How do sails work?

[Thistle]

I think I understand the theory of how sails on a conventional fore and aft rig work, even if I find it hard to believe that it's the pressure difference across a fraction of a millimetre of cloth that does the business upwind. But what about the sails on foiling craft that manage to go 2/3/4 times the windspeed both up and downwind?

 

[RJJ]

I think I understand the theory of how sails on a conventional fore and aft rig work, even if I find it hard to believe that it's the pressure difference across a fraction of a millimetre of cloth that does the business upwind. But what about the sails on foiling craft that manage to go 2/3/4 times the windspeed both up and downwind?

The same principle, different parameters. The angle of attack is much narrower - even a 49er is nearly close-hauled on all points of sail, an 18-footer more so, and presumably an AC75 most of all. So the sails have to be flatter and closer-sheeted.

At the speeds the sails face (for your 2/3/4 times the windspeed, read 3-6 times the apparent wind, there's no need for low-end grunt, and given foiling there's little need for "surplus power" to shove you through chop...so optimising the sails for top speed is very much about reducing drag. Now I am guessing: one of the design challenges must be about what the rig needs to do in marginal foiling - how to get your slippery, streamlined rig into acceleration mode without trashing its upper envelope.

 

[Thistle]

The same principle, different parameters. The angle of attack is much narrower - even a 49er is nearly close-hauled on all points of sail, an 18-footer more so, and presumably an AC75 most of all. So the sails have to be flatter and closer-sheeted.

At the speeds the sails face (for your 2/3/4 times the windspeed, read 3-6 times the apparent wind, there's no need for low-end grunt, and given foiling there's little need for "surplus power" to shove you through chop...so optimising the sails for top speed is very much about reducing drag. Now I am guessing: one of the design challenges must be about what the rig needs to do in marginal foiling - how to get your slippery, streamlined rig into acceleration mode without trashing its upper envelope.

Perhaps I need Sails 101.

 

[Euphonyx]

I think I understand the theory of how sails on a conventional fore and aft rig work, even if I find it hard to believe that it's the pressure difference across a fraction of a millimetre of cloth that does the business upwind. But what about the sails on foiling craft that manage to go 2/3/4 times the windspeed both up and downwind?

Apparent wind

 

[Thistle]

Apparent wind

I appreciate that the apparent wind will move forward as the speed increases but I still don't understand the forces involved which allow a boat to travel downwind on a broad reach at speeds considerably higher than the wind speed (thus bringing the apparent wind forward.)

 

[Buck Turgidson]

Lift =Cl 1/2 rho V^2 S

 

[Euphonyx]

I appreciate that the apparent wind will move forward as the speed increases but I still don't understand the forces involved which allow a boat to travel downwind on a broad reach at speeds considerably higher than the wind speed (thus bringing the apparent wind forward.)

How Do These Boats Sail Faster Than the Wind? | KQED

 

[Thistle]

How Do These Boats Sail Faster Than the Wind? | KQED

Thank you. That's the kind of thing I'm looking for. Much more helpful to me than Buch Turgidson's erudite contribution.

 

[Euphonyx]

Thank you. That's the kind of thing I'm looking for. Much more helpful to me than Buch Turgidson's erudite contribution.

Nobody likes a smart arse!

 

[RJJ]

How Do These Boats Sail Faster Than the Wind? | KQED

But even this isn't true:

"True wind always pushes a boat. If a boat sails absolutely perpendicular to true wind, so the sail is flat to the wind and being pushed from behind, then the boat can only go as fast as the wind—no faster. "

if you really want to confuse yourself, try this for size. What the wikipedia article doesn't tell you is that there was massive controversy - people familiar with sailing, land yachts and physics wrote this off as a prank and said the trials were faked; some still insist it's a fake fourteen years later. Enjoy!

Blackbird (land yacht) - Wikipedia

 

[TLouth7]

But even this isn't true:

"True wind always pushes a boat. If a boat sails absolutely perpendicular to true wind, so the sail is flat to the wind and being pushed from behind, then the boat can only go as fast as the wind—no faster. "

Presumably they meant to say parallel to the true wind i.e. dead downwind. In that case with a conventional sail (as opposed to a setup with moving parts) it is true that you cannot go faster than the wind.

There are various ways of thinking about how a sail works, pressure difference is the most familiar one for most people. The important thing to realise is that the sail needs some angle of attack relative to the apparent wind, and that the force generated on the sail by the air points slightly aft of right angles to the sail. As you point closer to the apparent wind you have to sheet in further to maintain that angle of attack, and so the arrow of the force points more sideways and less forwards. At some point the forwards contribution of this force is not enough to overcome the drag on the hull and foils, so the boat slows down.

So the limit on how close you can sail to the apparent wind is all to do with reducing drag of the rig to make that force arrow point as forwards as possible, and reducing drag on the hull and foils. The latter is best achieved by ice boats that can sail at tiny AWA.

If you have small amount of drag then you can keep accelerating on any point of sail until you reach that limiting AWA. The multiple of windspeed that you reach is a matter of trigonometry.

 

[ridgy]

I'm currently really enjoying this:

The Art and Science of Sails: Amazon.co.uk: Tom Whidden, Michael Levitt: 9780997392005: Books

It's a bit of an advert for North but super interesting all the same. Haven't read enough yet to answer the question though!

 

[TLouth7]

How Do These Boats Sail Faster Than the Wind? | KQED

On further reading, that article while not actually wrong is framed very oddly in a way that I am sure is confusing for the layperson.

“In physics we always talk about conservation of energy,” Collie said. “Sure, we can’t create energy. We can’t produce magic that way. But there’s no sort of parallel theorem of conservation of speed.”

This is the only really good bit of the article. Sailing boats extract energy from the difference in velocity between the air and the water. If a boat is very good at extracting this energy, and needs very little energy to overcome drag then there is no reason it couldn't keep getting faster indefinitely.

True wind is what you feel when you’re standing still and the wind is blowing. The wind an object feels when it’s in motion is apparent wind. Sailboats utilize both true wind and apparent wind. One force pushes the sailboat, and the other force pulls, or drags it forward.

Sorry what? This is an attempt to simplify the concept to the point where it is complete rubbish.

 

[TLouth7]

I think I understand the theory of how sails on a conventional fore and aft rig work, even if I find it hard to believe that it's the pressure difference across a fraction of a millimetre of cloth that does the business upwind.

As a simple demonstration of this, go sailing on a beam reach with the sails pulling nicely. Go round the leeward side of the boom and push your hand against the mainsail. You should find that you can push the cloth, but that it offers a certain amount of resistance. You are pushing that cloth against the pressure difference between the two sides of the sail. Note how you only move a small area of cloth, and that the force you experience is repeated across the massive area of your two sails. Is it really so hard to believe that between them they could pull your boat along? Remember boats are pretty slippery, as demonstrated by the fact that we, mere puny humans, can push them around when alongside.

 

[Buck Turgidson]

Nobody likes a smart arse!

That's the simplest expression of lift. If you want to understand how they sail so fast, that is I'm sorry to say, where you have to start.
At a Cl of 1 and 18kt apparent wind the mainsail makes about 7600 Newtons of force.
If that is enough to accelerate to 20kt apparent you will now be generating 9400 Newton's of force.
And at 32kt 30400Newtons.

That's 4 times the force at twice the apparent wind. Lift force increases at the square of the velocity. Get your hull out of the water and your drag is now mostly caused by the induced drag from the lift of the sail and hydrofoil.

Of course the formula for induced drag is the same so at some point the gains become marginal as you need lots of lift for righting moment which in turn makes lots of induced drag.

 

[Thistle]

That's the simplest expression of lift. If you want to understand how they sail so fast, that is I'm sorry to say, where you have to start.
At a Cl of 1 and 18kt apparent wind the mainsail makes about 7600 Newtons of force.
If that is enough to accelerate to 20kt apparent you will now be generating 9400 Newton's of force.
And at 32kt 30400Newtons.

That's 4 times the force at twice the apparent wind. Lift force increases at the square of the velocity. Get your hull out of the water and your drag is now mostly caused by the induced drag from the lift of the sail and hydrofoil.

Of course the formula for induced drag is the same so at some point the gains become marginal as you need lots of lift for righting moment which in turn makes lots of induced drag.

That may be fine if you understand the terms (CI, ρ , V, S) and want to quantify the result. I just want a superficial understanding of how a boat can sail upwind and / or downwind at speeds greater than the windspeed. The article linked to by Euphonyx may not be perfect but I find it more helpful at my current level of understanding.

 

[rotrax]

Getting back to the OP's Thread Title " How do sails work " I rather fear sails work like two stroke engines.

By accident......................................

 

[Buck Turgidson]

That may be fine if you understand the terms (CI, ρ , V, S) and want to quantify the result. I just want a superficial understanding of how a boat can sail upwind and / or downwind at speeds greater than the windspeed. The article linked to by Euphonyx may not be perfect but I find it more helpful at my current level of understanding.

I would be glad then if you could explain this to me:

True wind always pushes a boat. If a boat sails absolutely perpendicular to true wind, so the sail is flat to the wind and being pushed from behind, then the boat can only go as fast as the wind—no faster. That’s not because there’s no apparent wind; it’s because the apparent wind can’t help the boat when it’s hitting flat against a big sail.

Because it's utter nonsense.

 

[Thistle]

I would be glad then if you could explain this to me:


Because it's utter nonsense.

As TLouth7 suggested above, if you read it as the boat parallel (and the sails perpendicular) to the wind it becomes understandable.

 

[Buck Turgidson]

As TLouth7 suggested above, if you read it as the boat parallel (and the sails perpendicular) to the wind it becomes understandable.

It doesn't help this bit:

That’s not because there’s no apparent wind; it’s because the apparent wind can’t help the boat when it’s hitting flat against a big sail.

Because there is literally no apparent wind in the scenario they fail to correctly describe.