Helical Screw Mooring Procedure for Prototype (½-scale) Seastead

Below is a fleshed-out analysis of your capstan-driven helical mooring screw concept, with numeric checks, a suggested refined procedure, cost and weight estimates, and a scale-up discussion.

1. Summary of the Concept

• Three 6-inch-helix mooring screws, each with 1" hex shaft, 8 ft long.
• Each shaft carries a sliding 12" diameter capstan wheel.
• Rope is wrapped ~4 turns around the capstan; seastead drives away; capstan effect translates rope pull into rotation, which screws in the anchor.
• Target: 1000 lbf vertical hold per screw, 400 lbf seastead thrust available.

2. Capstan Wheel Sliding on the Hex Shaft

For smooth sliding and reliable torque transfer, the bore of the capstan hub must match the hex shaft with a small running clearance. Recommendations:

• Bore: a hex-shaped bore ~1/32" larger across flats than the shaft (e.g., 1.03" hex bore for 1.00" hex shaft).
• Bushing material: UHMW polyethylene or Delrin (acetal) sleeve pressed into the wheel hub. Both are self-lubricating, seawater-tolerant, and slide cleanly on stainless.
• Shaft finish: polished 316 stainless hex, slightly chamfered edges so the plastic bushing doesn't gall.
• Length of hub: ~4–6" so it doesn't cock and bind as it slides.
• Avoid metal-on-metal: a stainless hub on stainless hex will gall after a few cycles in sand.

3. Capstan Staying Down During Extraction

This is the real problem, as you identified. Three options ranked by practicality:

1. Stop-collar with one-way (ratchet) action on the shaft. The wheel has a sprag or pawl engaging tiny notches along the hex flats: wheel can slide down freely but grips the shaft when pulled up. During extraction, the shaft rises through the still-gripped wheel. This is the cleanest solution.
2. Slack-and-resume. Your own idea. Works, but slow, and in practice the wheel can wedge against the seabed and refuse to slide. Fine as a backup.
3. Diver-placed weighted collar. Since a swimmer must already descend to rig the rope for extraction, they can also clip a ~30–40 lb lead "keeper ring" onto the shaft above the wheel. Simple, low tech, reliable.

Recommendation for prototype: Combine option 1 (pawl/sprag) with option 3 (keeper weight) as redundancy.

4. Bottom-of-Wheel Contact System

Your two-layer idea is good. Concretely:

• Upper (soft) layer: 3 or 4 small spring-loaded casters or UHMW skid pads protruding ~1" below the rigid frame. Carries wheel weight on hard bottom.
• Lower (hard) layer: 6–8 angled steel teeth, raked so they bite the sand when the wheel tries to rotate into the bottom (during screw-in) but slip freely the other way (during extraction).
• When the wheel jams against the anchor eye at full depth, the teeth engage the seabed and resist rotation → rope tension exceeds the capstan's hold → rope slips → seastead is released. This gives you automatic disengagement.

5. Capstan Wheel Weight & Seastead Standoff Distance

You're right that during screw-in the wheel wants to be pulled down, not up — the torque reaction plus the shaft's downward migration both push the wheel toward the bottom. So wheel weight during installation isn't critical.

During normal pull (rope angle matters more for tooth engagement than for sinking):

With 400 lbf thrust, at 60 ft standoff that's only ~53 lbf upward on the wheel. A capstan wheel of ~25–30 lbs in water (so ~35–40 lbs dry) with downward-raked teeth will stay planted. Spec: steel hub with UHMW rim and stainless teeth, ~35 lb.

Recommended standoff: at least 60 ft, ideally 80–100 ft during the main screw-in pull.

6. Rope Length Calculation (½-scale prototype)

Your instinct of ~200+ ft is correct. Spec: 250 ft of ½" double-braid polyester (floating is not needed here — high grip and low stretch matter more). One rope is re-used for all 3 screws.

7. Holding Capacity in Caribbean Sand

Caribbean sand is typically medium-dense calcareous/coral sand. Empirical rule for small single-helix screws:

Qult ≈ Nq × σ'v × Ahelix

• 6" helix area ≈ 0.196 ft²
• At 7 ft embedment, σ'_v ≈ 7 ft × 60 lb/ft³ (buoyant sand) ≈ 420 lb/ft²
• N_q for medium-dense sand ≈ 20–40
• Q_ult ≈ 30 × 420 × 0.196 ≈ 2,470 lbf

With a safety factor of 2, working load ≈ ~1,200 lbf. This meets the 1000 lbf target, but just barely. Recommendations:

• Always verify by pulling to 1.5× working load (1,500 lbf) after installation.
• In soft patches, be prepared to relocate, or use a deeper embedment (10 ft shaft instead of 8 ft) as a margin.
• In coral-rubble bottoms, holding may drop 30–50% — do not use screws there.

8. Cost Estimates (Marine 316 Stainless, w/ capstan wheel)

Galvanized units are ~1/4 the price but won't survive repeated installation/removal cycles — stainless is the right call.

9. Weight Estimate (½-scale)

Manageable by one person on deck; easy to lay on side-mounted supports outside the rail.

10. Floating Eye Marker & Extraction

A 20 ft floating polypro line tied through the eye is ideal — it lies on the surface for easy grabbing. For extraction the swimmer:

1. Follows the floating line down to the eye / capstan.
2. Wraps the main rope ~4 turns around the capstan in the reverse direction.
3. Engages a spring clip or rope-gate that keeps wraps from peeling off until load is applied.
4. Surfaces; seastead applies thrust; teeth slip freely in the "out" direction; screw unwinds.

11. Cycle Time Estimate (experienced 2-person crew, 8 ft depth)

Faster than traditional anchor setting a tension-leg mooring, and well within a reasonable workday operation.

12. Scale-Up to Full-Size Seastead (8,000 lbf target)

Because of the weight, full-scale units must be deployed with a davit or small crane / pulley system — one person cannot manhandle a 200+ lb assembly. A simple mast-and-block system on each corner of the frame would work, stored flat on side rails as planned.

13. Viability Verdict