Extraction Challenge: Keeping the Capstan at the Bottom

During extraction, upward force on the screw tends to lift the capstan. The friction between the capstan hub and the hex shaft may not be enough to overcome this, especially if marine growth or corrosion increases the friction coefficient. To ensure the capstan slides down rather than riding up:

Slack-and-Slide Protocol: The primary method is for the captain to periodically go slack (release tension) during extraction. This allows the capstan to slide back down the shaft to the seabed. The crew then re-engages tension to unscrew another length. While slower, it is reliable and requires no complex mechanism.
Low-Friction Guide: Ensure the capstan hub has a smooth, polished bore with a marine-grade bushing (e.g., UHMW-PE) to minimize sliding friction. This makes the "slack-and-slide" more effective.
Diver Weight: In very shallow water (8 ft), a diver can manually push the capstan down if it sticks, though this is less ideal for automation.

A mechanical "one-way clutch" on the capstan is possible but adds significant cost and complexity. The slack-and-slide method is the most robust for a base system.

Summary Table: Prototype vs. Full Scale

Parameter	Prototype (1/2 Scale)	Full Scale	Scaling Ratio
Load per screw (lbs)	1,000	8,000	8x
Helix diameter	6 in	12 in	2x
Shaft length	8 ft	12 ft	1.5x
Capstan diameter	12 in	24 in	2x
Seastead thrust	400 lbs	2,000 lbs	5x
Screw turns (7 ft embedment)	~4.5 turns	~4.5 turns	Same
Rope used per screw	~90 ft	~180 ft	2x
Total rope (3 screws, shared)	~300 ft	~600 ft	2x
Estimated unit weight (screw + capstan)	100–140 lbs	250–350 lbs	~2.5x
Unit cost (3x, custom)	$800–$1,200	$2,500–$4,000	~3x
Unit cost (30x batch)	$300–$500	$800–$1,200	~2.5x
Install/remove time (3 screws, 2 crew)	25–40 min	35–60 min	~1.5x

Conclusion

The tension-leg mooring concept using a sliding capstan is mechanically sound and elegantly simple. It leverages the seastead's propulsion to drive screws without a motor, making it ideal for a low-cost base offering.

Key Takeaways:

Feasibility: The prototype system works in typical Caribbean sand for 1,000 lbs uplift per screw.
Cost: At $2,400–$3,600 for three custom screws, the base system is affordable. Batch production drops this to $900–$1,500.
Efficiency: Two crew can deploy or recover all three screws in under 40 minutes.
Scalability: Scaling to 8,000 lbs is feasible with a 12-inch helix and 24-inch capstan, though unit weight increases to ~300 lbs and may require a simple hoist.
Upgrade Path: The manual capstan system serves as a robust base. Customers wanting daily relocation can upgrade to an automated hydraulic or electric screw drive.

The main operational consideration is managing the capstan's position during extraction via the slack-and-slide technique. With proper crew training, this is a minor trade-off for the system's simplicity and low cost.

Disclaimer: These calculations are estimates for design-phase planning. Marine engineering analysis, including geotechnical surveys of specific seabed conditions and structural finite element analysis (FEA) of the capstan and screw, should be performed before construction. Mooring screw holding capacity varies significantly with soil type, embedment depth, and installation technique. Consult a marine engineer for final specification.