Seastead Propulsion – Large Low‑Speed Propeller Options

Goal: Move a ~30,000 lb seastead at ~0.5 mph (≈0.44 knots) using only ~2 kW of continuous electrical power (≈2 × 1 kW thrusters). The platform has 22 kW of solar, but we want to keep propulsion cost low. The design calls for two wide‑spaced thrusters that can be throttled independently for differential steering.

The most economical way to get a very large (≈2.5 m diameter) propeller that turns slowly (30‑50 rpm) is to repurpose low‑speed submersible mixers (often called “banana propellers”, “flow‑makers” or “sea‑water aerator pushers”). Below is a detailed comparison of the units you listed, plus some alternatives, pricing guidance, and a recommendation.

1. Low‑Speed Submersible Mixers – Comparison Table

Model (Manufacturer)	Diameter (mm)	Rated Power (kW)	Thrust (N)	Thrust (lb_f)	RPM (approx)	Seawater‑ready?	Price (USD) – Approx.	Variable‑Speed?	Notes
PTM Phantom‑2500	2500	3 kW (standard)	≈2000 N	≈450 lb	≈45	Option for SS (contact mfr)	$12,000 – $14,000	Yes – use VFD	European brand, robust, higher cost. Thrust figure is estimate – not published.
Flygt 4410 / 4430 (Xylem)	2500	2‑4 kW	≈2100 N	≈470 lb	≈45	SS impeller optional	$8,000 – $12,000	Yes – VFD	Well‑known brand, good support. Thrust published in some datasheets.
Landia POPL‑I	2300	1.5 / 2 / 4 kW	≈1500 N (est.)	≈340 lb	≈40‑50	Cast iron (not SS)	$5,000 – $7,000	Yes – VFD	No thrust data in the sheet – estimated. Good for shallow tanks.
Wilo EMU TR‑216	2100	2‑5 kW	≈1800 N (est.)	≈400 lb	≈40‑50	SS impeller optional	$6,000 – $9,000	Yes – VFD	European, reliable.
Sulzer ABS XSB 2750	2750	3‑7.5 kW	≈4000 N (max)	≈900 lb	≈30‑45	SS option	$8,000 – $15,000	Yes – VFD	High‑thrust, used in wastewater & aquaculture. Very robust.
ShinMaywa SME‑2500	2500	2‑3.2 kW	≈3200 N	≈720 lb	≈30‑45	Stainless steel	$5,000 – $8,000	Yes – VFD	Japanese marine‑grade, excellent corrosion resistance.
VIVAMIX VML‑2500/3 kW	2500	3 kW	≈2090 N	≈470 lb	≈45	Cast‑iron (SS upgrade possible)	$1,800 – $2,500	Yes – VFD	Low‑cost, widely sold on Alibaba. Thrust published.
ShinMaywa SM‑VRTN (2500 mm)	2500	2‑3.2 kW	≈3200 N	≈720 lb	≈30‑45	Stainless steel	$5,000 – $8,000	Yes – VFD	Same as SME series, data sheet shows 3200 N.
Henan Eco VML‑2500 (3 kW)	2500	3 kW	≈2100 N	≈470 lb	≈45	Cast‑iron	$1,800 – $2,500	Yes – VFD	Similar to VIVAMIX – low price, same specs.
QJB‑2500/3 kW	2500	3 kW	≈1960 N	≈440 lb	≈45	Cast‑iron	$1,800 – $2,500	Yes – VFD	Typical Chinese low‑cost mixer.
QDT‑5.5/4‑2500/2‑47	2500	5.5 kW	≈3840 N	≈860 lb	≈42	Cast‑iron (SS optional)	$3,000 – $4,500	Yes – VFD	Higher power – useful if you can supply 5 kW per unit.
QJB‑4/4‑2500/2‑42P	2500	4 kW	≈2500 N (est.)	≈560 lb	≈42	Cast‑iron	$2,500 – $3,500	Yes – VFD	Good balance of power & thrust.

All prices are “ballpark” EXW (Ex‑Works) and do not include shipping, customs, import duties, or inland freight. Typical landed cost (sea‑freight + 10‑15 % duty) adds $300‑$800 per unit.

Key Take‑aways from the Table

Thrust per watt: The 2‑3 kW Chinese mixers deliver ~2000 N (≈450 lb) – roughly 0.7 N per watt, which is good for a low‑speed propeller.
Seawater compatibility: Most Chinese units are cast‑iron (corrosion‑resistant only if painted or coated). For long‑term marine exposure you’ll want a stainless‑steel impeller or a protective coating (epoxy, hot‑dip galvanizing). The Japanese ShinMaywa and European Flygt & Sulzer have SS options.
Variable speed: All listed units use a three‑phase induction motor, so they can be throttled with a Variable Frequency Drive (VFD). The thrust scales roughly with the squared speed, giving you a wide range of thrust from a few hundred Newtons up to the rated value.

2. Do You Need a Separate Controller?

Most submersible mixers are sold without a controller. You’ll need a VFD that can:

Accept the input voltage you have (e.g., 220 V single‑phase from an inverter or 48 V DC → 220 V AC inverter → VFD).
Handle the motor’s rated current (typically 10‑15 A for a 3 kW unit).
Provide basic overload protection and speed‑reference input (0‑10 V or 4‑20 mA).

Typical VFD costs (2024‑2025):

Model (brand)	Power rating	Price (USD)
ABB ACS‑310	2.2 kW (3 HP)	$200 – $300
Schneider Altivar 312	2.2 kW	$250 – $350
Siemens SINAMICS V20	2.2 kW	$220 – $320
Invertek E2‑2‑2‑200	2.2 kW	$180 – $280

For a 5 kW unit you’ll need a 5‑5.5 kW VFD – usually $350‑$500. Many suppliers also sell “VFD + motor starter” packages for $400‑$600.

Bottom line: Budget $300‑$500 per thruster for the VFD. If you want a plug‑and‑play “motor‑controller‑assembly”, some Chinese manufacturers offer a “VFD‑integrated mixer” (e.g., “VIVAMIX VML‑2500‑VFD”), but those are usually custom‑built.

3. Alternative Solutions – Dedicated Marine Electric Thrusters

If you prefer a purpose‑built marine thruster (rather than a repurposed wastewater mixer), here are the main options:

Engineering effort for mounting, sealing, alignment

Type	Typical Thrust (N)	Power (kW)	Diameter (mm)	Cost (USD)	Pros	Cons
Electric tunnel thruster (e.g., Side‑Power, Vetus, Lewmar)	≈300‑600 N (≈70‑140 lb)	1‑3 kW	≈250‑400 mm	$2,000 – $5,000 per unit	Compact, marine‑rated, built‑in housing, easy install	Small propeller → higher rpm, less efficient at ultra‑low speed
Electric outboard (e.g., Torqeedo Deep‑Blue, ePropulsion Spirit)	≈300‑800 N (≈70‑180 lb)	3‑6 kW	≈300‑400 mm	$4,000 – $9,000	Complete system (motor + controller + propeller), tested for marine use	Propeller still relatively small; limited scalability for >2 m diameter
Custom axial‑flow pump (same as mixers) + motor	≈1500‑5000 N (≈340‑1125 lb)	2‑7.5 kW	≈1500‑3000 mm	$2,000 – $8,000 (motor + pump + VFD)	You can choose exact size, high thrust at low rpm

For a seastead that wants the largest possible propeller for the lowest cost, the submersible‑mixer route is still the most economical. The marine‑grade thrusters above are far more compact, but they can’t match the thrust per watt of a 2‑3 m diameter axial‑flow propeller.

4. Recommended Solution for Your Budget

Given the constraints (≈2 kW total, low speed, low cost, large propeller), the best “bang‑for‑buck” is to use two 2.5 m, 3 kW Chinese submersible mixers (VIVAMIX VML‑2500/3 kW or equivalent) and run each with a 2.2 kW VFD. This gives you:

≈2 × 2000 N ≈ 4000 N total thrust (≈900 lb) at full power – enough for 0.5 mph and plenty of control authority.
You can throttle each unit down to 1‑1.5 kW (via VFD) to match the 2 kW total budget.
Large propeller diameter → low rpm → high efficiency for ultra‑slow speeds.
Total hardware cost: $× $2,000 (mix2 er) + $2 × $300 (VFD) ≈ $4,600 (plus shipping, import duty, and any SS upgrade – maybe $1,000 extra).

If you can stretch the budget, the ShinMaywa SME‑2500 (Japanese, stainless‑steel) would give you a marine‑grade, corrosion‑proof unit with the same thrust, but at a higher price (≈$5,000 – $8,000 each). That would bring total cost to $12‑$16 k.

For the absolute lowest cost, you could also look at the QDT‑5.5/4‑2500/2‑47 (5.5 kW) – you would run it at reduced power (≈1 kW each) and still get >1500 N per unit, but the VFD must be sized for 5.5 kW, adding ~$400 per unit. Not cost‑effective unless you plan to upgrade later.

5. Marine‑Aquaculture Pushers with >400 lb Thrust

These are essentially the same axial‑flow pumps used in wastewater, but marketed for “aquaculture” (e.g., pond aeration, shrimp farming). Many manufacturers offer “high‑thrust flow‑makers” in the 2‑3 m range. Below are examples that exceed 400 lb (≈1800 N) thrust:

Model	Diameter (mm)	Power (kW)	Thrust (N)	Thrust (lb)	Typical Price (USD)	Notes
VIVAMIX VML‑3000/5.5 kW	3000	5.5 kW	≈4000 N	≈900 lb	$3,500 – $5,000	Larger diameter, high thrust. SS impeller optional.
Sulzer ABS XSB‑2750/7.5 kW	2750	7.5 kW	≈5000 N	≈1125 lb	$9,000 – $15,000	Heavy‑duty, marine‑grade, widely used in aquaculture.
Flygt 4470 (high‑thrust)	2500‑3000	4‑7 kW	≈3500‑4500 N	≈800‑1000 lb	$7,000 – $12,000	Premium brand, SS version available.
LianSheng LS‑3000/5.5 kW	3000	5.5 kW	≈3800 N	≈850 lb	$3,000 – $4,500	Chinese, lower cost, similar thrust.

All of the above can be driven with a VFD and can be mounted on the seastead’s hull in a similar fashion to the 2500 mm units.

6. How to Verify Prices & Sources

Alibaba / Made‑in‑China: The best way to get real‑world pricing is to request a quotation directly from the manufacturer. Most factories will respond within 24 h. Look for “Verified Supplier” and ask for the price including sea‑freight to your port.
Local distributors: In the US, “Grainger”, “McMaster‑Carr”, and “Zoro” sometimes carry low‑speed mixers, but they are often more expensive.
Trade shows: Events like “Aquaculture America” or “World Aquaculture” have many Chinese manufacturers exhibiting – you can negotiate discounts.
Used equipment: Sites like “eBay”, “EquipMatch” or “MachineryZone” occasionally list secondhand mixers at 30‑50 % of new price. Just be sure to inspect the motor and impeller for corrosion.

Remember that the “list price” on a manufacturer’s website is often the MSRP and can be negotiated, especially for orders of two or more units.

7. Practical Implementation Tips

Mounting: Secure the mixer to a sturdy steel or aluminum frame that can take the thrust. Use a “thrust bearing” or “pillow block” to isolate vibration from the hull. Many users simply bolt the mixer to a steel plate that is welded to the platform’s sub‑structure.
Direction control: The mixer’s motor can be reversed by swapping any two phases on the VFD. You can run two mixers on separate VFDs and use differential thrust for steering (one forward, one reverse).
Corrosion protection: If you buy the cast‑iron version, apply a two‑part epoxy coating (e.g., “Interline 704”) to the entire housing and shaft. Replace the standard shaft seal with a “double‑lip” seal rated for seawater.
Power‑management: Use a simple PLC or Arduino to read the VFD speed command (0‑10 V) and adjust the throttle based on solar input or battery state‑of‑charge. This keeps the propulsion within the 2 kW budget.
Noise & vibration: The mixers are not noise‑critical for a seastead, but you may want to add rubber isolators to reduce hull vibration.

8. Summary

Submersible mixers are the cheapest way to get a 2‑3 m, low‑rpm propeller that can deliver >400 lb thrust for ≈2 kW of power.
The Chinese “VIVAMIX VML‑2500/3 kW” (or equivalent) is the best low‑cost candidate – about $2,000 per unit plus $300 for a VFD.
All units can be throttled with a VFD, giving you variable thrust from a few Newtons up to the rated value.
If you need marine‑grade corrosion resistance, consider the Japanese ShinMaywa SME‑2500 or the European Flygt/Sulzer models – they cost more but are purpose‑built for seawater.
You can also look at “aquaculture pushers” (e.g., VML‑3000, Sulzer XSB) if you anticipate needing even higher thrust in the future.

With two of these mixers and the appropriate VFDs, you should be able to achieve a continuous speed of about 0.5 mph (0.4‑0.5 knots) while staying well within your 2 kW propulsion budget and keeping total hardware costs around $5 k‑$6 k (including VFDs, mounting hardware, and basic corrosion protection).

Next steps:

Contact the manufacturers (e.g., VIVAMIX, Henan Eco) for a formal quote, asking for the “stainless‑steel impeller” option and sea‑freight to your port.
Purchase two 2.2 kW VFDs (e.g., ABB ACS‑310) – budget about $300 each.
Design a simple thrust‑bearing mount and apply a marine‑grade epoxy coating.
Integrate the VFDs with your battery/inverter system and a simple speed‑command controller.

Good luck with the build! If you need more detailed mounting drawings or a parts‑list, feel free to ask.