1:10.5 Froude-Scale Model Dimensions and Test Notes

1. Froude scaling rules used

Scale ratio:

λ = full size / model size = 10.5

Quantity	Scaling rule	For 1:10.5 model
Length	Model = Full / λ	1 full-scale ft = 1.1429 model inches
Area	Model = Full / λ²	Divide by 110.25
Volume / displacement / weight	Model = Full / λ³	Divide by 1157.625
Time / wave period	Model = Full / √λ	Divide by 3.240
Velocity	Model = Full / √λ	Divide by 3.240
Linear acceleration, in g	Same model and full scale	Measured g-levels are directly comparable
Angles: pitch / roll	Same model and full scale	Degrees are directly comparable

Video timing: one second of model motion represents about 3.24 seconds full scale. To make model video look like full-scale time, slow it down by a factor of 3.24.

2. Main model dimensions in inches

Triangle living-area frame

Full-scale item	Full-scale dimension	1:10.5 model dimension
Left side of triangle	70 ft	80.00 in
Right side of triangle	70 ft	80.00 in
Back side / base of triangle	35 ft	40.00 in
Approximate triangle length from back center to front point	67.78 ft	77.46 in
Truss / living area height	7 ft	8.00 in

Main legs / floats / foils

Full-scale item	Full-scale dimension	1:10.5 model dimension
Vertical leg length	19 ft	21.71 in
Submerged lower half	9.5 ft	10.86 in
Upper half above water	9.5 ft	10.86 in
NACA 0030 chord	10 ft	11.43 in
NACA 0030 maximum thickness / width	3 ft	3.43 in
Built-in ladder length on upper front half	9.5 ft	10.86 in

RIM-drive thrusters

Full-scale item	Full-scale dimension	1:10.5 model dimension
RIM-drive diameter	1.5 ft	1.71 in
Height above bottom of leg	3 ft	3.43 in
Number of thrusters	6 total	6 total

Dinghy and aft deck

Full-scale item	Full-scale dimension	1:10.5 model dimension
RIB dinghy length	14 ft	16.00 in
Aft deck extension width	5 ft	5.71 in

Small airplane-like stabilizers

Full-scale item	Full-scale dimension	1:10.5 model dimension
Main stabilizer wingspan	12 ft	13.71 in
Main stabilizer chord	1.5 ft	1.71 in
Stabilizer body length	6 ft	6.86 in
Elevator wingspan	2 ft	2.29 in
Elevator chord	6 in	0.57 in
Approximate 25% chord notch depth into stabilizer wing	0.375 ft / 4.5 in	0.43 in

3. Target model weight / displacement

Assuming each main vertical leg is a NACA 0030 section with:

Chord = 10 ft
Maximum thickness = 3 ft
Submerged height = 9.5 ft
Three legs
Seawater density approximately 64 lb/ft³

A standard NACA 0030 section has an approximate cross-sectional area of:

20.55 ft² per leg

Submerged volume:

3 legs × 20.55 ft² × 9.5 ft = about 586 ft³

Full-scale displacement at the 50% submerged leg waterline:

586 ft³ × 64 lb/ft³ = about 37,500 lb

Model target weight:

37,500 lb / 10.5³ = about 32.4 lb

Recommended model all-up test weight: approximately 32 lb to 33 lb, including hull, legs, batteries, electronics, cameras, ballast, and any onboard test equipment.

This 32.4 lb estimate counts the buoyancy of the three main NACA-shaped legs only. If the model’s submerged stabilizers, thruster housings, brackets, or other appendages displace significant water, the correct all-up weight will be slightly higher. The best practical target is: ballast the model until the waterline is at the intended 50% leg-submergence mark.

4. Scale wave heights for 3 ft, 5 ft, and 8 ft full-scale waves

Full-scale wave height	Model wave height at 1:10.5
3 ft	3.43 in
5 ft	5.71 in
8 ft	9.14 in

Measure wave height as trough-to-crest height. If using a marked pole, video from shore can be used to read both wave height and wave period.

5. Doll / human scale

A full-size person scales as follows:

Full-scale person height	Model doll height
5 ft 6 in / 66 in	6.29 in
5 ft 8 in / 68 in	6.48 in
6 ft / 72 in	6.86 in

A doll around 6.5 inches tall will give a good visual sense of human scale.

6. Deep-water wave depth requirement

For the model to experience “deep water” waves, the water depth should be at least about:

Depth > one-half of the model wavelength

For deep-water waves, model wavelength can be estimated from measured model wave period:

L_model ≈ g T_model² / 2π

In feet, this is approximately:

L_model ft ≈ 5.12 × T_model², with T in seconds.

Therefore:

Required depth ft ≈ 2.56 × T_model²

Measured model wave period	Approx. deep-water wavelength	Minimum depth for deep-water behavior	Equivalent full-scale wave period
1.5 sec	11.5 ft	5.8 ft	4.9 sec
2.0 sec	20.5 ft	10.2 ft	6.5 sec
2.5 sec	32.0 ft	16.0 ft	8.1 sec
3.0 sec	46.1 ft	23.0 ft	9.7 sec

If the water is much shallower than half the wavelength, the waves are intermediate or shallow-water waves and may become steeper or more shore-influenced. For clean “deep water” model testing, try to test in water deep enough for the measured wave period, and avoid breaking waves.

7. Android apps for acceleration, pitch, roll, heave, and motion logging

Recommended Android options:

phyphox — Very good free science sensor app. Logs accelerometer, gyroscope, magnetometer, pressure, and other phone sensors. Can export CSV. Good for experiments and repeatable logging.
Physics Toolbox Sensor Suite — Also very good. Easy CSV export of accelerometer, gyroscope, linear acceleration, rotation, GPS, barometer, etc.
Sensor Logger type apps — Useful if you want simple high-rate CSV logging from accelerometer and gyro.
GoPro telemetry — GoPro cameras can record accelerometer and gyro data. Tools such as Telemetry Overlay or other GPMF telemetry tools can extract and display the data.

Best practical setup: put a phone or small IMU logger near the model’s center of gravity, waterproof it, mount it rigidly, and log accelerometer + gyro data while the shore camera records the visual motion.

Phone “linear acceleration” estimates can be filtered and may lag or drift. For serious analysis, save raw accelerometer and raw gyroscope data if possible. Also record the phone orientation carefully so you know which axis is fore-aft, side-to-side, and vertical.

8. Accelerations and “plates sliding on a table”

For Froude-scaled gravity-dominated testing, linear acceleration in g is the same at model scale and full scale. So if the model experiences a 0.15 g lateral acceleration, that corresponds to about 0.15 g lateral acceleration full scale.

A plate starts to slide when the sideways apparent acceleration exceeds available friction:

a_side / g > μ

where μ is the coefficient of static friction between the plate and table.

Surface condition	Approximate friction coefficient μ	Approximate lateral acceleration where sliding may begin
Very slippery / wet / smooth plastic	0.10	0.10 g
Smooth plate on smooth table	0.20 to 0.30	0.20 to 0.30 g
Higher-friction placemat	0.40 to 0.60	0.40 to 0.60 g
Rubber mat / non-skid	0.60+	0.60 g or higher

Pitch and roll also matter. A tilted table has a sideways gravity component:

a_tilt / g = sin(θ)

For small angles, sin(θ) is approximately θ in radians. For example:

5 degrees tilt gives about 0.087 g sideways component.
10 degrees tilt gives about 0.174 g sideways component.
15 degrees tilt gives about 0.259 g sideways component.

A useful combined sliding check is:

|a_horizontal/g + sin(θ)| > μ cos(θ)

In practical terms, plates on a smooth table may start sliding somewhere around 0.2 g to 0.3 g peak lateral apparent acceleration, especially if the motion lasts long enough and includes tilt.

Useful comfort and livability metrics

Peak lateral acceleration in g — good for “will things slide?”
RMS lateral acceleration — good for comfort over time.
95th percentile acceleration — better than a single extreme spike.
Vertical heave acceleration — important for seasickness and “elevator feeling.”
Pitch and roll angle — degrees are directly comparable model to full scale.
Pitch and roll rate — full-scale rate = model rate / 3.24.
Angular acceleration — full-scale angular acceleration = model angular acceleration / 10.5.
Jerk, change of acceleration — full-scale jerk = model jerk / 3.24.

9. Cup of water visual test

A transparent cup with water can be a good qualitative video indicator of apparent tilt and slosh.

Suggestions:

Put a grid or marked background behind the cup so the water angle is visible.
Use the same cup location in every test.
Video it from the onboard camera and/or shore camera.
Rocks will damp slosh and make it less splashy, but they also change the behavior.

The cup test is visually useful, but it is not perfectly Froude-scaled unless the cup is also geometrically scaled, which would make it very small. Treat it as a qualitative “what would it feel like?” indicator, not a precise hydrodynamic measurement.

10. Wave-height measuring pole

A marked pole is a good idea. For best results:

Use high-contrast inch markings.
Place it close to the model but not so close that it disturbs the waves.
Video the pole and model at the same time if possible.
Measure both wave height and wave period.
If possible, use two or three poles separated by known distances to estimate wave direction and wavelength.

11. Mooring or holding line during tests

A line can affect model motion, especially surge, pitch, and yaw. If the goal is only to keep the model from drifting away, the restraint should be as gentle as possible.

Recommendations:

Use a long, light, elastic line if station-keeping is needed.
Keep the line force small compared with wave forces.
Avoid a person actively pulling by hand during measurement runs, because human input is not repeatable.
If possible, add a small inline load cell or spring scale so you know how much restraint force was applied.
Run both “nearly free” and “moored” tests if you want to understand the difference.

If you later model the helical screw tension-leg mooring, the mooring stiffness should also be scaled. For Froude force scaling, force scales as 1/λ³ and length as 1/λ, so linear stiffness scales approximately as:

K_model = K_full / λ²

12. Other measurement methods worth considering

Visual motion tracking: Put high-contrast colored dots or AprilTag/ArUco markers on the model. Use shore video and software such as OpenCV to track heave, pitch, roll, surge, and yaw.
Known scale markings on the model: Put vertical marks on the legs so video can directly show immersion, heave, and waterline changes.
Synchronize all recordings: Clap, flash a light, or tap the model at the start so phone data, GoPro video, and shore camera video can be aligned in time.
Measure model trim before every run: Record total weight, center of gravity estimate, waterline, pitch trim, and roll trim.
Inclining test: Shift a known small weight sideways and measure roll angle to estimate model stability.
Wave period logging: Record time between crests at the pole. Period is just as important as height.
Mooring-line load cell: If the model is tied off, measuring line tension will help separate true wave response from restraint effects.
Multiple camera angles: Shore tripod camera for accurate analysis, onboard GoPro for subjective “ride feel,” and optionally an overhead drone shot for yaw/surge/sway.
Repeat runs: Do several runs in similar waves to avoid drawing conclusions from one unusually good or bad sequence.

13. Quick summary

Main triangle sides: 80 in, 80 in, 40 in.
Triangle front-to-back length: about 77.5 in.
Living-area height: 8 in.
Each main leg: 21.7 in tall, with 10.9 in submerged.
Main leg foil section: 11.4 in chord by 3.4 in thick.
Target model all-up weight: about 32.4 lb, then fine-tune ballast to the 50% leg-submergence waterline.
Scale wave heights: 3 ft = 3.43 in, 5 ft = 5.71 in, 8 ft = 9.14 in.
Slow model video by 3.24× to show full-scale time.
Use a 6.5 in doll for human scale.
For deep-water behavior, water depth should be greater than approximately half the measured model wavelength.