find
For 3 ft full-scale	6 inches
For 5 ft full-scale	10 inches
For 8 ft full-scale	16 inches

Time Scaling
Video playback speed	40.8% (or 60fps → 24fps)
Time multiplication factor	× 2.449

Acceleration Thresholds (Same in Model & Full Scale)
Objects start sliding	> 0.3 g
Difficult to walk	> 0.2 g
Comfortable limit	< 0.1 g

Cable Tension Scaling
Model to Full Scale Factor	× 216
Example: 10 lbs model	2,160 lbs full scale

11. Summary

Your experimental setup is well-conceived. The key points to remember:

Model dimensions: 80" × 32" living area, 8" diameter × 48" columns, target weight 166.7 lbs
Accelerations measured on the model equal full-scale accelerations - no conversion needed!
Wave heights scale directly: Find 6", 10", and 16" waves for your three test conditions
Use Phyphox for detailed sensor logging, with a separate camera for FPV video
The water glass indicator is excellent for visual communication of motion severity
Slow video to 40.8% (or use 60fps→24fps) for accurate full-scale time representation
Surgical tubing is reasonable for tension indication; camera-on-digital-scale is a practical alternative for data logging

Good luck with your testing in Sandy Hill Bay! Your methodical approach with multiple measurement methods will provide valuable data for your seastead design.