3 ms·

Sailboats are one I struggled with, especially sailing into the wind. I think my intuition there came from a boat going the same direction as the wind, say, no

by jfoutz 5y ago

Sailboats are one I struggled with, especially sailing into the wind. I think my intuition there came from a boat going the same direction as the wind, say, north at 1 meter/sec. now the boat can't go faster than 1 m/sec, but if you turn 45 degrees to the east you can travel sqrt(2) m/sec, because you're still going north at 1/ms and the boat gets pushed along to the east as well. the closer you get to perpendicular to wind, the faster you can go. I still don't quite get sailing into the wind, but I'd imagine this wheel and propeller system works the same way. The vehicle motion restricts moving to just a line, the wind pushes along the x axis, so the vehicle can sort of zoom along in the y direction, much faster than moving along x. __edit__ oh, I see, it's the difference between ground and wind. a sailboat couldn't do this but something clever with propellers and linkages probably could

mumblemumble 5y ago
It's hard to really explain how boats do it using only text and no good way to draw force diagrams. I blasted through this video pretty quickly, but it appears to explain things well, including how it's possible for a sailboat to exceed wind speed: https://www.youtube.com/watch?v=jJtvGF8vZbE https://www.youtube.com/watch?v=jJtvGF8vZbE The cart adds some moving parts, but I am pretty sure that all they're doing is ensuring that the blades' angle of attack is about the same as it would be for a sailboat's sail when it's sailing into the wind, by turning the propeller at an appropriate speed.
im3w1l 5y ago
I think sailing into the wind works because you divide the force of the wind into components twice. Dividing into components first one way and then another lets you pull some tricks. The first time comes from the sail, you divide the wind into a component flowing along the sail (doing nothing), and one that is perpendicular to it (pushing it) Then you take the perpendicular-to-the-sail part and divide that into a side-ways (drift) and a forwards component (propulsive). The side ways one is neutralized by the keel. Btw I kinda wonder if a rotatable keel tuned just right would allow sailing right into the wind...
jasonwatkinspdx 5y ago
A boat absolutely can sail downwind faster than the wind speed. The crazy cutting edge boats like you see in America's Cup or the giant trimarans that vie for various world records do this somewhat routinely. The key to understanding this is: sails are devices that work like wings, via lift, not via drag (exception, spinnakers and such). A sailboat has two wings: the sail, and the keel (or centerboard). The boat is extracting energy via how the lift vector of the sail projects onto the vector the keel keeps the boat tracking along. As you sail downwind the apparent direction of the wind begins to rotate forward in response to the boat's forward momentum. With extreme performance boats, this process can continue to the point where the boat is experiencing a forward apparent wind while sailing downwind. This is all pretty counter intuitive but valid physics. Speaking for myself, the most easy way to get an intuition for it all is to rent a windsurfer for an afternoon. It'll all make sense after that. People have tried sailboats with propeller rigs like this cart, but they end up being more trouble than they're worth. The current best anyone has figured out is solid wingsails, which have anywhere from 3x to 7x the lift drag ratio of traditional cloth sails. Hence their use in the crazy billionaire bragging contest races.
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by jfoutz 5y ago

mumblemumble 5y ago

It's hard to really explain how boats do it using only text and no good way to draw force diagrams. I blasted through this video pretty quickly, but it appears to explain things well, including how it's possible for a sailboat to exceed wind speed: https://www.youtube.com/watch?v=jJtvGF8vZbE https://www.youtube.com/watch?v=jJtvGF8vZbE The cart adds some moving parts, but I am pretty sure that all they're doing is ensuring that the blades' angle of attack is about the same as it would be for a sailboat's sail when it's sailing into the wind, by turning the propeller at an appropriate speed.

im3w1l 5y ago

I think sailing into the wind works because you divide the force of the wind into components twice. Dividing into components first one way and then another lets you pull some tricks. The first time comes from the sail, you divide the wind into a component flowing along the sail (doing nothing), and one that is perpendicular to it (pushing it) Then you take the perpendicular-to-the-sail part and divide that into a side-ways (drift) and a forwards component (propulsive). The side ways one is neutralized by the keel. Btw I kinda wonder if a rotatable keel tuned just right would allow sailing right into the wind...

jasonwatkinspdx 5y ago

A boat absolutely can sail downwind faster than the wind speed. The crazy cutting edge boats like you see in America's Cup or the giant trimarans that vie for various world records do this somewhat routinely. The key to understanding this is: sails are devices that work like wings, via lift, not via drag (exception, spinnakers and such). A sailboat has two wings: the sail, and the keel (or centerboard). The boat is extracting energy via how the lift vector of the sail projects onto the vector the keel keeps the boat tracking along. As you sail downwind the apparent direction of the wind begins to rotate forward in response to the boat's forward momentum. With extreme performance boats, this process can continue to the point where the boat is experiencing a forward apparent wind while sailing downwind. This is all pretty counter intuitive but valid physics. Speaking for myself, the most easy way to get an intuition for it all is to rent a windsurfer for an afternoon. It'll all make sense after that. People have tried sailboats with propeller rigs like this cart, but they end up being more trouble than they're worth. The current best anyone has figured out is solid wingsails, which have anywhere from 3x to 7x the lift drag ratio of traditional cloth sails. Hence their use in the crazy billionaire bragging contest races.

deleted 5y ago
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