Seastead Leg Battery Packaging Analysis
Feasibility Study: Low-CG Battery Stowage in NACA 0035 Foil Legs
Verdict: HIGHLY FEASIBLE. The battery bank required for 25% displacement (≈6,875 lbs LFP) occupies only the bottom ~1.5–2.0 ft of the 21.5 ft leg span. This leaves >19 ft of leg length for compartmentalization, access ladders, and progressive flooding safety margins. The design goal of "weight down low" is achieved perfectly.
1. Battery Requirements & Volume Calculation
Target Displacement (Design Waterline)27,500 lbs (Salt Water)
Battery Allocation (25% Displacement)6,875 lbs (Total across 3 legs)
Per Leg Battery Weight2,292 lbs
LFP Pack Density (Cells + BMS + Structure + Fire Suppression)~100–115 lbs/ft³ (Conservative Pack Level)
Per Leg Battery Volume Required20.0 – 23.0 ft³
Total Fleet Battery Volume (3 Legs)60 – 69 ft³
Why Pack Density ~110 lbs/ft³?
- Cell Level (LFP): ~130–150 lbs/ft³ (2.1–2.4 g/cc).
- Module Level (busbars, compression, thermal pads): ~115 lbs/ft³.
- Pack Level (BMS, contactors, fuses, coolant loops, fire barriers, structural tray, air gaps): ~100–110 lbs/ft³.
- Using 110 lbs/ft³ gives 20.8 ft³/leg. We design for 23 ft³/leg margin.
2. Leg Geometry & Internal Volume
Foil ProfileNACA 0035 (35% Thick)
Chord Length (Constant)8.5 ft (Root to Tip)
Max Thickness (30% Chord)2.975 ft (35.7 in)
Span (Length)21.5 ft
Tip TreatmentTrailing edge truncated last 0.5 ft of span (blunt tip)
Cross-Sectional Area (Theoretical, 8.5ft chord)~17.3 ft² (Area Coeff ≈ 0.24 × c²)
Internal Volume per Foot of Span~17.3 ft³/ft
Total Leg Internal Volume~372 ft³
Volume Verification (NACA 0035 Area Coefficient)
Standard 4-digit Area Coeff = 10 * t * ∫[0.2969√x - 0.126x - 0.3516x² + 0.2843x³ - 0.1015x⁴]dx
Integral (0 to 1) ≈ 0.0685
Area / c² = 10 * 0.35 * 0.0685 ≈ 0.23975
Area = 0.23975 * (8.5)² = 0.23975 * 72.25 ≈ 17.32 ft²
Key Insight: With ~17.3 ft³ per linear foot of span, the required 23 ft³ of batteries fits in just 1.33 vertical feet of leg height. Even with 30% packaging inefficiency (trays, cooling, bracing), it fits in ~1.75 ft.
3. Vertical Stack Layout (Bottom-Up)
The leg spans 21.5 ft total. Design Waterline (DWL) is at mid-span (10.75 ft from bottom). The "Top Half" (10.75 ft) is above water; the "Bottom Half" (10.75 ft) is submerged.
LEG ELEVATION (Side View - Leading Edge Forward)
↑ Z (Height from Keel Tip)
|
21.5 ft ◼ ◼ ◼ TRIANGLE ATTACHMENT POINT (Top of Leg)
◼ ◼ ◼ ┌─────────────────────────────┐
◼ ◼ ◼ │ UPPER LEG (DRY) │
◼ ◼ ◼ │ - Ladder Access (Front) │
◼ ◼ ◼ │ - Watertight Bulkheads │
◼ ◼ ◼ │ - Conduit/Wiring Chase │
◼ ◼ ◼ │ - ~8.5 ft usable height │
10.75 ft ◼ ◼ ◼ ├─────────────────────────────┤ ← DESIGN WATERLINE (DWL)
◼ ◼ ◼ │ LOWER LEG (WET) │
◼ ◼ ◼ │ - Heave Plates (Bolt-on) │
◼ ◼ ◼ │ - RIM Drives (2ft up) │
◼ ◼ ◼ │ - Access Compartment │
◼ ◼ ◼ │ - ~7.0 ft usable height │
3.5 ft ◼ ◼ ◼ ├─────────────────────────────┤ ← BULKHEAD #2 (Top of Access)
◼ ◼ ◼ │ BATTERY COMPARTMENT │
◼ ◼ ◼ │ - 3 LFP Modules Stacked │
◼ ◼ ◼ │ - Liquid Cooling Manifold │
◼ ◼ ◼ │ - Fire Suppression Nozzle │
1.5 ft ◼ ◼ ◼ ├─────────────────────────────┤ ← BULKHEAD #1 (Top of Batteries)
◼ ◼ ◼ │ VOID / TRIM TANK / FOAM │
0.5 ft ◼ ◼ ◼ │ (Truncated Trailing Edge) │
◼ ◼ ◼ └─────────────────────────────┘
0.0 ft ◼ ◼ ◼ KEEL TIP (Blunt)
Detailed Compartment Schedule (Per Leg)
| Compartment |
Vertical Extent (ft) |
Height (ft) |
Volume (ft³) |
Purpose / Notes |
| Tip Void |
0.0 – 0.5 |
0.5 |
~8.0 |
Truncated TE; foam-filled; collision buffer; no batteries. |
| BATTERY HOLD |
0.5 – 2.0 |
1.5 |
~26.0 |
Primary Target. Holds 23 ft³ pack + 15% margin. Watertight bulkheads top/bottom. Hatch at 2.0 ft level. |
| Lower Access / Ballast |
2.0 – 3.5 |
1.5 |
~26.0 |
Watertight. Ladder landing. Emergency pump. Water ballast trim tank option. |
| Wet Compartment 1 |
3.5 – 6.5 |
3.0 |
~52.0 |
Heave plate mounts (external). RIM Drive mount (2 ft up from tip = 2.0 ft). Cable glands. |
| Wet Compartment 2 |
6.5 – 10.75 |
4.25 |
~73.5 |
Ladder continues. Structural frames. Floodable for survival ballast if needed. |
| DESIGN WATERLINE (10.75 ft) |
| Dry Compartment 1 |
10.75 – 15.0 |
4.25 |
~73.5 |
Ladder well. Conduit penetration (watertight gland). Bilge sensor. |
| Dry Compartment 2 |
15.0 – 19.0 |
4.0 |
~69.0 |
Main access trunk. Equipment rack (leg inverter/charger). |
| Upper Attachment Bay |
19.0 – 21.5 |
2.5 |
~43.0 |
Structural connection to triangle vertex. Heavy bolting flange. Watertight hatch to living space floor. |
4. Battery Pack Physical Integration
Pack Dimensions (Per Leg)
- Available Envelope: 8.5 ft (Chord) × 2.97 ft (Max Thick) × 1.5 ft (High) = 37.8 ft³ gross.
- Usable Envelope (80% fill factor for curves/trays): ~30 ft³.
- Required: 23 ft³.
- Margin: 30% (Excellent).
Suggested Module Layout (3 Modules per Leg)
PLAN VIEW (Looking Down at Battery Compartment 0.5ft - 2.0ft)
Chord: 8.5 ft →
Thickness: 2.97 ft ↑
LEADING EDGE (Front / Ladder Side)
╔══════════════════════════════════════╗
║ ◼◼◼ ◼◼◼ ◼◼◼ ◼◼◼ ◼◼◼ ◼◼◼ ◼◼◼ ║ ← Module 1 (Forward)
║ ◼◼◼ ◼◼◼ ◼◼◼ ◼◼◼ ◼◼◼ ◼◼◼ ◼◼◼ ║ ~2.8 ft Chord × 2.5 ft Wide × 1.5 ft High
║ ◼◼◼ ◼◼◼ ◼◼◼ ◼◼◼ ◼◼◼ ◼◼◼ ◼◼◼ ║
╠══════════════════════════════════════╣
║ ◼◼◼ ◼◼◼ ◼◼◼ ◼◼◼ ◼◼◼ ◼◼◼ ◼◼◼ ║ ← Module 2 (Aft - Centered on Thickest Point)
║ ◼◼◼ ◼◼◼ ◼◼◼ ◼◼◼ ◼◼◼ ◼◼◼ ◼◼◼ ║ Max Width (2.97 ft) utilized here.
║ ◼◼◼ ◼◼◼ ◼◼◼ ◼◼◼ ◼◼◼ ◼◼◼ ◼◼◼ ║
╠══════════════════════════════════════╣
║ [COOLING / BMS / CONDUIT] ║ ← Center Chase (0.5 ft wide)
║ (Vertical Pass-Through) ║
╚══════════════════════════════════════╝
TRAILING EDGE (Aft / Thruster Conduit Side)
- Module Size: ~2.8 ft (Chord) × 2.5 ft (Width) × 1.5 ft (Height) = 10.5 ft³ × 3 = 31.5 ft³ Gross.
- Net Cell Volume: ~23 ft³. Fits with room for liquid cooling plates between modules and side walls.
- CG Height: ~1.25 ft above keel. Extremely low. Contributes massively to righting moment.
5. Compartmentalization & Safety Strategy
Watertight Bulkhead Schedule
| Bulkhead | Location (ft above keel) | Rating | Penetrations |
|---|
| BH-1 (Bottom) | 2.0 ft (Top of Batteries) | Watertight (SOLAS) | None (Solid). Battery monitoring via wireless/through-bulkhead comms (e.g., Bluetooth/LoRa) or feedthroughs. |
| BH-2 | 3.5 ft | Watertight | Ladder pass-through (flanged hatch), Bilge pump discharge, Sensor wires. |
| BH-3 | 6.5 ft | Watertight | RIM Drive motor cables (glanded), Heave plate bolts (external). |
| BH-4 | 10.75 ft (DWL) | Weathertight / Structural | Main Ladder, Main Power Cables (Conduit), Vent (with auto-seal). |
| BH-5 | 15.0 ft | Watertight | Equipment rack access. |
| BH-6 (Top) | 21.5 ft (Triangle Floor) | Watertight (Hatch) | Human Access Hatch (30" round), Main Power Feedthroughs. |
Progressive Flooding Safety
- Single Compartment Flooding: Any one compartment floods → Leg loses ~15-20% buoyancy. Remaining 2 legs + 2 intact compartments on damaged leg provide huge reserve buoyancy (Total Leg Vol 372 ft³ vs Req 175 ft³ submerged).
- Battery Compartment (BH-1 to Tip): If holed, only 8.7% of leg volume floods. Batteries are in sealed modules (IP67+) inside a sealed compartment. No "through-hulls" in legs. Thruster cables run in external conduit on trailing edge.
- Detection: Pressure sensors + humidity sensors in each compartment, monitored by leg-local MCU (triple redundant).
6. Human Access Design
Access Philosophy: "Inspect & Replace" not "Live In". Batteries are 10+ ft below waterline. Routine access is via the Internal Vertical Ladder on the Leading Edge (forward face).
Access Path
- Living Space Floor: Open BH-6 Hatch (30" dia, quick-acting dogs).
- Upper Leg (Dry): Descend ladder (Fall arrest rail + cable ladder). Landings at BH-5, BH-4.
- Waterline Transition (BH-4): Critical seal. Ladder passes through watertight flange. Hatch can be closed from above or below.
- Lower Leg (Wet - Pressurized Air): Compartments maintained at slight positive pressure (0.1-0.2 psi) via dryer/blower to keep dry and prevent corrosion. Descent continues.
- Battery Compartment (BH-1): Open upward-opening hatch (counterbalanced) at 2.0 ft level. Stand on BH-2 landing (grating).
- Module Removal: Modules on rails. Winch point on BH-2 overhead. Module slides up rails → through BH-1 → up ladder well → out BH-6. Requires 2 people, ~30 min/module.
Ladder Specification
- Location: Integral to Leading Edge (Forward) skin. Doubler plate + rungs welded/bonded internally.
- Width: 18" clear (between stringers).
- Fall Protection: Vertical cable rail (SRL compatible) + Rest platforms every 6 ft.
- Lighting: Explosion-proof LED strips (Class I Div 2) both sides.
7. Structural Integration with Triangle Frame
The leg attaches near the triangle vertex. The "Center of thickest part ±1.5 ft" constraint places the attachment bulkhead (BH-6) roughly at the max thickness station.
- Load Path: Hydrodynamic lift (up) + Weight (down) → Leg Spar Caps (Top/Bottom of foil) → Bulkhead BH-6 → Triangle Vertex Node.
- Battery Reaction: Battery trays bolted to longitudinal stringers on compartment floor (keel side). Inertia loads (slamming) reacted directly into leg skins via stringers.
- Conduit: Main DC+ / DC- / Comms conduit runs up Trailing Edge (Aft) external fairing → Penetrates BH-4 (DWL) via stuffing box → Up Dry Side → Triangle Beam Raceway.
8. Summary & Recommendations
| Parameter | Value | Status |
|---|
| Battery Height Required | 1.5 ft (0.5 – 2.0 ft above keel) | ✅ Fits easily |
| Battery CG above Keel | ~1.25 ft | ✅ Optimal Stability |
| Volume Margin in Battery Bay | ~30% | ✅ Room for Cooling/Fire Sys |
| Compartments below Waterline | 4 (Tip, Batt, Access, Wet) | ✅ Progressive Flooding Safe |
| Access Time (Top to Battery) | ~5 min descent | ✅ Acceptable |
| Module Swap Time | ~30 min / module | ✅ Feasible |
| Through-Hulls in Legs | ZERO | ✅ Design Requirement Met |
Critical Design Actions
- Detail BH-1 (Battery Top Bulkhead): Use composite sandwich (Carbon/Glass) with titanium feedthroughs for BMS antenna (wireless preferred) and temp sensor leads. Zero cable penetrations.
- Venting Strategy: Battery compartment needs thermal runaway vent duct (1-way rupture disc) → Overboard at DWL (BH-4) via dedicated small conduit. Not shared with ladder well.
- Corrosion Protection: Lower leg (wet) compartments: Sacrificial anodes + Epoxy/Ceramic internal coating. Dehumidified air supply (solar powered) from top.
- Heave Plate Bolting: Ensure bolt pattern on lower leg (Wet Comp 1) does not penetrate Battery Compartment (BH-1 is at 2.0 ft, Heave plates typically start ~3-4 ft). Verify clearance.
- RIM Drive Cable Glands: At 2.0 ft above keel (inside Wet Comp 1). Use "Cable Transit System" (Roxtec/Hawke) for multiple cables, gas-tight, fire-rated.
Conclusion
The NACA 0035 foil leg provides vastly more volume than needed for the battery bank. The batteries occupy only the bottom 7-9% of the leg length. This allows a robust safety architecture: a dedicated, double-bulkhead-isolated battery vault at the very bottom (maximizing stability), followed by a dry access trunk, then wet machinery spaces, all fully compartmentalized. Human access via an internal ladder on the leading edge is standard marine practice and entirely feasible for maintenance intervals. This design works exceptionally well.
Generated for Seastead Design Review | Containerized Trimaran Foil Platform