Seastead Leg Cross-Section Study

30′ legs · equal displacement (≈358 ft³, ≈22,900 lb buoyancy) · duplex 2205 vs. 5083 marine aluminum · conceptual estimates (±30%)

Key findings

Drag: any streamlined section cuts leg drag 70–85% vs. the cylinder. At 2 MPH the 4-leg propulsion load drops from ≈13.4 kW (cylinder) to ≈2.0–2.8 kW (Kamm / extended-Kamm / airfoil family).
Best value shapes: the Kamm-tail teardrop and lenticular sections capture most of the aerodynamic benefit at modest fabrication premium. A slightly longer “extended Kamm” is near the practical optimum.
Material: marine aluminum legs are ≈45% lighter and ≈45–50% cheaper than duplex at these sizes. Duplex wins on dent/abrasion tolerance and thinner walls; both are credible.
Packing: your 3–4-per-container estimate works only in a high-cube box (or open-top). In a standard 40′ the 7′5″ door aperture blocks stacked loading — plan 2 per standard container, 3 per high-cube for most shapes.
10 PSI internal pressure: agreed — it works for every closed section including ellipse and lenticular, and is most valuable there. See §7.

1. Assumptions

All legs 30.0 ft long, half-submerged in service (15 ft wetted). Seawater ρ = 64 lb/ft³, ν = 1.1×10⁻⁵ ft²/s.
Cross-sections normalized to the cylinder’s displacement (≈11.9 ft² section area) except where noted.
Skin-friction coefficient C_f ≈ 0.004 (turbulent, Re_L ≈ 2–4×10⁶). Section drag coefficients C_d from typical 2-D section data at Re 5×10⁵–1×10⁶; see table in §4. Biofouled surfaces push the cylinder toward C_d ≈ 1.2 and degrade foils 50–100% — budget periodic cleaning.
Calm-water, steady flow aligned with the section’s long axis; no wave-making, wind, or interaction between legs. In a seaway expect 2–4× these figures.
Structure: rolled/welded plate shell + light internal ring frames @ ~5 ft + thickened end cans with forged/machined hard-point lugs (included in weights/costs as a ~12% adder).
Propulsion: 4 legs’ drag only, overall drivetrain efficiency 60% (large slow mixers). Hotel loads, appendages, and wave resistance excluded.
Costs: FOB Asia (China/Korea/Vietnam), fabricated, ex-works, excluding ocean freight, duties/tariffs, and tooling. Material pricing volatile — duplex especially (Ni/Mo content).

2. Candidate cross-sections (to scale)

1 · Cylinder∅3.90′

2 · Airfoil6.0′ × 2.9′

3 · Stadium4.70′ × 2.95′

4 · Ellipse4.75′ × 3.20′

5 · Lenticular5.00′ × 3.30′

6 · Ovate4.50′ × 3.20′

7 · Kamm-tail5.10′ × 3.30′

8 · Extended Kamm5.60′ × 3.00′

Shape 8 (“extended Kamm”) is the “something else” candidate: same blunt nose and truncated-tail philosophy as your Kamm, with a longer taper. Beyond this, extra chord buys <10% more drag reduction while costing container slots — diminishing returns. A cheap intermediate step: retrofit foam/GFRP fairing sleeves over existing cylinder legs to test the concept at sea before committing to new tooling.

3. Geometry & displacement

Table 1 — Section properties (30 ft length)
Shape	Dims (ft)	Area (ft²)	Volume (ft³)	Buoyancy (lb)	Perimeter (ft)	Frontal width (ft)
Cylinder	∅3.90	11.95	358	22,900	12.25	3.90
Airfoil	6.00 × 2.90	12.2	365	23,400	14.0	2.90
Stadium	4.70 × 2.95	12.0	360	23,000	12.77	2.95
Ellipse	4.75 × 3.20	11.94	358	22,900	12.59	3.20
Lenticular	5.00 × 3.30	11.9	357	22,900	12.69	3.30
Ovate	4.50 × 3.20	11.3	339	21,700	12.4	3.20
Kamm-tail	5.10 × 3.30	11.9	357	22,900	13.29	3.30
Extended Kamm	5.60 × 3.00	11.9	357	22,900	13.2	3.00

Your 3.10′ × 4.90′ Kamm sketch computes to ≈10.8 ft² (−10% volume); dims above are stretched slightly to hold displacement. The ovate runs ≈5% light — stretch to 4.6′ if exact parity matters.

4. Drag per leg (calm water, half submerged)

Table 2 — Drag per leg (lbf). Wetted length 15 ft, C_f=0.004
Shape	Assumed C_d	1.0 MPH	1.5 MPH	2.0 MPH	vs. cylinder @2 MPH
Cylinder	1.00	127	285	507	—
Stadium	0.62	60	136	241	−52%
Ellipse	0.38	41	91	163	−68%
Lenticular	0.30	33	75	134	−74%
Ovate	0.25	27	61	109	−79%
Kamm-tail	0.23	26	59	104	−79%
Airfoil	0.20	20	46	82	−84%
Extended Kamm	0.18	19	43	76	−85%

C_d notes: the cylinder sits in the drag-crisis Re band (5×10⁵–1×10⁶); smooth paint could dip to ~0.7, biofouling pushes toward 1.2 — 1.0 is a fair design value. Thick (≈50%) sections like the airfoil risk trailing-edge separation; C_d 0.20 assumes a fairly clean section. Yaw warning: asymmetric sections (ovate, Kamm, airfoil) assume flow aligned with the long axis; at 20–30° yaw their drag can rise 1.5–3× and they generate side forces. If your seastead weathervanes on a mooring, the symmetric lenticular/ellipse sections are the forgiving choice; asymmetric ones want active heading control.

Drag @ 2 MPH, per leg (lbf)

Cylinder

507

Stadium

241

Ellipse

163

Lenticular

134

Ovate

109

Kamm-tail

104

Airfoil

Extended Kamm

5. Propulsion power — 4 legs

Table 3 — Electrical power, 4 legs, 60% drivetrain efficiency
Shape	1.0 MPH (W)	1.5 MPH (W)	2.0 MPH (W)	kWh / statute mile @1.5 MPH
Cylinder	1,680	5,670	13,440	3.8
Stadium	800	2,700	6,400	1.8
Ellipse	540	1,810	4,320	1.2
Lenticular	440	1,490	3,550	1.0
Ovate	360	1,210	2,890	0.8
Kamm-tail	350	1,170	2,760	0.8
Airfoil	270	910	2,170	0.6
Extended Kamm	250	860	2,020	0.6

Solar-budget read: a typical seastead solar array sustains perhaps 2–6 kW continuous. Cylinder legs make 2 MPH a 13.4 kW proposition (not sustainable on solar); any streamlined section brings 2 MPH under ~3 kW. Your two 2.5 m mixers are low-speed, high-thrust machines — well matched to 1–2 MPH once drag is cut; verify their rated shaft power (large mixers are commonly 4–8 kW each). Figures exclude wave resistance; in a seaway multiply by 2–4×.

6. Weight & cost per leg

Table 4 — Fabricated leg, incl. ring frames, end cans, hard points (FOB Asia, ±30%)
Shape	Wall — duplex (in)	Wall — alum (in)	Weight duplex (lb)	Weight alum (lb)	Cost duplex	Cost alum
Cylinder	0.20	0.31	3,350	1,850	$15,000	$7,900
Stadium	0.24	0.36	4,180	2,180	$20,100	$10,000
Ellipse	0.28	0.40	4,800	2,380	$25,000	$12,000
Lenticular	0.26	0.38	4,500	2,280	$23,000	$11,200
Ovate	0.26	0.38	4,400	2,230	$23,800	$11,700
Airfoil	0.24	0.36	4,580	2,390	$25,200	$12,900
Kamm-tail	0.24	0.36	4,350	2,260	$22,600	$11,400
Extended Kamm	0.24	0.36	4,320	2,250	$22,900	$11,600

Why non-circular walls are thicker: the cylinder is the ideal pressure shell; flatter panels (ellipse, stadium) and pointy rims (lenticular) lose external-pressure buckling capacity and pay for it in gauge. Internal pressure (§7) buys some of this back.
Cost basis: duplex ≈$2.5/lb material + $2.0/lb baseline conversion; aluminum ≈$2.0/lb + $2.25/lb (aluminum welding is slower). Shape complexity multiplies conversion labor: cylinder 1.0×, stadium 1.15×, lenticular/Kamm 1.3–1.35×, ellipse 1.35×, ovate 1.45×, airfoil 1.5×. US/EU fabrication would run roughly 2.5–3.5× these numbers.
Not included: ocean freight (~$300–800/leg in a shared container), anodes ($100–300/yr), coatings ($500–1,500/leg if specified separately), duties. Check tariff exposure — steel/duplex articles from Asia can attract significant duties; aluminum and Vietnam/Korea routing change the math materially.
Take: aluminum saves ~$8–13k and ~2,000 lb per leg. For a buoyancy-dominated, low-speed platform, weight is not precious — cost usually decides. Duplex earns its premium where ice, debris, or docking abrasion threaten thin skins.

7. Structural checks — the 4 MPH case and the 10 PSI question

Held-at-ends, 4 MPH, any direction

Broadside flow is the worst case: even the streamlined sections present C_d ≈ 1.0–1.2 flat-on, giving a distributed load of roughly 2,000–2,600 lbf over the wetted half. End-fixed bending moments land around 9–12 kip·ft → bending stresses of only 1–3 ksi in the gauges above — far below yield for both materials. Global Euler buckling is a non-issue (P_cr is orders of magnitude above working loads). The governing modes are local: shell buckling under external pressure, ovalization under bending, and bearing at the hard points. Hence the specified ring frames @ ~5 ft, end cans at 2× wall over the last 2–3 ft, and forged/machined lug hard points. Sized this way, all eight shapes meet the 4 MPH requirement with margin.

10 PSI internal pressure — yes, and it helps the ellipse/lenticular most

Agree. At 15 ft depth the external head is ~6.7 psi; 10 psi internal flips the net load to slightly positive — external-pressure buckling essentially disappears at service depth, and ovalization under bending is resisted by pre-tension.
It is especially valuable for ellipse/lenticular/stadium sections, whose flatter panels are the first to buckle. Hoop stress added is trivial here (≈750–1,200 psi).
Leak detection works exactly as you imagine: a $20 pressure switch alarmed to the bus gives instant warning of weld cracking or puncture — arguably the biggest practical benefit.
Caveats: the shell becomes a pressure boundary — weld QA/NDT to match; fit a ~15 psi relief valve; allow for thermal swing (≈0.6 psi per 30 °F, or use a small compensation bladder); seal hard-point penetrations properly. For the lenticular, round or reinforce the sharp rim seams where membrane stresses concentrate.
Recommendation: 5–10 psi working, 15 psi shop test, alarm at −2 psi.

8. 40-foot container packing

Table 5 — Legs per container (30 ft legs, loaded lengthwise)
Shape	Standard 40′	High-cube 40′	Arrangement
Cylinder ∅3.90′	1–2	2	Two stacked = 7.80′ high: clears 7.83′ interior by 0.03′ but cannot pass the 7.42′ door stacked; three-across needs 7.67′ — zero margin, not assemblable inside
Stadium	2	3	2 upright + 1 nested on top (7.65′)
Ellipse	2	3	2 upright + 1 nested (7.95′)
Lenticular	2	3	2 upright + 1 nested (8.30′)
Ovate	2	3	2 upright + 1 nested (7.70′)
Airfoil	2	2	Chords horizontal, 2 layers (5.8′); 3rd layer misses door clearance
Kamm-tail	2	3	2 upright + 1 nested (8.40′)
Extended Kamm	2	2	Chords horizontal, 2 layers (6.0′)

The door is the constraint, not the box. A standard 40′ door is ~7′5″ high × 7′8″ wide. Any leg that must end up resting on another leg has to transit the door above it — impossible in a standard cube for these diameters. Hence 2/standard, 3/high-cube for the nestable shapes.
If your 3–4-per-box estimate came from pure cross-section geometry, high-cube (or open-top, crane-loaded) containers recover it. Open-top also solves the cylinder: 3 fit a 40′ high-cube open-top with overhead loading.
Shrinking the cylinder to ∅3.83′ (same length, −2% volume) allows 3 in a standard high-cube with ordinary floor loading — worth considering before freezing the diameter.
Every layout leaves ~9.5 ft of container length free for hard points, cradles, and spares. Payload is never binding (heaviest load-out ≈12.5 tonnes vs. ~27.6 t limit).

9. Recommendations

Short-list the Kamm-tail and lenticular sections. The Kamm captures ~80% drag reduction with benign fabrication (gentle curves, one dish-formed nose, one small tail cap); the lenticular matches it with a symmetric section that tolerates yaw — pick based on whether the platform holds heading.
Default to marine aluminum (5083/H116) unless abrasion/ice drives duplex; the ~$10k/leg saving funds a lot of solar panel.
Adopt 5–10 psi internal pressure with relief valve and alarmed pressure switch — cheapest buckling margin and leak detection you can buy.
Book high-cube (or open-top) containers and re-check the cylinder diameter against the 3.83′ threshold before freezing drawings.
De-risk cheaply: fairing sleeves on one cylinder leg as an at-sea A/B drag test before committing to new section tooling.

Method & disclaimer. Conceptual (Class 4) estimates ≈±30%. Drag: D = ½ρV²(C_d·b·L_wet + C_f·P·L_wet), ρ=1.99 slug/ft³, C_f=0.004, L_wet=15 ft; C_d from published 2-D section data at Re 5×10⁵–1×10⁶. Weights: shell perimeter × gauge × length × density (duplex 487 lb/ft³, 5083 169 lb/ft³) + 12% for frames, end cans, hard points. Power: 4 × drag × speed ÷ 0.60. Costs: FOB Asia material + conversion rates as of early 2025; commodity-price and tariff exposure not modeled. This study is for screening only — confirm with a naval architect and a fabricator RFQ before detailed design. Wave loads, vortex-induced motion, leg-to-leg interference, and grounding/impact cases are excluded and will govern some details.