Subject: Under-Leg Anchoring System & Duplex Stainless Materials Feasibility
Displacement
30,000 lbs / 13.6 tonnes
Float Depth
~7 ft below surface
Float Array
44' × 68' rectangle
Column Angle
45° (20 ft length)
1. Anchoring System Feasibility
The Verdict: Your proposed "under-leg" anchoring scheme is mechanically possible but presents significant operational challenges regarding cable interference, scope management, and retrieval safety.
Geometric Constraints
Based on your specifications:
Waterline intersects column at midpoint (10 ft from float)
Float vertical depth = 10 ft × sin(45°) = 7.07 ft below surface
Horizontal offset from hull = 20 ft × cos(45°) = 14.14 ft
Your cable layout creates a 3D obstacle course:
Diagonal Cables: Two cables per float ascend to adjacent living area corners, crossing the space where an anchor rode would travel.
Perimeter Cable: The rectangular cable connecting float bottoms sits at the 7-foot depth, creating a snag hazard for any rode deployed near the float perimeter.
Clearance: If the anchor deploys vertically through the center of the float (not off the edge), it may avoid the perimeter cable.
Proposed System: Pros & Cons
Aspect
Feasibility
Technical Notes
Anchor Storage Below Float
Difficult
Anchor hanging below float increases drag during 1 mph transit and risks entanglement with the perimeter cable if it swings.
Rode Path Under Leg
Manageable
Requires a reinforced fairlead or hawse pipe at the float top to prevent chafe on the 45° column. The angle creates point-loading.
Cable Avoidance
Critical
The rode must pass through a dedicated central chute in the float to miss the diagonal cables ascending to the living area.
Scope Requirements
Problematic
With only 7 ft of "freeboard" to the float, achieving 5:1 scope in 20 ft depth requires 135 ft of rode which must stow on the float or leg.
Primary Risk: If the anchor drifts during retrieval, it can foul the rectangular cable between floats or the diagonal cables. A fouled anchor in 7-foot seas could transmit shock loads directly to your cable network, risking structural failure.
Recommended Modifications
Install a Centerline Anchor Tube: Instead of hanging below, integrate a vertical tube (moon pool) through the center of each float. This protects the rode from cables and guides the anchor during deployment.
Use a Retractable System: House the anchor in a pocket on the top of the float (above water), with a hinged deployment arm that lowers it clear of the structure. This eliminates the dangling anchor drag issue.
Chain vs. Rope: For the "under leg" section, use chain (chafe-resistant) transitioning to rope above the waterline. The chain weight helps the rode drop clear of the cables.
Scope Management: Install a small electric windlass on the living area deck with a fairlead aligned with the column centerline. Store rode on a drum at the deck, not on the float.
2. Duplex Stainless Steel Availability
Duplex 2205 Anchors
Status: Available but specialty order.
High-end anchor manufacturers (e.g., Ultra Anchor, Mantus, Rocna) offer 2205 duplex stainless as a custom option.
Cost factor: Approximately 6–10× the price of galvanized steel.
Weight: Similar to 316 stainless; suitable for your 30,000 lb displacement (approximate anchor weight needed: 60–80 lbs / 27–36 kg).
Duplex 2205 Chain
Not Recommended. Duplex stainless chain is essentially unavailable as a catalog item. Marine chain is typically:
Hot-dip galvanized Grade 43 or Grade 70 alloy steel (standard)
316 stainless steel (rare, used for轻奢 yachts)
Duplex wire rope is available, but chain links in 2205 are specialty-forged items requiring custom fabrication.
Material
Availability
Relative Cost
Galvanic Compatibility
Duplex 2205 Anchor
Special Order
$$$$$
Perfect (same material)
Duplex 2205 Chain
Unobtainable/Custom
$$$$$$
Perfect
316 SS Chain
Limited (import)
$$$$
Acceptable with precautions
Galvanized G7 Chain
Universal
$
Requires isolation (see below)
Corrosion Strategy Recommendations
Instead of all-duplex, consider this hybrid approach:
Anchor: High-test galvanized steel (or 2205 if budget allows).
Isolation: Use a thrubolt isolator or composite bushing where the chain passes through the duplex float/leg. This breaks the galvanic circuit.
Sacrificial Anodes: Zinc anodes on the floats will protect the galvanized chain/anchor if they touch, though ideally keep them isolated.
Engineering Note: Duplex 2205 is excellent for resistance to crevice corrosion in seawater, but its primary advantage is strength (roughly 2× yield strength of 316). For an anchor rode, the advantage is minimal since chain is sized by wear, not tensile strength. Galvanized G7 chain at 30% the cost will outlast the seastead.
3. Propulsion Context (Brief)
Your specification of 2.5 meter diameter propellers on submersible mixers is unusual for this displacement:
At 1 mph (0.45 m/s), a 30,000 lb barge-form hull requires only ~200–400 watts of propulsive power.
2.5m props are typically found on 100+ ton vessels. They will create massive thrust (likely >500 kgf) which is unnecessary for 1 mph but useful for station-keeping against strong currents.
Warning: Submersible mixers (e.g., Flygt/Sulzer types) are designed for tank mixing, not open-water propulsion. Their efficiency and bearing durability in continuous transit may be poor. Ensure the mixers are rated for thrust loading, not just torque.
Summary Checklist
Item
Recommendation
Anchor storage below float
Change to "anchor pocket on float top" or "centerline deployment tube" to avoid cable fouling.
Duplex 2205 Anchor
Feasible; order 6–8 months advance.
Duplex 2205 Chain
Abandon; use galvanized G7 with isolation bushings.
Rode Management
Electric windlass on deck, chain-rope combo, 150 ft minimum scope.
Cable Obstruction
Install fairleads to force rode through centerline of float assembly.