# Seastead Helical Screw Mooring System — Design Analysis
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Seastead Helical Screw Mooring System — Design Analysis
Seastead Helical Screw Mooring System
Design analysis for a capstan-driven, manually deployed helical screw anchor system — half-scale prototype and full-scale seastead.
1. Design Overview
The system uses helical (screw-type) mooring anchors driven into the seabed by converting the seastead's forward thrust into capstan torque. Each screw has:
- A helical screw on a hexagonal shaft
- A sliding capstan wheel that rides on the hex shaft
- A float attached to the screw eye to keep it upright in the water
- A rope system that transmits seastead thrust through the capstan to the screw
Half-Scale Prototype Parameters
| Parameter | Value |
|---|
| Helix diameter | 6 inches |
| Hex shaft length | 8 feet |
| Capstan wheel diameter | 12 inches (1 foot) |
| Design load (vertical) | 1,000 lbs per screw |
| Seastead thrust | 400 lbs |
| Number of mooring screws | 3 |
| Screw embedment depth | ~7 feet |
| Working depth | ~8 feet (shallow protected water) |
Full-Scale Parameters
| Parameter | Value |
|---|
| Helix diameter | 12 inches (1 foot) |
| Hex shaft length | 12 feet |
| Capstan wheel diameter | 24 inches (2 feet) |
| Design load (vertical) | 8,000 lbs per screw |
| Seastead thrust | 2,000 lbs |
| Number of mooring screws | 3 |
2. Capstan Wheel Design — Sliding on the Hex Shaft
2.1 Bushing System for Low-Friction Sliding
The capstan wheel must slide freely along the hex shaft while being keyed to it (no independent rotation). The recommended approach:
Recommended: UHMWPE (Ultra-High Molecular Weight Polyethylene) bushing inserts
- Press a hex-bore UHMWPE bushing into the capstan wheel's center bore
- UHMWPE is self-lubricating, extremely abrasion-resistant, and impervious to saltwater
- It slides easily against polished stainless steel even when wet and sandy
- Clearance of ~1/32" (0.8 mm) per flat provides smooth sliding without excessive play
- Replaceable — a new bushing can be pressed in when worn
Alternative approaches:
- Nylon (PA6) bushings: Cheaper than UHMWPE, slightly less durable. Good for prototype.
- PTFE (Teflon)-coated bore: Lowest friction but the coating can wear off with sand abrasion.
- Delrin (Acetal) bushings: Excellent machinability, good saltwater resistance, slightly stiffer than UHMWPE.
Hex shaft finish: The hex shaft should be polished to at least 120-grit finish (Ra ≤ 0.8 µm). Marine-grade 316L stainless steel provides corrosion resistance. If budget allows, hard-chrome plating on the flats further reduces wear.
2.2 Two-Tier Bottom Design
The bottom of the capstan wheel serves two purposes that conflict: it must slide easily across the seabed during positioning, but grip firmly during screwing and extraction. The solution is a two-tier design:
Tier 1 — Outer sliding ring (UHMWPE or Delrin plate)
- A flat ring of UHMWPE around the outer perimeter of the wheel bottom
- Bears the wheel's weight and allows it to slide across sand during positioning
- Thickness: ~1/2 inch
Tier 2 — Retractable angled pegs (spring-loaded)
- 6–8 stainless steel pegs arranged in a circle inside the outer ring
- Each peg is spring-loaded upward (retracted flush during sliding)
- When the wheel is pressed into the sand under load, the springs compress and the pegs extend downward ~1–2 inches into the seabed
- Peg tips are angled (like tent stakes) — they slide over the shaft when the wheel rotates in the driving direction, but dig in when the wheel tries to rotate the other way or when pulled upward
- For extraction specifically: the pegs resist upward pull (the angled faces prevent the wheel from climbing)
Peg detail: Each peg is a ~3/8" diameter stainless rod, ~3" long, with a 30° chisel tip. The spring force should be calibrated so the pegs retract under the wheel's static weight (~10 lbs) but engage when the sand pushes back with ~20+ lbs of resistance. Commercial spring-loaded toggle latches or retractable plunger pins can be adapted for this purpose.
2.3 Rope Surface Texture
The outer circumference of the capstan wheel (where the rope wraps) should have a cross-hatched or diamond-knurled texture machined into the stainless steel, with ~1/16" deep grooves. This dramatically increases the rope-to-metal friction coefficient from ~0.2 (smooth) to ~0.4–0.5 (textured), which is critical for the capstan effect. For even more grip, bond a layer of rubber or vulcanized coating to the rope-contact surface.
3. Capstan Effect Analysis & Rope Length Calculation
3.1 How the Capstan Mechanism Works
The rope setup for screwing in:
- ~4 turns of rope are pre-wrapped around the capstan wheel
- End A goes to the seastead (~80 feet away at the surface)
- End B trails along the seabed (provides the "fixed" end)
- The capstan wheel is keyed to the hex shaft via the UHMWPE bushing
- As the seastead pulls End A, the capstan wheel rotates, turning the hex shaft and driving the helix into the seabed
3.2 Capstan Effect Calculation
The capstan (Euler–Eytelwein) equation:
T_tight = T_loose × e^(μ × θ)
Where:
T_tight = tension on the seastead side (End A)
T_loose = tension on the seabed trailing end (End B)
μ = friction coefficient (rope on textured stainless ≈ 0.4)
θ = total wrap angle in radians
e = 2.718 (base of natural log)
For 4 complete turns:
θ = 4 × 2π = 8π ≈ 25.13 radians
e^(0.4 × 25.13) = e^(10.05) ≈ 23,100
So: T_tight = T_loose × 23,100
Key insight: With 4 wraps, the capstan effect is enormous (~23,000:1). This means:
- The seastead's 400 lbs of pull requires only 0.017 lbs of resistance on the trailing end
- The trailing rope's friction along the seabed (far more than 0.017 lbs) is more than sufficient to engage the capstan
- Nearly 100% of the seastead's pull converts to torque on the capstan
3.3 Torque Available vs. Torque Required
TORQUE AVAILABLE (half-scale):
Capstan radius = 12 in / 2 = 6 in = 0.5 ft
Rope tension from seastead ≈ 400 lbs (at full thrust)
Net torque on capstan ≈ (T_tight - T_loose) × r
≈ (400 - 0.017) × 0.5
≈ 200 ft-lbs
TORQUE REQUIRED to drive 6" helix in sand:
Using soil mechanics: T = c × A_blade × r_avg × k
For medium-dense Caribbean sand:
Undrained shear strength at depth ≈ 400 psf
Blade area per turn ≈ π × 6" × 0.375" ≈ 7.07 sq in ≈ 0.049 sq ft
Effective radius ≈ 0.15 ft
k (cutting factor) ≈ 10–20
Estimated torque ≈ 80–200 ft-lbs (varies with sand density)
RESULT: Available torque (200 ft-lbs) ≥ Required torque (80–200 ft-lbs) ✓
Margin of safety: 1.0× to 2.5× depending on sand density.
Note on sand type: Loose coral sand (common in shallow Caribbean) may require only 50–80 ft-lbs. Dense calcareous sand may require 150–250 ft-lbs. The system has adequate margin for typical conditions but may struggle in very dense or cemented sand. Test in your specific location.
3.4 Rope Length Calculation
ROPE CONSUMPTION DURING SCREWING:
Helix pitch: 6 inches per turn (assumed — see Section 3.5)
Screw embedment: 7 feet = 84 inches
Number of turns: 84 / 6 = 14 turns
Each turn of capstan pulls rope from seastead:
Rope per turn = π × D_capstan = π × 12" = 37.7 inches ≈ 3.14 ft
Total rope consumed from seastead side:
14 turns × 3.14 ft/turn = 44 feet
INITIAL SETUP:
Distance from seastead to screw: ~80 feet
(Needed for reasonable pulling angle — see below)
TRAILING END (seabed side):
Must be long enough to stay on the seabed
Minimum: ~30 feet
ROPE NEEDED FOR ONE SCREW INSTALLATION:
Seastead distance: 80 ft
Rope consumed (screwing): 44 ft
Trailing end: 30 ft
Recovery margin: 20 ft
────────────────────────────────
Total per screw: 174 ft
SAME ROPE, ALL 3 SCREWS (sequential):
After each screw, recover ~44 ft of rope from seastead side
and re-spool trailing end
Effective rope needed: ~174 ft works for all 3 screws
(rope length doesn't accumulate — you re-use it)
RECOMMENDED ROPE LENGTH: 200 feet (with margin for setup/adjustment)
Rope type: 3/4" double-braid nylon (working strength ~12,000 lbs)
3.5 Critical Assumption: Helix Pitch
IMPORTANT: The helix pitch is the single most critical parameter affecting timing and rope consumption.
| Pitch | Turns per 7 ft | Rope consumed | Time @ 2 knots | Notes |
|---|
| 3 inches | 28 | 88 ft | ~80 min | Standard for helical piers — VERY SLOW |
| 6 inches | 14 | 44 ft | ~40 min | Recommended for this application |
| 9 inches | 9.3 | 29 ft | ~27 min | Fast but needs more torque |
| 12 inches | 7 | 22 ft | ~20 min | Very aggressive — may not hold well |
Recommendation: Specify a
6-inch pitch for the helix. This is steeper than typical helical pier pitches (3") but appropriate for a mooring screw that will be installed and removed repeatedly. The steeper pitch:
- Cuts installation time in half compared to 3" pitch
- Reduces rope consumption
- Still provides adequate holding capacity (see Section 5)
- Requires slightly more torque, but the capstan provides sufficient margin
If your helix manufacturer only offers 3" pitch, plan for
~80 minutes per screw at 2 knots and
~300 feet of rope.
4. Installation and Extraction Procedures
4.1 Installation Procedure
- Preparation (on deck):
- Screw assembly stored horizontally on supports outside the railing
- Float attached to screw eye via short tether
- ~4 turns of rope pre-wound on capstan; spring-loaded retainer holds wraps in place
- End A of rope led to seastead (through fairlead)
- End B of rope laid out along deck, ready to trail
- Deployment:
- Release screw assembly over the side — float keeps the eye end up, blunt end (helix) hangs down
- Crew member in dinghy positions the screw vertically at the desired location
- Positioning:
- Seastead pays out rope to ~80 feet from the screw (slack)
- Trailing end B of rope lies along the seabed
- Screwing in:
- Seastead captain applies forward thrust (gradually increasing to ~200–400 lbs)
- Rope tension engages the capstan — capstan wheel rotates, hex shaft turns, helix drives into sand
- Capstan wheel slides down the hex shaft as the screw descends
- Seastead continues moving away at ~2 knots, consuming ~44 feet of rope
- When the screw eye reaches the capstan wheel (shaft fully embedded), the wheel is pressed into the sand, creating high resistance → rope releases
- Securing:
- Retrieve trailing rope end from seabed (or leave it for extraction)
- 20-foot floating rope on the screw eye marks the location on the surface
- Repeat for screws #2 and #3
4.2 Extraction Procedure
- Diver preparation:
- Swimmer/snorkeler dives to the capstan (at ~7 ft depth in 8 ft water)
- Pulls up floating rope from screw eye
- Wraps ~4 turns of rope around the capstan wheel
- Engages spring-loaded retainer to hold wraps until tension is applied
- End A goes to seastead; End B trails along seabed (or is tied to a small anchor/kedge)
- Pulling out (intermittent method):
- Seastead slowly approaches the screw position (~1–2 knots)
- The capstan converts the seastead's pull into reverse torque on the hex shaft
- The screw unscrews from the sand
- Critical: The seastead pauses periodically (every few turns) to let the capstan wheel slide back down the shaft. This prevents the capstan from riding up with the screw.
- When the helix is fully out of the sand, the screw floats up (aided by the float)
- Recovery:
- Dinghy retrieves the screw assembly
- Hoist back onto storage supports using pulley system
- Repeat for screws #2 and #3
4.3 Capstan Stays Down During Extraction — Design Solutions
This is the most challenging aspect of the design. Three complementary mechanisms ensure the capstan stays near the seabed during extraction:
Mechanism 1 — Spring-loaded locking collar (primary)
- A one-way locking collar on the hex shaft above the capstan wheel
- Allows the capstan to slide downward freely
- Locks against upward movement when the shaft is pulled up
- Spring-loaded pins engage the hex flats when the shaft tries to move up relative to the wheel
- This is the same concept as a self-locking carabiner or a one-way cable clamp
Mechanism 2 — Angled pegs in seabed (secondary)
- The angled pegs on the wheel bottom dig into the sand
- The angle is chosen so they resist upward pull (the chisel face bites into the sand) but slide when turning in the extraction direction
- Provides ~50–200 lbs of upward resistance depending on sand density
Mechanism 3 — Intermittent pulling (backup)
- If the capstan still rides up, the operator pauses pulling
- During the pause, the capstan's weight (heavier than water) slides it back down the shaft
- Resume pulling — repeat as needed
- Adds ~20–30% to extraction time but ensures it works
5. Screw Capacity — Will It Hold 1,000 lbs in Caribbean Sand?
5.1 Bearing Capacity Analysis
The pullout capacity of a helical screw comes from two components: helix bearing and shaft friction.
HELIX BEARING CAPACITY (half-scale, 6" helix):
Helix area: A = π × (3")² = 28.3 sq in = 0.196 sq ft
Effective vertical stress at helix depth (6 ft below seabed):
Dry sand above water table (0–2 ft): γ_dry = 110 pcf
Saturated sand below water table (2–6 ft): γ_sub = 55 pcf
σ'_v = 110×2 + 55×4 = 440 psf
Bearing capacity factor for sand (φ = 32°): Nq ≈ 23
Helix pullout: Q_helix = A × Nq × σ'_v
= 0.196 × 23 × 440
= 1,987 lbs (ultimate)
SHAFT FRICTION:
Shaft diameter: ~1.5" (across flats of hex)
Shaft surface area in soil: π × 1.5" × 72" = 339 sq in = 2.36 sq ft
Friction coefficient (steel in sand): β ≈ 0.3
Average effective stress: ~330 psf
Shaft friction: Q_shaft = 2.36 × 0.3 × 330 = 234 lbs
TOTAL ULTIMATE PULLOUT:
Q_ultimate = 1,987 + 234 = 2,221 lbs
With safety factor of 2.0:
Q_working = 2,221 / 2.0 = 1,111 lbs ≥ 1,000 lbs ✓
Result: A single 6-inch helical screw at 6–7 ft embedment in medium-dense Caribbean sand provides approximately 1,000–1,100 lbs of working pullout capacity (SF = 2.0). This meets the 1,000 lb design requirement. ✓
5.2 Sand Quality Matters
| Sand Type | φ (friction angle) | Nq | Ultimate Pullout | Working Load (SF=2) |
|---|
| Loose coral sand | 28° | 13 | ~1,200 lbs | ~600 lbs ⚠️ |
| Medium sand | 32° | 23 | ~2,000 lbs | ~1,000 lbs ✓ |
| Dense calcareous sand | 36° | 38 | ~3,200 lbs | ~1,600 lbs ✓✓ |
Recommendation: Before committing to the 1,000 lb load, do a pullout test at your specific site. Drive one screw to 7 ft and apply a measured vertical load until failure. If the sand is loose coral, you may need to:
- Increase embedment to 8–9 ft
- Increase helix diameter to 8 inches
- Use a multi-helix screw (two 6" helices, 2 ft apart)
- Reduce the design load to 600 lbs per screw
6. Marine Stainless Steel Screws — Cost & Weight Estimates
6.1 Weight Estimates
Half-Scale (6" helix, 8 ft shaft, 12" capstan)
| Component | Material | Weight (lbs) |
|---|
| 6" helical blade (single turn, 3/8" thick, + hub) | 316 SS | 8–12 |
| Hex shaft, 8 ft long, 1.5" AF | 316 SS | 5–7 |
| Capstan wheel (12" dia × 3" thick, with hex bore) | 316 SS | 8–12 |
| UHMWPE bushing | UHMWPE | 0.5 |
| Spring-loaded pegs (8 pcs + springs) | 316 SS | 2–3 |
| Eye / attachment hardware | 316 SS | 1 |
| Rope retainer clips | 316 SS | 0.5 |
| TOTAL per assembly | | 25–36 lbs |
| 3 assemblies | | 75–108 lbs |
Full-Scale (12" helix, 12 ft shaft, 24" capstan)
| Component | Material | Weight (lbs) |
|---|
| 12" helical blades (2 helices, 1/2" thick, + hub) | 316 SS | 50–75 |
| Hex shaft, 12 ft long, 3" AF | 316 SS | 20–30 |
| Capstan wheel (24" dia × 4" thick, with hex bore) | 316 SS | 50–65 |
| UHMWPE bushings | UHMWPE | 1 |
| Spring-loaded pegs (10 pcs + springs) | 316 SS | 4–6 |
| Eye / hardware / retainer | 316 SS | 2 |
| TOTAL per assembly | | 127–179 lbs |
| 3 assemblies | | 381–537 lbs |
6.2 Cost Estimates
Half-Scale — 6" Helical Screw with Capstan Wheel (316 Stainless)
| Order Size | Per Unit | Total | Notes |
|---|
| 1 unit (prototype) | $1,800–$3,000 | $1,800–$3,000 | Custom fabrication, domestic machine shop |
| 3 units | $1,400–$2,200 | $4,200–$6,600 | Setup costs shared; slightly better pricing |
| 30 units (China) | $350–$600 | $10,500–$18,000 | Alibaba/Made-in-China; MOQ ~10–30 pcs |
Full-Scale — 12" Dual-Helix Screw with Capstan Wheel (316 Stainless)
| Order Size | Per Unit | Total | Notes |
|---|
| 1 unit (prototype) | $5,000–$8,000 | $5,000–$8,000 | Heavy-duty fabrication, domestic |
| 3 units | $4,000–$6,500 | $12,000–$19,500 | Shared setup |
| 30 units (China) | $900–$1,500 | $27,000–$45,000 | Requires dedicated tooling for large helices |
Cost breakdown notes:
- 316 stainless steel raw material is roughly $4–6/lb (domestic) or $2–3/lb (China)
- Machining a hex bore into a large stainless disc is the most expensive single operation
- The spring-loaded peg mechanism adds ~$50–150 per unit in parts and assembly
- For Chinese suppliers, search for "helical screw anchor manufacturer" or "marine anchor factory" on Alibaba.com; specify 316L stainless, not 304
- Quality control is critical — insist on material test certificates and dimensional inspection for Chinese orders
7. Operation Timing Estimates
7.1 Half-Scale Prototype (8 ft water, experienced 2-person crew)
| Phase | Per Screw | ×3 Screws | Notes |
|---|
| Retrieve assembly from storage, prep rope | 3 min | 9 min | Pre-wrapping can be done in advance |
| Lower into water, dinghy positions vertically | 3–5 min | 9–15 min | Current and wind dependent |
| Seastead moves to position (~80 ft), pays out rope | 2–3 min | 6–9 min | At 1–2 knots |
| Screwing in (6" pitch, 2 knots) | 5–10 min | 15–30 min | Including speed-up, slow-down, verification |
| Verify set, recover rope, transition | 2 min | 6 min | |
| INSTALLATION TOTAL | 15–23 min | 45–69 min | |
| Phase | Per Screw | ×3 Screws | Notes |
|---|
| Diver descends, wraps rope on capstan | 5–8 min | 15–24 min | This is the bottleneck — visibility matters |
| Extraction (intermittent pulling) | 8–15 min | 24–45 min | Slower than insertion due to pauses |
| Recover screw to deck, stow | 3–5 min | 9–15 min | |
| EXTRACTION TOTAL | 16–28 min | 48–84 min | |
Full cycle (install 3 + extract 3): approximately 1.5–2.5 hours
An experienced crew that has done this 10+ times could get the full cycle under 1.5 hours. The diver's rope-wrapping speed is the biggest variable — practice and good visibility cut this dramatically.
7.2 Scaling to Full-Scale Seastead
With the full-scale system (12" helix, 24" capstan, 12 ft shaft), the main time increases come from:
- Heavier assembly (130–180 lbs) — requires pulley system, adds 2–5 min per screw
- Longer shaft — more turns to drive (14 turns for 7 ft at 6" pitch, same as half-scale if same pitch)
- Diver depth — if water is deeper, diver time increases; in 12–15 ft water, still manageable for a free-diver
- Larger capstan — more rope per turn (π × 24" = 6.3 ft vs. 3.1 ft), but same number of turns
Estimated full-scale timing: 20–35 min per screw installation, 25–40 min per extraction. Full cycle for 3 screws: 2.5–4 hours.
8. Full-Scale Feasibility Assessment
8.1 Does the Scaling Work?
FULL-SCALE PARAMETER CHECK:
Helix diameter: 12" (2× half-scale) ✓
Shaft length: 12 ft (1.5× half-scale) ✓
Capstan diameter: 24" (2× half-scale) ✓
Seastead thrust: 2,000 lbs (5× half-scale) ✓
CAPACITY CHECK (two 12" helices, 6 ft embedment):
Helix area each: π × (6")² = 113 sq in = 0.785 sq ft
σ'_v at 4 ft depth ≈ 380 psf (saturated sand)
σ'_v at 6 ft depth ≈ 490 psf
Nq ≈ 23 (medium sand)
Q_helix1 = 0.785 × 23 × 380 = 6,886 lbs
Q_helix2 = 0.785 × 23 × 490 = 8,879 lbs
Q_shaft ≈ 800 lbs (friction along 3" shaft)
Q_ultimate = 6,886 + 8,879 + 800 = 16,565 lbs
Q_working (SF=2.0) = 8,283 lbs ≥ 8,000 lbs ✓
TORQUE CHECK:
Available: 2,000 lbs × 1.0 ft radius ≈ 2,000 ft-lbs (max)
With capstan effect (4 wraps): nearly 2,000 ft-lbs available
Required (12" helix, dense sand): 300–800 ft-lbs
Margin: 2.5× to 6.7× ✓
ROPE LENGTH:
Rope per turn: π × 24" = 75.4" = 6.28 ft
Turns for 7 ft embedment (6" pitch): 14
Rope consumed: 14 × 6.28 = 88 ft per screw
Initial distance: ~120 ft (larger seastead)
Total rope needed: ~250–300 ft (3/4" or 1" nylon)
Full-scale verdict: YES, the system scales reasonably well.
- A dual-helix design (two 12" helices, spaced 2–3 ft apart) is needed to reliably achieve 8,000 lbs working capacity. A single 12" helix is marginal.
- The capstan torque scales favorably — the 2,000 lb thrust provides ample margin.
- Rope consumption roughly doubles (88 ft vs. 44 ft per screw).
- Weight triples (130–180 lbs vs. 25–36 lbs) — manageable with a pulley system.
- The fundamental operating principle is identical — only the hardware is bigger.
8.2 Full-Scale Design Recommendations
| Parameter | Your Proposed | Recommended | Reason |
|---|
| Number of helices | 1 | 2 | Single 12" helix gives only ~5,700 lbs working — short of 8,000 |
| Helix spacing | — | 24–36 inches | Minimum 1× diameter spacing for independent bearing |
| Shaft length | 12 ft | 12 ft ✓ | Adequate for 7 ft embedment + above-surface portion |
| Capstan diameter | 24 in | 24 in ✓ | Good balance of torque and rope consumption |
| Seastead thrust | 2,000 lbs | 2,000 lbs ✓ | Provides adequate margin |
| Rope | — | 300 ft, 1" nylon | Working strength ~18,000 lbs; handles the loads with margin |
8.3 Product Tiers — Base vs. Premium
Base Offering (what you're designing):
- Manual capstan-driven helical screw system
- 2-person crew, ~30 min per screw
- Simple, robust, low cost
- Target price: $3,000–5,000 per screw assembly (full-scale, 316 SS)
- Ideal for: customers who move weekly or less, shallow protected anchorages
Premium Option (future development):
- Powered hydraulic or electric screw driver mounted on a deployable arm
- Automated rope/cable management
- Single-person operation, ~10 min per screw
- Target price: $15,000–30,000 per screw station
- Ideal for: customers who move daily, open-water anchorages, commercial operators
9. Additional Design Considerations
9.1 The 20-Foot Floating Retrieval Rope
The floating rope on each screw eye should be polypropylene (floating) or polyethylene, 1/2" to 3/4" diameter, bright orange or yellow for visibility. Attach with a stainless steel thimble and shackle to the screw eye. The rope should have a small float (foam ball) every 5 feet to keep it visible on the surface. This rope serves double duty — it's also the rope the diver wraps around the capstan for extraction (if the same rope is used, it needs to be long enough; 20 ft is marginal — consider making it 30–40 ft).
9.2 Dinghy Positioning Aid
Consider attaching a short (~3 ft) rigid pole or PVC pipe to the dinghy that the screw shaft can slide through vertically. This acts as a guide to keep the screw vertical during deployment without the dinghy operator having to hold it by hand. A simple 2" PVC pipe clamped to the dinghy's tube works well.
9.3 Rope-to-Capstan Attachment for Extraction
For extraction, the diver needs to quickly wrap rope around the capstan. Consider adding rope guide channels — shallow grooves machined into the capstan's outer surface that the rope naturally seats into. This makes wrapping faster and more reliable in low-visibility conditions. With 4 guide channels at 90° spacing, the diver only needs to wrap the rope once around and seat it in each channel.
9.4 Corrosion and Galvanic Compatibility
Important: All metal components should be 316L stainless steel — no mixing with aluminum, carbon steel, or zinc. The hex shaft, capstan wheel, pegs, fasteners, and screw must all be the same alloy to prevent galvanic corrosion in saltwater. Use 316L (low carbon) specifically, as it resists sensitization (intergranular corrosion) better than standard 316.
9.5 Sand vs. Other Seabeds
This system is designed for sand. In other seabed types:
| Seabed Type | Feasibility | Notes |
|---|
| Sand (medium-dense) | Excellent | Primary design case |
| Mud / silt | Poor | Very low bearing capacity; screw may not hold |
| Seagrass over sand | Good | Roots may add holding; clear grass from helix path |
| Coral rubble | Moderate | May jam during installation; high holding if it sets |
| Rock | Not feasible | Cannot penetrate; use conventional anchor |
10. Summary of Key Numbers
| Parameter | Half-Scale | Full-Scale |
|---|
| Helix diameter | 6" | 12" (dual) |
| Shaft length | 8 ft | 12 ft |
| Capstan diameter | 12" | 24" |
| Working pullout capacity | ~1,000 lbs | ~8,000 lbs |
| Seastead thrust required | 400 lbs | 2,000 lbs |
| Capstan torque available | ~200 ft-lbs | ~2,000 ft-lbs |
| Rope per screw | ~44 ft | ~88 ft |
| Total rope needed | ~200 ft | ~300 ft |
| Weight per assembly | 25–36 lbs | 127–179 lbs |
| Cost per assembly (3 units) | $1,400–$2,200 | $4,000–$6,500 |
| Cost per assembly (30 units, China) | $350–$600 | $900–$1,500 |
| Install time per screw (2-person crew) | 15–23 min | 20–35 min |
| Extract time per screw | 16–28 min | 25–40 min |
| Full cycle (3 in + 3 out) | ~2 hours | ~3–4 hours |
Overall assessment: The capstan-driven helical screw mooring system is a mechanically sound, low-cost approach for a tension-leg seastead anchor. The design works at half-scale and scales to full-scale with the addition of dual helices. The main operational constraint is the labor-intensive diver-assisted extraction, which is acceptable for customers who move infrequently. For the base offering price point, this is an excellent starting design.
Analysis prepared for seastead mooring system development. All calculations assume medium-dense Caribbean sand with φ ≈ 32°. Site-specific pullout testing is strongly recommended before committing to operational loads.
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