1:4 Froude-Scale Model of the Seastead / Solar USV

Important: This is a preliminary engineering estimate, not a final design review. A non-self-righting unmanned vessel intended for open-ocean operation should be reviewed by a naval architect, especially for stability, reserve buoyancy, watertight subdivision, structural fatigue, electrical safety, and COLREGS/navigation compliance.

1. Froude Scaling Rules for 1:4 Model

Scale ratio:

If full scale displacement is 36,000 lb:

Target 1:4 model displacement = 36,000 / 64 = 562.5 lb

The model should be ballasted to approximately 560 lb all-up weight if you want it to float at the same fractional draft as the full-scale design. However, based on the foil-leg displacement, 560 lb leaves very little margin if the legs are exactly as described. More reserve buoyancy would be desirable.

2. 1:4 Scale Dimensions

Item Full Scale 1:4 Scale Feet / Inches
Triangle left side 70 ft 17.5 ft 17 ft 6 in
Triangle right side 70 ft 17.5 ft 17 ft 6 in
Back side / beam of triangle 35 ft 8.75 ft 8 ft 9 in
Triangle length, front point to back side 67.78 ft 16.95 ft 16 ft 11 in
Frame height / living-area height 7 ft 1.75 ft 1 ft 9 in
One foil leg vertical length 19 ft 4.75 ft 4 ft 9 in
Leg submerged portion, 50% 9.5 ft 2.375 ft 2 ft 4.5 in
Leg above-water portion, 50% 9.5 ft 2.375 ft 2 ft 4.5 in
NACA 0030 leg chord 10 ft 2.5 ft 2 ft 6 in
NACA 0030 leg max thickness / width 3 ft 0.75 ft 9 in
RIM/thruster diameter if scaled 1.5 ft 0.375 ft 4.5 in
Thruster height above bottom 3 ft 0.75 ft 9 in
Stabilizer wing span 12 ft 3 ft 3 ft
Stabilizer wing chord 1.5 ft 0.375 ft 4.5 in
Stabilizer body length 6 ft 1.5 ft 1 ft 6 in
Elevator span 2 ft 0.5 ft 6 in
Elevator chord 0.5 ft 0.125 ft 1.5 in
Back decks, if included 5 ft wide 1.25 ft 1 ft 3 in

3. Triangle Area and Solar Panel Fit

The exact 1:4 triangle has:

The proposed BougeRV 200 W flexible panel:

By raw area, 74.1 ft² / 11.36 ft² = 6.5 panels, but triangular packing is inefficient. On the exact 8.75 ft back-width triangle, a practical layout is likely only 4 panels, possibly 5 with careful staggering and overhangs.

Recommended Small Increase for 6 Panels

If you increase the 1:4 model back side from 8.75 ft to approximately 10 ft, while keeping the side lengths about 17.5 ft, the triangle becomes much easier to pack with panels.

With the panels oriented lengthwise front-to-back, you can likely fit rows of:

That gives:

For the USV version, I would strongly consider using the slightly wider 10 ft rear beam. The solar gain from 4 panels to 6 panels is very significant: 800 W to 1,200 W STC.

4. Buoyancy Check of the 1:4 Legs

Each 1:4 leg is a vertical NACA 0030 foil shape:

The cross-sectional area of a NACA 0030 foil with 2.5 ft chord is approximately:

1.28 ft²

Submerged volume per leg:

1.28 ft² × 2.375 ft = 3.05 ft³

For three legs:

3 × 3.05 = 9.15 ft³

Using seawater at about 64 lb/ft³:

Buoyancy at 50% submergence ≈ 9.15 × 64 = 586 lb

Target Froude-scaled model weight is 562.5 lb.

At the intended 50% leg immersion, the three legs provide only about 586 lb buoyancy. That is only about 23 lb above the 562.5 lb target. This is a very small margin. The craft may float, but it has little reserve before it sits deeper than intended.

For an operational ocean drone, I would prefer one or more of the following:

5. Approximate Weight Budget

Assuming marine aluminum construction and 6 solar panels:

Component Estimated Weight Comments
Triangle perimeter tube, 3 in OD × 1/8 in wall aluminum ~58–60 lb About 1.32 lb/ft, around 44–45 ft perimeter for the widened version.
Additional cross tubes, brackets, mounts 30–60 lb Depends strongly on how much structure is added.
Three aluminum foil legs, 1/8 in shell ~140–160 lb Could be roughly half this if made from 1/16 in aluminum with internal stiffeners.
Three stabilizer assemblies 20–45 lb Wings, pivots, actuators, linkages.
Six BougeRV 200 W panels 47.4 lb 1,200 W STC.
Six Blue Robotics T200 thrusters with mounts and wiring 15–30 lb Thrusters are light, but wiring, guards, mounts, ESCs add weight.
LiFePO4 battery pack, 30% of 562.5 lb ~169 lb Approximately 8–10 kWh nominal depending on cells and packaging.
Electronics, Starlink Mini, cameras, AIS, lights, enclosures 20–40 lb Includes waterproof boxes and connectors.
Cables, rope netting, hardware, sealants, coatings 25–50 lb Small items add up quickly.

Estimated total:

The current design can hit the Froude target only if weight is aggressively controlled. With 1/8 in aluminum legs, 6 panels, batteries, and six thrusters, the model could easily exceed the 562.5 lb target.

6. Battery Capacity If 30% of Weight Is Batteries

30% of the 562.5 lb Froude target:

0.30 × 562.5 = 168.75 lb batteries

For practical LiFePO4 packs, including BMS, compression, enclosure, bus bars, and wiring, assume approximately:

Nominal energy:

A reasonable planning number:

~9 kWh nominal LiFePO4 battery

If you avoid using the last 20%:

Usable energy ≈ 7.2 kWh

7. Hotel Load Estimate

Load Estimated Power
Starlink Mini 25–45 W typical
Raspberry Pi / onboard computer 4–12 W
Cameras 5–15 W depending on number and type
AIS transmitter 2–8 W average, intermittent transmit
Navigation lights 2–8 W
Autopilot, sensors, GPS, network gear 5–15 W

Recommended planning number:

Hotel load ≈ 60 W average

Daily hotel energy:

60 W × 24 h = 1.44 kWh/day

8. Solar Energy and Motor Power Available

With 6 × 200 W panels:

Using a planning value of 5.0 kWh/day solar harvest:

However, during bright daytime the boat may have:

At night, using battery:

For long-duration energy-neutral operation, average motor power may be only 100–200 W. For short periods, especially with a full battery, you could use 500–1,500 W propulsion power.

9. Speed Estimate

The leg shape is favorable because the NACA foil sections have low drag when aligned with the flow. But real-world drag will include:

Assuming a reasonably clean hull and aligned foils, these are rough operating estimates:

Motor Electrical Power Calm / Light Wind Into 15–20 kt Wind Across Wind Downwind
150 W total 1.8–2.3 kt 1.3–1.8 kt 1.5–2.0 kt 2.0–2.6 kt
300 W total 2.2–2.9 kt 1.7–2.4 kt 2.0–2.6 kt 2.5–3.3 kt
700 W total 3.0–4.0 kt 2.3–3.3 kt 2.7–3.6 kt 3.5–4.5 kt
1,500–2,000 W burst 4.0–5.5 kt 3.0–4.5 kt 3.5–5.0 kt 4.5–6.0 kt

These numbers are uncertain, but they are plausible for a lightweight solar USV with three streamlined submerged struts.

10. Stabilizers and Hydrofoil / Semi-Foiling Potential

At 1:4 scale each stabilizer wing is:

Approximate lift from the stabilizers:

Speed Lift at CL = 0.5 Lift at CL = 1.0
3 kt ~43 lb ~86 lb
4 kt ~77 lb ~154 lb
5 kt ~120 lb ~240 lb
6 kt ~172 lb ~344 lb
7 kt ~235 lb ~470 lb
8 kt ~307 lb ~614 lb

To lift most of a 560 lb craft, you need roughly:

The six T200 thrusters probably cannot push this craft into efficient full foiling for long. They may support partial lift / semi-foiling, reducing leg drag and improving ride, but full stable hydrofoiling is unlikely without larger foils and/or more propulsion power.

Burst Foiling Range Estimate

If the battery has about 7.2 kWh usable and the craft uses:

If a future higher-power version used 4 kW and achieved 7 kt:

I would treat hydrofoil lift first as a ride-control and drag-reduction tool, not as the main normal operating mode. For real foiling, increase horizontal foil area and design the control system around pitch, roll, and heave stability from the start.

11. Wave-Capsize Risk and “999 Days out of 1000” Avoidance

The model has good static form stability because the three legs are widely spaced. However, it is probably not self-righting. That is the main risk.

Approximate concern thresholds for the 1:4 model:

Because the model’s frame underside is only about 2.4 ft above the water at design trim, steep chop and breaking wave crests matter a lot.

I would not assume that a 1:4 model surviving real ocean waves proves that the full-scale vessel is safe in waves four times as high. Froude scaling helps, but wind loading, Reynolds number, material stiffness, control response, sensor noise, and breaking-wave details do not scale perfectly.

Is 999 Days out of 1000 Practical?

For open Caribbean operation, 999/1000 avoidance of capsize-producing waves is ambitious. It may be possible for short-range missions with strict weather windows and rapid recovery, but it is not something I would promise commercially.

For a customer product, I would describe operating limits conservatively, for example:

12. Thrusters: Blue Robotics T200 Reliability

I am not aware of a published, formal MTBF rating for the Blue Robotics T200 in continuous months-long ocean-surface service. The T200 is widely used and well-regarded for ROVs and marine robotics, but continuous open-ocean USV use adds failure modes:

Reliability Estimate With 6 Thrusters

You said the craft needs at least 2 working thrusters not on the same leg to make forward progress using differential thrust.

With 6 thrusters total, 2 per leg, and assuming independent exponential failures, the expected time until the system can no longer satisfy that condition is approximately:

System mean time ≈ 1.35 × single-thruster MTBF

Examples:

Assumed T200 Single-Thruster MTBF Estimated Mean Time Until Propulsion Geometry Fails
1,000 hours ~1,350 hours = ~56 days
2,000 hours ~2,700 hours = ~112 days
3,000 hours ~4,050 hours = ~169 days
This calculation ignores common-mode failures, especially seaweed, fishing line, bad connectors, corrosion, and lightning/surge events. In the Caribbean, Sargasso may dominate the reliability problem more than motor MTBF.

Thruster Alternatives

The T200 is attractive because it is affordable, documented, and easy to control. However, for Sargasso-heavy waters, I would also consider:

Cheap small RIM drives would be nice, but affordable, reliable, saltwater-rated RIM drives are not yet common.

13. Rope Netting and Hooks on 3-Inch Aluminum Tube

The 3 in OD × 1/8 in wall aluminum tube has good axial strength but can still bend if the rope net applies large inward loads along long unsupported spans.

Approximate tube properties:

If the side tube is treated as a simply supported 17.5 ft beam and every hook pulls inward, the global bending can become important. With no intermediate support, even rope tensions of 10–20 lb per rope can create noticeable bending/deflection.

For the solar panel net, you probably do not need very high rope tension. A 6 in grid supporting lightweight flexible panels can work with:

That should be enough to keep the panels from sagging badly if the grid is well designed.

Recommendation: Do not rely on individual hooks screwed directly into thin tube wall as structural hard points. Use welded tabs, through-bolted saddle clamps, or an independent perimeter tension cable. The aluminum tube should carry frame loads; the rope net should carry panel support loads.

Also design for wind uplift. A 200 W flexible panel has about 11.4 ft² area. At 40–50 kt wind, uplift loads can be tens of pounds per panel even if the panel is low and mostly horizontal.

14. Recovery System Ideas

Your three-part recovery plan makes sense conceptually.

1. Upwind Testing / Self-Rescue

Testing mostly upwind of home is smart. If propulsion fails, the vessel can use wind drift, keel-like leg area, and active stabilizers to maintain a helpful attitude.

Using the stabilizers asymmetrically as drag devices for emergency steering is plausible, but should be tested early in shallow/local water.

2. Automatic Emergency Water Brake / Drogue

The hinged drag device that deploys when the drone drifts backward is a good idea. It is similar in spirit to a self-deploying drogue or sea anchor.

I would design it so that:

3. Drone-to-Drone Rescue Hook

The front floating tow rope and stern V-capture system is practical and worth prototyping.

Suggested improvements:

With Starlink video and repeated attempts, remote capture should be possible in calm to moderate water.

15. Salt Spray on Cameras, Solar Panels, and Starlink

Salt accumulation will be a major operational issue.

Cameras

Solar Panels

Starlink Mini

16. Raspberry Pi / Computer Recommendation and Potting

For reliability, I agree that eMMC is better than booting from a microSD card.

Recommended Raspberry Pi Option

For this application, I would prefer:

The Raspberry Pi 5 is more powerful, but it uses more power and makes more heat. For a long-endurance marine drone, lower power is usually better.

Alternatives

I would use a two-computer architecture:

Potting

Potting can help, but full potting of a Raspberry Pi has drawbacks:

Better approach:

Best compromise: conformal coat the electronics, put them in a sealed serviceable enclosure, and use a thermal bridge to the water-cooled aluminum leg. Fully pot only simple, cheap, non-serviceable modules.

17. Sargasso Avoidance

Sargasso is likely one of the main practical hazards around Anguilla.

Recommendations:

18. Approximate Parts Cost for 5 Sets Made in China

Very rough estimate for a batch of 5 kits, excluding your assembly labor:

Item Approximate Cost per Drone
Custom aluminum legs, frame tubes, stabilizers, brackets $3,000–$7,000
Six solar panels $1,200
Six T200 thrusters, ESCs, mounts $1,500–$2,200
LiFePO4 battery cells, BMS, enclosure, fusing $1,500–$3,000
MPPT solar controllers, DC/DC converters, wiring $500–$1,200
Starlink Mini hardware ~$600, service extra
Computer, cameras, GPS, IMU, autopilot, AIS, lights $800–$2,000
Actuators for stabilizers $300–$1,000
Marine connectors, glands, sealants, coatings, hardware $700–$1,500
Shipping, customs, spares allocation $1,000–$3,000

Estimated parts cost per drone:

If sold for twice parts cost:

At that price, it could be highly competitive if it actually achieves multi-week to multi-month autonomous operation.

19. Markets for This Drone

Potential markets:

The strongest early niche may be:

Low-cost solar USV for island governments, marine parks, and research groups that cannot afford Saildrone / Wave Glider class systems.

20. Comparable USVs

1. Saildrone

2. Liquid Robotics Wave Glider

3. Open Ocean Robotics DataXplorer / Similar Solar USVs

Competitiveness

If your USV sells for $25k–$40k, has Starlink, 1,200 W solar, 8–10 kWh battery, user-programmable control, and useful payload capacity, it could be very attractive.

The main weaknesses versus professional systems:

21. Overall Assessment

The 1:4 model is a very interesting platform. The best features are:

The main risks are:

My Top Design Recommendations

  1. Increase rear beam to about 10 ft to fit 6 solar panels cleanly.
  2. Increase reserve buoyancy or reduce target operating weight below 562 lb.
  3. Use six panels for 1,200 W STC; this is a major advantage.
  4. Do not depend on full hydrofoiling initially; use stabilizers for ride control and partial lift.
  5. Design for Sargasso from day one: cameras, routing, guards, line cutters, and easy cleaning.
  6. Use a serviceable sealed electronics enclosure rather than fully potting the main computer.
  7. Add a backup low-power tracker and failsafe controller independent of Starlink and the Raspberry Pi.
  8. Prototype recovery docking early; your tow-rope and V-capture concept is promising.