Helical Mooring Screw Analysis – Seastead Tension Leg Mooring

1. Context & Assumptions

We are evaluating a dinghy-powered lever technique for installing helical mooring screws as tension-leg anchors for both a half-scale prototype and the full-scale seastead. The seastead may relocate every couple of weeks, so a rapid, low-infrastructure installation method is desirable. The concept is:

Manually start the helical screw a few turns (by hand from the water or from the dinghy with a lever).
Attach a long lever bar to the mooring screw eye.
Run a rope from the far end of the lever to the dinghy.
Drive the dinghy in circles around the mooring point, pulling the lever and screwing the helix into the sand.

Key assumptions for this analysis:

Dinghy: 14-foot RIB with a 10 HP electric outboard (Yamaha HARMO equivalent). Bollard pull ≈ 250–350 lbf (electric motors deliver excellent low-speed torque).
Water depth (prototype): ~8 ft. Helix depth target: 7 ft into sand.
Water depth (full-scale): Helix depth target: 11 ft into sand.
Soil: Typical Caribbean sand – medium-dense, well-graded, submerged unit weight ~60–65 pcf (buoyant), friction angle ~32°–36°.
Helix type: Single-helix, central shaft with eye at top.
Pitch: Assumed ~4–5 inches per revolution (typical for marine helical anchors).

2. Estimated Installation Torque

Installation torque for helical piles in sand can be estimated using the empirical relationship:

T ≈ K_t · D · A · N_q · γ' · z where: K_t = torque factor (~0.6–0.9 for single helix in sand) D = helix diameter A = helix plate area = π·(D/2)² N_q = bearing capacity factor (~25–40 for sand) γ' = buoyant soil unit weight z = depth to helix plate

Based on this and field data from similar marine helical anchor installations, we estimate the peak installation torque (at final depth) as follows:

Parameter	6″ Helix (Prototype)	12″ Helix (Full-Scale)
Helix diameter	6 inches (0.5 ft)	12 inches (1.0 ft)
Helix plate area	~0.196 ft²	~0.785 ft²
Target depth into sand	7 ft	11 ft
Estimated peak torque	800–1,500 ft·lb	3,500–6,500 ft·lb
Typical mid-range estimate	~1,100 ft·lb	~5,000 ft·lb

* Torque values are for the final portion of driving. Torque increases roughly linearly with depth. The first few feet require significantly less torque.

⚠ Important: The 12″ helix at 11 ft depth pushes well beyond what a simple dinghy-and-lever setup can reliably deliver. We discuss mitigations below.

3. Lever Bar Analysis

The torque delivered to the mooring screw equals the dinghy pulling force × lever arm length (assuming the rope pulls perpendicular to the lever). With a 10 HP electric outboard, we estimate a sustained pulling force of ~250–300 lbf during slow circling (the dinghy is not planing; it's essentially doing a "bollard pull" in a circular path).

3.1 For the 6″ Helix (Prototype)

Lever Length	Torque Available (at 275 lbf avg)	Sufficient for 1,500 ft·lb peak?
8 ft	2,200 ft·lb	✅ Yes, with margin
10 ft	2,750 ft·lb	✅ Comfortable margin
12 ft	3,300 ft·lb	✅ Large margin (but heavier bar)

✅ Recommended: A 10-foot lever bar provides a solid ~2.6× margin over the mid-range torque estimate, keeping the required dinghy pull well within the outboard's capability. Even at peak torque (1,500 ft·lb), the dinghy only needs to pull ~150 lbf — about half its capacity.

3.2 For the 12″ Helix (Full-Scale)

Lever Length	Torque Available (at 275 lbf avg)	Sufficient for 5,000 ft·lb peak?
10 ft	2,750 ft·lb	❌ Insufficient
15 ft	4,125 ft·lb	⚠️ Marginal
18–20 ft	4,950–5,500 ft·lb	⚠️ Barely sufficient

⚠ For the 12″ helix: You would need an 18–20 ft lever to have a chance at the peak torque. This bar would be extremely heavy and unwieldy (see below). Additionally, at 20 ft the dinghy's turning radius becomes very large (~125 ft circumference), making tight circles difficult. A geared torque multiplier or hydraulic drive is strongly recommended for the full-scale 12″ helix.

4. Ideal Lever Bar Specifications

The lever bar experiences significant bending stress at the attachment end. The bending moment is maximum at the mooring eye and tapers to zero at the rope end. The bar must resist this moment without yielding.

4.1 Material Options

A36 Structural Steel: Yield strength ~36,000 psi. Readily available, easily welded. Good for the prototype.
4140 Chrome-Moly Steel (normalized): Yield strength ~60,000–95,000 psi. Lighter bar for the same strength. More expensive but ideal for a refined tool.
Schedule-80 or Schedule-160 Steel Pipe: Hollow, lighter than solid round bar of same outer diameter. Stiffness is good. Ends can be reinforced with welded inserts.

4.2 Solid Round Bar – Weight & Strength

Application	Bar Diameter	Length	Material	Approx. Weight	Bending Capacity at Eye End
6″ Helix (Prototype)	1.5″ solid round	10 ft	A36 Steel	~60 lb	~3,000 ft·lb (safe)
6″ Helix (lighter option)	1.25″ solid round	10 ft	4140 Steel	~42 lb	~3,200 ft·lb (safe)
6″ Helix (pipe option)	1.5″ Sch-80 pipe (1.9″ OD)	10 ft	A53 Steel	~35 lb	~2,200 ft·lb (adequate)
12″ Helix (Full-Scale)	2.5″ solid round	18 ft	4140 Steel	~300 lb	~9,000 ft·lb
12″ Helix (pipe alternative)	2.5″ Sch-160 pipe (2.875″ OD)	18 ft	A53 Steel	~190 lb	~7,500 ft·lb

✅ For the 6″ prototype helix: A 1.5″ diameter solid A36 steel bar, 10 ft long, ~60 lb is highly manageable. It can be handled by one strong person or easily by two. A Schedule-80 pipe at ~35 lb is even easier to handle and still strong enough.

⚠ For the 12″ full-scale helix: Even the "light" pipe option weighs ~190 lb at 18 ft. This is not practical to maneuver from a small dinghy. The solid bar at 300 lb is essentially a crane lift. For full-scale, a motorized torque head or hydraulic drive system is strongly recommended.

5. Reinforced Attachment End

The end that attaches to the mooring screw eye needs to be stronger than the rest of the bar because it experiences:

Maximum bending moment (the full lever-arm force × distance to the eye)
Stress concentration at the pin/eye interface
Potential for localized yielding if the eye connection is sloppy

5.1 Recommended Bar-End Designs

Design	Description	Fabrication Difficulty	Best For
Forged Eye Bar	Commercially available. One end has a forged circular eye. The other end is plain. Simply cut to length.	None (off-the-shelf)	6″ helix (prototype)
Welded Clevis End	Weld a heavy forged clevis to the end of the bar. The clevis pin goes through the mooring eye. Distributes load well.	Moderate (welding shop)	Both applications
Reinforced Collar + Eye	Slide a thick-walled pipe collar over the bar end, weld it in place, then weld an eye plate or D-ring to the collar. This doubles the cross-section at the critical zone.	Moderate	12″ helix (if attempting lever method)
Tapered Forged End	Custom forging where the bar tapers from full diameter down to a thicker eye section. Maximizes strength-to-weight.	High (custom forge work)	Production tool

✅ Practical answer: For the prototype, buy a forged eye bar (readily available from industrial suppliers, ~$80–$150 for a 1.5″×10′ A36 eye bar). Alternatively, have a local welding shop fabricate a bar with a welded clevis end for approximately $200–$400. Both are very reasonable approaches. The reinforced collar design is also straightforward to fabricate and adds significant strength at the eye.

6. Estimated Installation Time

The total installation time depends on the number of revolutions required and the time per revolution (one dinghy circle = one screw rotation).

6.1 Revolutions Required

Helix	Target Depth	Assumed Pitch	Revolutions Needed
6″	7 ft (84″)	4″ per rev	~21 revolutions
6″	7 ft (84″)	5″ per rev	~17 revolutions
12″	11 ft (132″)	4″ per rev	~33 revolutions
12″	11 ft (132″)	5″ per rev	~27 revolutions

6.2 Time Per Revolution

With a 10 ft lever, the dinghy's circular path has a radius of ~10–12 ft (rope adds some). The circumference is roughly 65–75 ft. At a slow, steady pulling speed of 2–3 knots (3.4–5 ft/sec), one full circle takes about 15–25 seconds. In practice, maintaining perfect tension and a clean circle requires some finesse, so we estimate 25–40 seconds per revolution including minor adjustments.

6.3 Total Active Driving Time

Scenario	Revolutions	Time per Rev	Active Driving Time
6″ helix, 7 ft (4″ pitch)	21	30 sec avg	~10–12 minutes
6″ helix, 7 ft (5″ pitch)	17	30 sec avg	~8–10 minutes
12″ helix, 11 ft (4″ pitch)	33	35 sec avg	~18–22 minutes
12″ helix, 11 ft (5″ pitch)	27	35 sec avg	~15–18 minutes

6.4 Total Operation Time (Including Setup)

✅ 6″ Helix (Prototype) – Total: ~25–40 minutes

• Initial hand-starting: 5–10 min
• Attach lever & rope: 5 min
• Active driving: 8–12 min
• Adjustments & checks: 5–10 min
• Remove lever, tension line setup: 5 min

⚠ 12″ Helix (Full-Scale) – Lever Method Total: ~40–70+ minutes

• Initial hand-starting: 10–15 min
• Rig heavy lever: 10–15 min
• Active driving: 15–22 min
• Frequent torque stalls likely: +10–20 min
• Remove lever: 5–10 min
Motorized drive strongly recommended.

7. Practical Considerations & Recommendations

7.1 For the 6″ Prototype Helix (7 ft into sand, 8 ft water depth)

Lever: 10 ft long, 1.5″ solid A36 forged eye bar, ~60 lb. Highly manageable.
Rope: Use a low-stretch rope (e.g., polyester double-braid, ½″–⅝″) to maintain efficient torque transfer. Nylon stretches too much and wastes energy.
Dinghy operator: Maintain steady, slow circles. Avoid jerking. The electric outboard's smooth torque is ideal here.
Lever stabilization: The lever will want to tilt. A simple float collar or a second person in the water can keep it horizontal.
Total time: Budget 30–40 minutes per screw for the prototype. With 3 screws, that's roughly 1.5–2 hours of installation work.

7.2 For the 12″ Full-Scale Helix (11 ft into sand)

Lever method is borderline at best. A 190–300 lb lever bar is dangerous to handle from a dinghy.
Recommended alternative: Use a hydraulic torque motor mounted on the dinghy or a small work float, powered by the dinghy's battery bank or a small generator. Helical pile drive heads that produce 5,000–8,000 ft·lb are commercially available and can be rented.
Geared torque multiplier: A manual torque multiplier (e.g., 5:1 ratio) with a reaction arm could allow a manageable input force, but still requires a stable platform.
For the full-scale seastead: Consider investing in a modular hydraulic drive system that can be deployed from the seastead's deck. This would enable quick installation and removal when the seastead relocates every couple of weeks.

7.3 General Tips

Pre-survey the bottom: Ensure you're not hitting coral or rock. Caribbean sand can have buried coral heads.
Test the torque: During prototype testing, measure the actual torque required (a simple torque gauge on the lever can provide valuable data for refining the full-scale design).
Practice: The dinghy-circle technique requires coordination. Do a few practice runs in shallow water where you can see the screw.
Safety: Keep limbs away from the rope and lever during operation. Sudden release of tension can cause snap-back.

8. Summary Table

Question	6″ Helix (Prototype)	12″ Helix (Full-Scale)
Time to drive to depth	8–12 min active driving ~25–40 min total	15–22 min active driving ~40–70+ min total (lever method) Motorized drive recommended
Ideal lever bar	10 ft × 1.5″ solid A36 forged eye bar	18–20 ft × 2.5″ 4140 solid or Sch-160 pipe Impractically heavy (~190–300 lb)
Lever weight	~60 lb (solid) ~35 lb (pipe option)	~190–300 lb Too heavy for dinghy handling
Reinforced end needed?	Yes – forged eye or welded clevis Readily available / easy to fabricate	Yes – reinforced collar + eye or custom forging Fabrication is reasonable but the whole bar is unwieldy
Overall feasibility	✅ Highly feasible	⚠ Marginal – consider motorized drive

Seastead Tension Leg Mooring Analysis · Prototype & Full-Scale Helical Screw Installation
Assumes Caribbean sand bottom, 10 HP electric outboard dinghy, single-helix mooring screws.
For engineering discussion only. Consult a geotechnical engineer for site-specific soil data.