How Does a Sailboat Work?

[Ex-Calif]

Maybe this is why there aren't a lot of 3 year olds sailing.

The laws of physics don't apply until you understand them.

Here's how I explained it to my then 6 year old.

See that yarn attached to the shroud? That tells you where the wind is coming from.

Anytime the wind is behind the middle of the boat the wind is pushing the sails.

Anytime the wind is in front of the middle of the boat the wind is pulling the the sails.

How does that happen?

Take a piece of string and stretch it from the mast to the end of the sail on the "curved in side" of the sail. You can stretch the string straight to the back right?

Now take the string and stretch it on the "curved out" side. The string isn't long enough or you need more string to reach the end of the sail.

Now the tricky part - Imagine wind molecules are buddies that have to stay together. When the wind comes along and one buddy goes down the front and one goes down the back, the wind going around the curved out side has to go a lot faster than the wind on the curved in side to meet at the back end. Because the wind on the front is going faster there is less pressure.

The faster the wind the less the pressure and the stronger the pull on the sails.

Why doesn't the pull on the sails make the boat go sideways? The keel stops the boat from going sideways so instead it goes forward.

[Dick Pluta]

I sympathize. 45 years ago I had the same problem. I bought a sailboat in the late fall and spent the entire winter reading every book I could find. I read about center of effort, sail flattening and mast bending. I still could't make the damned thing go. I then went to the local library and got my hands on the Cub Scout sailing merit badge manual. The best treatise on basic sailing I ever saw. Get it. Read it. Go sailing.

Dick Pluta
AEGEA

[anjou]

Aerodynamics are the same, whether you sail or fly

In this case, the sail is an aerofoil section.

For simplicity...........As air travels over an eliptical section with an asymetrical aspect, the air going over the side with the greater aspect ratio is first compressed and the air passing the other side looses pressure.
This differential of high and low pressure creates lift or suction, and its this which provides the propulsion.

Next time it snows, look for drifting behind obstructions like fence posts, or on a beach, look for the sand bars behind rocks as water has similar properties.

This flow of air or water is subject to varying resistance which is calculated by a formulae known as Reynolds numbers, and its these which are used to calculate the design of airfoil sections for various purposes like economy, stability, aerobatics, speed etc.

[crosscr]

Originally Posted by Bash

the wind wants to push the boat sideways. the boat provides lateral resistance to that sideways force in the form of a keel (or centerboard/dagger board/whatever). the lateral resistance transforms the sideways push into forward thrust because it's so much easier to push a boat forward than sideward.

a simple experiment will illustrate the point, one that doesn't require the sails to be up. Go out on a windless day, and lock the rudder of the sailboat. Now, take the tender (this experiment works best with an inflatable) and line it up abeam to the boats center of lateral resistance. Now using the tender's outboard, attempt to push the sailboat sideways. Note that how the sailboat quickly transitions from moving sideways to moving forward.

This is the correct answer, I believe. Those explanations that involve comparisons to wings and lift are wrong when applied to sailboats. If those were true then you should be able to sail straight into the wind.

[crosscr]

Originally Posted by Knowazark

OK, I've got a simpler expanation and its one my dad used to use to explain to people how a yacht sails forward. It may not be technically correct and certainly isn't detailed, but it generally gives folks an inkling.

The wind exerts force on the yachts sails in one direction. The water exerts an opposite force on the keel (or centreboard) of the yacht. The direction of movement that results from these two forces depends on the balance of them relative to each other.

A practical example: Take an apple seed (looks a little like a yacht hull). Get the seed wet (to make it slippery) and pinch it between thumb and forefinger. This represents the opposing forces of wind and water. The apple seed will shoot forward as a result of the two forces. The greater the forces, the further the apple seed will shoot.

Even if folks still don't understand you can watch them spit on apple seeds and fire them at each other.

That's the same explanation I gave my wife a few years ago, except I used a grape

[Randy]

I think the more this question is in ones mind indicates the more he/she maybe ought to rethink what boat he's sailing on - or says he's sailing on.

[qwer5r]

Originally Posted by crosscr

This is the correct answer, I believe. Those explanations that involve comparisons to wings and lift are wrong when applied to sailboats. If those were true then you should be able to sail straight into the wind.

While I'm no aeronautical engineer, I don't think this analogy is quite right, crosscr. After all, a plane uses its wing for lift, but relies on a propeller or a jet engine for thrust. But a sailboat, while underway but not under power, uses its "wing" for propulsion.

In other words, if you wanted to rely on something other than the "wing" for propulsion the way a plane does (like, for example, a prop attached to a diesel engine), then you can sail straight into the wind.

The bottom line, it seems to me, is that the explanations involving comparisons to wings and and lift are in fact correct.

[Randy]

When sailing to windward the laws of lift hold sway as Bash has pointed out. When sailing downwind the wind is merely pushing. We could get in to a physics discussion at which point aerodynamic lift changes to simply pushing (there must be a scientific term) but that would take someone like GordMay to figure out.

[Sandero]

On a beam reach you can either be sailing up wind or down wind or even both if one sail is trimmed to lift and the other is trimmed to "catch". hahahaha

[GordMay]

Originally Posted by Randy

When sailing to windward the laws of lift hold sway as Bash has pointed out. When sailing downwind the wind is merely pushing. We could get in to a physics discussion at which point aerodynamic lift changes to simply pushing (there must be a scientific term)

Sailing is about the vectorial addition of forces - driving, sideways (leeway), and heeling.

In downwind sailing (running) we add the positive force (push) of wind-speed to the the negative resistance force (drag) to obtain boat-speed (net).

If the “driving force” (wind) is 10 knots, and the boat makes 6 knots in the same direction, then the crew feels a wind of 4 knots coming over the stern of the boat. The true wind equals the speed of the boat plus the relative wind. As the boat speed approaches the wind speed, the relative wind drops towards zero and so there is no force on the sail. So you can't go faster than the wind.

The physics of sailing ~ by Bryon D. Anderson
➥ Cookies Required

[GordMay]

As an interesting aside ...

Solar Sailing Breaks Laws Of Physics
➥ Solar Sailing Breaks Laws Of Physics

Update (April 2009): Cosmos 1 never accomplished its stated goals, but this does not spell the end of The Planetary Society's involvement in solar sailing. The Society, with Cosmos Studios, is currently exploring the possibility of launching another spacecraft in the near future.
➥ Solar Sailing - What We Do | The Planetary Society

[crosscr]

Originally Posted by qwer5r

While I'm no aeronautical engineer, I don't think this analogy is quite right, crosscr. After all, a plane uses its wing for lift, but relies on a propeller or a jet engine for thrust. But a sailboat, while underway but not under power, uses its "wing" for propulsion.

In other words, if you wanted to rely on something other than the "wing" for propulsion the way a plane does (like, for example, a prop attached to a diesel engine), then you can sail straight into the wind.

The bottom line, it seems to me, is that the explanations involving comparisons to wings and and lift are in fact correct.

If you look at the geometry involved I don't think that they are correct explanations. When sailing close to the wind the sail is sheeted in tight and since any pull or lift created by the sail would be perpindicular to the boom( if we are assuming the wing analogy) then the force created would be a lateral pull, not inline with the hull. If we have a lateral force on the the we go back to the explanation given by bash in post #10. To make use of any "pull' by a sail(wing) then the boom would have to be let all the way out. Seems logical to me. Also, to me if the engine is on then your are not sailing

[KairosKona]

Three year old answer:

Da wind blows, da boat moves!

David

[Dick Pluta]

First, the sail is a wing and the boat is "lifted" to windward. No propellor is needed because, unlike an airplane, the air (wind) moves past the boat. The airplane needs to move through the air and must be pushed. BTW, airplanes take off into the wind to get a little extra boost from the motion of the air. In high winds, aircraft tied down to the ground can fly without moving. Look it up.

Second, the reason you can't sail directly into the wind is that the sail (wing, airfoil) must have an "angle of attack". It can't point right into the wind. If it points directly into the wind it flaps like a flag (luffs). The sail must be presented to the wind at an angle to work. This is why your boat will only point at some angle to the wind. The wing stops working.

Now, that's just the sail or the "motor" of the boat. You have a similar effect with the keel. The keel does more than just preventing the boat from sliding sideways. Just like the sail, it "lifts" the boat to windward.

In principle it's easy. Unfortunately you have to throw in heeling, current, sideslip and a whole lot of other things. That's what makes mokes like me cruisers and guys like Paul Cayard racers.

A long time ago I bought a banjo. I also bought Pete Seegers book, "How to play the 5 string banjo". On the back cover there is a story. It says "I once asked an old time picker how to find the notes on a banjo. He said, Hell, son! There ain't no notes on a banjo. You just play the darned thing". There ain't no wings on a sailboat. You just sail the darned thing.

Dick Pluta
AEGEA

[Herbseesmoore]

Its magic, sorry can't be explained. Just go with it. I've heard it all and believe none of it. Pinch a marble and it squirts out from your fingers. Just finished reading Tom Whidden's book on the art and science of sails, he states the wing explanation does not fly.

Anyway, when sailing it feels like magic.