Project: 40 ft × 16 ft living area, ~30 000 lb displacement, four 4 ft‑wide columns at 45° (≈½ underwater). Floats form a 44 ft × 68 ft rectangle. Desired cruise speed ≈ 1 mph (≈ 0.45 m s⁻¹). The platform is intended to be a “tiny oil‑platform” with relatively high drag.
For a propeller moving water, the ideal (momentum) relations are:
where ρ ≈ 1025 kg m⁻³ (seawater), A = propeller disk area (m²), and vi is the induced water speed through the propeller.
Because the required thrust scales with the square of speed while the required power scales with the cube, large‑diameter, low‑rpm propellers give the highest static (bollard) thrust per watt – the principle behind the “low‑speed submersible mixers”.
Using a flat‑plate drag approximation:
Power to overcome this drag at constant speed:
Assuming a propulsive efficiency η ≈ 0.5, the required input power is ≈ 25 kW. This is the baseline figure for any propulsion system.
Two 2.5 m‑diameter mixers, each with a 4.9 m² disk, could be mounted on the platform’s underside. From the momentum relations, to deliver the required static thrust (≈ 28 kN total) the induced speed per mixer would be:
This is essentially the same power budget as the drag‑limited case, meaning the mixers could both push the platform at 1 mph and provide a comfortable static thrust margin.
Pros: Simple installation, proven technology, no tether, low development cost. Cons: Direct vibration/noise transfer to the living quarters, limited maneuverability (fixed direction), harder to service underwater.
The “underwater quadcopter” is a tethered, four‑propeller unit that hangs below the platform (or can be deployed a few metres away). Each propeller could be the same 2.5 m‑diameter unit used in the mixer concept, giving the same thrust‑per‑propeller numbers above.
| Number of props | Thrust per prop (≈ 7 kN) | Induced speed vi | Power per prop | Total electrical power (η≈0.5) |
|---|---|---|---|---|
| 4 | ≈ 7 kN | ≈ 0.83 m·s⁻¹ | ≈ 5.8 kW | ≈ 23 kW |
The quadcopter can produce the same total thrust as the two mixers while also providing differential thrust for steering (like an aerial quadcopter). Because all four props are identical, the control algorithm is straightforward:
To supply ~25 kW to the quadcopter, a high‑voltage DC tether is practical:
Alternative: use a 48 V battery onboard the quadcopter and recharge via the tether – but that would require very thick conductors for the same power, so high‑voltage is preferred.
| Feature | Submersible Mixers (2 × 2.5 m) | Underwater Quadcopter (4 × 2.5 m) |
|---|---|---|
| Total static thrust (≈) | ≈ 28 kN (same as required) | ≈ 28 kN (same) |
| Electrical power for 1 mph cruise | ≈ 23 kW | ≈ 23 kW |
| Installation complexity | Mount two units on hull; straightforward | Build/buy quadcopter + tether + control system |
| Maintenance | Requires divers or dry‑dock for removal | Can be pulled up for service; modular |
| Noise/Vibration on living area | Direct transmission – higher | Isolated – lower |
| Maneuverability | Fixed direction; would need separate bow‑thrusters for yaw | Built‑in differential thrust for yaw & translation |
| Redundancy | Loss of one mixer = 50 % thrust | Loss of one prop still leaves 75 % thrust; can still maneuver |
| Multi‑use | Only propulsion | Can serve as a “tug” for other vessels, a diving‑platform, etc. |
| Development cost & time | Low (off‑the‑shelf mixers) | Higher (custom frame, electronics, control software) |
The underwater quadcopter adds complexity, extra cost, and a tether to manage. However, it offers:
If the project’s primary goal is a simple, reliable, low‑cost propulsion system, the submersible mixers are the pragmatic choice. If the design philosophy values flexibility, low‑impact operation, and future extensibility, investing in the underwater quadcopter is justified – especially given that the power required is essentially the same as the mixer solution.
In practice, a hybrid system (mixers + quadcopter) may give the best of both worlds: primary thrust from the mixers, with the quadcopter providing backup, fine‑control, and utility beyond propulsion.