⚓ Modular Helical Mooring Screws
PROTOTYPE 1/2 scale
FULL SCALE
Tension leg capable
This system uses a detachable above‑water torque unit and stainless‑steel helical screws designed for frequent installation / removal.
The seastead holds position via GPS & thrusters, countering installation torque. Both prototype and full‑scale versions are optimised
for shallow water (8 ft prototype, up to 16 ft full scale) and modular deployment.
🔹 Prototype (1/2 scale) – 1000 lbs working load per screw
📐 Screw & Shaft
| Parameter | Specification |
|---|
| Helix diameter | 6 inches (single helix) |
| Shaft length | 8 ft (≈7 ft into sand) |
| Material | Stainless steel 316 (marine grade) |
| Shaft outer diameter | 1.5 inch Schedule 40 |
| Pitch (helix) | 4 inches per revolution |
| Top coupling | Hex or spline adapter (1 inch square drive) |
⚡ Above‑water torque unit
| Component | Detail |
|---|
| Motor type | Brushless DC, 48 V, 2.5 kW (peak) |
| Gearbox | Planetary 50:1, output ~60 RPM |
| Torque rating | ~350 Nm continuous |
| Drive shaft | 1 inch hex, 8 ft length, SS304 |
| Quick coupler | Stainless claw / pin coupling |
| Power source | Seastead solar/battery (48 V DC) |
💰 Prototype cost estimate (China sourcing, per screw set + shared torque unit)
| Item | Qty | Unit cost (USD) | Total (USD) |
|---|
| 6" SS316 helical screw + 8' shaft (with top adapter) | 3 | $220 | $660 |
| Modular torque drive unit (motor+gearbox+frame) | 1 (shared) | $1,150 | $1,150 |
| 8' hex drive shaft SS304 + quick coupler | 1 | $130 | $130 |
| Mounting bracket / torque reaction arm | 1 | $85 | $85 |
| Electrical controller & remote (48V) | 1 | $310 | $310 |
| Spare coupling pins / hardware | kit | $40 | $40 |
| Total system (3 screws + drive) | | | $2,375 |
| Additional screw only (if needed): $220 each. All prices FOB China, approximate. |
🔸 Full Scale – 5000‑8000 lbs working load per screw
📐 Screw & Shaft
| Parameter | Specification |
|---|
| Helix diameter | 12 inches (single helix, heavy plate) |
| Shaft length | 12 ft (up to 16 ft water depth) |
| Material | Duplex stainless steel (2205) / super duplex |
| Shaft OD | 2.5 inch Schedule 80 |
| Pitch | 6 inches per revolution |
| Top adapter | 1.5 inch square drive, hardened |
⚡ Above‑water torque unit (full scale)
| Component | Detail |
|---|
| Motor | Brushless DC, 72 V, 7.5 kW |
| Gearbox | Planetary 80:1, output ~45 RPM |
| Torque | ~1200 Nm continuous |
| Drive shaft | 1.5 inch hex, 12 ft, SS304/316 |
| Coupling | Heavy‑duty quick pin coupler |
| Power | Seastead 72 V battery bank, solar |
💰 Full scale cost estimate (China sourcing, per set of 3 + shared drive)
| Item | Qty | Unit cost (USD) | Total (USD) |
|---|
| 12" duplex SS helical screw + 12' shaft | 3 | $680 | $2,040 |
| Heavy torque unit (motor, gearbox, frame) | 1 (shared) | $2,850 | $2,850 |
| 12' hex drive shaft + heavy coupler | 1 | $240 | $240 |
| Mounting & reaction arm (adjustable) | 1 | $190 | $190 |
| Controller & remote (72V, waterproof) | 1 | $490 | $490 |
| Backup shear pins / hardware | kit | $65 | $65 |
| Total system (3 screws + drive) | | | $5,875 |
| Additional screw only: ~$680 each. Duplex stainless ensures durability with frequent cycling. |
🛠️ Installation & Removal Procedure
Seastead holds precise GPS position using its RIM drive thrusters. The torque reaction is absorbed by the massive triangular structure; a small reaction arm on the torque unit braces against the deck or leg attachment point.
📥 INSTALLATION (per screw)
- Position seastead over target mooring point. Thrusters active. ~2 min
- Lower the helical screw with the attached drive shaft through the leg guide tube (or over the side) until tip touches seabed.
- Engage torque unit onto top of drive shaft; secure reaction arm to deck bracket.
- Start motor – screw penetrates sand. Monitor current/torque. Typical rotation ~60 RPM (prototype) / 45 RPM (full scale).
- Screw advances ~4" per rev (prototype) or 6" per rev (full). 7 ft penetration ≈ 21 revs (prototype) / 14 revs (full). ~25‑40 sec
- Once design depth reached (torque plateau), stop motor. Disconnect torque unit from drive shaft.
- Remove drive shaft (leaves screw in seabed). Attach mooring line to screw top eye / pad‑eye.
- Repeat for remaining two screws. Total per screw ~5‑7 min
📤 REMOVAL (per screw)
- Detach mooring line; attach drive shaft to embedded screw (diver or remote guide).
- Connect torque unit, set reverse rotation.
- Unscrew helix from seabed. ~20‑35 sec
- Lift screw and shaft aboard; stow.
- Move to next position. ~4‑5 min per screw
⏱️ Time summary (3 screws): Prototype installation: ~18‑22 minutes total. Full scale: ~18‑25 minutes (deeper penetration).
Removal is faster (no careful positioning). Experienced crew can reduce time. The electric drive allows clean, quiet operation directly from solar.
🧾 Additional system details
🔩 Modular philosophy
One torque unit services all screws sequentially. After each screw is installed, the drive shaft is decoupled and moved to the next location. The helical screw remains in the seabed with a top connection eye. This keeps underwater hardware simple and inexpensive.
⚙️ Torque reaction
The reaction arm transfers torque to the seastead’s leg attachment points or deck tri‑beam. The structure easily handles the peak torque (prototype ~350 Nm, full scale ~1200 Nm) without shifting position, assisted by dynamic positioning.
📊 Quick comparison
| Feature | Prototype (1/2 scale) | Full Scale |
|---|
| Working load per screw | 1,000 lbs | 5,000–8,000 lbs |
| Helix diameter | 6″ | 12″ |
| Shaft length | 8 ft | 12 ft |
| Max water depth | ~8 ft | ~16 ft |
| Material | SS316 | Duplex 2205 |
| Drive motor | 48 V, 2.5 kW | 72 V, 7.5 kW |
| Install torque | ~350 Nm | ~1200 Nm |
| System cost (3 screws + drive) | $2,375 | $5,875 |
Design for seastead with foil legs • v1.0 • Cost estimates based on typical China manufacturing, subject to market fluctuation.
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