Large low-RPM “submersible mixer” propulsors for a slow seastead (1

High-level take

Conceptually workable: big diameter + low RPM is exactly how you get efficient thrust at very low boat speed.
Main risks: corrosion/seals/bearings in salt water, marine growth, safety guarding, and whether the unit is rated for continuous variable speed + frequent reversing (propulsion-like duty).
If you can accept some engineering and prototyping, a large mixer/flowmaker can be a cost-effective “cheap azimuth-less thruster” for ~0.5–1.0 mph.

Reality check: does ~2,000 W for ~0.5 mph make sense?

Important: This only says what thrust you can “afford” at that power. Whether you actually need 400–800 lbf depends on your true hydrodynamic resistance (drag). Your 45° columns may reduce waterplane but can increase wetted area and form drag. A tow test or a CFD/empirical resistance estimate is worth doing early.

Are submersible mixers a “good plan” vs marine thrusters?

Pros

Cons / engineering issues (the “gotchas”)

Salt water suitability: many are cast iron with coatings intended for wastewater, not seawater. Long-term seawater needs correct metallurgy (316L/duplex), proper fasteners, anodes, seal materials, and coatings.
Duty cycle mismatch: mixers are commonly “run at one speed for months.” Propulsion needs variable speed, reversing, and sometimes rapid changes.
Cooling at low Hz: if you slow an induction motor far below rated speed using a VFD, motor cooling can suffer. Some mixers are fine; others need oversizing or forced cooling—manufacturer approval matters.
Mounting / structure: you must design for several kN of thrust and bending loads, plus vibration. These units are heavy (200–400+ kg).
Safety: a 2.5 m prop is extremely hazardous. You’ll want guards and operational rules. Also consider debris/line entanglement.
“Thrust” ratings are not standardized for marine propulsion: mixer thrust (N) is usually measured in still water in a test rig; it’s a good starting point, but not identical to “bollard pull” standards used for marine thrusters.

Speed control: VFDs, variable power, reversing

Most low-speed mixers use 3‑phase induction motors. Variable speed is typically done by a VFD (variable frequency drive).

Typical VFD costs (order-of-magnitude)

Pricing: why Made-in-China prices are often unreliable & how to confirm

Comparison of the candidates you listed (focused on ~2.5 m, 1–5 kW, “push”)

Which looks “best” for your use case?

Item	Typical budget range (USD)	Notes
3–5 kW industrial VFD (ABB/Danfoss/Schneider/Delta/etc.)	$250 – $1,200	Basic drive only, not marine-sealed.
EMI filter / line reactor / dv-dt filter (optional but often needed)	$100 – $600	Depends on cable length to thruster and EMC requirements.
NEMA 4X / IP66 enclosure + glands + disconnect + breakers	$250 – $1,500	Salt air is brutal; sealed enclosure is strongly recommended.
Marine-grade cabling/connectors	$200 – $1,000+	Depends heavily on length and current.

Vendor / model	Prop dia	Power	Thrust / “push”	Salt water suitability	Variable speed (VFD)	Budgetary price (each)	Links
ShinMaywa (SM-VRTN / related)	~2.5 m	2–3.2 kW	3200 N (~720 lbf) per your note	Strong candidate if configured in stainless for seawater (you noted stainless + seawater OK)	Likely VFD-capable (confirm frequency range, reversing, cooling)	Likely mid/high (often far above “China pricing”); request quote	Product page PDF
Flygt / Xylem 4410 / 4320 series	Large (series-based)	Varies	Often excellent thrust/kW (but depends on exact configuration)	Can be specified for aggressive environments; verify seawater package	Many installations use VFDs; “adaptive” versions may have special controls	High (commonly among the most expensive)	4410 4320
Sulzer ABS XSB 900–2750	Up to ~2.75 m	Varies	Model-dependent	Industrial-grade; verify seawater metallurgy and coatings	Typically VFD-integrated in plants (confirm limits)	High	XSB series
PTM Phantom 2500	2.5 m	Varies	Depends on configuration	Verify seawater materials	Likely VFD-capable if 3‑phase induction	You estimated ~$13k (needs confirmation)	Phantom 2500
Landia POPL-I	~2.3 m	1.5 / 2 / 4 kW	Not shown in your linked datasheet (ask for thrust curve)	Verify seawater configuration	Likely VFD-capable (confirm)	Likely mid/high	Datasheet
Wilo EMU TR series	Series-dependent	Varies	Model-dependent	Verify seawater metallurgy	Likely VFD-capable (confirm)	Likely mid/high	TR series
Chinese (VIVAMIX VML2500/3kW)	2.5 m	3 kW	2090 N (~470 lbf) per your note	Cast iron version is not ideal for seawater; stainless version cost unknown—verify grade (316L?) + fasteners + anodes	Likely VFD-capable (3‑phase). Confirm insulation/seal rating for VFD use.	You saw ~$2k (cast iron). Stainless likely significantly more.	VML series page
Chinese (QJB/QDT 2500mm units)	2.5 m	3–5.5 kW	Examples you listed: 1960 N (~440 lbf), 3840 N (~863 lbf)	Often cast iron unless upgraded; seawater requires careful spec	Likely VFD-capable; confirm	Often low upfront but higher risk/unknown lifecycle cost	(Examples) QJB 2500 example QDT 5.5kW example

Pragmatic recommendation: If cost is the driver, buy one low-cost unit first (in the exact material you would actually deploy in seawater), build a test mount, measure thrust (bollard pull), power draw, noise/vibration, and check seal temps and corrosion after a few weeks. Then scale to two units.

Do you need a nozzle (Kort nozzle / duct) to improve low-speed efficiency?

Other manufacturers / product categories to investigate (additional links)

I can’t live-browse to verify every model’s maximum diameter/thrust, but these are commonly used industrial mixer/propulsor families worth checking for large low-speed units and seawater options:

“Marine aquaculture pushers” (> 400 lbf) — what to look for

Many aquaculture “circulators” quote flow rate (m³/h) instead of thrust (N/lbf), which makes it hard to confirm “>400 lbf push” without a datasheet. The units most likely to exceed 400 lbf are the same large industrial “flow booster / low-speed mixer” products you already found.

Confirmed (from your list) at or above ~400 lbf

Aquaculture-adjacent keywords that often lead to “pushers” with published thrust

What to ask vendors so you don’t get burned

Budgetary pricing guidance (what I’d expect, not a quote)

Question	Why it matters
Is this exact model warrantied for seawater immersion? What materials are wetted parts (prop, hub, fasteners, housing)?	Cast iron + coatings often fail in seawater; fasteners can galvanically destroy the assembly.
Seal type (double mechanical seal?), seal face materials, oil chamber, leak detection options?	Seal failure is the #1 killer in submerged rotating machines.
Can it run on a VFD? Allowed frequency range? Continuous at low Hz? Any special inverter-duty motor requirements?	Some motors overheat or have insulation issues on VFDs.
Is reverse rotation permitted? Any limits on reversing frequency?	For docking/positioning you may reverse often.
What is the test condition for the thrust number (N)?	Ensures apples-to-apples comparisons.
Spare seal kit cost and lead time? Prop replacement cost?	Lifecycle cost matters more than purchase price offshore.

Price uncertainty warning: Without a formal quote that specifies metallurgy + seals + motor/VFD duty, the “$2,000” class listings are frequently not realistic for seawater + propulsion-like use.

Next step if you want: I can turn this into a short “shopping spec”

…I can output a one-page RFQ/spec sheet you can email to ShinMaywa/Flygt/Sulzer and 2–3 Chinese suppliers to get directly comparable quotes.

Using “submersible mixers / flowmakers” as ultra-low-speed seastead thrusters