Feasibility physics • Review of your 12 candidate units • VFD & controller answers • Realistic pricing • Aquaculture pusher search results • Recommendations & RFQ template
Short answers to your headline questions.
YES — strong fit Your instinct is correct. A 2,500 mm prop at 42–47 RPM is almost exactly the right actuator for 0.5–1 MPH. Nothing in the marine market gives you this disk area per dollar.
You need far less Calm-water drag at 1 MPH is only ≈ 70 lbf (310 N). One 2,500 mm mixer delivers 440–860 lbf. Two units = huge redundancy + differential steering margin, but 1.5–2.2 kW versions would already meet spec.
Yes, nearly all of them These are ordinary induction motors — VFD-friendly. Flygt “Adaptive” is factory variable-speed; Chinese units are commonly bundled with VFD control cabinets. Cap output at rated frequency (usually 50 Hz) unless the maker approves more.
Verify — listed prices are ceilings Chinese 2,500 mm stainless: ≈ $1,800–3,500/unit ex-works (negotiate 10–30% off listings). EU brands: ≈ $9,000–20,000+. Confirm via Alibaba Trade-Assurance RFQs to 5+ vendors — method below.
Value: Eco/VIVA VML2500-3kW or Lanshen QJB3/4-2500 in SS316 ×2.
Quality: Shinmaywa SM 2,500 mm (confirmed stainless + 3,200 N thrust).
Premium: Flygt 4410 SST.
A complete 2-thruster, VFD-controlled, saltwater-ready system lands around $8–10k delivered — comfortably inside a “⅓ the cost of a yacht” philosophy.
Estimated for a ~30,000 lb hull with four 45° columns (small waterplane, large wetted area). Assumes equivalent flat-plate drag area CdA ≈ 3 m². Your real number should be measured (see callout).
| Condition | Speed | Thrust required | Hull power | Electrical draw (η≈45%) |
|---|---|---|---|---|
| Calm water cruise | 0.5 MPH (0.22 m/s) | ≈ 17 lbf (77 N) | ≈ 17 W | ≈ 40 W |
| Calm water cruise | 1.0 MPH (0.45 m/s) | ≈ 70 lbf (310 N) | ≈ 140 W | ≈ 310 W |
| Station-keeping vs 20 kt headwind + 1 kt current | 0 | ≈ 230 lbf (≈1,030 N) | — | <1.5 kW transient |
Yes — with eyes open about three things: corrosion, seals, and the fact that these are wastewater-industry products adapted to a marine duty.
| Approach | Example | Cost (installed pair) | Duty | Marine-ready | Verdict for you |
|---|---|---|---|---|---|
| WWTP low-speed mixer (your plan) | QJB / Flygt / Shinmaywa 2,500 mm | $5–10k (CN) · $20–40k (EU) | S1 continuous | With SS316 + anodes | Best value |
| Custom pod: BLDC + planetary reduction + sailboat prop | 5 kW 48V motor, 10:1, 28″ feathering/fixed prop on strut | $8–14k parts + integration | Continuous | Yes (marine parts) | Best efficiency & control, most DIY effort |
| Electric bow/stern thrusters | Sleipner/Side-Power SE210 (~460 lbf), Vetus, Max-Power | $6–12k | Intermittent! Thermal duty cycle ~2–10 min | Yes | Fails continuous-run requirement unless hydraulic version |
| Large trolling motors / pods | Torqeedo Cruise 12, ePropulsion X-24 | $10–16k | Continuous | Yes | Small props (≤13″), wrong optimization, pricey |
| Hydraulic drive + big prop | Pump/motor + custom prop | $10k+ | Continuous | Yes | ~25–35% loop efficiency — wasteful on solar |
| DC-bus VFD trick (enhancement, not alternative) | PV → DC/DC to VFD DC terminals, skipping inverter | +$300–800 | — | — | Nice upgrade later |
Bow thrusters deserve emphasis: they look perfect on paper (marine, sealed, ~300–500 lbf) but virtually all electric models are thermally limited to minutes of runtime — a dealbreaker for continuous transit. Mixers are rated S1 (continuous).
| Unit | Variable speed? | Notes |
|---|---|---|
| Flygt 4320 “Adaptive” | Factory feature | Xylem’s adaptive mixers self-regulate speed; also accepts external control. The cleanest factory answer. |
| Flygt 4410 / 4430 | Yes, via VFD | Order the VFD-duty variant (reinforced insulation, sensor kit). Xylem publishes permissible frequency windows — stay inside them. |
| Sulzer ABS XSB | Yes, via VFD | Commonly applied with VFDs in plants; specify at order. |
| Landia POPL-I | On request | Offered with variable-speed drives; the 1.5/2/4 kW figures are separate models, not one variable unit. |
| Wilo EMU TR | On request | Standard units fixed-speed; VFD packages available through Wilo. |
| Shinmaywa SM series | Ask | Some SM-family models are inverter-rated; confirm in writing for the 2,500 mm size. |
| All Chinese QJB/QDT types | Yes — routine | Sold daily with matched VFD control cabinets (speed pot + run/stop + protections). Easiest ecosystem for VFD bundling. |
| PTM Phantom 2500 | Likely on request | German builder; ask for the variable-speed option and thrust curves. |
| Source | Included? | Typical controller cost |
|---|---|---|
| EU brands (Flygt, Sulzer, Wilo, Landia, PTM) | No — motor unit only; VFD and protection modules are line items. | Brand VFD 2.2–4 kW: $600–1,200 each; plus Xylem/Sulzer monitoring relays if wanted. |
| Japanese (Shinmaywa) | No — panel sold separately. | $500–1,000 (or use third-party VFD). |
| Chinese (Eco/VIVA, Lanshen, Qiankun, Lanling, Lingchun…) | Often bundled — ask for the “control cabinet” line item; many quote it free or ~$150–400 with the mixer. | Cabinet w/ VFD, breaker, speed pot, thermal+leakage relays: $150–450. |
Nameplate VFD examples if buying separately: ABB ACS580, Danfoss FC-102, Yaskawa GA500 (premium); Veichi AC70, Sunfar, BEST (budget, $150–350 at 3 kW). For a 3 kW motor fed from single-phase 220–240 V, select a VFD rated ~4–5 kW to cover single-phase input derating, or use a 3-phase inverter upstream.
PV 22 kWp ──► MPPT chargers ──► 48 V LiFePO₄ (10 kWh) ──► 230 VAC inverter/charger ─┬─► VFD #1 ──► Mixer STBD (2,500 mm)
└─► VFD #2 ──► Mixer PORT (2,500 mm)
Later upgrade (skips inverter losses):
PV strings ──► DC/DC to ~540 V bus ──► VFD DC+ / DC− terminals (many VFDs accept common DC bus)
You’re right to distrust listed prices. Made-in-China/Alibaba figures are ex-works “starting at” numbers, negotiable, and often exclude stainless upgrades, sensors, cabinets, and freight.
| Class | Unit type | Realistic unit price (ex-works) | Delivered pair estimate* |
|---|---|---|---|
| Chinese, cast iron | 2,500 mm, 2.2–3 kW | $1,300–2,200 | $4–6k |
| Chinese, SS316 | 2,500 mm, 2.2–3 kW | $1,800–3,500 | $6.5–9k (incl. freight, duty, cabinets, spares) |
| Chinese, 5.5 kW / heavy | 2,500 mm, 47 RPM, 3,840 N | $2,500–4,000 | $8–11k |
| Japanese (Shinmaywa SM 2,500) | Stainless, 3,200 N, 2–3.2 kW | $7–12k (quote required) | $18–28k |
| European (Flygt 4410 SST, Sulzer XSB 2750, Wilo TR, Landia, PTM) | 2,300–2,900 mm class | $9–20k+ each, options-heavy | $25–45k |
*Delivered = 2 units + 2 VFD cabinets + anodes/spares + LCL sea freight (units weigh 200–310 kg each) + ~3–5% US duty + local guards/cabling. Treat all figures as ±30–40% until you hold written quotes.
Freight reality-check tool: freightos.com for LCL quotes (expect ~$300–700/unit from a CN port to US Gulf/East Coast for 200–310 kg crates).
Specs as published on the pages you supplied; blanks = not published (chase them via RFQ). Prices are my estimates, not quotes.
| # | Unit | Prop Ø | Power | Rated thrust | Material | VFD | Est. price | Verdict |
|---|---|---|---|---|---|---|---|---|
| 1 | PTM Phantom 2500 (DE) | 2,500 mm | ~3 kW class | Request data | Marine-quality builds | Option | ~$11–15k | Excellent, pricey German quality; get thrust curves + VFD option in writing. |
| 2 | Flygt 4410 / 4430 low-speed (Xylem) | ~2,300–2,900 mm | 1.5–7.5 kW range | High; best-in-class N/kW (your note — plausible) | SS variants avail. | Yes | $10–20k | Premium benchmark The quality ceiling. Only worth it if budget doubles. |
| 3 | Flygt 4320 Adaptive | ~1,600–2,400 mm class | variable | Model-dependent | SS options | Built-in | $8–16k | Nice tech Factory adaptive speed; confirm largest prop size offered. |
| 4 | Landia POPL-I | 2,300 mm | 1.5 / 2 / 4 kW models | Not published | SS option | On request | ~$8–13k | Data gap Agree: hiding thrust is disqualifying until they send curves. RFQ it. |
| 5 | Wilo-EMU TR 216 / TR 326 | Confirm per datasheet | Confirm | Confirm | CI std, SS opt. | Package opt. | $7–14k | Middle tier Solid brand; make them earn it with thrust + SS316 pricing. |
| 6 | Sulzer ABS XSB 900–2750 “flow booster” | up to 2,750 mm | ~1.5–4 kW class | Published per size — pull the XSB 2750 row | SS options | Yes | $9–16k | Strong EU alt The 2,750 mm disk is attractive; price is the hurdle. |
| 7 | Shinmaywa SM 2,500 (JP) | 2,500 mm | 2–3.2 kW | 3,200 N (720 lbf) | Stainless, seawater-rated ✔ | Ask (inverter models exist) | $7–12k | Quality pick The only unit on your list with confirmed seawater stainless + published thrust. My premium recommendation. |
| 8 | Eco/VIVA VML2500/3kW | 2,500 mm | 3 kW | 2,090 N (470 lbf) | CI base; SS extra (price TBD) | Cabinet opt. | $2.0k CI / ~$2.6–3.2k SS | Budget pick 220 V 3φ 60 Hz available. Pin down the exact SS316 surcharge and sensor list. |
| 9 | Lanshen QJB3/4-2500/2-29B | 2,500 mm | 3 kW | 1,960 N (440 lbf) | Confirm (likely CI) | Cabinet opt. | $1.6–2.8k | Budget alt Same class as #8; RFQ both and play them against each other. |
| 10 | Chinalanling QDT5.5/4-2500/2-47 | 2,500 mm | 5.5 kW @ 47 RPM | 3,840 N (863 lbf) | Confirm | Cabinet opt. | $2.5–4k | Overkill Lovely margin (863 lbf × 2!), but 310 kg each and more power than your solar story needs. Consider ONE as a boost unit. |
| 11 | Qiankun QJB4/4-2500/2-42P | 2,500 mm | 4 kW @ 42 RPM | Not published | Confirm | Cabinet opt. | $2–3.5k | Hold No thrust figure = can’t score. Same RFQ fixes that. |
| 12 | Lingchun QJB (water-purification listing) | 2,500 mm class | per model | Not published | CI typical | Cabinet opt. | $1.4–2.6k | Thin info Generic trader listing; only engage if they answer the RFQ properly. |
Two thrusters, wide spacing, differential + pivot steering, VFD speed matching to solar. Three budgets.
| Item | Est. cost |
|---|---|
| 2 × 2,500 mm / 2.2–3 kW submersible mixers, SS316 prop+shaft+hardware, PTC + leakage sensors, S1 duty, 380 V 50 Hz motor (VFD-fed) | $3,600–7,000 ($1.8–3.5k ea) |
| 2 × VFD control cabinets w/ speed pot, breaker, sensor relays (bundled or sourced) | $300–900 |
| Zinc anodes, spare seal kits, oil charges, coatings | $400–700 |
| LCL sea freight + duties (2 crates @ 200–310 kg) | $800–1,400 |
| Local fabrication: pivot yokes + SS bar-screen guards + cabling | $900–1,600 |
| Total | ≈ $6.0–11.6k (plan ~$9k) |
Suppliers to duel: Henan Eco/VIVA (you have the contact), Lanshen, plus 3–5 more surfaced by the RFQ blitz. Play written quotes against each other.
Confirmed stainless + seawater + 3,200 N published thrust — the lowest-risk saltwater choice on your list, at ~2.5–3× the price. Worth it if you never want to think about the drivetrain again.
The industry benchmark for thrust-per-kW and service network. Hard to justify for 0.5–1 MPH, but unbeatable resale/documentation culture.
Compiled without live web access — treat as leads to verify, not guaranteed live pages. Root domains given where I can’t vouch for deep links.
REQUEST FOR QUOTATION — Submersible Low-Speed Mixer (marine adaptation) Quantity: 2 units (repeat orders possible) Application: continuous saltwater propulsion of a floating structure (NOT wastewater) Required specification: - Impeller diameter: 2,500 mm (or larger) - Motor power: 2.2 – 3.2 kW, duty rating S1 (continuous) - Speed: ≤ 50 RPM preferred - Rated static thrust: please state certified value in N (minimum acceptable: 1,900 N) - Materials: ALL wetted parts AISI 316 stainless steel minimum — impeller, shaft, bolts, lifting hardware. NO 304. Material certificates required. Any remaining housing parts: fusion-bonded epoxy coated. - Sealing: mechanical seal with oil chamber, PLUS winding thermal sensors (PTC) and leakage/moisture sensor, wired out. - Sacrificial zinc anodes fitted (quote extra sets). - Motor: 380–400 V 3-phase, 50 Hz nominal, VFD duty (insulation suitable for inverter supply). Please confirm permissible VFD frequency window. - Submersible cable 10 m included. - IP68, continuous submergence in seawater. Please quote (FOB port, itemized, USD): 1) Unit price with all above options 2) Control cabinet with VFD, speed potentiometer, run/stop, thermal + leakage relay protection — price each 3) Spare-parts kit: mechanical seal set, O-rings, gaskets, oil charge, anodes 4) Packing, gross weight/dimensions per crate, lead time 5) HS code, warranty terms, photos of SS316 material certificate Answers to any item marked "confirm" above are mandatory for quotation validity.
Suppliers who won’t answer the thrust/material/sensor questions are telling you something. Eliminate them.
| Risk | Likelihood | Impact | Mitigation |
|---|---|---|---|
| Cast-iron unit corrodes in seawater | High (if CI accepted) | Unit loss in 1–3 yrs | SS316 spec + anodes + coatings; reject 304 |
| Mechanical seal failure floods motor | Medium over years | Motor loss | Leakage sensor interlock; spare seals aboard; annual oil change |
| Listed price ≠ invoice price | High | Budget overrun | Itemized FOB RFQs, Trade Assurance, duel 5+ vendors |
| VFD run outside maker’s frequency window damages gearbox/motor | Low-Medium | Warranty void | Written frequency window; cap at 50 Hz; PTC trip enabled |
| Entanglement / injury from open 2.5 m blades | Low | Severe | Bar screens, kill switches, lockout procedure, signage |
| Actual hull drag exceeds estimate (column interference) | Medium | Undersized thrust | Spring-scale tow test BEFORE ordering; the 3,840 N QDT option is the insurance pick |
| Supplier QC variance (motor grade, seal brand) | Medium | Early failures | Pre-shipment inspection photos/video, nameplate photo, sensor bench-test video |