Monday, February 13, 2006

Why Do Sails Work?

Why do sails work?

In particular why does the air flowing around a sail generate lift?

The usual answer given to this question is that a sail is like an airplane wing and that it generates lift for much the same reason. Because of the curvature of the upper surface of the wing, the air passing over that side has to travel a greater distance than that passing under the wing. Since it has to go farther, it has to
go faster in order to reach the trailing edge at the same time as the air flowing past the underside of the wing. Because of the Bernoulli effect the faster flowing air on the upper surface has a lower pressure than the slower moving air on the underside of the wing; and the pressure difference generates the lift.

Does this makes sense? Is it the correct explanation? If not, why not?

10 comments:

Anonymous said...

That is really only true when sailing on reaches and close-hauled. When sailing on a run, the sails act as kites, not airfoils, and the primary consideration is surface area presented. Maximum speed on a dead run is limited to wind speed.

Many boats, especially multihulls, will make more speed over ground towards their goal if sailing on broad reaches and essentially "tacking" down wind, rather than sailing on a dead run, as their speed on the reach is high enough to more than compensate for the extra distance travelled.

A good page for the physics of sailing is located here.

Litoralis said...

During the summer I worked with Young America and Professor Milgram at MIT designing sails, I learned alot about the fluid dynamics of sailboats. Here is my attempt at an explanation.

A sailboat on a reach and especially upwind generates lift more like a paper airplane than a conventional airplane. What I mean is that most of the lift is generated as a result of the angle of attack between the sail and the wind (and the foils and the water). A powered airplane wing operates at a much lower angle of attack than a sailboat sail. The direction in which the lift force acts on an airplane wing is essentially perpendicular to the wing (straight up - perpendicular to the chord line). The increased angle of attack of a sail allows the lift force to provide a component that drives the boat forward.

Of course, on a beat, a large component of the lift force generated by the sail is pushing the boat sideways. The foils balance this force and are also operating at an angle of attack in the water.

On a reach, the angle of attack between the sail and the wind is similar to the angle of attack on a beat. However the angle between the wind and the heading of the boat is now much greater because the sail is eased out. Now the component of the lift force that is driving the boat forward is larger than the component acting sideways and that's why reaches are faster than beats.

Diagrams would be useful here but I can't use HTML in comments, so I'll have to post a quick tutorial in my blog at some point.

Anonymous said...

"Since [the air on the upper side of the wing] has to go farther, it has to go faster in order to reach the trailing edge at the same time as the air flowing past the underside of the wing."

The problem with this part of the explanation is: why does the air layer on the top of the wing have to reach the trailing edge at the same time as the air layer on the underside, anyway? They don't have an appointment. ;-)

The explanation for the speeding-up of the air on the upper side of the wing must lie elsewhere. I think it speeds up because it's forced into a narrower channel or cross-section. That is, it's confined to move between the curved wing (beneath it) and an unperturbed, flat layer of air (above it). This space gets rapidly narrower at the front edge of the wing, which rises dramatically (compared to the gentler downwards slope that follows). This forces the air at the front of the wing to either speed up or compress in order to get through the narrower space. It speeds up rather than compressing for reasons that someone who understands physics could explain :-)

Anonymous said...

I've recently done an RYA Instructors course, as part of it I did a presentation on Basic aerodynamic theory. I basically did the same explanation as mentioned at the beginning, for ease of comprehension.

However, I am on an engineering degree so take an interest in things like this, and I know that a sail does not act like a wing on its side.. simply put.. draw a wing cross section.. and draw a sail from above.. where is the so called longer distance that the air has to flow around on the sail diagram.. its not there

I havent yet bothered to work out the details, yet I think it is to do with deflecting the wind, in a laminar flow pattern, when going upwind. This links a bit with Litoralis' comment about angle of attacks slightly, Though then of course when heading downwind the sail acts more like a kite and is just pushed.

Unknown said...

I'm with "litoralis". I know more about aeroplane wings than sails but, yep, it's all about angle of attack. The shape of the sail is a red herring, a coincidence of its construction (fabric on a frame). I suspect, though have no evidence, that a flat sail would work just as well, but it's not possible to create that in fabric. Has anyone designed/built more-or-less-solid sails, does anyone know?

Anonymous said...

I used to believe the travels further in the same time explanation. Sitting in a dinghy as an instructor - it is clear that it is not significantly further around the outside of the sail compared with the inside.
I believe the best explanation is based on circular motion. The air has to follow a curved path, on the inside it is pushed towards the sail by centrifugal forces ( you can convert to centipetal if you prefer). On the outside the centrifugal forces throw the air away from the sail. This causes the high pressure on the windward side and lower on the leeward side. The pressure differences then give the drive. (Force = pressure * area).
Air as it approaches the low pressure area is "sucked in " and speeds up - this is a consequence of the low pressure - not the cause. Obviously the high pressure slows the air down on the other side of the sail. Tension in the outhaul, angle of attack - you are just changeing the radius of the circle (chord) the air is moving in. The tighter the circle the greater the centrifugal force.

Anonymous said...

I have heard that on the leeward side of the sail is low pressure, and the windward side is high pressure. When those two meet just aft of the sail, it creates thrust. how do a low and high pressure system provide thrust. (regardless of keel etc.)

Anonymous said...

or are the keel/ centerboard etc. the key? but don't they work on the same principle?

Anonymous said...

I have just been doing some research into this field, and the answer is that no one really knows why sails work. I am also a dinghy instructor, and agree with the comment that it is clear that the distance around each edge of the sail is not significantly different.

The most concise explaination that I can give is this:
as the wind reaches the mast or the edge of the sail, half of it goes to windward, and half to leeward. The air that goes to windward slows down and as a result, the pressure increases (see Bernoulli's principles). On the leeward side, the opposite happens and the air speeds up. The pressure decreases and a vacuum is essentially created with the pressure gradient between one side of the sail and the other. This vacuum 'sucks' the sail towards it and the boat moves forward and to leeward. This is the 'lift' that is so commonly mentioned. The leeward movement is counteracted by the keel and the overall movement is mostly forward.

A commonly held misconception is that the air on the leeward side of the sail speeds up because it has furthur to travel and has to meet up with the relevant particles that it was with when it first met the sail. This is not true - the particles make no effort at all to meet eachother again and the leeward particles remain ahead until some other external force stops them from doing so. A good demonstration of this can be found at http://www.av8n.com/how/htm/airfoils.html.

No one is really sure why the leeward air speeds up, but part of it is to do with the unaffected air above it pulling it along and preventing the friction from slowing the air down. Boundary and Laminar flow are also to do with it.

Richard Preston said...

I know this thread is fairly ancient, but I was puzzled by Penguin's "no-one really understands why the leeward air speeds up", because quite clearly people do.

As it passes through the air, the sail is an obstacle. Air does not instantaneously move out of the way when presented with an obstacle, so air on the leeward side of the sail is effectively "squeezed" and therefore speeds up as it passes over the sail.

Anyway, there's a good article here that explains it much better than I can

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