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Sunday, February 13, 2011

The physics of sailing link


From KQED

We have a storied, 500-year history of sailing.
But despite this rich heritage, scientists and boat designers continue to learn more each day about what makes a sail boat move
Contrary to what you might expect, the physics of sailing still present some mysteries to modern sailors.

To make a sailboat move, you need both wind and water, in addition to a sail, of course!
When a sailboat sails downwind, wind blows into and pushes against the boat's sails.
Because the wind is faster than the boat, the air pushing into the sails is decelerated.
As the sails push back against the wind, the wind pushes forward on the sails and the boat moves.

But working with just the sail and the wind in this way, the boat will move slowly and only in the direction the wind carries it.
To really make a sailboat move quickly, a sailor needs to know how to harness aerodynamic and hydrodynamic lift force.
Bernoulli's principle is a scientific principle stating that as the speed of a moving fluid or gas increases (or decreases), the pressure within the fluid decreases (or increases).
It’s the guiding principle behind the physics of lift.

By sailing closer to the wind, a boat will generate more aerodynamic lift.
To move around the sails, the wind will have to change direction.
This creates a change in wind velocity and harnesses lift force.
But instead of being fully forward of the boat, the force is now primarily sideways on the boat. This means that the sailboat will move sideways if left unchecked.

This is where the keel comes into play.
Unlike the keel in other kinds of boats, a sailboat keel is typically very large and uses the forward momentum of the boat to generate hydrodynamic lift and counter the lateral force coming from the air interacting with the sail.
Instead of lying flat or nearly flat against the boat hull, a sailboat keel drops down into the water beneath the boat like a large rudder.
When the boat moves sideways, the keel must push a lot of water sideways.
Like the interaction between the wind and the sail, the water resists the push from the keel to generate lift.
Because of the similar roles the keel and sail play in generating lift, the keel on a sailboat is sometimes regarded as a "second sail."
Between the sail and the keel, a boat generates enough lift to accelerate forward.

In fact, in this way a sailboat can even move faster than the wind!
When moving, a sailboat generates its own wind, often called apparent wind or relative wind.
This is the flow of wind acting upon the sail.
The faster a boat travels, the more of this kind of wind occurs and the more force there is acting upon the sails.
This means greater acceleration, and the boat will travel faster than the true wind speed.
Many modern, extreme design sailboats and larger skiffs are built especially with this purpose in mind -- to sail faster than the wind.

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24 Comments:

At November 1, 2011 at 7:56 PM , Blogger Robertson said...

Hello, Sailing gives examples of physics: Newton's laws, vector subtraction, Archimedes' principle and others. This comment asks from Physclips asks

How can a boat sail upwind?
How can boats sail faster than the wind?
Why are eighteen foot skiffs always sailing upwind?
We introduce the physics of sailing to answer these and some other questions. But first:
A puzzle.

A river runs straight from West to East at 10 knots. A 10 mile race is held: the boats sail downstream, from West to East. The first heat is held in the morning, when there is no wind. The second heat is held in the afternoon, when there is a 10 knot wind from the West. In which heat are the faster times recorded?

(Answer below.)
Answer to the first question is Sailing close to the wind uses the shape of the sails to generate lift. To flow around the sails, the wind has to deviate in direction, as shown by the arrows for initial velocity vi and final velocity vf, which are given with respect to the boat. The change of velocity dv is in the direction shown. The acceleration aa of the air is dv/dt, so the force Fa that sails exert on the air is in the same direction. (Newton's first and second laws: F = ma.) The force Fw that the wind exerts on the sails is in the opposite direction. (There is also a Bernoulli effect, which contributes in a secondary way.)

Sailing downwind (parallel to the wind, like the boat at left) is easy to understand: the wind blows into the sails and pushes against them. The wind is faster than the boat so the air is decelerated by the sails. The sails push backwards against the wind, so the wind pushes forward on the sails. But for a boat with normal sails, the catch is that, downwind, you can only ever sail more slowly than the wind. Which is usually boring, even with a spinnaker.
You know this force: In a strong wind, it is easier to walk, run or bicycle with the wind pushing on your back. Usually, the wind pushes you in the direction it is going.

Sailing directly upwind (exactly anti-parallel to the wind, like the boat at right) is also easy to understand: it's impossible. You just sit there with your sails flapping. This is also usually boring.

But boats can sail at say 40° to the wind and, by tacking (alternate lines on either side of the wind direction) they can go where they like. So let's think about....Get here for more blogging flat mate finders

 
At January 11, 2012 at 4:45 AM , Blogger Shagor said...

This blog is nice and amazing. I love your post! It's also nice to see someone who does a lot of research and has a great knack for ting, which is pretty rare from bloggers these days.
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At January 13, 2012 at 7:00 AM , Blogger Robertson said...

Hi,
Sailing is the propulsion of a vehicle and the control of its movement with large (usually fabric) foils called sails. By changing the rigging, rudder, and sometimes the keel or centre board, a sailor manages the force of the wind on the sails in order to move the vessel relative to its surrounding medium (typically water, but also land and ice) and change its direction and speed. Mastery of the skill requires experience in varying wind and sea conditions, as well as knowledge concerning sailboats themselves and a keen understanding of one's surroundings. The most common form of commercial T-shirt decoration is screen-printing that can be obtained by werbemittel or alike store or market point. In screen-printing, a design is separated into individual colors.

 
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At January 26, 2012 at 3:56 AM , Blogger Unknown said...

Sailing has been instrumental in the development of civilization, affording mankind greater mobility than travel over land, whether for trade, transport or warfare, and the capacity for fishing. Botox is a drug made from a toxin produced by the bacterium Clostridium botulinum.

 
At January 27, 2012 at 6:01 AM , Blogger mkohlmann23 said...

That's right! The sailboat cannot move if there is no wind and water. Better if the wind is not that fast and beware of the sharks! Lol. Just wondering, still the sailboat moves depending on the direction of the wind right? Thanks!
- Mike from waxing company or hair removal company.

 
At January 28, 2012 at 6:06 PM , Blogger Robertson said...

Hi,
As you know that the sailboat depends upon air speed, as a car depends upon petrol. So the direction in which air blows, sail boat moves in the same direction. And it is also a natural fact that it can never move at a speed faster than air, since it has to face friction issues also. To enhance the beauty I have tried many methods, you can also trust in using the services or Botox without any fear and you must consult your practitioner before applying.

 
At January 31, 2012 at 4:27 AM , Blogger Sierramay said...

I see, so there are various forces to be taken into consideration when riding a sailboat. I do need some help comprehending Bernoulli's principle though. I know that it is responsible for creating the lift in aircraft but I don't quite understand why an increase in velocity would create an upward force.

sailing florida

 
At July 12, 2012 at 9:02 AM , Blogger Erica White said...

I'm so impressed with this physics explanation of how sailboats move across the ocean. It's pretty amazing how it floats too and how well they could resist gusts. I work with billy joel yachts company and now I'm glad about how things are.

 
At December 19, 2012 at 10:10 AM , Blogger Jude Crowe said...

This looks great, My neighbor finally got his boathouse when he retired from his office job. He got a team that did furniture removals in Perth to move his oven and book cases. He calls every month to check up on us. I hope to see him on the holidays.

 
At December 27, 2012 at 6:55 AM , Blogger Monty Pennington said...

My uncle went to the Hamptons one summer to splurge his retirement fund. He had help from sydney airport transfer to get there too. He says that sailing there was great but he still got homesick on his third week there.

 
At December 28, 2012 at 7:06 PM , Blogger eagle one said...

Is there any good reference for understanding this in a more deep way? Like That a book for an undergrad -grad in engineering or physics or something like that, with more mathematical tools.

 
At January 8, 2013 at 9:00 AM , Blogger Zane Hovell said...

I don't know anything about navigation. Your site gave me a fair amount of idea on what its all about. However, I would still entrust my voyage with a boat service perth. I don't want to get lost because I wanted to go fishing at sea.

 
At January 11, 2013 at 6:03 AM , Blogger bella bell said...

These would come very handy as I am eager to man my own single person yacht for my upcoming tanzania safaris marine. I feel like a swash-buckling pirate.

 
At January 21, 2013 at 10:31 PM , Blogger Theodore Van said...

Thanks for this information. I would benefit taking down your explanation as I am looking into property for sale in portugal and planning to migrate.

 
At January 30, 2013 at 2:19 AM , Blogger Damian Bathory said...

Thing is, like any alaska cruise ship, most vessels these days have engines to propel them. Using sails is, sadly, a dying art.

 
At January 31, 2013 at 11:16 PM , Blogger Mark Jones said...

Proper rigging installation is the key to maintaining propelling with the wind. Many incidents happen because of lack of rigging training, and improper installation.

-Marchello

 
At February 11, 2013 at 8:41 PM , Blogger Theodore Van said...

With that in mind, maybe it's better to let the sailing enthusiasts practice a toy boat on one of the pool in Tampa Bay as long as to make sure that the fences are already installed.

Theodore Van

 
At March 21, 2013 at 3:32 PM , OpenID carriemcclain said...

It is now understandable why furniture that needs to be shipped from abroad costs much more than those ones available locally, especially during those times that the plane haven't been discovered yet. Everything has to be shipped and transported through ship vessel, quite a long travel plus the uncertainty of the weather makes it more expensive.

Carrie McClain

 
At March 27, 2013 at 6:35 AM , Blogger Cadence Vaughan said...

Seriously, I'd rather go on a road trip and drive all day than go sailing. I have some serious issues with the sea as I get seasick easily.

-Cadence Vaughan

 
At April 15, 2013 at 11:39 AM , Blogger Mischna Ong said...

I love sailing ever since I was a kid. It had been a fascination that I have shared with my dad. I can't wait for the spray hood installation to be done and over with. The sea is calling me.

Best,
Mischna Ong

 
At May 8, 2013 at 11:06 AM , OpenID conxion4332 said...

Impressive video. I always envied people who can sail. The video is not just an ordinary one, it is clear that research is done. Great work.

 
At June 17, 2013 at 9:46 AM , Blogger swetha jain said...

So glad I found this page. Thanks a ton buddy. Keep reading to keep updated, great read here.
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At October 10, 2013 at 10:45 PM , Blogger Anne said...

Awesome video! Sailing can provide adventure, relaxation, recreation, and just good old fun. To enjoy yourself on a sailboat, you need to know the basic sailing maneuvers, sail positions, and rules of the waterway for when you encounter other vessels. And, it pays to know what to bring aboard and what to leave on shore. thanks!
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