This report provides engineering answers and analysis for the 1:4 scale model derived from the full-scale seastead design using Froude scaling laws, along with component selection, performance estimates, cost budgets, and operational strategies.
The full‑scale vessel displaces 36,000 lb. The scale factor λ = 4. By Froude scaling, weight (displacement) scales with λ³.
| Component | Full‑scale | 1:4 Model (ft‑in) |
|---|---|---|
| Triangle Frame | ||
| Side length (port / stbd) | 70 ft | 17 ft 6 in |
| Base (athwartships) | 35 ft | 8 ft 9 in |
| Height (fore‑aft altitude) | 67.8 ft (calculated) | 16 ft 11.3 in |
| Truss depth (full‑scale living area) | 7 ft | 21 in (not used – model flat) |
| Legs / Foils (each) | ||
| Length (vertical) | 19 ft | 4 ft 9 in |
| Submerged length (50%) | 9.5 ft | 2 ft 4.5 in |
| Chord (NACA 0030) | 10 ft | 2 ft 6 in |
| Max thickness | 3 ft | 9 in |
| Ladder (top front half) | yes | Not needed |
| RIM drive thrusters (scaled for ref.) | ||
| Diameter | 1.5 ft | 4.5 in |
| Height above leg bottom | 3 ft | 9 in |
| Stabilisers (each, like a small airplane) | ||
| Main wing span | 12 ft | 3 ft |
| Main wing chord | 1.5 ft | 4.5 in |
| Body length | 6 ft | 1 ft 6 in |
| Elevator span | 2 ft | 6 in |
| Elevator chord | 6 in | 1.5 in |
| Leading‑edge notch (for balance) | 25% of chord | ~1.125 in into wing |
With 30% of the 562.5 lb total weight dedicated to batteries:
Remaining weight budget (393.75 lb) must cover: structure, solar panels, thrusters, electronics, actuators, wiring, etc. A detailed weight breakdown is given in Section 8.
Lightweight semi‑flexible or rigid walk‑on panels that tolerate occasional salt spray. Good options:
Triangle area (flat deck) = 0.5 × base × height = 0.5 × 8.75 ft × 16.95 ft ≈ 74.1 sq ft. Filling ~80% with panels gives about 59 sq ft of active cell area.
At a typical 200 W/m² (≈18.6 W/sq ft), this yields ≈ 59 × 18.6 = 1100 W peak. To maximise solar, the triangle can be slightly enlarged: increasing the base to 9 ft and sides to 18 ft gives altitude ≈ 17.3 ft, area ≈ 78 sq ft, resulting in ~1400 Wp. We recommend 1200 Wp as a realistic, well‑packed array using off‑the‑shelf 100 W panels.
The Blue Robotics “M200” motor is the brushless motor inside the popular T200 Thruster. The complete T200 Thruster (with nozzle and propeller) is the unit we will use. Three pairs (one on each side of each leg) give six thrusters total, providing exceptional redundancy.
Blue Robotics does not publish MTBF. Community experience indicates T200 thrusters routinely last 2000–5000 hours in clean water with proper maintenance. Continuous power at 100–200 W per thruster is well within ratings. For planning we assume a conservative MTBF of 3000 h per thruster.
We require at least two working thrusters on different legs to maintain forward motion and steer via differential thrust. With three independent legs, the system survives if at least two legs each have at least one operational thruster.
Using a simple exponential reliability model (MTBF=3000 h), the probability that a thruster fails by time t is 1−e−t/3000. The configuration can fail only if thrusters on two whole legs are lost. The probability of losing a leg (both thrusters fail) by time t is (1−e−t/3000)². The system fails if two or three legs are lost. This gives a very high reliability: after 500 h, probability of being unable to steer ≈ 0.5%. The drone could operate for months with scheduled thruster replacements.
Conclusion: With 6 thrusters, one can expect thousands of hours of safe operation before loss of steering, far exceeding a typical USV mission length.
The stabiliser uses a “servo‑tab” on the elevator. Moving the elevator changes the wing’s angle of attack via aerodynamic/hydrodynamic balance, so no wing‑angle sensor is needed (the wing follows the tab).
We want a spring‑loaded pin that automatically engages when the stabiliser lines up (probably trailing‑edge‐down) to lock it as a fixed heave plate. To unlock, the pin must be retracted.
Mechanism:
Prices are rough estimates for small‑batch orders; labour for assembly is zero.
| Item | Qty per unit | Unit Cost (US$) | Subtotal per unit |
|---|---|---|---|
| T200 Thruster (Blue Robotics) | 6 | 240 | 1440 |
| ESC (Basic 30A BlueESC) | 6 | 40 | 240 |
| Stabiliser Servos + Locking Solenoids | 3 sets | 80 | 240 |
| Solar Panels (1200 Wp flexible) | 1 set | 1200 | 1200 |
| LiFePO₄ Battery Pack (8.4 kWh) | 1 | 840 | 840 |
| Marine Aluminium Extrusions (2″ angle) | various | 500 | 500 |
| Pivot & attachment hardware (stainless) | 1 set | 200 | 200 |
| Raspberry Pi CM4 + carrier board | 1 | 150 | 150 |
| Starlink Mini | 1 | 1500 | 1500 |
| 360° Cameras (2×) | 2 | 150 | 300 |
| IR Night Camera | 1 | 200 | 200 |
| AIS Transmitter + LED Nav Lights | 1 set | 200 | 200 |
| Navigator Board (IMU, compass, PWM) | 1 | 250 | 250 |
| Wiring, connectors, junction boxes | 1 set | 150 | 150 |
| Potting compound (Sylgard 184) | 1 kit | 50 | 50 |
| Misc (rope, float, rescue hook) | 1 set | 100 | 100 |
| Total per unit | $7,560 | ||
If sold for twice the parts cost, the retail price would be around $15,000 – dramatically lower than existing ocean USVs.
| Component | Weight (lb) | Notes |
|---|---|---|
| LiFePO₄ Battery | 168.8 | 30% of target |
| 6× T200 Thrusters | 12.0 | ~2 lb each |
| ESCs + cables | 3.0 | est. |
| Solar Panels (1200 W) | 24.0 | ~2 lb/100 W |
| Aluminium framing (triangle + legs) | 250.0 | Major structure |
| Stabilisers (3x) with linkages | 15.0 | Lightweight aluminium |
| Electronics (RPi, Navigator, cameras, Starlink, AIS) | 8.0 | Starlink Mini ~1.1 lb |
| Wiring, connectors, junction boxes | 8.0 | |
| Mast for 360 cam & other | 5.0 | |
| Hardware, pins, springs | 10.0 | |
| Contingency (10%) | 50.0 | |
| Estimated Total | 553.8 lb |
The estimate is slightly under the 562.5 lb target, leaving a small margin. The design can be easily adjusted (e.g., thicker framing) to hit the exact displacement.
The three‑stage recovery plan is sound:
We can move faster during the day when solar is directly powering the motors, and slower at night on battery.
Drag dominated by the three submerged foil legs (frontal area 3 × 0.75 ft × 2.375 ft = 5.34 ft²). Using a drag coefficient CD ≈ 0.25 (conservative for NACA 0030), the electrical power required (assuming 50% propulsive efficiency) is:
Pelec (W) ≈ 3.6 × V³ (V in knots)
| Scenario | Motor Power (W) | Speed (knots) |
|---|---|---|
| Night cruise (battery, 8 h) | 200 | ~3.8 |
| Day cruise (solar direct, 6 h) | 500 | ~5.2 |
| Sprint (short battery burst) | 1200 | ~6.9 |
Wind added as vector; upwind speed will be 0.5‑0.8 kt less, downwind up to 1 kt faster. Crosswind performance is close to the values above because the legs act as excellent daggerboards.
If we deliberately angle the three stabilisers to generate lift, they could partially or fully lift the hull out of the water, removing most of the leg drag.
Foiling endurance: With 6.7 kWh usable energy and 400 W, the drone could foil for ~16.8 hours, covering ~120 nm at 7 knots. With solar supplement during the day, a foiling range of 200+ nm in 24 h is plausible. This requires fine control, but the architecture makes it feasible. Putting thrusters below the stabiliser wing improves propulsive efficiency when foiling.
The ship is inherently stiff against capsize because the three legs (keels) provide a wide waterplane area when immersed, but the superstructure is low. A capsize (roll over) would require a breaking wave of height comparable to the beam or a steep impulsive load. The triangle beam is 8.75 ft; breaking waves ≥ 4 ft could flip it if broadside. However, the drone can always run downwind and take waves from the stern. With weather forecasting, the drone can actively avoid regions with significant wave height > 2 m (6.5 ft). In the Caribbean, trade‑wind waves are typically 1‑2 m with occasional 3 m, but tropical storms are well forecast. By moving away from predicted high‑wave areas and using the “run downwind” strategy, 999 days out of 1000 are perfectly safe. The full‑scale vessel is four times larger, so it will be even safer.
Recommended SBC: Raspberry Pi Compute Module 4 (CM4) with eMMC, because it avoids the unreliability of an SD card. If lower power is desired, the newer Raspberry Pi 5 (4 nm, idle very low) is also good. Orange Pi or other clones offer no advantage in marine reliability.
Potting the entire carrier board in Sylgard 184 is excellent. Leave the tall heat‑sink exposed, with a tight seal where it passes through the potting. Mount the sealed unit inside a leg where it is water‑cooled by the seawater. This gives a very rugged, corrosion‑proof computer.
Daytime: visible‑light cameras + AI can detect Sargassum mats. At night, an uncooled LWIR camera (8‑14 µm) can see the temperature difference between water and floating seaweed (seaweed slightly warmer). Options: FLIR Boson (640×512, ~$2000), but a cheaper 320×240 core is sufficient. This feeds into the Raspberry Pi’s AI module.
Thrusters: T200’s nozzle can be fouled by thick seaweed, but M200‑based thrusters with an open propeller (like the “M200 Thruster” version sold by Blue Robotics) are more weed‑shedding. The user is right to prefer that configuration.
Flexible solar panels typically have a junction box on the underside. For additional wave‑splash protection:
This USV fills a niche for affordable, solar‑powered long‑endurance monitoring. Markets: marine research, fisheries patrol, border/EEZ surveillance, oceanographic data collection, search & rescue.
| Vehicle | Propulsion | Speed (knots) | Endurance | Cost (USD) | Self‑righting | Open platform? |
|---|---|---|---|---|---|---|
| Saildrone Explorer (7 m) | Wind (wing‑sail) + solar | 2‑5 | Up to 12 months | ~$500k | Yes | Limited (SDK provided) |
| Liquid Robotics Wave Glider | Wave‑glider (surface + sub) + solar | 0.5‑2.0 | Years | ~$400k | Yes | Limited proprietary OS |
| SeaTrac SP24 | Solar + electric thruster | 2‑5 | Months | ~$250k | Yes (weighted keel) | Some customisation |
Our 1:4 scale model, sold for $15,000‑$20,000, would be 10‑30× cheaper. The full‑scale version could be produced for far less than a Saildrone. The price difference is due to:
Our USV does not have a self‑righting capability, which is a significant trade‑off but is acceptable if the operational envelope is managed via weather avoidance and the inherent stability of the trimaran layout.
The 1:4 scale model is an exceptional test platform: it has a higher power/weight ratio and more active stabilisation authority than the full‑scale, allowing validation of control algorithms in waves that are effectively 4× larger. With the hardware outlined above, a per‑unit cost near $7500 and a huge amount of operational flexibility, the drone can prove the concept and open a market for a completely new class of affordable ocean USVs.
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