Rotor ships, aka Flettner ships

The "wind" caused by the ship's forward movement is not useful (it would be perpetual motion if it were).

The motion of a ship shifts the apparent wind forward (it is a vector addition of the wind's velocity vector and the ship's velocity vector). The faster a ship goes, the less useful the real wind will be since the apparent wind shifts forward, making the "sails" less efficient. The Wikipedia article had an anecdote that the Flettner rotors worked to 20-30 degrees from dead ahead. That is pretty good for a sailing ship of 1924 vintage, but still leaves large angles where the wind is not useful. It also didn't say how efficient the rotor was at 20 degrees apparent wind. :-O

Obviously, a Flettner rotor sailing ship is vulnerable to all the problems of a traditional sailing vessel: wind is unreliable (except in the trades) and often comes from a direction that is not useful (astern, forward). Note that the trade winds are east/west and a ship generally wants to go east or west to cross the ocean. Square rigger ships like to sail downwind. Flettner rotors, not at all.

It seems that in the e-ship they capture the heat from the ordinary ship engines (which drive the propeller) and then use that heat to drive the magnus effect cylinders. So it seems they get the savings from capturing waste heat, mostly.

But who knows if you would be able to get the same or more savings if you capture the heat and just use it to drive another propeller.

Huh, that does get around that problem. Interesting!