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Seastead Propulsion Analysis: RIM Drives vs Submersible Mixers
Seastead Propulsion: RIM Drives vs. Low-Speed Mixers
The Design Shift: Upgrading from 1 MPH massive submersible mixers to 3 MPH hydrodynamic wing-hulls changes your propulsion physics entirely. Large mixers excel at bollard pull (0 MPH static thrust) but create immense hydrodynamic drag at higher speeds. Smaller, distributed active units like RIM drives operating with differential thrust will dramatically improve maneuverability and allow you to reach your 3 MPH target efficiently.
The Search for True Chinese RIM Drives (RDTs)
Finding a true Rim-Driven Thruster (where the stator coils are enclosed in the outer duct and the permanent magnets are embedded in the propeller's outer ring) out of China requires careful sorting.
Caution: Many Chinese marine suppliers on sites like Alibaba advertise "RIM drives" when they are actually just traditional hub-motors with a Kort nozzle (duct) bolted around the propeller.
To find true RDTs in China with >100 lbs of thrust (approx. 45-50 kgf), you must look at the commercial ROV and USV (Unmanned Surface Vessel) market, rather than the recreational boating market. Companies operating in Shenzhen and Changzhou manufacture heavy-duty hubless thrusters for industrial underwater robotics. While Yamaha's HARMO brings RDT to recreational boating, Chinese makers are keeping it strictly industrial for now.
Theoretical Physics: Thrust & Power vs. Advance Speed
For a fixed-pitch propeller, such as those found on RIM Drives, performance changes dynamically as the seastead moves through the water. We assume constant voltage (maximum throttle) for these models.
- Thrust: Peaks at 0 MPH (Bollard Pull). As the seastead speeds up, "advance speed" increases. The relative angle of attack of water hitting the blades decreases, so the propeller cannot push as hard. Once the water speed matches the geometric pitch speed of the prop, thrust becomes zero.
- Power Draw: At 0 MPH, the prop bites hard into stationary water. This mechanical load holds the motor RPM down slightly, meaning lower back-EMF and maximum current (wattage) draw. As the vessel speeds up towards 3 MPH, water rams into the prop. The prop "unloads," allowing the RPM to spin up closer to its theoretical maximum. Higher RPM yields higher back-EMF, which resists the current. Therefore, power draw naturally drops as speed approaches the zero-thrust point.
Propulsion Comparison Table
Below is a comparison of realistic options. Keep in mind that "lbs per kW" strictly measures static bollard pull. Submersible mixers will always win this metric because of their massively oversized slow propellers, but they will fail at pushing a vessel to 3 MPH due to geometric pitch limits and drag.
| Type / Model |
Lbs Thrust (approx) |
Watts |
lbs/kW (Efficiency) |
$ Price (Est) |
$/lbs-thrust |
URL / Search Term |
| True RIM: Yamaha HARMO |
227 lbs |
3,700 W |
61.3 lbs/kW |
~$3,500 |
$15.41 |
Yamaha Marine |
| True RIM: Chinese USV Hubless Thruster (Generic 50kgf) |
110 lbs |
3,000 W |
36.6 lbs/kW |
~$1,800 |
$16.36 |
Alibaba: "Hubless Thruster ROV" |
| Fake RIM (Ducted Hub): ePropulsion Navy 3.0 Evo |
~130 lbs |
3,000 W |
43.3 lbs/kW |
~$2,100 |
$16.15 |
ePropulsion |
| Submersible Mixer: Chinese QJB2.2 Low Speed |
~250 lbs (at 0 mph only) |
2,200 W |
113.6 lbs/kW |
~$800 |
$3.20 |
Alibaba: "QJB Mixer" |
| Ducted ROV: Blue Robotics T500 (Baseline) |
35.5 lbs |
1,000 W |
35.5 lbs/kW |
$360 |
$10.14 |
Blue Robotics |
Conclusions for the Seastead
- The Yamaha HARMO is highly impressive: It was specifically engineered for low-speed marine boat traction. Achieving 61.3 lbs/kW on a smaller rim drive is exceptional engineering, largely due to acoustic tuning and very tight tolerances in the stator/prop gap.
- Mixers are out: If you want 3 MPH, low-speed mixers (like QJB series) won't work. By the time you hit 1.5 MPH, the massive low-pitch propeller blades effectively become a waterbrake, and the power draw will plunge because the flow is pushing the prop faster than the motor wants to spin it.
- Sourcing in China: Instead of searching for "Rim Drive Outboard" on Chinese sites, you must search B2B manufacturing ports for "Hubless Thruster AUV" or "ROV Thruster 50kgf". Keep an eye out for manufacturers in Shenzhen. Alternatively, consider using high-efficiency pod drives with Kort nozzles (like ePropulsion); while they have a central hub, they offer similar hydrodynamics to a rim drive at 3 MPH and are fully commercialized and reliable in China.
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