# Seastead Leg Battery Placement Analysis
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Seastead Leg Battery Volume Analysis
How High Up the Legs Do the Batteries Need to Reach?
1. Leg geometry recap
- Foil shape: NACA 0030, chord c = 8.5 ft, with the last 0.5 ft of trailing edge truncated.
- Leg length (vertical, the "span" of the foil standing on end): 14.5 ft, but only 13 ft is in the water + above water column (the 0.5 ft truncation is on the trailing edge, not the height — but you noted the truncated foil fits the 8.9 ft container height, so effective foil height when standing = 13 ft of submersion-relevant length, with 14.5 ft total leg).
- Half submerged: 6.5 ft underwater, 6.5 ft above water (using your "0.5 × 13 ft" figure).
- Max thickness of a NACA 0030 with c = 8.5 ft: tmax = 0.30 × 8.5 = 2.55 ft.
2. Cross-sectional area of one leg
For a NACA 4-digit symmetric foil, area ≈ 0.685 × c × tmax (full foil with sharp TE).
Afoil ≈ 0.685 × 8.5 × 2.55 ≈ 14.85 ft² per horizontal slice through the leg.
(Truncating the last 0.5 ft of the trailing edge subtracts only a tiny triangular sliver — under 2% — so the working number is about 14.6 ft².)
3. Total displaced volume per leg (submerged half)
Vsubmerged ≈ 14.6 ft² × 6.5 ft ≈ 94.9 ft³ per leg.
Across 3 legs: ~285 ft³ of displacement.
4. How much weight is the seastead?
Seawater ≈ 64 lb/ft³. Total buoyant capacity from 3 legs half-submerged:
285 ft³ × 64 lb/ft³ ≈ 18,200 lbs total displacement.
If batteries are ~25% of displacement: battery weight ≈ 4,550 lbs total, or ~1,520 lbs per leg.
5. Volume needed for batteries
Good LiFePO4 packs (cell-level) reach ~90–100 Wh/kg and ~180 Wh/L. Translating to volumetric density of packed modules: roughly 55–65 lbs/ft³ for a realistic installed pack (including BMS, cases, busbars, mounting).
Using ~60 lbs/ft³:
Vbatt per leg ≈ 1,520 / 60 ≈ 25.3 ft³.
6. Usable cross-section as a function of height in the leg
For a NACA 0030, the thickness distribution along the chord peaks at ~30% chord. The chord is horizontal (8.5 ft long), and we're stacking batteries vertically. The cross-sectional area is constant up the leg — what changes with height is just whether that level is underwater or not.
What matters for "how high do batteries reach" is: starting from the bottom of the leg and filling upward, how many feet of leg do we need to fit 25.3 ft³?
But not the whole 14.6 ft² cross-section is usable for batteries. We need to subtract:
- Thin trailing-edge region (rear ~30–40% of chord where the foil is too thin for a battery module). Treat this as a sealed dry compartment. This removes roughly 30–35% of the cross-section.
- Thin leading edge tip (front ~5% of chord). Small loss, ~3%.
- Human access space — a vertical chase about 1.5 ft × 1.0 ft (1.5 ft²) for a person to climb down and reach clamps/connections.
- Wiring, BMS, structure — call it 10% overhead.
| Item | Area used (ft²) |
|---|
| Gross foil cross-section | 14.6 |
| − Sealed thin trailing-edge compartment (~32%) | −4.7 |
| − Sealed thin leading-edge tip (~3%) | −0.4 |
| − Human access chase | −1.5 |
| − Structure, wiring, BMS (~10% of remainder) | −0.8 |
| Net usable for batteries | ~7.2 ft² |
|---|
7. Height of battery stack
Height needed = 25.3 ft³ / 7.2 ft² ≈ 3.5 ft of stack height per leg.
Result: Batteries only need to fill roughly the bottom 3.5 ft of each 13-ft tall leg. That keeps the center of gravity very low — well below the waterline (which sits at 6.5 ft up from the bottom). Excellent for stability.
8. Suggested compartment layout per leg (bottom → top)
| Height from bottom | Compartment | Purpose |
|---|
| 0 – 0.5 ft | Sump / bilge | Catches any leak; sensor + small pump; sealed below batteries |
| 0.5 – 4.0 ft | Battery bay (sealed, dry) | Holds ~25 ft³ of LiFePO4; access via overhead hatch |
| 4.0 – 6.5 ft | Lower service bay (sealed) | Charge controller, inverter, BMS master. Underwater region — fully sealed bulkhead above. |
| 6.5 ft (waterline) | Waterline bulkhead | Fully watertight floor with bolted/gasketed hatch. Critical safety boundary. |
| 6.5 – 10 ft | Upper access shaft | Dry, ladder for human entry from the living area above. Cable conduit pass-through. |
| 10 – 13 ft | Top junction to triangle frame | Structural attachment, top hatch into living area floor. |
| Entire chord length | Sealed thin-TE compartment | Runs full leg height. Unused dead air = buoyancy reserve if other compartments flood. |
9. Multi-chamber safety check
If any single compartment floods:
- Battery bay flooded (3.5 ft × 7.2 ft² ≈ 25 ft³ × 64 lb/ft³ = 1,600 lbs of water in). Lost buoyancy ≈ 1,600 lbs. The other two legs and the sealed TE compartment in the affected leg still provide ~16,600 lbs of buoyancy — seastead stays well afloat (only ~10% capacity lost).
- Lower service bay flooded (~2.5 ft × 7.2 ft² ≈ 18 ft³ = 1,150 lbs of water). Same story — easily survivable.
- Sealed TE compartment flooded — only affects the dry/above-water portion above 6.5 ft; below waterline it was already providing buoyancy and would lose ~6.5 × 4.7 = 30.5 ft³ ≈ 1,950 lbs. Still survivable with 2 other legs intact.
- Entire leg lost — remaining 2 legs provide ~12,100 lbs buoyancy vs. ~13,650 lbs of remaining seastead+batteries+payload. Marginal — but with mooring screws or rapid jettison of the dead leg's batteries (drop hatch), recoverable.
Recommendation: add at least one horizontal watertight bulkhead inside the battery bay (split it into two ~1.75 ft tall sub-bays), each with its own hatch. That way a single-point hull breach near the bottom only floods half the battery bay (~800 lbs of water), and you keep three-out-of-three legs functional.
10. Does the plan work?
Yes — comfortably.
- Batteries need only the bottom ~3.5 ft of each 13-ft leg — well under the 6.5-ft waterline, giving an excellent low center of gravity.
- The wide part of the leg (front 60–65% of the chord) gives ~7 ft² of usable horizontal area, which is plenty for both batteries and a ~1.5 ft² human access chase.
- The thin trailing-edge region naturally becomes a separate, sealed, unused compartment — costs nothing and adds redundant buoyancy.
- Adding 1–2 horizontal watertight floors with bolted hatches on the way down gives strong puncture survivability without preventing human access for maintenance.
- Putting the inverter/charge-controller in a dedicated dry compartment above the batteries but still below the waterline keeps wire runs short and isolates electronics from any battery thermal event.
11. Numbers at a glance
| Quantity | Value |
|---|
| Foil cross-section (gross) | 14.6 ft² |
| Submerged volume per leg | 94.9 ft³ |
| Total displacement (3 legs) | ~18,200 lbs |
| Battery weight (25% of disp.) | ~4,550 lbs total / ~1,520 lbs per leg |
| Battery volume per leg @ 60 lb/ft³ | ~25 ft³ |
| Usable cross-section for batteries | ~7.2 ft² |
| Battery stack height per leg | ~3.5 ft |
| Waterline height in leg | 6.5 ft |
| Vertical margin between batteries and waterline | ~3 ft |
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