Seastead Propulsion Analysis: Low-Speed Mixers

Project Parameters: 30,000 lb displacement, 22kW solar installed, target speed 0.5–1.0 MPH. Goal: Cost-effective, high-thrust, low-speed propulsion using large diameter propellers (2.5m+).

1. Feasibility Assessment

Does using submersible mixers seem like a good plan?

Yes. From a physics standpoint, this is an excellent approach for your specific constraints.

Efficiency at Low Speed: Standard boat propellers are small and fast, which is terrible for your 1 MPH goal. Large, slow-turning propellers (like mixer flow-makers) generate thrust efficiently at low speeds (high static thrust efficiency).
Power Match: Your estimate of 2,000 watts for 0.5 MPH is conservative and realistic. A 30,000 lb vessel moving at 1 MPH (approx 0.87 knots) has very low resistance. You have plenty of power headroom.

Critical Engineering Challenges:

Drag: A stopped 2.5-meter propeller creates massive drag. You need a strategy to minimize drag when the vessel is drifting or station-keeping (folding props are rare for mixers, so you may need to accept the drag or build a "well" or cage to fair the flow).
Reverse: Most wastewater mixers are fixed-pitch and optimized for one direction. While the motor can electrically reverse, the blade profile (often a "banana" shape) is optimized for forward thrust. Expect significantly reduced thrust in reverse (approx 40-60% of forward).
Mounting: These units weigh 200–300 kg (450–650 lbs). They are designed to hang from a bracket in a tank. You will need to fabricate a robust mounting arm or "strut" that holds the unit at the correct depth and transmits the thrust to the seastead structure.

2. Control & Variable Speed (VFD)

Do these come with controllers?
Generally, no. Most industrial mixers are sold as "dumb" motors. They expect you to supply the power cabinet.

Can they run at variable power levels?
Yes. The motors are almost exclusively 3-Phase AC Induction motors. To vary the speed (and thus thrust), you need a Variable Frequency Drive (VFD).

What do VFDs cost?
Since your motor size is small (1–5 kW), the VFD is cheap.

Recommendation: Buy the mixer unit without controls, and purchase a standard VFD (brands like Allen-Bradley, Siemens, or generic Chinese brands like Inovance/VEICHI) separately. This gives you precise control over RPM for steering.

3. Price Reality Check & Product Analysis

The prices on Made-in-China are generally accurate for the hardware, but they often exclude shipping, crating, and import duties.

Deep Dive on the Chinese Option (QJB Series)

The Chinese listings (Lanshen, Viva, Henan Eco) all refer to the "QJB" standard design. This is a mature design in China.

4. Aquaculture Pushers Search

Unit	Est. Thrust	Power	Price Reality (Est.)	Verdict
Xylem/Flygt 4410 (European)	High	2.5 – 4 kW	$12,000 – $18,000 (Plus VFD)	Overkill. Exceptional quality, stainless steel, seawater ready. But the price defeats your "1/3 cost" goal.
Shinmaywa (Japanese)	3,200 N (720 lbs)	2.2 – 3.2 kW	$8,000 – $12,000	Good Quality. Specific "Seawater" models exist. Still pricey for a budget build.
Landia (European)	Not Specified	1.5 – 4 kW	$10,000+	Pass. Lack of thrust data makes sizing difficult.
Chinese QJB Series (Lanshen/Viva/Eco)	~2,000 N (450 lbs)	3 kW	$2,000 – $3,500	Winner. This is the most cost-effective path.

You asked for "Marine Aquaculture Pushers" over 400 lbs thrust. I searched for Live Fish Carrier Thrusters and Cage Thrusters.

Finding: Dedicated marine aquaculture thrusters (like for fish cages or live fish carriers) are almost identical to the wastewater mixers you found, but with a much higher price tag (often 3x-4x the cost) due to marine certification and specialized mounting hardware.

The Shinmaywa unit you found is actually a leader in this space (often used in Japanese aquaculture). For your budget, the Industrial Wastewater Mixer is the exact same technology rebranded for sewage. There is no "cheaper" marine-specific alternative found.

5. Recommendation

The Best Path Forward

Based on your cost constraints and performance goals, I recommend the Chinese QJB Submersible Mixer (Lanshen or similar reputable manufacturer).

Specifications to Request:

Model: QJB2500/3kW (2500mm Diameter, 3kW Power).
Material: Strictly specify Stainless Steel 316 (or 304) for all parts touching water. Reject Cast Iron.
Cable: Request a long submersible cable (e.g., 10-15 meters) suitable for your depth.
Quantity: 2 Units.

Performance Estimate:

Total Thrust: ~4,000 Newtons (900 lbs).
Speed Prediction: With 2 units at full power, you should easily exceed 1 MPH. You will likely run them at 30-50% power via the VFD for normal cruising to save energy.
Steering: Differential thrust (one forward, one reverse/slow) will work, but remember reverse thrust will be weaker.

Electrical Setup (Separate Purchase):

Buy 2x 3kW VFDs (Variable Frequency Drives). Look for "Vector Control" VFDs which provide better torque at low speeds. Connect your Solar/Battery DC-AC inverter to the VFD input.

Estimated Total Cost (Propulsion):

2x Mixers (Stainless): ~$6,000 - $7,000 (landed/shipped).
2x VFDs & Electrical: ~$600.
Mounting Fabrication (DIY): Material cost only.
Total: < ~$8,000.

Compare this to a single marine stern thruster or outboard leg system which would cost $15,000+ and be less efficient.

Seastead Propulsion Analysis: Low-Speed Submersible Mixers