Based on your description, your seastead is a floating platform resembling a small oil rig, with a 40x16 ft living area supported by four 4-ft wide, 20-ft long columns angled at 45 degrees from the corners. The submerged bottoms form a 44x68 ft rectangle, stabilized by cables connecting the floats and providing redundancy. The total weight is around 30,000 lbs, and you're aiming for low-speed propulsion (about 1 MPH) using solar power, potentially assisted by ocean eddies.
Your primary idea is low-speed submersible mixers with 2.5-meter propellers for efficient static thrust by moving large volumes of water slowly. As an alternative, you're proposing a reciprocating wing system mounted on two parallel cables (1.5 meters apart) between the bottom of the back two floats. This wing would slide back and forth using wheels or pulleys, flipping its orientation to generate forward thrust in both directions. It could also enable steering by biasing movement to one side.
I'll provide my thoughts on this alternative system, including potential advantages, challenges, and comparisons to your propeller-based approach. This is based on fluid dynamics principles, marine engineering concepts, and practical considerations for low-speed, solar-powered propulsion.
You're spot on with the physics: Thrust comes from momentum transfer (mass of water × velocity imparted), while energy input scales with the square of that velocity (kinetic energy = ½mv²). For maximum thrust per unit energy, especially at low speeds or static conditions (bollard pull), it's ideal to accelerate a large mass of water by a small velocity—hence large, slow-turning props or, in your idea, a broad wing sweeping through water.
The reciprocating wing could indeed "push on a lot of different water" by traversing fresh volumes on each pass, potentially improving efficiency over a fixed propeller that recirculates the same water. Integrating wheels inside the wing could minimize drag and hydrodynamic losses. Flipping the wing (e.g., via a simple hinge or servo) to maintain thrust direction is clever and mimics bio-inspired propulsion like penguin flippers or oscillating foils in some experimental marine tech.
Your original idea of low-speed, large-diameter (2.5m) propellers on submersible mixers is proven technology, used in wastewater treatment and some marine applications. It's likely simpler to implement, with fewer moving parts exposed to water motion. Propellers can provide steady thrust and easy steering via differential speed or azimuthing.
The wing system might edge out in theoretical efficiency for bollard pull but could underperform in practice due to losses. I'd recommend hydrodynamic simulations (e.g., using CFD software like ANSYS or OpenFOAM) or scale-model testing in a pool/tank to compare thrust output per watt. For your 30,000 lb seastead, estimate required thrust: At 1 MPH (0.447 m/s), drag force might be around 500-2000 N (rough guess based on shape; calculate properly using drag equations).
Overall, this is an innovative and promising idea that fits your efficiency goals and could work well for a slow-moving seastead. It has real potential for both propulsion and steering with minimal added complexity. However, the underwater mechanics introduce risks that propellers avoid. Start with back-of-the-envelope calculations: Wing area, reciprocation speed, expected thrust (using momentum flux), and power draw. If feasible, build a small prototype to test in water.
If you provide more details (e.g., wing dimensions, materials, or power budget), I can refine this analysis. This could be a cool addition to seasteading tech—keep iterating!