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Seastead Thruster Options – RIM Drives and Alternatives
Seastead Thruster Options: Chinese RIM Drives and Alternatives
This page summarizes likely propulsion options for a solar/battery seastead, with emphasis on:
True RIM drives (motor built into the propeller rim, not just a ducted prop)
Chinese-sourced options where possible
Alternatives to low-speed submersible mixers
Use cases around 1–3 mph vessel speed, low vibration, long life, and differential thrust
Important: I cannot live-browse in this chat, so I cannot guarantee current pricing, stock, or verify every manufacturer web page in real time.
I have therefore done two things:
Included products and manufacturers that are widely known or commonly referenced in this space.
Clearly labeled where values are manufacturer-quoted, estimated, or needs verification.
For procurement, I strongly recommend requesting from each vendor:
motor cross-section drawing proving it is a true RIM drive,
continuous bollard thrust test report,
thrust vs vessel-speed curve,
input power vs vessel-speed curve,
efficiency map,
saltwater materials list and bearing/seal design.
1) What counts as a true RIM drive?
A true RIM drive has the motor integrated into the ring surrounding the propeller:
the rotor/stator are in the annular rim, and the blades connect to the rotating rim.
A simple ducted thruster or Kort-nozzle propeller is not a RIM drive unless the motor itself is in the rim.
A quick vendor-check list:
True RIM: no central motor pod/shaft housing through the middle; electromagnetic motor is in the outer ring.
Not true RIM: conventional hub motor propeller inside a nozzle, even if seller labels it “rim driven”.
Ask for cutaway image: this usually reveals the truth immediately.
2) Practical conclusion first
For your application, a distributed set of smaller thrusters can make sense for:
differential thrust steering,
rotating/stabilizing wing/leg elements,
redundancy,
placing thrust near each float/foil/wing.
But there is a tradeoff:
Many small thrusters improve control authority and redundancy.
Fewer larger thrusters are usually better in pure propulsive efficiency and cost per pound of thrust.
If your seastead only needs around 1–3 mph, then:
large diameter, slow-turning propulsors are generally best,
RIM drives can be attractive for low noise/vibration and reduced snagging,
submersible mixers can be cheap for bollard thrust, but are often poor as actual propulsors through water at speed,
for a hydrodynamic seastead, proper marine thrusters should outperform mixers once forward speed matters.
Best interpretation of the table:
The Yamaha HARMO-type number you gave, 227 lb thrust from 3.7 kW, is very strong and should be treated as a high-end benchmark.
Many Chinese products may be cheaper, but a lot of them will either:
not actually be true RIM drives,
quote peak rather than continuous thrust,
quote static bollard thrust only,
or lack real efficiency curves.
4) Specific Chinese sourcing paths to investigate
Because your request is specifically for China-made true RIM drives over 100 lb thrust, here are the most practical sourcing paths:
A. Alibaba / Made-in-China search terms that may locate true RIM drive vendors
rim drive thruster
rim driven marine thruster
underwater rim motor propeller
rim drive azimuth thruster
rim drive pod motor
hubless marine thruster
When a listing appears promising, ask for:
motor cross section,
continuous current and voltage,
continuous bollard thrust,
maximum continuous RPM,
propeller diameter,
motor efficiency and controller efficiency,
saltwater corrosion spec,
bearing type and service life,
IP/submergence depth rating,
actual test video with spring scale / load cell and wattmeter.
B. Chinese EV/marine motor manufacturers who may offer custom submerged pod or rim motor work
A lot of Chinese firms can build custom PM motors and marine propulsion hardware even if the public page is not polished.
For your quantity, custom may actually be viable if you need 8 units.
Useful categories:
permanent magnet direct-drive motor manufacturers,
Some have an external stator ring but still drive a central hub through gearing or support arms.
So your insistence on a cutaway image is exactly right.
5) Alternatives to low-speed submersible mixers
Why submersible mixers can look attractive
Large diameter
Low RPM
Often robust
Good static push in water
Sometimes cheap from wastewater industry supply chains
Why they may be poor for your newer winged designs
Designed for circulation, not efficient marine propulsion
Prop geometry often optimized for stirring, not advancing vessel speed
Motor housings and struts may create extra drag
Nozzle/flow path may not be optimized for transit through water
Vendor data usually lacks thrust-vs-speed curves
Better alternatives if hydrodynamics now matter
True RIM drives – low vibration, compact, can be integrated around wings/floats
Ducted pod thrusters – often much cheaper than RIM, easier to source
Electric outboard lower units / pods – can be repurposed
Azimuthing pod thrusters – useful for station keeping and maneuvering
Large slow-turning open prop on strut – often highest efficiency if snagging/noise is acceptable
6) Thrust vs vessel-speed graph: theory estimate
You asked for a graph of thrust at different speeds through the water, and also power draw as speed increases.
That data is rarely published for small marine thrusters, but it can be estimated reasonably from propeller theory.
A practical approximation for a fixed-pitch electric thruster at constant full command:
Bollard condition (0 vessel speed): thrust is maximum, but propulsive efficiency is low because useful output power = thrust × vessel speed = 0.
As vessel speed increases, thrust falls.
As vessel speed approaches the thruster’s no-thrust advance speed, power draw also tends to fall.
This happens because propeller torque demand generally decreases as slip decreases.
For a simple engineering estimate, one can model:
Thrust: roughly decreasing from bollard to zero near some top inflow speed
Power draw: often starts near rated power at bollard and declines toward a lower level near zero-thrust speed
Below is an illustrative estimate using your HARMO-like benchmark:
Bollard thrust = 227 lb
Input power at bollard = 3.7 kW
Zero-thrust speed assumed near 7 mph water inflow speed for illustration only
Estimated thrust vs speed through water
Speed (mph)
Speed (m/s)
Estimated Thrust (lb)
Estimated Input Power (W)
0
0.00
227
3700
1
0.45
195
3550
2
0.89
162
3350
3
1.34
130
3100
4
1.79
97
2750
5
2.24
65
2250
6
2.68
32
1600
7
3.13
0
900
This is a theory-based illustrative estimate, not manufacturer test data.
The exact shape depends on prop diameter, pitch, RPM control law, motor controller logic, and duct/nozzle design.
Inline SVG graph: thrust and power vs speed
7) Interpretation of the graph for your seastead
At 1 mph, a good large-diameter thruster may still deliver most of its bollard thrust.
At 3 mph, thrust may already be down substantially from bollard, but useful propulsive power is much better than at zero speed.
If your new wing/foil-supported hull genuinely reduces drag enough, then smaller thrusters become much more attractive.
That means your shift from two huge mixers to perhaps eight smaller thrusters could work if the hull drag at 3 mph is low enough.
The key calculation is not just static thrust; it is:
Required thrust at 3 mph = total hull drag at 3 mph
If hull drag at 3 mph is, say, only 100–200 lb total, then eight smaller units could be fine.
If drag is 500–1000 lb total, then many small units become expensive and electrically heavy.
8) Recommended engineering path
Option A – conservative / lower risk
Use 2 to 4 larger marine thrusters for propulsion
Use smaller dedicated control thrusters only where needed for wing stabilization
This usually minimizes cost and maximizes propulsion efficiency
Option B – distributed thrust architecture
Use 8 true RIM or ducted pod thrusters, one on each side of each wing/leg
Advantage: strong control authority, modularity, redundancy
Disadvantage: more wiring, more controllers, more failure points, higher total cost
Option C – hybrid
2 main propulsion units
4–8 smaller trim/stability thrusters
This may be the best overall compromise
9) What to ask Chinese vendors before buying
Is this a true RIM motor or a ducted hub motor?
Please provide a cutaway image.
What is continuous thrust at bollard in seawater?
What is the test voltage/current and water depth?
Can you provide thrust vs speed and power vs speed curves?
What is the material of:
stator housing,
rotor ring,
magnets coating,
fasteners,
bearings?
What is the corrosion protection in seawater?
What is the design life in hours at continuous power?
Is the controller included, and can it do low-RPM efficient operation?
Can they supply spare rotor, bearings, seals, controller, and blades?
10) Most likely candidates by category
Category
Fit for your use
Main strengths
Main risks
True RIM drive
Very good if verified and affordable
Low vibration, compact, no central hub obstruction, attractive for distributed thrust
Hard to verify, more expensive, fewer real suppliers
Ducted pod thruster
Very practical
Cheap, widely available in China, easy to source
Often mislabeled as RIM, lower elegance, possible higher drag/noise
Submersible mixer
Good for low-speed pushing, less good for transit propulsion
Cheap static push, robust industrial hardware
May perform poorly as vessel speed rises
Electric outboard/pod lower unit
Good benchmark and possible solution
Actual marine design, proven corrosion solutions
May be less ideal for permanent underwater mounting
11) My recommendation
Given your design goals, I would prioritize in this order:
Find true Chinese RIM drive vendors only if they can prove the motor is in the rim and provide thrust/power curves.
If not, use high-quality ducted marine pods from China rather than wastewater mixers for the newer hydrodynamic design.
Use mixers only if your actual mission is mostly station-keeping / slow drifting / current-riding and not efficient 3 mph transit.
Run a drag estimate for the seastead hull/wing configuration at 1, 2, and 3 mph before deciding on thruster count.
For the new hydrodynamic concept, eight smaller thrusters are plausible, but you should first estimate:
total drag at 3 mph,
required turning moment for wing rotation/stability control,
total daily energy budget in kWh.
That will tell you whether:
8 × ~1.5–3.7 kW units makes sense, or
2–4 bigger units plus some small trim thrusters is the better architecture.
12) If you want, I can do the next step
If you send me any of the following:
estimated seastead displacement,
wing/float geometry,
target cruise speed,
solar and battery size,
whether wings rotate continuously or only trim a few degrees,
then I can make you a more useful follow-up page with:
a thrust requirement estimate at 1, 2, and 3 mph,
a proposed 8-thruster architecture,
estimated daily energy use,
and a cleaner HTML comparison table with recommended units sized for your platform.