Review of Your Seastead Propulsion and Kite Concepts

1) Do rim drives have a "spin freely" mode to reduce drag?

Sometimes, but not automatically. A rim-drive thruster is an electrically driven propulsor where the propeller blades are supported by a rotating rim, often with the motor integrated into the rim. Whether it "spins freely" when unpowered depends on several design choices:

• Motor/controller behavior: Some motors create electrical braking or cogging drag when unpowered or when the controller is off.
• Bearing and seal friction: Even if electrically free, mechanical drag can still be significant.
• Blade pitch and duct shape: A fixed propeller inside a ring still creates drag in through-flow.
• Controller programming: In some systems, the motor can be set to coast; in others it may resist motion.

Practical possibilities

For your design, because the thrusters are mounted on the sides of the submerged foil-legs, their drag may be substantial whenever the craft is moving under kite power or drifting in current. So the right question is not only "can they spin freely?" but:

• What is the total drag of 6 thrusters at expected transit speed?
• Does freewheeling reduce drag enough to matter?
• Would a fairing, shutter, or retractable mount be better?

2) What I think of the kite idea

The kite-robot-on-track idea is creative and has real advantages, but it is not simple. In principle it could provide:

• Backup propulsion independent of the main electrical thrusters
• Reduced fuel dependence if you otherwise need a generator backup
• Useful downwind and beam-reach movement
• Possibly some upwind progress if the underwater foils provide enough lateral resistance

Conceptually, your 3 submerged foil-legs could indeed act somewhat like giant daggerboards, resisting sideways drift while the kite supplies a side-and-forward force. Moving the kite anchor point fore and aft along the track could also create a yawing moment, similar in spirit to shifting a sail’s center of effort.

Why the idea is promising

1. Separated failure modes: a wind-based propulsion system is genuinely different from electric thrusters.
2. Track-mounted tow point: moving the tow point is a clever way to steer without huge exposed spars or masts.
3. Downwind deployment side: keeping the lines away from the occupied structure is good.
4. Modular kite stack: changing kite area can match wind conditions.

Main concerns

3) Most important engineering issue: overturning moment

This is probably the biggest concern.

A kite can create a large horizontal force, and that force acts at a high point compared to the center of buoyancy of the floats. That creates a roll moment:

roll moment = horizontal kite force × vertical lever arm

Even a moderate line force becomes serious if the lever arm is large. Your platform is 40 feet wide, which helps, and the 3 separated buoyancy legs also help, but the real question is:

• How much righting moment do you have at 5°, 10°, 15°, and 20° heel?
• What is the maximum sustained kite pull in those conditions?
• What happens in a gust 2× that force?

4) Will the 3 foil-legs act like daggerboards?

Yes, to some extent. Since they are long vertical-ish submerged foil bodies aligned fore-aft, they will resist sideways motion and generate hydrodynamic side force. That is helpful for sailing-like behavior.

But there are caveats:

• Depth is limited: only about half of each 19-foot leg is submerged, so approximately 9.5 feet underwater by your description.
• Wave effects near the surface: near-surface foils are less ideal than deeper ones.
• Interference from thrusters: side-mounted thrusters and supports may disturb flow.
• Low-speed control: if boat speed is low, lateral foil effectiveness is lower just when gust loads may still be high.

So yes, they may act like daggerboards, but not as cleanly or efficiently as purpose-designed deep centerboards or leeboards.

5) The robot-on-track steering idea

The steering logic is plausible:

• Move tow point forward → bow pulled off/downwind differently
• Move tow point aft → stern influenced differently
• Keep kite to leeward side → avoid crossing over the structure

But in practice, steering response will depend on:

• kite altitude,
• line length,
• apparent wind angle,
• current,
• wave-induced yaw,
• underwater foil sideforce distribution among the 3 legs.

The result may feel less like a simple sailboat and more like a slow, highly damped, nonlinear towing system.

6) About the stack of 20–50 kites

This is one of the parts I would simplify first.

A large stack of many kites sounds adjustable, but operationally it may be difficult:

• more rigging complexity,
• more tangles,
• harder launch/retrieval,
• more uneven loading,
• more maintenance,
• greater risk in squalls.

A smaller number of larger, better-controlled traction kites may be more practical than dozens of small units. Another possibility is a single parafoil traction kite with reefing or multiple preset sizes.

7) Safety systems I would consider essential

1. Instant depower/release: one-command dump of kite force.
2. Mechanical fuse / weak link: sacrificial overload protection.
3. Load cell on line: measured tension at all times.
4. Heel-angle cutoff: automatic depower if heel exceeds threshold.
5. Robot jam detection: if the carriage stops moving, depower.
6. Line-cutter or emergency release: if entanglement occurs.
7. Exclusion zone: no crew near line path during deployment/recovery.
8. Protected dinghy position: ensure no line can snag the RIB, outboard, ropes, or davit supports.

8) Design suggestions to improve the kite concept

• Use a low-friction, very strong track and carriage designed for large lateral shock loads, not just average loads.
• Place the tow point as low as practical to reduce roll moment.
• Model the righting moment carefully before choosing kite size.
• Limit maximum kite force in software and hardware.
• Start with one kite, not a big stack.
• Use the thrusters and kite together: thrusters can help maintain heading while the kite adds net propulsion.
• Consider a dedicated towing bridle rather than relying only on one moving point load on the rail.
• Keep solar panels protected from lines and failed kite landings.

9) About your stabilizers shaped like little airplanes

The idea of active hydro stabilizers on the aft part of each leg is interesting and could help with pitch/roll damping. But for this application, key questions are:

• Do they stay fully submerged in waves and heel?
• Can they generate enough force at low speed?
• Will they ventilate near the surface?
• Are the pivot and notch region strong enough for cyclic loads?
• What is the failure mode if an actuator sticks hard-over?

They may be useful as a ride-control system. I would not rely on them as the sole answer to large kite heeling loads unless you do full hydrostatic and dynamic analysis.

10) Overall judgment

Rim drives: possibly free-spinning, but you must verify with the exact manufacturer. For your design, drag when unpowered is important enough that you should request hard data or consider retractable/fairing solutions.

Kite concept: clever and potentially valuable as backup propulsion, especially because it has independent failure modes from the electric thrusters. The basic physics are sound enough that it is worth studying. But it is not a small add-on; it is a serious control and safety subsystem.

11) Recommended next steps

1. Do a hydrostatics study:
   • displacement,
   • waterplane area,
   • center of buoyancy,
   • center of gravity,
   • righting moment vs heel angle.
2. Estimate kite line force ranges for realistic wind speeds.
3. Compare kite overturning moment to platform righting moment.
4. Estimate drag of 6 thrusters when unpowered and underway.
5. Run a simple dynamic model for heading control with moving tow point.
6. Prototype the kite robot at small scale before integrating with the full structure.

If you want, I can next help you with any of these:

• a rough buoyancy/displacement estimate for the 3 foil-legs,
• a righting moment estimate for the triangular platform,
• a rough kite force calculation for different wind speeds, or
• a failure-modes table for the thruster + kite system.