However, it should be treated as a real marine/offshore engineering project, not as a simple “bolt-together” accessory. The hard parts are:
You mentioned an expected seastead weight of about 25,000 lb and a desired down-pull/pre-tension of about 3,500 lb per leg. With three legs, the total downward pretension would be about:
3 × 3,500 lb = 10,500 lb
Each Auto Screw Unit, or ASU, would therefore normally hold around 3,500 lb working vertical tension. Since each ASU has two screws, the nominal working load per screw is about:
3,500 lb / 2 = 1,750 lb per screw
That is not a large load for a properly embedded helical anchor in decent sand. But because waves, wind gusts, motion-control errors, wake loads, and imperfect seabeds can create dynamic loads, I would not design each screw merely for 1,750 lb. A more sensible target would be:
| Item | Recommended Concept-Level Value |
|---|---|
| Normal working vertical load per ASU | 3,500 lb |
| Normal working vertical load per screw | 1,750 lb |
| Recommended minimum ultimate pullout per screw in good sand | 8,000 to 12,000 lb |
| Recommended ultimate pullout per ASU | 16,000 to 24,000 lb |
| Practical safety factor against normal pretension | Approximately 4.5 to 7 |
This gives margin for imperfect installation, sand variability, dynamic loading, and capacity reduction because the two screws are near each other.
For your size vessel and desired 3,500 lb pretension per corner, I would start with something approximately like this:
| Component | Recommended Size | Comment |
|---|---|---|
| Material | 2205 duplex stainless steel preferred | Better than 316L in warm seawater, especially for crevice/pitting resistance. 316L can work but is less attractive for repeated use in sand and seawater. |
| Shaft type | Solid hex shaft | Fits your “Kelly rod” concept. Avoid hollow shafts unless carefully designed for buckling, torque, and sealing. |
| Hex shaft across flats | 1.5 to 1.75 in | I would lean toward 1.75 in AF for robustness and torsional margin. |
| Total shaft length | 9 to 11 ft | Allows 7 to 9 ft embedment while leaving enough upper shaft for the load-transfer mechanism. |
| Helix diameter | 14 to 16 in | 16 in gives more holding capacity but needs more installation torque. 14 in is easier to drive. |
| Number of helices per screw | 1 or 2 | For reliability, I would likely use two helices per screw, for example 12 in lower and 14 or 16 in upper. |
| Helix plate thickness | 3/8 in minimum; 1/2 in preferred for durability | Sand abrasion and repeated installation favor thicker plate. |
| Helix pitch | 4 to 6 in per revolution | A moderate pitch reduces required torque and helps controlled installation. |
| Spacing between helices on same shaft | Approximately 3 × helix diameter | For a 14 to 16 in helix, roughly 42 to 48 in spacing. |
| Tip | Forged or machined conical/chisel tip | Should start in sand reliably but not be so sharp that it is fragile. |
A good first prototype would be:
If the seabed is reliably clean medium/dense sand, this should be much stronger than your normal 1,750 lb per screw working load. If the seabed is loose sand, shell hash, grass, or silty sand, the required embedment and helix diameter may need to increase.
The two screws in one ASU should be far enough apart that their soil failure zones do not strongly overlap. For helical anchors, a practical rule is:
For 14 to 16 in helices:
| Helix Diameter | 2.5D Spacing | 3D Spacing | Recommended ASU Screw Spacing |
|---|---|---|---|
| 14 in | 35 in | 42 in | 42 to 48 in |
| 16 in | 40 in | 48 in | 48 to 54 in |
I would recommend about 4 ft center-to-center as a good minimum, and 4.5 to 5 ft if the geometry allows it. This gives good torque cancellation while preserving anchor capacity.
The ASU could be arranged as a compact seabed machine with two vertical helical screws, a central drive carriage, a load-transfer frame, and a top lifting/tensioning cable.
| Subassembly | Description |
|---|---|
| Two helical screws | One right-hand helix and one left-hand helix. They rotate opposite directions so both screw downward while cancelling torque. |
| Central seabed frame | A welded duplex or isolated stainless/aluminum frame with feet/skids. It lands on the sand and keeps the screw spacing fixed. |
| Sliding Kelly-drive carriage | Contains the motor, reduction gearbox, and two hex-drive sleeves. The hex shafts slide vertically through the sleeves while the sleeves rotate them. |
| Counter-rotating geartrain | Could use spur gears, bevel gears, timing belt, chain, or a dual-output gearbox. Needs to tolerate sand and seawater. |
| Load-transfer yoke | After the screws are installed, the mooring tension must transfer into the screw shafts through mechanical collars/clamps, not through the gearbox. |
| Guide floats | Small syntactic foam or sealed HDPE floats near the upper part of each screw keep the shafts upright before they bite into the sand. |
| Camera and lights | One small low-light camera and LED lights per ASU, aimed at the screw tips and frame feet. |
| Umbilical | Power, control, video/data, and possibly a small mechanical safety line. |
| Topside winch | Mounted near each seastead corner. Used to lower, recover, and tension the ASU. |
This is critical. If the tension load goes through the drive sleeves, gear teeth, or motor bearings, the system will wear, jam, or fail.
The top of each screw should have a structural load feature. Possible options:
For repeated automated use, I would favor a positive mechanical collar or pin system rather than relying only on friction clamps. A good arrangement would be:
For helical piles, a rough relationship often used is:
Ultimate anchor capacity ≈ Kt × installation torque
For small helical anchors in sand, Kt varies significantly, but values around 7 to 10 ft-1 are often used for preliminary thinking. If you want 8,000 to 12,000 lb ultimate capacity per screw, installation torque may be in the range of:
For a conservative small marine machine, I would design each ASU drive for:
| Parameter | Recommended Value |
|---|---|
| Continuous torque per screw | 1,000 to 1,500 ft-lb |
| Peak torque per screw | 2,000 to 2,500 ft-lb |
| Rotation speed | 5 to 15 rpm |
| Peak electrical power per ASU | 5 to 12 kW |
| Normal installation power | 2 to 6 kW depending on soil and speed |
A practical architecture would be a 48 V, 96 V, or higher-voltage brushless motor driving an oil-filled reduction gearbox. Higher voltage is attractive because the current is lower. For example, a 10 kW motor at 48 V draws over 200 A, which is inconvenient underwater. A 96 V or 144 V system is easier, but requires more electrical safety design.
If the helix pitch is 4 to 6 in/rev and the drive rotates at 5 to 15 rpm, the theoretical penetration rate is fast. In real sand, alignment, starting, torque limits, and verification dominate the time.
| Operation | Estimated Time per ASU | Estimated Total for 3 ASUs |
|---|---|---|
| Lower ASU, land on sand, check camera | 3 to 10 min | Can be partly parallel if using three operators/cameras; otherwise sequential. |
| Start screws and verify bite | 1 to 3 min | Operator should watch each screw start. |
| Screw in to full depth | 3 to 10 min | Depends heavily on sand and torque limit. |
| Lock load yoke and test pull | 2 to 5 min | Should include load-cell confirmation. |
| Tension to working load | 1 to 3 min | Use load control, not just winch position. |
| Total installation | 10 to 30 min per ASU | 20 to 60 min for the whole seastead, depending on how parallelized the operation is. |
| Screw-out / recovery | 5 to 20 min per ASU | 15 to 45 min total in normal conditions. |
In very clean sand and with mature hardware, the three units could probably be installed in about 20 to 30 minutes total. Early prototypes will be slower.
Your idea of storing each ASU horizontally near floor level in a rubber-lined cradle is reasonable. I would add:
Even with rubber isolation, seawater can create electrical paths. A corrosion engineer should review the aluminum/duplex/stainless/anode system.
For this application, I would prefer 2205 duplex stainless for the screws and structural load parts.
| Material | Advantages | Disadvantages |
|---|---|---|
| 316L stainless | Widely available, easier to weld, cheaper than duplex | Can pit/crevice corrode in warm seawater; lower strength; less attractive for repeated seabed abrasion |
| 2205 duplex stainless | Higher strength, better pitting resistance, good seawater performance compared with 316L | More expensive; welding requires correct procedure and filler; supply chain less common |
| 2507 super duplex | Excellent seawater corrosion resistance and strength | Much more expensive; probably overkill unless reliability/certification demands it |
For warm Caribbean seawater and repeated use, 316L is not my first choice. It may be acceptable for noncritical parts, but the screw shafts, helices, collars, and load yokes should preferably be 2205 duplex or better.
Yes, hex-bore drive parts exist under names such as:
However, most commercial ones are carbon steel, alloy steel, or agricultural/industrial parts, not subsea duplex-stainless components. You may be able to buy standard hex-bore hubs for prototyping, but for the real marine ASU I would expect a custom-machined drive sleeve with:
Small stainless screw anchors exist for boats, aquaculture, docks, and moorings. Large duplex stainless helical anchors are less common and may be custom-order items. Most ordinary mooring screws are galvanized carbon steel because they are installed once and left in place.
For your repeated-use abrasion case, custom 2205 screws are likely the right answer.
For a prototype, maybe. For the final ASU, probably not directly.
Reasons:
A good development path would be to use commercial helical pile tooling to learn torque/depth behavior in sand, but design a purpose-built marine ASU for production.
The following numbers are rough ex-works China estimates for an order of 60 ASUs for 20 seasteads, meaning:
| Part | Low Estimate | Likely/Target Estimate | Conservative Estimate |
|---|---|---|---|
| Two 2205 duplex helical screws | $1,600 | $2,800 to $4,000 | $5,000+ |
| Subsea motor and reduction gearbox | $2,000 | $4,000 to $7,000 | $10,000+ |
| Counter-rotating drive sleeves/gears/bearings | $800 | $1,500 to $3,000 | $4,500+ |
| Seabed frame, feet, yoke, collars, floats | $1,200 | $2,000 to $4,000 | $6,000+ |
| Camera, lights, sensors, connectors | $500 | $1,000 to $2,500 | $4,000+ |
| Topside winch, load cell, controls share | $1,000 | $2,000 to $4,000 | $6,000+ |
| Assembly, testing, QA allowance | $1,000 | $2,000 to $4,000 | $7,000+ |
| Total per ASU | $8,000 to $10,000 | $15,000 to $25,000 | $35,000+ |
| Basis | Cost for 3 ASUs per Seastead |
|---|---|
| Very low-cost China build, minimal certification, aggressive sourcing | $24,000 to $30,000 |
| More realistic target for robust marine hardware | $45,000 to $75,000 |
| Conservative, high-quality, heavily tested subsea hardware | $100,000+ |
For planning, I would use $50,000 to $80,000 per seastead for the complete three-ASU anchoring package, excluding nonrecurring engineering, prototypes, destructive tests, shipping, import duty, and spares.
| Scenario | Total for 60 ASUs |
|---|---|
| Aggressive low-cost build | $480,000 to $600,000 |
| Realistic robust build | $900,000 to $1,500,000 |
| High-end subsea-quality build | $2,000,000+ |
Before committing to 60 units, I would strongly recommend a staged test program:
The test program should include a load cell on each tension leg and torque/depth recording for each screw. The torque/depth curve is one of the best indicators of whether the screw is properly set.
A safe automated sequence would be:
You likely need a small team rather than one individual. The ideal lead could be a marine/offshore mechanical engineer who has experience with subsea tooling, mooring systems, or helical anchors.
Write a short request for proposal, or RFP, asking for:
| Work Package | Typical Fee Range | Typical Duration |
|---|---|---|
| Concept review and feasibility memo | $5,000 to $20,000 | 2 to 4 weeks |
| Preliminary engineering and sizing | $20,000 to $60,000 | 1 to 3 months |
| Detailed ASU mechanical design package | $60,000 to $180,000 | 3 to 6 months |
| Prototype build support and test supervision | $30,000 to $150,000 | 2 to 6 months |
| Full classification/certification-level package | $250,000+ | 6 to 18 months |
For your immediate purpose, I would expect a useful preliminary engineering package for the ASU to cost roughly $30,000 to $80,000. A production-ready design with prototype test support could easily be $100,000 to $250,000.
I agree that the automatic paired-helical-screw concept is worth developing. The most sensible preliminary design is:
For budgetary planning, assume roughly $50,000 to $80,000 per seastead for the three complete ASUs when ordering for 20 seasteads, plus engineering, prototypes, spares, shipping, and testing.
The concept is especially reasonable if the operating rule is: use it in shallow, protected, sandy locations; do not use it on coral, rock, seagrass, unknown bottoms, or in severe storms without a much more conservative offshore-grade mooring design.