Seastead Propulsion Analysis: Rigid Mixers vs. Underwater Drone Tug

1. The Physics of Your Seastead

Your understanding of fluid dynamics and thrust is incredibly accurate. Moving a 15-ton (30,000 lbs) extreme-drag structure at 1 MPH using strictly solar power requires treating the seastead less like a boat and more like a barge or oil platform.

The Momentum Efficiency Principle: As you noted, Thrust (T) is proportional to mass (m) × velocity (v), while Kinetic Energy (E) is proportional to ½ × m × v². To maximize bollard pull per watt of solar energy, you must move a massive volume of water as slowly as possible.

Therefore, utilizing large 2.5-meter propellers operating at low RPM is arguably the most mathematically sound approach for your specific use case.

2. The "Underwater Quadcopter" Tug Concept

Your idea is essentially a heavy-duty, tethered ROV (Remotely Operated Vehicle) that acts as an omnidirectional cyber-tugboat. Using four rotors (two CW, two CCW) gives it differential thrust and yaw control, identical to an aerial drone.

Is it worth the trouble? From an engineering standpoint, it is a fascinating and highly capable idea, but it comes with severe practical tradeoffs.

Theoretical Advantages (The "Pros")

Decoupled R&D: You are absolutely right. You can build, test, break, and iterate the propulsion drone in a local pond without touching the seastead. This drastically speeds up development.
Zero NVH (Noise, Vibration, and Harshness): Because the connection is a flexible cable, zero mechanical vibration from the motors transfers to the seastead columns or living quarters.
Ideal Propeller Environment: The drone can dive 15 to 30 feet deep to pull. This places the propellers in undisturbed water, away from surface wave action, cavitation risks, and the turbulent wake of the seastead.
Omnidirectional Towing: By swimming to the front, side, or rear of the seastead, the tug can turn the structure with massive leverage, making intricate maneuvering possible.

Engineering Challenges (The "Cons")

The Scale Paradox: To keep your incredibly efficient low-RPM physics, the drone must house four 2.5-meter (8-foot) propellers. This means the drone itself will be a massive 5x5 meter (16x16 foot) structure floating independently. If you shrink the drone and propellers down to normal ROV sizes, you lose the mass-over-velocity physics advantage and will drain your solar batteries instantly.
The Plunge and Snap: In ocean swells, an uncoupled tug and seastead will heave and pitch at different frequencies. This creates aggressive shock-loading on the tow cable, which can snap the tether or violently jerk the drone.
The "Running Over" Problem: Cables only transmit tension (pulling), not compression (pushing). If you are towing the seastead at 1 MPH and need to stop, the seastead's 30,000 lbs of momentum will continue forward. It will run over the tow drone, potentially tangling the tether or destroying the drone. Braking requires the drone to quickly navigate behind the seastead and pull backward, which takes too much time in an emergency.
Cable Fouling: Your seastead design features an underwater rectangle (44' x 68') of structural tensioned cables holding the columns. Operating a loose tow tether near this complex web of structural cables is a massive entanglement risk.

3. Conclusion and Alternative Recommendation

Verdict: While conceptually brilliant and excellent for rapid R&D, deploying an uncoupled "drone tug" on a permanent basis adds too much operational complexity (tether management, braking inability, and scale issues) compared to rigid mounting.

The Compromise: Azimuth Thruster Pods

You can achieve the benefits of both systems by developing your dual submersible mixers as independent Azimuth Pods. Mount the massive 2.5m props on vertical pipes that slide down the outside of your seastead columns.

Easy R&D: The pods can be tested independently on a pontoon boat before attaching them to the seastead.
Directional Control: Mount the vertical pipes on bearings so they can rotate 360 degrees. This provides the omnidirectional "drone-like" steering without differential thrust.
Rigid Braking: Because they are rigidly attached to the columns, they can provide instant reverse thrust to stop the 30,000 lb structure.
Vibration Isolation: Using rubber motor mounts between the pipes and the seastead columns will eliminate the noise advantage of the tethered drone.