Seastead Auto Screw Unit (ASU) — Feasibility & Design Recommendations
Short answer: Yes, the concept is sound and engineerable for a reasonable price. The mechanical
problem (two counter-rotating helical screws driven by a sliding "Kelly drive" carriage) is a well-understood
pattern in oil-field and geotechnical drilling, just at a much smaller scale. Nothing exotic is required.
The biggest engineering risks are corrosion management, motor sealing, and reliably getting the screws
started straight in sand — all solvable.
1. Loads and Sizing the Screws
You want roughly 3,500 lbf hold-down per leg, with a generous safety factor (say 3×) → design each
pair for ~10,500 lbf working uplift, or ~5,000–6,000 lbf per individual screw. In Caribbean
calcareous/silica sand (medium-dense) a single helical anchor's uplift capacity is approximately:
Q_ult ≈ A_helix × N_q × γ' × H
For a 10" (250 mm) helix at 5–6 ft embedment in medium-dense sand, you typically get 4,000–7,000 lbf
ultimate uplift per helix. So:
| Parameter | Recommendation |
|---|
| Helix diameter | 10 in (254 mm), single helix per screw |
| Shaft (hex) size | 1.25 in (32 mm) across-flats hex, solid |
| Total screw length | 7 ft (2.13 m) — gives ~6 ft embedment + stick-up |
| Helix pitch | 3 in (76 mm) |
| Material | 2205 Duplex stainless (preferred; ~2× yield of 316L, much better in chloride/sand abrasion) |
| Number of screws per ASU | 2 (counter-rotating) |
| Spacing between the 2 screws | ~30 in (760 mm) center-to-center = 3× helix diameter, the standard rule to avoid stress-bulb interference |
Why 2205 over 316L: 316L's yield is only ~25 ksi annealed; a 1.25" hex shaft at 6,000 lbf
uplift plus driving torque is fine, but driving torque in dense sand can spike. 2205 (yield ~65 ksi)
gives much better margin and far better pitting resistance (PREN ~35 vs ~25). Cost premium in China
is only ~20–30%.
2. The Auto Screw Unit (ASU) — Mechanical Layout
- Two hex shafts ("Kelly rods"), 1.25" hex, 7 ft long, with the helix welded near the bottom and a closed-cell foam float bonded at the top.
- Central carriage ("Kelly drive"): a 316L stainless frame ~36 in × 12 in × 10 in holding:
- One sealed BLDC gearmotor (~1.5 kW, 48 V, ~150 rpm output) driving a chain or spur-gear set that turns both shafts in opposite directions.
- Two hex bore bushings / hex drive sleeves (the "Kelly bushings") — these are the rotating element that lets the shaft slide vertically through the carriage while transmitting torque.
- An LED + small camera so the captain can confirm bite.
- Load cell / current sensor on the motor for torque feedback (auto-stop at target torque).
- Landing legs: 3 short tripod legs (~18 in) on the carriage so it sits stably on the seabed before screws engage. Once both screws have penetrated ~6 in, the carriage rides up the shafts as they auger down.
- Load-transfer head: as the carriage reaches the top of the shafts, a pair of spring-loaded dogs or a simple shoulder on each shaft engages a hardened seat in the carriage. This is what transfers the 3,500 lbf uplift from the cable (which attaches to the carriage) into the screws.
- Power/comms tether: single armored cable from seastead leg to the carriage only. No connection ever made to the screws themselves.
3. Off-the-Shelf vs. Custom — What You Can Just Buy
| Part | Off the shelf? | Notes |
|---|
| Hex bore bushings / hex drive sleeves (1.25" hex) | Yes |
Common for PTO drives and ag equipment. Search "1-1/4 hex bore bushing", "hex broach bushing", "PTO hex adapter". McMaster, Misumi, Bondioli & Pavesi, Weasler. In China: Alibaba has many under "hex bore sprocket" or "hex broach hub". $15–60 each in carbon steel; ~$80–150 in stainless. You'll likely want stainless or sealed bronze with stainless hard-chrome shaft contact. |
| Hex-shaft helical anchors in galvanized steel | Yes |
"AB Chance", "MagnumPI", "Hubbell" sell them widely with 1.5" hex. Standard sizes. |
| Hex-shaft helical anchors in 2205 duplex or 316L | Rarely |
You'll almost certainly have to have these custom made. Chinese stainless fabricators (Wenzhou, Jiangsu) make them readily from drawings. Budget ~$180–280 each for 1.25" hex × 7 ft with 10" helix in 2205. |
| Handheld/skid hex anchor drivers (hydraulic) | Yes |
e.g., Pengo, Eskridge, Digga "anchor drives." But they're single-screw and react torque through an operator/skid steer. You can't just bolt two together and call it done — you need the counter-rotating gear coupling, the sliding bushings, the load-transfer head, and a submersible motor. So use them as inspiration, not as a kit. |
| Submersible BLDC gearmotor 1–2 kW | Yes |
ROV thruster suppliers and Chinese subsea motor makers (e.g., Shenzhen-area) have IP68 units for $400–900. |
4. Time to Screw In / Out
With a 3 in pitch and ~150 rpm at the shaft (after gear reduction), the screws advance at
150 × 3 in = 450 in/min = 37 ft/min in free rotation. In sand, real penetration
rate is limited by torque/feed balance; typical helical-anchor installs run at 15–30 rpm under load.
| Phase | Time per ASU |
|---|
| Lower ASU from winch to seabed (15 ft) | 1–2 min |
| Start & verify bite (human-supervised first few seconds) | 30–60 sec |
| Screw in ~6 ft at ~20 rpm × 3 in pitch = 60 in/min | ~1.5–2 min |
| Tension & lock | 30 sec |
| Total install per ASU | ~4–6 min |
| Unscrew (reverse, usually faster, no feed force) | ~2–3 min |
| Retrieve to deck | 1–2 min |
Three ASUs running simultaneously after start-up: full anchoring in roughly 10–15 minutes;
full retrieval in ~10 minutes.
5. Cost Estimate (Chinese Manufacture, Quantity 20 Seasteads = 60 ASUs, 120 Screws)
| Item | Qty per ASU | Unit cost (CNY → USD) | Per ASU (USD) |
|---|
| 2205 duplex helical screw, 1.25" hex × 7 ft, 10" helix | 2 | $210 | $420 |
| Float (closed-cell PE foam, encapsulated) | 2 | $20 | $40 |
| Hex bore bushings (stainless, sealed) | 4 (2 per shaft, top & bottom of carriage) | $110 | $440 |
| Submersible BLDC gearmotor, ~1.5 kW, 48 V | 1 | $650 | $650 |
| Counter-rotation gearset (2 spur gears + idler, stainless) | 1 | $220 | $220 |
| Carriage frame, 316L, machined & welded | 1 | $380 | $380 |
| Tripod landing legs, 316L | 1 set | $90 | $90 |
| Load-transfer dogs / hardened seats | 2 | $70 | $140 |
| Camera + LED (IP68) | 1 | $80 | $80 |
| Load cell / current sensor + wiring harness | 1 | $120 | $120 |
| Subsea power/comms tether, 50 ft armored | 1 | $180 | $180 |
| Assembly, test, QC | — | — | $180 |
| ASU subtotal | ~$2,940 |
|---|
| Deck winch (12V/24V, 2000-lb pull, stainless drum) per ASU | 1 | $280 | $280 |
| Rubber-lined cradle (isolating Al hull from duplex SS) | 1 | $110 | $110 |
| Shipping & export packaging (allocated) | — | — | $120 |
| Per-ASU delivered cost | ~$3,450 |
|---|
| Per seastead (3 ASUs) | ~$10,400 |
|---|
These prices assume a real production run of 60 ASUs from one Chinese supplier with full drawings in hand,
FOB Shanghai/Ningbo. Prototypes will run roughly 2.5–3× per unit. Add ~10–15% if the US dollar weakens or
if you require third-party material certs (PMI + mill certs) — and you should require those for
the duplex parts.
6. Spacing of the Two Screws in a Pair
Standard helical-anchor practice is 3 × helix diameter center-to-center as the minimum
to avoid overlapping stress bulbs and reduced capacity. With a 10" helix that gives 30 in (760 mm).
This is also a comfortable distance for the counter-rotation gearbox and lets the float-stabilized
screws stay upright independently before the motor descends.
7. Corrosion & Galvanic Notes
- Duplex screws + aluminum hull is a serious galvanic mismatch (Al becomes the sacrificial anode and dissolves). Your rubber-lined cradle is correct — extend it to anywhere the screws transit (chocks, guides).
- Use 316L, not 304, for any stainless that touches the duplex screws — the duplex will be cathodic to lesser stainless too.
- Add sacrificial zinc anodes on each ASU carriage; cheap insurance.
- Rinse the screws with fresh water when retracted; sand abrasion is your real long-term wear mechanism, not corrosion.
8. Hiring an Engineer to Productionize This
Who you want
You need a mechanical engineer with marine / subsea mechanism experience, ideally
also comfortable with motor sizing, finite-element load analysis, and sending production drawings to Chinese
shops. Roughly three options:
- Independent contract engineers — find them on Upwork (filter "PE" + "subsea"), LinkedIn ProFinder, or the SNAME (Society of Naval Architects and Marine Engineers) member directory. Look for prior helical anchor, ROV tooling, or aquaculture mooring projects in their portfolio.
- Small naval-architecture / marine engineering firms — e.g., in the US: Glosten, Bristol Harbor Group, BMT; in the UK: Houlder; in NL: C-Job; in China: CSSC subsidiaries. Overkill for this part alone but useful for the whole seastead.
- Chinese ODM partner with in-house engineering — many Jiangsu/Zhejiang stainless fabricators will do the drawings included in the unit price if you commit to a production run. Lower cost, but you must inspect carefully and own the IP contractually.
Fees and timing
| Scope | Typical fee | Duration |
|---|
| Concept review + sizing memo (1 engineer) | $1,500–4,000 | 1–2 weeks |
| Full mechanical design + production drawings + BOM, ready for Chinese fab | $12,000–35,000 | 6–12 weeks |
| Plus prototype build & sea trial supervision | +$8,000–20,000 | +4–8 weeks |
US/EU rates are roughly $120–200/hr; experienced offshore freelancers $60–100/hr; Chinese engineering houses $30–60/hr (with the caveat that English-language documentation and detailed tolerancing standards vary).
How to vet
- Ask for two prior projects involving submerged actuated mechanisms, not just static marine hardware.
- Require they specify materials by ASTM/EN designation, not just "stainless."
- Require GD&T on machined parts and weld symbols per AWS D1.6 (stainless) on welded parts.
- Have them sign an NDA + IP assignment before any drawings are shared with Chinese fabs.
- Get fixed-price milestones, not pure hourly, for the drawing package.
9. Risks and Things to Prototype First
- Getting the screw started straight in soft sand. The float-stabilized + tripod-landed approach is good, but you'll want to confirm in a test tank or a Caribbean beach trial that the screws don't "walk" sideways before biting.
- Load-transfer dog engagement when the carriage reaches the top of the shafts — must be reliable in zero-visibility silty water. Spring-loaded with positive mechanical indication via load cell is the right path.
- Sand intrusion into the hex bushings. Use labyrinth seals + grease cups, and orient the carriage so sand falls away from the bushings during operation.
- Tether management — 50 ft of armored cable behaves badly in current. Consider a small clump weight just above the carriage.
10. Summary
- Feasible: yes, and at the price point you'd hope for (~$10K per seastead in volume).
- Mostly off-the-shelf for: hex bushings, motors, winches, cameras, sensors.
- Custom-fabricated: duplex screws, carriage frame, counter-rotation gearset, load-transfer head. All within easy reach of competent Chinese stainless fab shops.
- No, you can't just bolt two single-screw drivers together — the counter-torque coupling, the sliding Kelly-bushing arrangement, and the load-transfer-to-screw-tops mechanism are the real engineering content of the ASU.
- Engineering effort: budget $15–35K and ~2–3 months for a full drawing package, plus prototype.
- Install time: ~5 min per ASU; full seastead anchored in ~15 min; retrieval ~10 min.