Seastead Leg Battery & Compartment Analysis

Analysis of battery stowage height, watertight compartmentalization, and human access inside the NACA 0030 trimaran legs.

1. Design Parameters

Leg Geometry
Profile	NACA 0030 (symmetric)
Chord	8.5 ft
Max thickness	2.55 ft (30% of chord)
Total leg length	14.5 ft
Protrusion below hull	~13 ft
Submerged draft (per leg)	~6.5 ft
Gross cross‑sectional area	~14.9 ft²

Displacement & Power
Seawater density	~64 lb/ft³
Buoyancy (3 legs @ 6.5 ft)	~18,600 lb
Battery fraction	25% of displacement
Total battery weight	~4,650 lb
Battery weight per leg	~1,550 lb
Target location	Lowest possible in each leg

2. Battery Volume Requirements

Marine LiFePO₄ packs (cases, BMS, and busbars included) typically occupy 0.014 – 0.018 ft³ per pound. Using a conservative mid‑range value:

Volume per leg ≈ 1,550 lb × 0.015 ft³/lb ≈ 23 ft³

So all three legs together need roughly 70 ft³ of packed battery volume.

3. Usable Internal Cross‑Section

The leg is not a perfect rectangle. After subtracting shell, frames, and the unusable trailing‑edge wedge, the practical deck area for battery racks is reduced.

Reduction factor	Reason	Approx. penalty
Shell & ring frames	Aluminum/steel plating, stringers, and conduit	~10%
Trailing‑edge wedge	Last ~0.7 ft of chord where thickness < 6 in	~2%
Rectangular packing	Rectangular battery trays inside a curved foil	~20–30%

  Net usable battery deck area per leg ≈ 9 – 11 ft².

  (If you build curved/custom battery trays that hug the walls, you can push this toward 12 ft².)

4. How High Must the Batteries Go?

Dividing the required volume by the usable deck area per leg:

Scenario	Usable Area	Required Stack Height
Optimized curved trays	11 ft²	2.1 ft
Standard rectangular boxes	9 ft²	2.6 ft
Conservative (loose packing)	7 ft²	3.3 ft

  Bottom line: The battery bank for each leg requires roughly 2.5 to 3.0 vertical feet of leg height.  
  This means the entire battery load fits comfortably inside the bottom of the leg—well below the waterline—giving you the low center of gravity you want.

5. Recommended Compartment Layout (per leg)

With ~6.5 ft of the leg underwater and ~6.5 ft above, stacking horizontal compartments from the bottom up keeps water out and people safe.

Level	Height above leg bottom	Status	Contents & Notes
1. Battery Bay	0 – 3.5 ft	Fully submerged Watertight	• LiFePO₄ bank for this leg • Slide‑out or fixed aluminum trays • Bilge sensor & small backup pump • Removable intake duct for external thruster cables
2. Utility Bay	3.5 – 7.0 ft	Spans waterline Watertight	• Charge controller & inverter (per leg redundancy) • Junction box for RIM‑drive power feeds • Tool stowage & spares
3. Dry Access Bay	7.0 ft to top	Above water Splash‑tight	• Floor hatch from the living area down into the leg • Internal ladder or rungs • Active stabilizer actuator access

Why this works

Reserve buoyancy: Because the batteries only occupy the bottom ~2.5–3 ft, the bay above them (3.5–6.5 ft) remains a dedicated air compartment even if the lowest bay floods.
Trailing‑edge buoyancy pocket: The very thin tail (last ~0.7 ft of chord, thickness < 6 in) can be sealed off as its own longitudinal air chamber. It is useless for storage but provides extra reserve buoyancy and a safe chase for the external wiring conduit.
No through‑hulls: The shell below the waterline is never penetrated. Thruster wires run in the external welded pipe; power enters the leg through a sealed penetrator located in the dry access bay above the waterline.

6. Human Access & Maintenance

You asked whether a person can get down inside to tighten a clamp or swap a battery. The answer is yes, with careful hatch placement.

Feature	Recommendation
Living‑area hatch	24 in round (or 20×28 in oval) marine hatch in the triangle floor, opening into the dry access bay.
Watertight deck hatches	20 in clear opening in the floor at 3.5 ft and again at 7.0 ft. Dogged, gasketed, and rated for the hydrostatic head.
Internal ladder	Welded rungs on the forward (leading‑edge) wall or a removable aluminum ladder.
Working room	In the widest part of the foil the internal cross‑section is roughly 8 ft (fore‑aft) × 2.3 ft (side‑to‑side). A technician can crouch or sit while working; for heavy lifts, use a 3:1 tackle from the living‑area hatch rather than hand‑carrying modules up a ladder.

7. Verdict

Yes — this approach is sound.

Volume fits: The batteries need only 2.5–3.0 ft of vertical space per leg. They can be housed entirely in a dedicated submerged bay at the bottom.
Compartmentalization works: Two watertight floors (at ~3.5 ft and ~7.0 ft) divide each leg into three isolated zones. Flooding is contained to a single zone.
The thin trailing edge should be sealed: Exactly as you proposed, making it a dedicated air compartment eliminates a hard‑to‑use volume and adds reserve buoyancy.
Human access is feasible: Hatches every ~3.5 ft let a person climb down, and the wide section of the NACA profile provides enough crawl‑space for maintenance.

Practical tip: Build the battery bay 3.5 ft tall even though the stack is only ~2.5 ft. That extra foot gives you aisle room, cooling space, and a place to route low‑voltage harnesses without creating pinch points.

8. Design Checklist & Notes

Weight budget discipline: These numbers assume a finished displacement of ~18,500 lb. If the living‑area structure grows heavier, draft will increase past the 6.5 ft mark. Keep a running weight log; the legs have straight, constant sections so a few extra inches of immersion costs you reserve buoyancy but does not change the battery layout.
Container shipping: Ship the legs as empty shells. Install the batteries (and inverters) after assembly so you stay well inside the 62,000 lb container limit and avoid shipping hazardous cells under tight IMDG rules if not needed.
Battery retention: Use elastomeric mounts or padded aluminum racks. In a seaway the foil will flex slightly; rigidly bolted cells can crack their welds.
Corrosion: 5086‑H116 aluminum for the shell and 316 stainless for hatch dogs. Isolate dissimilar metals where the battery racks touch the hull.
Stabilizer clearance: The little “airplane” stabilizer is attached near the back of the leg in the upper (above‑water) region. Make sure its pivot hardware does not interfere with the 7 ft utility‑bay wall.

Calculations based on NACA 0030 area coefficient ≈0.685, seawater at 64 lb/ft³, marine LiFePO₄ pack density ≈60–65 lb/ft³, and a 13 ft leg protrusion with 50% immersion.