"For 15 cm thickness, we need a tube tower 13 meters (43 feet) in diameter.

.... which is admittedly pretty big. But not impossible. "

Are you saying a wall thickness of 15 cm, for a pipe 13 meters in diameter and length of around 600 meters?

A 6" wall thickness of steel is pretty sturdy stuff- without doing the math that's probably well over 10,000 tons of steel- my pipelauncher thingy was about 1/3 of this length about same diameter and using about 1/2" wall thickness and some where less than 1000 tons. Hmm probably closer to 20,000 tons. That would be very strong- though I don't know how you could make it. It would only be conceivable if you built it in the location in something like 10' or 20' sections. It's own weight would pile drive it into the ground - or if you made a concrete foundation it would have to be pretty impressive. The guy wires would need to be fairly massive and require a large anchors.

Hmm, I don't think you need the wall thickness so thick, you could probably do it with about half that thickness or less. And you might forget about steel tubing and use reinforced concrete. You use the towers like you use tower in a suspension bridge- using compressional loads. Use the guy lines mainly to deal with horizontal forces- but they would need to be quite massive. These guy lines would connected directly to your "sail" and held up by the tower.

But this is massive scale which would make "normal" bridge making look pretty easy. And as far as I see, you still don't have a roof. And 100 kph is simply too low of a wind speed to design a structure to withstand.