This is a preliminary engineering review of the proposed removable helical screw mooring system for the half-scale seastead prototype, with comments on scaling to the full-size system. The basic idea is workable in principle, but several details should be changed before testing:
| Item | Prototype assumption | Comment |
|---|---|---|
| Number of anchors | 3 | One per tension leg point. |
| Target working load | 1,000 lb vertical load per anchor | This should be treated as working load, not ultimate load. |
| Seastead available thrust | 400 lb | Enough torque for a small helical anchor if the capstan works efficiently. |
| Helix diameter | 6 inch single helix | Marginal for 1,000 lb working load in variable sand. |
| Shaft length | 8 ft | Reasonable for shallow protected water, assuming about 7 ft embedment. |
| Capstan diameter | 1 ft | Gives 0.5 ft radius, so 400 lb line pull gives about 200 ft-lb torque. |
| Rope wraps | About 4 wraps | Enough if rope/wheel friction is controlled and the tail has modest tension. |
Yes, probably. With a 1 ft diameter capstan:
Torque = line pull × capstan radius
With 400 lb thrust and a 0.5 ft capstan radius:
Torque = 400 lb × 0.5 ft = 200 ft-lb
A common empirical relation for helical anchors is:
Ultimate tension capacity ≈ Kt × installation torque
For small helical anchors, Kt is often roughly 8 to 12 ft-1, although it varies by soil and anchor geometry.
Using Kt = 10 ft-1:
Estimated ultimate capacity ≈ 10 × 200 = 2,000 lb
So, if the system can actually apply close to 200 ft-lb of torque, it should be able to install an anchor capable of roughly 1,000 lb working load in decent sand. But this is not enough margin to skip proof-testing.
The capstan equation is:
Thigh / Tlow = eμθ
Where:
μ = friction coefficient between wet rope and capstan surfaceθ = wrap angle in radians8π radians = about 25.1 radians| Assumed wet friction coefficient | Holding multiplier with 4 wraps | Tail tension needed for 400 lb pull |
|---|---|---|
| 0.15 | About 43:1 | About 9 lb |
| 0.20 | About 153:1 | About 3 lb |
| 0.25 | About 535:1 | Less than 1 lb |
Four wraps should be enough, but the system should not depend only on random seabed drag of the tail line. Use a controlled tail-tension method.
The number of capstan turns depends on the helix pitch. A 6 inch helix commonly has about 3 inch pitch, but this should be specified with the manufacturer.
| Assumed helix pitch | Embedment travel | Turns required | Rope movement with 1 ft capstan |
|---|---|---|---|
| 3 inch pitch | 7 ft = 84 inch | 28 turns | 28 × 3.14 ft = 88 ft |
| 4 inch pitch | 7 ft = 84 inch | 21 turns | 21 × 3.14 ft = 66 ft |
So the prototype should be designed around roughly 70 to 90 ft of rope movement per anchor installation.
A practical rope layout would be:
Recommended total rope length for the prototype:
350 to 500 ft of rope
Your proposed 80 ft on the pulling side plus 200+ ft on the tail side is close, but I would treat 300 ft as tight. A 400 to 500 ft rope gives much better operating margin.
The farther away the seastead is, the smaller the upward component of the rope force at the capstan. For installation this may not be critical because the screw tends to pull itself downward. For extraction it matters more, because the capstan may want to ride upward with the screw.
Assume:
| Horizontal distance | Approximate rope angle, assuming 10 ft vertical rise | Upward force at 400 lb pull |
|---|---|---|
| 40 ft | 14 degrees | About 97 lb |
| 80 ft | 7 degrees | About 50 lb |
| 120 ft | 4.8 degrees | About 33 lb |
| 160 ft | 3.6 degrees | About 25 lb |
For the prototype, I would start the pull at around 100 ft from the anchor if space allows. The seastead will move another 70 to 90 ft during installation, so it may finish roughly 170 to 200 ft away.
To make the capstan slide up and down the hex shaft reliably, avoid metal-on-metal stainless sliding contact. Wet stainless-on-stainless can gall badly, especially with sand present.
Recommended construction:
Example clearance:
The rope-contact surface should grip but not cut the rope.
Recommended:
The spring-loaded rope keeper is a good idea, but it should be designed to avoid trapping fingers or jamming the rope. Use:
Your concern is valid. During extraction, the capstan may want to move upward with the screw instead of staying on the seabed. Repeatedly slacking the line may help, but it is not a robust primary method.
Use a non-rotating seabed shoe / mudplate under the rotating capstan. The capstan rotates on a low-friction thrust surface, while the mudplate rests on or slightly bites into the sand.
The assembly would have:
The mudplate does not need to hold the full anchor load. It only needs to keep the capstan near the seabed during reverse rotation.
For the prototype, a mudplate around 18 to 24 inches diameter would be reasonable. Add 3 or 4 small skids/teeth that bite lightly into sand but do not prevent retrieval.
Target submerged weight of capstan plus mudplate:
I would not rely on the eye reaching the capstan and then forcing the capstan into the sand to release the rope. That could work sometimes, but it is unpredictable and can overload the shaft, rope, or fittings.
Better options:
A rough bearing-style estimate for a 6 inch helix at about 7 ft embedment in sand gives:
This gives a very approximate ultimate uplift capacity range:
About 1,200 to 3,500 lb ultimate
With a safety factor of 2, that becomes:
About 600 to 1,750 lb allowable working load
So a single 6 inch helix may hold 1,000 lb in good dense sand, but it is marginal as a general design.
| Anchor option | Assessment |
|---|---|
| Single 6 inch helix | Usable for experiments, but marginal for reliable 1,000 lb working load. |
| Single 8 inch helix | Better. About 1.8 times the area of a 6 inch helix. |
| Single 10 inch helix | Much better. About 2.8 times the area of a 6 inch helix. |
| Two 6 inch helices on one shaft | Good option if spaced properly, for example 2.5 to 3 helix diameters apart. |
For a serious 1,000 lb working anchor in variable sand, I would prefer either:
Assuming:
| Component | Estimated dry weight |
|---|---|
| 8 ft stainless hex shaft | 35 to 45 lb |
| 6 inch helix and weldment | 5 to 10 lb |
| Eye fitting / top hardware | 5 to 10 lb |
| 1 ft capstan, hub, bushing, mudplate | 20 to 40 lb |
| Total per anchor assembly | Approximately 60 to 90 lb dry |
Underwater apparent weight may be roughly 75% to 85% of dry weight for stainless steel parts, less if there are buoyant parts.
Marine stainless is reasonable, but be careful with 316 stainless in warm seawater and oxygen-poor sand. 316 can pit or crevice-corrode. For repeated insertion/removal in sand, ordinary coating will wear off, so stainless or duplex stainless is sensible.
Recommended material hierarchy:
Use passivation after fabrication, avoid crevices, and avoid stainless-on-stainless sliding without bushings.
These are broad estimates for custom fabricated marine stainless hardware. Actual cost depends heavily on shaft size, stainless grade, machining method, passivation, welding quality, inspection, shipping, and supplier.
| Quantity | Likely ex-factory cost per assembly | Total for 3 | Comment |
|---|---|---|---|
| 3 custom units | $800 to $2,000 each | $2,400 to $6,000 | Small custom order, high setup cost. |
| 30-unit batch from China | $300 to $700 each | $9,000 to $21,000 | Ex-works estimate; add shipping, duty, QC, and spares. |
If using duplex 2205 instead of 316L, add perhaps 20% to 60%, depending on supplier.
For a trained two-person crew in shallow protected water, after practice:
| Operation | Time per anchor | Time for 3 anchors |
|---|---|---|
| Install, not including proof-load | 10 to 20 minutes | 30 to 60 minutes |
| Install including proof-load and adjustments | 15 to 30 minutes | 45 to 90 minutes |
| Remove after crew is practiced | 10 to 25 minutes | 45 to 90 minutes |
| First few trials | 30+ minutes per anchor | 2 to 4 hours is possible |
The swimmer/diver step is likely the slowest and most safety-sensitive part. For repeated use, a tool that wraps or engages the capstan without a swimmer would be a major improvement.
Your proposed full-scale changes:
With a 2 ft diameter capstan, radius is 1 ft. With 2,000 lb thrust:
Torque = 2,000 lb × 1 ft = 2,000 ft-lb
Using the rough torque correlation:
Estimated ultimate capacity ≈ 10 × 2,000 = 20,000 lb
That suggests the seastead has enough thrust to install a much larger anchor, if the rope and capstan are strong enough.
A 12 inch helix has 4 times the area of a 6 inch helix. The deeper embedment also helps. Going from about 7 ft to about 11 or 12 ft embedment may add another factor of roughly 1.5 to 1.7 in sand stress. So capacity may increase by about:
4 × 1.5 = 6 times
That is helpful, but the working load target increases from 1,000 lb to 8,000 lb, which is 8 times higher. Therefore a single 12 inch helix is not a comfortable scale-up if the 6 inch prototype is already marginal.
For an 8,000 lb working tension-leg anchor, I would prefer one of these:
A single 12 inch helix may work in dense sand after proof-loading, but I would not make it the base design for all customers and all sites.
Assume:
| Embedment travel | Pitch | Turns | Rope movement with 2 ft capstan |
|---|---|---|---|
| 10 ft | 6 inch | 20 turns | 126 ft |
| 11 ft | 6 inch | 22 turns | 138 ft |
Recommended full-scale rope length:
Total recommended full-scale rope:
700 to 1,000 ft
Assuming:
| Component | Estimated dry weight |
|---|---|
| 12 ft stainless hex shaft | 90 to 160 lb |
| 12 inch helix and weldment | 20 to 40 lb |
| Eye/top hardware | 15 to 30 lb |
| 2 ft capstan, hub, mudplate | 70 to 150 lb |
| Total per full-scale assembly | Approximately 200 to 380 lb dry |
So “triple the pounds” may be too low. A full-scale unit could easily be 3 to 5 times the prototype weight. Side storage outside the railing is possible, but it should have a small davit, pulley, or A-frame because manual handling will be difficult and dangerous.
The method is still generally workable at full scale, but it becomes more operationally serious:
For a base low-cost system, the capstan method could be acceptable if customers move rarely and anchor only in benign shallow sand. For customers moving often, an automated or semi-automated system would be worth offering.
A more refined system could use:
The proposed system can probably be made to work for a half-scale prototype in shallow protected sand, especially as an experimental removable anchoring method. The thrust and capstan torque are plausible. The biggest changes I would make are:
For the full-size seastead, the same basic idea can scale, but the equipment becomes heavy and the loads become dangerous. It is still possible as a low-cost base system for occasional moves, but frequent users will probably want a more automated anchor installation/removal system.