Seastead "Auto Screw Unit" (ASU) — Feasibility & Design Notes
Short answer: Yes, this is realizable and can be built for a reasonable price. Nothing here requires exotic technology — it is a combination of well-understood parts (helical piles, Kelly-drive rotary systems, hydraulic/electric motors, winches). The engineering challenges are real but ordinary: corrosion, load transfer, alignment on a soft/uneven seabed, and controls.
Big-picture caution first. Your holding-load math is the thing to nail down before spending on hardware. A helical anchor's uplift capacity depends on soil, helix diameter, and embedment depth — not on the screw being stainless. In loose-to-medium Caribbean sand, a single 8"–10" helix at ~4–6 ft embedment gives roughly 2,000–5,000 lbf of uplift; deeper/multi-helix gets you more. You want ~3,500 lbf per leg, and you have a pair of screws per leg, so ~1,750 lbf each — very achievable. But confirm with a geotech capacity calc, because "nice sand" varies a lot.
1. Overall ASU Concept — Refinement
Your concept is sound. Here is how I would define it more precisely:
- Two helical screws per unit, counter-rotating, driven simultaneously by one motor. Counter-rotation cancels reaction torque — good instinct, this is the key trick that lets the unit react its own torque instead of spinning the frame.
- Hex "Kelly" shafts on each screw, with a sliding drive head (your Kelly bushing / hex drive sleeve) so the motor stays near the seabed while the screws advance.
- Floats at the top of each shaft to keep the unit upright while descending. Good.
- A load-transfer collar at the top of each screw that engages once the screw is at depth, so the tension-leg cable pulls on the screws (not on the sliding motor). This is the most important custom part to get right.
Recommended Sizes
| Item | Recommendation | Reasoning |
|---|
| Helix diameter | 8 in (200 mm), possibly a second 10 in helix higher up on each shaft (double-helix) | Balances install torque vs. holding capacity in sand |
| Shaft (Kelly rod) size | 1.5 in (38 mm) hex, solid | Strong enough for both torque and the ~1,750 lbf axial per screw with big safety factor |
| Shaft length | ~8 ft each (allows ~5–6 ft embedment + stick-up + float) | Fits your ≤50 ft water depth use, target 15 ft. Note: cable, not shaft, spans water column |
| Screw pitch | 3 in per revolution | Standard for helical piles; controls advance rate |
| Spacing between the 2 screws in a pair | 3 to 4 helix-diameters center-to-center = ~24–32 in; I'd use 30 in | Close enough for a compact unit and shared motor, far enough that the two helices don't shear the same soil cone and reduce each other's capacity |
| Motor | See section 3 | |
On spacing: The main constraint is soil interaction. Helical anchors that are too close "overlap" their failure cones and lose capacity. The rule of thumb is ≥3× helix diameter center-to-center. With 8" helices, 30" is comfortable. It also gives the counter-torque a good moment arm and keeps the two shafts from binding the shared drive head.
2. Materials & Corrosion
Your instinct to avoid galvanized coating for a repeatedly-cycled screw is correct — abrasion in sand strips zinc quickly, then you get rapid steel loss. Solid corrosion-resistant alloy is the right call.
- 2205 Duplex is the better choice than 316L for the helices and shafts: roughly double the yield strength (so thinner/lighter for the same torque) and better chloride pitting resistance. For seawater + sand abrasion + cyclic load, 2205 is the sweet spot on cost/performance.
- Galvanic isolation: Your rubber-lined cradle is good. Also isolate the duplex screws electrically from the aluminum hull everywhere (insulating bushings at the winch/cable attachment). Aluminum is anodic to stainless in seawater — if they're electrically connected, the aluminum hull will corrode preferentially. This is one of the most important details in the whole design.
- Use a non-metallic (Dyneema/HMPE) or jacketed tension line where possible to further break the galvanic path, with the metallic connection only at the isolated fitting.
3. Motor Sizing, Install Time, and Removal Time
The torque to install a helical pile in sand is estimated by the empirical relation Qult ≈ Kt × T, where T = installation torque and Kt ≈ 10 ft⁻¹ for ~1.5" shafts. To reach ~2,000 lbf ultimate capacity per screw you need on the order of 200 ft·lbf of torque at the screw. Add margin for hard patches and the fact you drive two at once.
| Parameter | Value |
|---|
| Torque per screw (design) | ~250–350 ft·lbf |
| Total drive torque (two screws) | ~700 ft·lbf capability desired |
| Rotation speed | ~15–25 RPM |
| Motor power | 1.5–2.2 kW (2–3 hp) electric gearmotor, or a hydraulic drive head |
Why that wattage: Power = torque × angular speed. 700 ft·lbf (≈950 N·m) at 20 RPM (≈2.1 rad/s) ≈ 2,000 W of useful output. Add drivetrain and stall-margin losses → a 2.2 kW motor is a reasonable spec, and you already have big battery banks on board to feed it.
Install / Remove Time
- Target embedment ~5 ft. At 3 in/rev that's ~20 revolutions.
- At 20 RPM → ~1 minute of actual turning per pair.
- Realistically, allow 3–5 minutes per pair including start-up care, watching the camera, slow-start, and soil variability. It goes slower at first to get the screw "biting."
- Screw-out is typically similar or a bit faster (say 2–4 min) — the soil is already disturbed, but suction/setup can add resistance, so size the motor to reverse at full torque.
4. Off-the-Shelf vs. Custom Parts
Available off the shelf
| Part | Availability |
|---|
| Hex-shaft helical piles / mooring screws | Yes — common (galvanized) from helical pile suppliers (US: Ram Jack, Chance/Hubbell; boat-mooring "sand screw" / "manta ray"-type anchors). Standard hex is often 1½". |
| Hex drive heads / torque motors for helical piles | Yes — hydraulic "helical pile drive heads" are a stock construction item. Electric versions exist but hydraulic is more common. |
| Hex bore bushings / sleeves ("Kelly bushing" analogs) | Yes — hex bore hubs, hex broach bushings, hex sprockets, and PTO hex adapters are all catalog items (McMaster-Carr, agricultural PTO suppliers, sprocket makers). A 1½" hex bore is a standard PTO size, so parts exist. |
| Winches (12/24V electric) | Yes — abundant. |
| Duplex/316L helical piles | Rarely off-the-shelf. These are normally galvanized steel. Solid stainless helical piles are usually a custom/semi-custom fabrication. Expect to have these made. |
"Can I just bolt two hex pile drivers together?"
For a prototype — yes, essentially. Two standard hex pile drive heads, mounted on a shared frame with a rigid crossmember so their reaction torques oppose, is a legitimate way to build ASU v1. You'd want them geared/timed to counter-rotate, or simply mount them in opposite orientation and accept that the frame takes some net torque. For a first water test this is the fastest path.
Parts you'll almost certainly need to custom-make
- Load-transfer collar / capture mechanism at the top of each screw (the part that hands the tension load from the frame to the embedded screw). This is genuinely custom.
- The sliding drive carriage that keeps the motor near the seabed while shafts advance (unless you accept the shafts sticking up above a fixed motor).
- Floats & guides — could be off-the-shelf marine floats adapted.
- The aluminum frame / cradle — locally welded, as you said.
- The isolated cable termination — semi-custom.
- Solid duplex/316L screws with hex shafts — custom fabrication (weld a duplex hex bar to a duplex helix plate; do it with proper duplex welding procedure so you don't ruin the corrosion resistance in the heat-affected zone).
3D printing vs. machine shop
- Structural load-bearing steel/duplex parts: do not 3D print — machine/weld/cast them. A metal shop (waterjet plate + machined bosses + welding) is right.
- 3D printing IS useful for non-structural prototyping: fit-check jigs, float bodies, cable guides, alignment fixtures. Prints in these sizes are cheap.
- The load-transfer collar prototype could be 3D-printed in plastic for fit/motion checking, then made in metal for the real test.
5. Cost Estimates
These are order-of-magnitude figures for planning, not quotes. Chinese manufacturing, 2024-ish pricing, medium volume. Duplex raw material and stainless welding are the cost drivers.
Per-ASU production cost (at volume of 60 units / 120 screws)
| Component (per ASU) | Est. cost (USD) |
|---|
| 2 × duplex/316L helical screws w/ hex shaft (solid, welded, ~8 ft) | $700 – $1,400 |
| Drive motor (2.2 kW, marine-ized, gearbox) | $300 – $700 |
| Hex drive sleeves / bushings (2) | $80 – $200 |
| Sliding carriage + frame (stainless/aluminum) | $300 – $600 |
| Load-transfer collars (2, custom machined stainless) | $200 – $500 |
| Floats, guides, hardware | $100 – $250 |
| Cable, isolated termination, connectors | $150 – $350 |
| Assembly, test, misc | $200 – $400 |
| Subtotal per ASU | ~$2,000 – $4,400 |
|---|
Per seastead (3 ASUs): roughly $6,000 – $13,000 in ASU hardware at a 20-seastead volume.
Add the 3 corner winches + cradles per seastead: ~$1,500 – $3,000.
Total mooring system per seastead ≈ $7,500 – $16,000. Call it ~$10,000–$12,000 for planning.
A single one-off prototype ASU built with as many off-the-shelf (galvanized, non-stainless) parts as possible, plus a few locally welded/machined custom pieces, is likely $3,000 – $7,000 for the one unit — the per-unit cost is much higher at quantity one because of the custom collar machining and one-off welding setup.
6. Hiring an Engineer / Firm
What discipline you need
This spans several specialties. You want either one generalist marine/mechanical engineer who can coordinate, or a small team:
- Marine / naval architect — for the mooring loads, tension-leg dynamics, and how ASU loads feed into the hull. (This is the lead you want.)
- Geotechnical engineer — for helix capacity in sand and install torque. Even a short consult is worth it.
- Mechanical / machine designer — for the drive head, sliding carriage, load-transfer collar, and manufacturing drawings.
- Corrosion / materials input — duplex welding procedures and galvanic isolation review.
Where to find them
- Firms/individuals who do helical pile / marine foundation or mooring & anchoring engineering (search those terms + "consulting engineer").
- Freelance platforms for the CAD/drawing production: Upwork, Cad Crowd, Engineer.ai-type networks — good for the drafting once concepts are set.
- Professional bodies: SNAME (naval architects) and RINA member directories.
- University ocean/marine engineering departments — professors or grad students for the capacity analysis.
- Chinese manufacturers of helical piles often have in-house engineering; you can partly co-develop with your chosen factory.
Fees and timeline
| Scope | Rough fee | Time |
|---|
| Concept review + geotech capacity + torque calc (a "does this work?" study) | $3,000 – $8,000 | 2–4 weeks |
| Detailed mechanical design + manufacturing drawings for ASU | $10,000 – $30,000 | 1–3 months |
| Full package incl. controls, load-transfer mechanism design, FEA on critical parts | $25,000 – $60,000 | 3–6 months |
Senior marine consultants run roughly $120–$250/hr in the US/EU; competent CAD drafting via freelance $25–$70/hr. A pragmatic path: pay for a small feasibility/capacity study first (cheap, de-risks everything), then commission drawings only after that confirms the sizes.
7. Recommended Prototype Path
- Get a 1-page geotech capacity + torque memo for 8" helices in Caribbean sand. ($3–8k)
- Buy two off-the-shelf hex-shaft helical anchors (galvanized is fine for the test) and two catalog hex drive heads or one drive head + hex bushings.
- Have a local shop weld an aluminum test frame and machine one prototype load-transfer collar (metal for real load test; 3D-printed plastic first for fit).
- Bench/pool test the counter-rotation + sliding drive, then a real sand test in shallow water.
- Only after that works, commit to duplex production drawings and the China order for 20 seasteads.
One design flag to resolve early: your unit needs to reliably self-right and land the two screws vertically on an uneven seabed while a ~2 kW motor reacts torque through the frame. If one screw hits a hard spot and the other doesn't, the frame sees net torque and can twist. Design the frame and its guides for the worst-case single-screw stall torque (~700 ft·lbf), not the balanced case. This is the single most likely thing to bite you in testing.
Everything you've described is buildable with known techniques. The prototype-first, then-volume approach keeps your risk and cost low, and the per-seastead mooring hardware cost in the ~$10k range is very reasonable given what it does.