[jessriedel]

> Rotor ships take advantage of this same effect by spinning a large vertical cylinder, typically using an electric motor, and using the resulting force for propulsion.

I always thought sails were partially defined by being passive. I wonder why these aren't called "rotor fans" if they're electrically driven?

Is there anywhere it's discussed when rotors are expected to outperform normal fans? They seem to be used only on large ships, but I can't see a first-principles explanation for that.

[kitd]

"Fan" implies the propulsion is in response to air being forced backwards.

These rotors create lift in a forwards direction from the wind blowing across them, a bit like having a fixed wing pointing up.

[kenneth]

Regular sails actually work similarly to a wing, in that pressure differentials on either side create a force we call lift on a plane, rather than simply being "pushed from the back" by directly catching the wind. That's why a sailboat can go in almost any direction regardless of which direction the wind is blowing.

[lostlogin]

Watching bleeding edge yachts sail is always interesting. Hydrofoils achieve speeds greater than 3x wind speed though their stability doing it is a bit alarming.

https://www.odt.co.nz/sport/yachting/how-fast-can-americas-c...

[usrusr]

The imaginary future where huge cargo ships foil under city-sized kites is more impressive than anything I've seen with spaceships.

[jessriedel]

Edit: sorry, nevermind, I was confused as to what you were claiming.

[kdmytro]

>> A point.

> No. Literally the same point, but in different words and with a feel of authority.

You should consider using words other than "No" if you agree with the point.

[jessriedel]

I misinterpreted his original point, which I intended to disagree with.

[jessaustin]

Reading the parent carefully, I find nothing that contradicts what you have written in response. Pressure is just force over area.

[jessriedel]

Thanks, upon re-reading I agree. I misinterpreted the "pushed from the back" statement.

[samstave]

I'm really interested in how dimpling would affect the drag/lift properties of these.

Years ago on /. I saw a post about how wales bumps on the leading edges of their fins/flippers were to increase/modify the eddies around their fins in such a way as to provide them more thrust (or something, it's been years since I read the article) - and I was curious what it would look like to have helicopter blades with leading edge bumps on them similar to wales...

So, I'd like to see what dimples both convex and concave would have on the surfaces if these - like a golf ball.

Also, if you have never done it, take a cylindrical plastic cup and, in a bathtub full of water spin the cup as hard as you can on the surface of the water - like a top - and see how long you can get it to stand up. Kids like this trick.

I always thought it would be interesting to have a barge like flat boat on top of several cylindrical feet like floats, which can spin rapidly along the Z access and use it as thrust.

[antoncohen]

Mythbusters tested golf ball like dimples on a car, and apparently is help fuel economy.

https://www.autoblog.com/2009/10/22/mythbusters-golf-ball-li...

[mirimir]

Yeah, but they are motor driven. So how much of the propulsion comes from the motors, and how much from the wind? Given the relatively small reduction in fuel cost, I'm guessing that most comes from the motors.

[russdill]

But you want to make sure you aren't just creating drag you have to fight against. So even with the rotors, the wind direction still matters.

[sandworm101]

Which is why these things can be stopped and spun the other way.

[agumonkey]

vortex wings ?