Seastead Backup Propulsion Analysis
Configuration: 30,000 lb platform with 3 angled columns (4' wide, 13' submerged at 45°),
powered by 2× Shinmaywa submersible mixers (2.5m propellers) with differential thrust steering.
Performance Estimates
Estimated equilibrium speeds when operating at limited power (cruising mode):
| Power per Thruster |
Total Power |
Speed |
Total Thrust |
Drag Force |
| 1.2 kW (37% power) |
2.4 kW |
0.93 MPH (0.81 kt) |
1,636 N (368 lbf) |
1,636 N |
| 2.2 kW (69% power) |
4.4 kW |
1.15 MPH (1.00 kt) |
2,510 N (564 lbf) |
2,510 N |
| 3.2 kW (100% power) |
6.4 kW |
1.30 MPH (1.13 kt) |
3,175 N (714 lbf) |
3,175 N |
* Calculations assume equilibrium (cruise) conditions where thrust equals drag.
Acceleration would require additional power. Speeds are theoretical maximums for steady-state operation.
Key Assumptions
- Drag Model: Estimated drag coefficient Cd ≈ 1.2 with effective frontal area ~15.4 m²
(3 columns × 4' × 13' at 45° angle plus platform appendages). Drag force ≈ 9,500 × v² (Newtons, v in m/s).
- Thruster Performance: Based on Shinmaywa SME series data. Thrust decreases linearly with speed
from 3,200 N at 0 MPH to 740 N at 1.5 MPH when at full power. Power required also decreases with speed
(unloading effect).
- Power Limiting: At limited power (e.g., 1.2 kW), available thrust at speed v is calculated as
T_available = T_table(v) × (P_limit / P_table(v)). This assumes efficiency remains similar at reduced power.
Operational Notes:
- At 1.2 kW (37% power), the system operates at very low thrust margins. Any significant current (>0.5 knots) or headwind would overcome propulsion.
- These speeds represent "steaming" capability for station-keeping or slow relocation, not transit speeds.
- Differential thrust steering will be less effective at lower speeds (1.2 kW mode) due to reduced flow over the rudders (if any) or reduced moment arm effectiveness.
Power vs. Speed Summary
For solar management planning:
- Minimum viable propulsion (1.2 kW × 2): ~0.9 MPH — Useful for maintaining position against light currents or very slow docking maneuvers. Consumes ~2.4 kW total.
- Efficient cruise (2.2 kW × 2): ~1.15 MPH — Reasonable for relocating within a bay or calm conditions. Consumes ~4.4 kW total.
- Maximum backup speed (3.2 kW × 2): ~1.3 MPH — Full power for emergency relocation or fighting moderate currents. Consumes ~6.4 kW total.
Methodology: Equilibrium speeds calculated by solving 2×T(v)×(P_limit/P_table(v)) = ½ρv²CdA,
where ρ=1025 kg/m³, CdA≈15.4 m² (effective), T(v)=3200-3670v N, P_table(v)=3.2-1.79v kW.
```