Seastead Anchoring System Analysis

Disclaimer: This analysis is based on the provided specifications and general engineering principles. It is not a substitute for professional marine engineering certification. Any offshore structure, particularly one intended for habitation, should be reviewed by a qualified naval architect to ensure safety and stability.

1. Evaluation of the Anchor Deployment Plan

Your proposed anchoring system involves storing the anchor below the end of the leg/float and deploying it from there. While this keeps the anchor clear of the living area and the structural cables above, there are significant geometric risks involved due to the "cable rectangle" described.

Critical Risk: Cable Fouling

You mentioned there is a cable creating a rectangle between the bottoms of all the floats. If you deploy an anchor line from the bottom of a float, and the seastead swings (due to wind or current change), the anchor line risks tangling with or chafing against these structural cables.

Because your structure is rigid in the water (high drag, low speed), it will act like a weathervane. If the wind shifts, the structure rotates. If the anchor line exits from a corner float and the structure rotates, the anchor line could sweep across the underside of the structure and snag the cross-cables.

Recommendations for the Anchor System:

Chafe Protection: If the rope/chain goes "under" the leg, it will likely bear against the steel structure. Stainless steel is hard, but rope under tension is abrasive. You would need a dedicated chafe guard (like a stainless steel thimble or U-bolt roller) at the exit point to prevent the rope from sawing through the leg or fraying.
Catenary Management: To avoid the cross-cables, you may need a "bridle" system. Instead of a single line from one float, run lines from two opposing floats that meet at a shackle below the structural cables, then drop the anchor from that shackle. This keeps the tension away from the cross-bracing.
Depth Consideration: Ensure the anchor, when stored "below the end of the leg," is deep enough that it does not create drag while you are trying to move at 1 MPH, yet accessible enough for deployment without diving.

2. Material Selection: Duplex Stainless Steel

Your intuition to match the materials to avoid galvanic corrosion is correct. Using dissimilar metals (like a galvanized steel anchor on a stainless steel leg) in a saltwater environment creates a battery effect, leading to rapid corrosion of the less noble metal.

Feasibility: Yes, it is possible.

Duplex stainless steel (such as grades 2205 or 2507) is an excellent material for marine construction due to its high strength and superior resistance to chloride pitting compared to standard 316 stainless steel.

Availability of Components:

Chain: Yes, you can source duplex stainless steel chain. It is a specialty item and significantly more expensive than standard 316 or galvanized chain, but it is manufactured for high-end marine and oil & gas applications. You will likely need to order it from industrial marine suppliers rather than a standard boat chandlery.
Anchors: This is more challenging. Most commercially available anchors (e.g., Rocna, Manson, Bruce) are made from cast steel (galvanized) or standard 316 Stainless. Duplex anchors are not typically "off-the-shelf" items.

Solutions for Sourcing Duplex Anchors:

Custom Fabrication: You would need to commission a custom anchor from a fabrication shop specializing in duplex steel. This ensures metallurgical compatibility.
Isolation: If a duplex anchor cannot be sourced, you can use a high-quality 316 stainless anchor and electrically isolate it from the duplex structure using a specialized swivel or shackle with an isolator sleeve. However, 316 is slightly less noble than Duplex; in a galvanic couple, the 316 would act as the anode. Given the massive surface area of your structure (cathode) compared to the anchor (anode), this could lead to accelerated corrosion of the anchor.
Crevice Corrosion Warning: Stainless steel (even Duplex) requires oxygen to maintain its passive layer. If the anchor buries deep in the seabed (mud/sand), the oxygen-starved environment can lead to crevice corrosion. This is why many permanent moorings still use heavy galvanized steel or specialized alloys. Duplex is better than 316 for this, but it is not immune.

Summary

Your plan to anchor from the legs is structurally sound in theory, but poses a high risk of fouling on your sub-structure cables during swings. Switching to a bridle system that clears the cross-cables is recommended.

While Duplex stainless chain is available, finding a matching Duplex anchor will likely require custom fabrication. Given the expense and the risk of crevice corrosion in seabed burial, you should consult with a marine corrosion specialist to see if isolation bushings with a galvanized anchor might be a safer, more economical long-term solution, provided the galvanic pair is managed correctly.

This analysis is for informational purposes only. Offshore engineering involves life-safety risks. Please consult with a certified naval architect before finalizing construction or anchoring plans.