This is a creative engineering problem. Your intuition regarding "high mass of water at low velocity" is physically correct and is the basis for high-efficiency propulsion. However, the specific mechanism you proposed (a reciprocating wing on a cable) faces a significant physics hurdle: **Reversal Loss.**
Here is the breakdown of the physics involved, followed by an interactive HTML visualization of your concept.
### The Physics Analysis
**1. The Efficiency Problem (The "Stop-Start" Penalty)**
For your wing to act as a thruster, it must accelerate water backward.
* **Forward Stroke:** The wing pushes water back, pushing the seastead forward. This takes energy.
* **Return Stroke:** The wing must fly back to the starting position.
* If the wing is streamlined perfectly during the return so it offers *zero* drag, it simply coasts without creating thrust. You are now a "pulsed" engine. At 1 MPH, the intervals between thrust would be long, causing the massive 30,000 lb seastead to decelerate and accelerate repeatedly (high stress on cables, uncomfortable ride).
* To maintain constant speed, the return stroke must also provide thrust. But to provide thrust while moving forward (relative to the seastead), the wing must create drag in the forward direction. This **cancels out** the thrust gained on the backward stroke.
**2. The Hydrodynamic Advantage of Screw Propellers**
A propeller is essentially a wing that moves in a circle.
* By rotating continuously, the blades never have to "stop" and "reverse."
* They maintain a constant angle of attack relative to the water.
* This allows them to act on a massive amount of water, slowly accelerating it (high efficiency), without the massive kinetic energy losses associated with reciprocating parts (starting/stopping the moving mass of the wing mechanism).
**3. Drag on the Cables**
Running two 1.5m apart cables underwater between the floats creates significant "parasitic drag." As the seastead moves at 1 MPH, those two long cables act like fishing nets, creating resistance that your propulsion system has to overcome.
**Verdict**
While the "Linear Wing" allows for steering, the **rotary propeller** is superior for open ocean station-keeping or cruising because it avoids the energy penalty of reversing direction. A better alternative for your specific "low RPM / high water mass" requirement is a **Kort Nozzle** or **Ducted Propeller**. This uses a shroud to prevent tip vortices and allows a larger diameter prop (like your 2.5m plan) to move even more water at lower RPMs, increasing efficiency without the mechanical complexity of a rail system.
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### Interactive HTML Visualization
Here is a visual simulation of your design. It shows the layout of the seastead and animates the "Reciprocating Wing" concept you described to visualize the stroke cycle. You can save the code below as an `.html` file (e.g., `seastead.html`).
```html
Seastead Linear Thruster Concept
Seastead Linear Wing Thruster Concept
Top-down view. Green = Platform. Blue = Rails. Yellow = Reciprocating Wing.
Design Specifications
Platform Size: 40' x 16' (Living Area)
Float Rectangle: 44' x 68'
Weight: ~30,000 lbs
Target Speed: ~1 MPH
Thruster Location: Between rear floats
Physics Analysis:
The simulation shows the wing moving back and forth. While this moves a lot of water (good for efficiency), the energy required to reverse the wing's momentum and the drag created on the return stroke often reduces net efficiency compared to a continuous screw propeller.
Benefit: Excellent steering by biasing stroke length or speed to one side. Challenge: High mechanical stress on the cables due to constant acceleration/deceleration.