Have you ever wondered how a sailboat sails upwind?
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Sailboats racing upwind. Credit: Public domain |
The sun is out, the wind is blowing, and I've been busy taking some sailing lessons. Turns out there's an interesting bit of physics that allows sailboats to not only travel downwind, being pushed by the wind, but also to travel upwind, or nearly so.
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An 18th-century square-rigged ship sailing downwind. Credit: Public domain |
This is the most basic point of sail, and was often used by ancient Egyptian, Greek, and Roman sailors. When they needed additional speed or wanted to travel upwind, they rowed.
The large square-rigged boats popular in the 18th and 19th centuries (the classic pirate ship, for instance) were also most effective on a downwind sail.
Modern sailboats can sail in any direction that is greater than about 45 degrees with respect to the wind. They can't sail exactly upwind but with a clever boat design, a well-positioned sail, and the patience to zig-zag back and forth, sailors can travel anywhere.
To explore this, let's draw a diagram that labels all the forces on the sailboat. If you haven't seen a force diagram before, not to worry—it's just a few arrows and triangles. By adding the forces together we will get the total force on the boat and thus the direction in which it will move.
Here's a basic sailboat. The two parts we will focus on are the sail above the boat and the keel below the boat. The keel keeps the boat from tipping over and, as we shall see, plays a crucial role in moving the boat forward.
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A basic sailboat. Diagram credit: Tamela Maciel |
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Looking down on a sailboat, showing the equal and opposite forces on the wind and sail. Credit: Tamela Maciel |
The wind fills the sail into the shape of a wing, but because the sail is held fast at both ends, the wind can't push it out of the way. Instead the wind must change direction to flow parallel to the sail. The taut sail has created a force on the wind that causes it to change direction and Newton's third law tells us that there is an equal and opposite force on the sail by the wind, as shown by the red arrows in the diagram above.
If this was the only force acting on the boat, then we would be in trouble: the boat would move forward but also to the right. But sailboats have a secret weapon hidden below decks: the keel.
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Keel and rudder below a sailboat. Credit: Paul Schultz |
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Forces acting on a sailboat cancel each other such that the total force moves the sailboat forward. The downward pointing keel is outlined by the dashed rectangle. Credit: Tamela Maciel |
Some sailboats can even move faster than the wind itself. When sailing upwind, the relative speed of the wind on the sails is greater than the actual speed of the wind and this relative wind creates a larger force on the sails that can push sailboats faster than the actual wind speed.
There is a limit to how fast sailboats can move forward, of course. I have ignored boat drag in this example, but the boat also has an inherent friction as it moves forward through the water. The boat will accelerate until the force pushing the boat forward is balanced by the drag force pulling the boat back, and then the boat will travel at a constant speed.
Further reading:
This sailing simulator by the National Geographic allows you to adjust the sails and rudder to find the fastest point of sailThe Physics of Sailing by the University of New South Wales
How Sailboats Sail Against the Wind by Physics for Architects
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By Tamela Maciel, also known as "pendulum"
Sailing without an engine is really admirable. In the past sailing for months to reach a shore must have been really difficult task !
ReplyDeleteDim
www.cosmos-yachting.de
test
ReplyDeleteWhat is even more counter-intuitive is sailing directly downwind. faster than the wind:
ReplyDeletehttp://en.wikipedia.org/wiki/Sailing_faster_than_the_wind#Sailing_dead_downwind_faster_than_the_wind
Thank you.Now i might understand pirates of caribbean a little more :p but seriously thank you. Got a good basic idea from this article.
ReplyDeleteIm sooooo tried rn and this is not easy to read so i think that you should make it simpler, maybe with bullets.
ReplyDeleteHa Ha
DeleteYou can't fix stupid
DeleteHA HA HA !!!!!!
DeleteMaybe more pictures so it's better to understand.
ReplyDeleteAgreed
DeleteVery thorough (and understandable!) explanation of upwind sailing. My several sailing friends were starting to grow tired of me asking how it works, but this lays it out perfectly. Even they are impressed by the article. Thanks for sharing the knowledge in a relatable way.
ReplyDeleteThank you very much, I knew this already but this is a very clear explanation that I can use as a guide for my friends who need this explained.
ReplyDeleteDoes the position of the mast relative to the center of the boat (it is usually in the front) matter since all of the force from the sail is transmitted to the point of the mast?
ReplyDeleteHey, amazing read. This article really helps to shed light on the mechanics of sailing through the deep sea. I commend Columbus and the crew for being able to navigate the water. We are so spoiled these days with our engines to guide us. :)
ReplyDeleteI've never sailed in my life and tbh I'm here to learn the physics of sailing for use in a video game. However, I found this info concise and clear. Thankyou.
ReplyDeleteThank you for this article
ReplyDeletegoooood
ReplyDeletehmmm good article !!!!!!
ReplyDeleteI am confuzzled
ReplyDeletesame here brother
Deleteyou so good that my teacher is using this for school.
ReplyDeleteyes
ReplyDelete