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Comment by krschultz | Hacker News Reader

A little improvement in the graphics would make this much more intuitive.

#1) Which is the front end of the boat? Not that hard to have a pointed end and a not pointy end.

#2) The sail shouldn't go all the way around, nor rotate around its center. If you are approximating something like a Laser it can go through about a 210 degree arc. Other boats, more like 120-160 degrees.

#3) So the wind is coming from the left? Note that almost all things generally show the fluids as streamlines, so at first glance it seemed to me that the wind was coming from the top and the boat was moving to the left.

#4) It's missing the force vector from the boat in the water. That's what really explains what is going on. The force vectors don't add up to the motion shown.

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Great suggestions.

>It's missing the force vector from the boat in the water. That's what really explains what is going on. The force vectors don't add up to the motion shown.

Yes. You see, I couldn't really isolate that vector. What I am doing is taking the force vector on the sail and taking the projection of it in the direction that the boat is facing. Actually, the forward force comes from the boat trying to move in the direction of the force of the sail. But ends up moving a bit forward because the keel is in a slanted direction to that movement. So even if I show that vector, the viewer probably wont still have any idea regarding where it came from.

Aren't you forgetting about the keel/center board? The keel only produces lift when in motion, thus it is very important, as you can only start sailing upwind once you have a bit of speed. It's not the projection of sail force in direction of the boat that causes forward motion, but the addition of sail and keel forces.

This is a neat idea, but it's super confusing without the concept of the sail's curvature.

AFAIK, sails work like airplane wings (not like socks), creating lift on the convex surface by virtue of faster moving air and lower pressure. Perhaps your model assumes the same, but it's not clear from the way the sail is drawn, and it looks like sailing down wind is just as fast as sailing somewhat across the wind, which is in fact the fastest way to sail.

Airplane wings don't work like that; it's a common myth and does not really make sense. Wings generate lift by a rather straightforward utilization of Newton's third law of motion: they deflect air downward. Sails work the same way: they deflect air sideways.

http://www.cam.ac.uk/research/news/how-wings-really-work

Video that (i think) explains it better: https://www.youtube.com/watch?v=aFO4PBolwFg

This doesn't explain what is going on very well. A sail generates lift and drag (the power is perpendicular to the sail angle). The keel generates an opposing force that converts the lift into upwind travel.

The only way I know how to explain a keel, is like ice skating. you plant one foot, and kick the other foot to the side and back, but the result is forward motion.

Ice skating is impossible with just one leg, and sailing upwind is impossible without both a keel and a sail.

> Ice skating is impossible with just one leg ...

Not true, it's just a little more difficult because it requires better balance. You can fairly easily do a swizzle on one foot shifting from your inside edge to your outside edge, similar to how a boat would tack. Yes, the blade of skate is acting similar to a keel, but no, you don't need two legs to feel the effect.

I vote for the following UI improvement: a clear indication of where the mast is---and sail pivoting around it. Bonus points for curved sail graphic ;)

I have made the boat pointed and an indication for the point around which the sail pivots..)

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