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Seastead Propulsion Analysis: SME-VRTN Mixer Performance
Performance Table (Estimated)
Speed
(MPH) |
Speed
(m/s) |
Thrust
(Newtons) |
Thrust
(lbs) |
Power Draw
(kW) |
Thrust vs
Static |
| 0.0 |
0.00 |
3,200 |
720 |
3.2 |
100% |
| 0.5 |
0.22 |
~2,850 |
~640 |
~2.9 |
89% |
| 1.0 |
0.45 |
~2,400 |
~540 |
~2.5 |
75% |
| 1.5 |
0.67 |
~1,900 |
~430 |
~2.1 |
59% |
Important Limitation: Based on your seastead's estimated drag (high-drag platform hull), the equilibrium speed (where thrust equals drag) is approximately 1.2-1.4 MPH. At 1.5 MPH, the required thrust to overcome drag (~3,800 N) exceeds what this propeller can provide at that speed (~1,900 N). The 1.5 MPH figures represent the propeller's capability, but your vessel cannot actually reach this speed with a single unit.
Physics Explanation
Exit Velocity Calculation:
For an ideal propeller, power (P) = ½ × Thrust (T) × Exit Velocity (Ve)
Therefore: Ve = 2 × P / T = 2 × 3,200W / 3,200N = 2.0 m/s (4.5 MPH)
This means the propeller accelerates water to approximately 4.5 MPH relative to the vessel. As your seastead approaches this speed, the thrust drops toward zero.
The thrust decreases because as the vessel moves forward, the water entering the propeller already has forward momentum. The propeller's ability to create a pressure differential diminishes as the advance speed increases. For this slow-speed mixer design:
- Thrust curve: Decreases roughly linearly with speed, reaching zero near 4.5 MPH
- Power curve: Decreases as the propeller "unloads" (less torque required at higher advance ratios)
- Efficiency: Peaks at some intermediate speed (likely around 1-2 MPH for this design)
Design Implications for Your Seastead
Hull Drag Considerations:
Your 30,000 lb platform with four 4-foot diameter columns at 45° presents significant drag:
- Projected column area: ~13.7 m²
- Drag at 1 MPH: ~1,700 N (380 lbs)
- Drag at 1.5 MPH: ~3,800 N (850 lbs)
At 1.5 MPH, drag exceeds available thrust, making 1.5 MPH unreachable with one unit. You would need
two units (port and starboard) to achieve 1.5 MPH against calm water resistance.
Operational Recommendations:
- Station Keeping: The 3,200N static thrust is adequate for holding position against moderate currents (up to ~0.8 m/s / 1.8 MPH current with one unit, assuming good directional alignment)
- Transit: Expect practical cruising speed of 0.8-1.2 MPH with one unit
- Battery/Power: At 3.2 kW, you need ~3.2 kWh per hour of operation at full thrust. Solar sizing should account for this continuous draw during maneuvering
- Redundancy: Consider that if one unit fails, your maximum speed drops significantly due to the non-linear drag curve
Technical Notes on the Mixer Selection
The Shinmaywa SME-VRTN is designed for wastewater mixing (stationary water), not marine propulsion. However, its characteristics suit your application:
- Large diameter (2.5m) provides high thrust at low RPM (high efficiency)
- Stainless steel construction suitable for seawater
- Low tip speed reduces cavitation risk at the slow speeds you're operating
Caution: Verify that the unit's thrust bearings and shaft seals are rated for continuous axial loading in the direction of travel, as tank mixers sometimes operate with different load distributions than propulsion units.
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