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In my world, 500 years in the future, my population fits anti-gravity engines to salvaged boats as a form of transport. These transports fly close to the Earth's surface and are powered by a highly efficient solar panel system rigged as sails.

I'm struggling to come up with a design to propel the boats along, and the only concept that I can think of is propellers driven by electric engines.

Is there a more efficient/effective engine design that I can utilise?

Limitations:

  • Fossil Fuels are depleted, and cannot be used.
  • The largest of transports being Cargo Ships must travel at a minimum of 25 knots, and the smallest at a minimum of 45 knots.
  • Fuel sources required for the engine must be in abundant supply and easy to manufacture or contain or, if a reactor is required - able to be stationed onboard the vessel and be maintained by an engineer.
  • The engine must run as cleanly as possible. (As few emissions as possible)

EDIT: This question has been flagged as a duplicate due to another question that I have asked in the past. I believe that this question is not a duplicate as I am looking for a propulsion system, not an anti-gravity engine that could propel my transports. (In this case they cannot due to how my anti-gravity engines work.)

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    $\begingroup$ "Cargo ships must travel at a minimum of 25 knots": you do realize that this makes them go three times as fast as our present cargo ships powered by burning fuel oil? No cargo ship ever has maintained an average speed of 25 knots: sailing ships couldn't, and steamers wouldn't because of the vast amounts of fuel required. $\endgroup$ – AlexP Mar 9 at 1:23
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    $\begingroup$ So usually the answer is "depends on what technology is available." However, in this case, I see a discontinuity that gives me pause. They have anti-gravity engines and ultra-efficient super-thin solar cells for sails, but they salvage boats to put them on instead of designing crafts built for the purpose? I have a feeling there's a reason behind that, and the reason may dramatically affect which answer is best. $\endgroup$ – Cort Ammon Mar 9 at 5:20
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    $\begingroup$ Possible duplicate of Airship Propulsion System $\endgroup$ – dot_Sp0T Mar 9 at 8:29
  • $\begingroup$ Please do not create multiple accounts to ask the same question. $\endgroup$ – dot_Sp0T Mar 9 at 8:29
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    $\begingroup$ @Cyn Thanks for the grammar check! $\endgroup$ – Lucas A. Mar 13 at 22:52
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  • Fossil Fuels are depleted, and cannot be used.

  • The largest of transports being Cargo Ships must travel at a minimum of 25 knots, and the smallest at a minimum of 45 knots.

  • Fuel sources required for the engine must be in abundant supply and easy to manufacture or contain, or if a reactor is required - able to be stationed onboard the vessel and be maintained by an engineer.

  • The engine run as cleanly as possible. (As little emissions as possible)

Hydrogen powered jet engines.

In respective order to each requirement:

  • Not fossils;
  • More powerful than diesel, and most other fuels. This is literally rocket fuel. So you may go much faster or farther for the same amount of fuel mass;
  • Just electrolyze water. About 0.0003% of the mass of the planet is just oceans. You may use nuclear plants power to break water into fuel and oxidizer at large scale, serving millions of vehicles regionally - or car-sized and larger vessels may use small, solar powered electrolyzers (might take days to refill a tank in this way, though).
  • Your exhaust is literally steam.
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    $\begingroup$ The OP wanted low/no emissions. Your plan spews Dihydrogen Monoxide everywhere!!!! $\endgroup$ – ShadoCat Mar 9 at 0:07
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    $\begingroup$ @ShadoCat Not the dreaded Dihydrogen Monoxide, it will kill us all one day! $\endgroup$ – Lucas A. Mar 9 at 0:17
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    $\begingroup$ @LucasA., The water gets ripped into the two gasses when it is separated. However, if you can condense the waste steam from burning it, that is as close to chemically pure water as you are going to get. $\endgroup$ – ShadoCat Mar 9 at 0:22
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    $\begingroup$ @LucasA. adding to what ShadoCat said: that water is so pure it may actually harm living beings due to osmosis. You may need to add salts to it. $\endgroup$ – Renan Mar 9 at 0:29
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    $\begingroup$ @LucasA., Running the water through a "sand filter" should fix the too pure water issue. Just change the sand frequently to have a fresh supply of dissolvables. $\endgroup$ – ShadoCat Mar 9 at 1:03
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From OP. /and are powered by sails/

They have sails!

I know you are using your sails as solar panels which is excellent. They can still be regular sails. Boats have a long history of successfully being propelled by sails.

A problem with steering a sail airship is lack of contact with the water. Regular sailboats have a rudder and I am not sure that will work 100% in the air. You could have a super long blade like air keel.

If you are wind powered that leaves open the question of what you are using your electricity to do. Keg fridge?

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  • $\begingroup$ Windspeeds at high altitdue do reach hundreds of mph, so sails might work. Also airplanes do have rudders and so do airships. $\endgroup$ – Efialtes Mar 9 at 0:59
  • $\begingroup$ You can steer with electric propellers. Some solar panels and batteries, and tail propellers line a helicooter and you are set. $\endgroup$ – Renan Mar 9 at 1:00
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    $\begingroup$ I guess the keel is the main problem, as without it, you can only move in the general direction the wind does. But you could make the keel be part of the way the anti-gravity engine works. Say instead of generating lift (which you could then use for propulsion anyway), it just disables movement in certain directions. For antigravity, you disable up and down movement and for a keel-like effect, you simply additionally disable left and right movement. $\endgroup$ – mlk Mar 9 at 9:28
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    $\begingroup$ A keel and a rudder does nothing when there's not a significant movement differential between what you are on and what you're in. Using wind to power your travels while you're entirely airborne means you move with the wind. Something like mlk's variant engine would allow one to be able to tack, but anti-grav would not be an appropriate name for something that worked like that. Also, the electricity would certainly be used to power anti-grav engines or mlk's thing. $\endgroup$ – Ed Grimm Mar 9 at 14:33
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    $\begingroup$ Sails only work because a ship is at the interface between two different fluid media. A floating ship is functionally the same as a balloon, and no sail will do anything to propel it. $\endgroup$ – Thucydides Mar 10 at 3:31
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In my world, 500 years in the future; my population fit anti-gravity engines to salvaged boats as a form of transport

Use your anti-grav engines.

Gravity exerts a force on your sail-ships towards the center of the earth. In order for your ships to fly the anti-gravity engines must exert some force in the opposite direction. To move forward just add another engine that is 90° rotated.

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  • $\begingroup$ I'd go for this. To maintain the constant low altitude the "anti-gravity" would actually be a repulsor that pushes on the ground below. If it can be balanced enough to be stable it can also be tilted enough to provide forward thrust. $\endgroup$ – Ville Niemi Mar 9 at 6:47
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Not long a go a paper was released with the concept of an solid state propulsion for aeroplanes.

It works by creating a electical field in the wings, as they contain high-voltage electrodes. Nitrogen in the air gets ionized by the electic field and transports the charge from the front of the wing to the back of it. By that, the Nitrogen can collide with the rest of the air and pushes the air backwards, creating a backwards force and thus propels the aeroplane forward.

Biggest Problem might be to get the voltage to create enough propulsion for bigger ships, but else this is as carbon neutral as it gets.

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