How Does A Sailboat Actually Work?

[Applause] So my question to you is, uh, uh, let's say the wind is coming from over there. I want you to position the boat in whatever direction you think will make it go the fastest. How would you set it up? You can set the sail how you want, something like that.

Take this way. You want to point the boat this way. What would you want to do? So how do most people think a sailboat works? What would you say most people think? A sailboat gets some wind and manages to move; they don't really know. Or maybe that they like get pushed from behind.

Yeah, the pushing from behind is somewhat easy to visualize. Your fastest direction would be straight forward; we'll be going with the wind. Yeah, with the wind. I want you to talk to me about what the wind is doing to the boat here to make it go forwards.

Well, with the wind behind, it'll push into the sail. It pushes against the sail, and it pushes the sailboat forward. So the slowest way to position your boat is... well, the slowest is straight into the wind because you're not generating any lift. The second slowest, like the slowest actually sailing, it would probably be close to dead away because you're very limited.

So the wind's coming from the back, and you just got your sails full out? Yeah, because when you're going directly away from the wind, your boat wind cancels out the actual wind. So you can actually never exceed it; you would only be able to match. You would have nothing, no force in your sails. But that would only be wind, but that would be frictionless too, which couldn't happen.

Could you sail upwind? Probably with difficulty, very slowly. Uh, that's doing this little zigzag. How does it go forwards if the wind is coming from here? How do you get it? You'd have to bring it right round like that to bring it to the side and then flick it right back around to bring it in against the wind as you go.

But that's how I've seen the boats go, so I'm trying to figure out what could make the boat go forwards if the wind's coming from that side. You know what I mean? An engine. Which way is the force on these sails? What do you think?

Uh, I think it tends to be a little, little, little back and sidewards. So then the force is back and sidewards? Yeah. How does the boat go forwards? Yeah, it's okay, but, well, again, I know, I know it does, but again I don't have all the...

So how does a sailboat move? The sails actually act like wings, generating high and low pressure on each side. You generate high pressure on the inside of your sail and low pressure on the outside. So the air is going faster around the outside, right? It has a longer way to travel in a sense, right?

Okay, and then, um, you generate a bit of lift. So the lift will actually end up being in kind of this direction towards me, which is mostly sideways and a little bit, mostly sideways and a little bit forwards. But that's why we need to have the centerboard and rudder, and they do... they kind of counteract that force because they generate high and low pressure as well as the boat's trying to be pushed sideways and kind of balances it out so the boat goes forwards.

Okay, but it seems like... I mean, if there's a force pushing that way on the sail and the opposite way on the centerboard, it seems like the whole thing would tend to... it would absolutely try and turn over, which is why the body weight and the leverage of the people is so important.

How can a sailboat go faster than the wind? Because it can generate... it can generate more pressure by its motion. Okay, right, like it's... there's only... it can actually happen quite easily in light wind because, let's say you only have three knots going this way, and then you generate a knot of motion.

Well, all of a sudden, you're sailing with more than three knots of breeze over your sails, and then it can build and compound on itself. The limiting factor is the drag on the hull. I see, okay, yeah, yeah, so, and the drag on the sails actually, that's actually very large as well.

So in fact, the fastest way to sail a boat is not downwind; it's actually kind of, uh, like 90° or 45 degrees to the boat. Something like that, that makes the boat go faster.

Okay. Thank you very much. You're very welcome. I hope you enjoyed this video on the physics of sailing. If you want to see more with Hunter, click the first link. If you want to see more about how a wing generates lift, click the second link.

And if you haven't subscribed to Veritasium yet, then click the third link, and I'll see you next. [Music] [Applause] Time. [Music]