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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. These transports fly close to the earths surface, and are powered by sails made of 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 run as cleanly as possible. (As little emissions as possible)

• 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.

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?

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.

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 to fly the gravity engines must exert some force in the opposite direction. So in order to move forward just add another engine that is 90° rotated.