# A sail is not a wing

Discussion in 'Hydrodynamics and Aerodynamics' started by Sailor Al, Feb 7, 2021.

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### gonzoSenior Member

You have been replying with insults and personal attacks to comments on the flaws in your reasoning. Perhaps going back to school or spending time with books in basic physics could make you understand the basic principles. You should start by learning about free body diagrams.

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### GlueandcoffeeJunior Member

It is adverse if no moments or forces oppose leeway and heel. But we do have centerboards and we do sit on the weather side of the boat. Any problem will break down if you take away 1 of the components which make it up. Take away gravity, boats and planes float away. Take away drag boats and planes accelerate to infinity. Take away thrust and they go nowhere. Take away lift from either the centerboards rudder wing or sail and leeway becomes an awful lot more adverse. Take away the crew and bulb on your keel you get a capsize.
But this is not how you use your boat(Hopefully). If the forces adverse or not are balanced while in normal use they are then wing vs sail is an impeccable analogy.

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### Sailor AlSenior Member

But don't you see, whether it is opposed or not, leeway and heel are adverse.
Leeway is adverse for two reasons:
1. It slows the boat down by generating hydrodynamic drag from the keel and the hull.
2. It reduces VMG by increasing the boat's angle to the wind
Heel is adverse for three reasons
1. it compromises the underwater symmetry of the hull, generating additional parasitic drag and windward moment
2. it reduces the effective sail area, reducing the available thrust from the sail.
3. It drags the centre of lift outboard, also generating an adverse windward turning moment

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### Sailor AlSenior Member

I'm getting tired of responding to you.
If nothing else, my university education (in mathematical physics) taught me that it was possible, and preferrable to conduct a philosophical argument without resorting to personal attacks. I'm pretty sure I have been following that principle in this thread.

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### GlueandcoffeeJunior Member

That is all true but only after the fact of the boat heeling over. Take ETNZs AC72 doing a bow down around the mark caused by a pitching moment from the thrust force of the sail. Does that make thrust an adverse force? No. Only when it is not opposed by another force like the foils of an ac72.
Now... this is the same argument turned another 90°. The FORCE is not adverse . The heeling MOMENT is. Dont aim to remove power by reducing lift to drag ratio. Increase the righting moment to use the force better. You said it yourself drag is adverse. But so is lift? And you want to decrease the lift to drag ratio. Which equals increase drag to lift ratio. So do you want to increase or decrease your drag or lift. If the answer is decrease both then why do sailboats have sails. Lift and drag are the only forces on the sail. You can't say that the aerodynamics of a sail are adverse when that is physically what makes them work.

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### gonzoSenior Member

This is not a philosophical argument, but a scientific one. If you have a degree in mathematical physics, it baffles me that you claim to have no knowledge of free body diagrams. They are taught in basic classes in physics. Also, you have not been following the principle you claim to; you have been attacking people instead of their ideas.

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### Sailor AlSenior Member

Come on!
The AC72's, were radical, foiling, winged catamarans and are a very extreme example of a sailboat. It's hardly valid drag them in as a slam dunk to the argument.
I don't think you can make your point about no heel using a conventional dinghy or keelboat which always heels in a breeze.

Again: Come on!
It's the Heel force that creates the moment. How can the force not be adverse if the resulting moment is adverse?

It's easy: I want to decrease Leeway and Heel and increase Thrust.
By excluding the wing analogy we get away from the convoluted discussion about Lift and Drag on a sail.

I DON'T.
I say the Lift component of a sail is adverse.

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### Sailor AlSenior Member

All arguments, except "No I didn't", "Yes you did" are philosophical.
I Do.
I'm sorry you're baffled.
No, they're not. Not by that name anyway.
But anyway, why introduce them as an objection to my argument since it the diagrams I presented in Post#15 would appear to correspond to their description in Wikipedia.
That falls into the "No I didn't", "Yes you did." kind of argument above. I don't think it is supported by evidence.

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### gonzoSenior Member

That is wrong. Heel is the inclination created by opposing forces.

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### KeithOSenior Member

Thrust is thrust and it acts in some arbitrary direction based on the scenario that lead to its existence. How you choose to decompose that thrust (into vectors in the direction of motion and at 90 degrees to it) are up to you.

If you are as smart and educated as you say, then learn to operate 3D CFD and the software will show you exactly how the various forces are created through velocity and pressure distribution. Most sailboats have 2 sails, with 2 different thrusts produced at different locations, different heights, all based on apparent wind and sail trim. This is not a high school level physics problem and I doubt there are many people who are able to calculate accurately (without resorting to CFD) the exact speed and slip angle of a sailboat in a particular sail trim and particular true wind speed.

The diagrams etc posted on sites like Wikipedia are simply for people to have an understanding of what makes sailboats move, not to help them do hull, sail or rigging design. You seem to fail to understand the most basic explanation, which is that a sailboat will reach its highest speed in a beam reach and be slower when pointing into the wind and that sailing downwind one cannot ever exceed the wind speed. Furthermore, you wish to emphatically state that all that has been written so far is a lie ? Have you had any success at exploiting the unstated truth about sailboats that others will not disclose ? What exactly do you have to contribute to this subject except to say that you don't like the terms that authors have used in the past ?

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### gonzoSenior Member

Wrong again. In post #44 you call me pedantic because I show your mistakes in terminology and the basic understanding on Newton's laws.

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### Sailor AlSenior Member

Let me make it quite clear:
I am not decomposing Thrust.
I am proposing that, in the language of physics and for the purpose of describing how boats use the wind on sails to work upwind, it is more appropriate to resolve the "total aerodynamic force" (vector) around the centreline of the boat into "Thrust" and "Leeway/Heel" (scalar) components than it is to to use the aircraft wing analogy which resolves the total aerodynamic force around the chord of the wing into "Lift" and "Drag".
Let me repeat my thesis:
The wing analogy is wrong because:
• Unlike a wing, a sail does not generate adverse Drag
• Unlike a wing, a sail does not generate useful Lift
• Unlike a wing, a sail does generate useful Thrust
• Unlike a wing, a sail does generate adverse Leeway and Heel

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### KeithOSenior Member

I'd like to see you stand in the water, hold up a windsurfer mast and sail and tel me a sail does not generate drag.... hahahahha.. To generate minimum drag, a sail has to produce minimum thrust and you are not going to get lower than a certain amount of drag simply based on the windage of the mast, spreaders and the flogging sail.

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### GlueandcoffeeJunior Member

Your entire argument (pardon the pun) hinges on the forces acting through the center line of the boat. You do not use the force perpendicular to the centre line. You do not use the force perpendicular to the chord . You use the force perpendicular to the airflow..
Heel is not a force it is caused by a moment which is not being opposed. Heel reduced lift in the sail. This is adverse. Heel reduces the effective area of the keel. Heel reduces symmetry of the hull. Heel is the cause of the problem. But the cause of heel in your case is trying to resolve the thrust force perpendicular to your hull centre line not parallel to it.
Ease the sheet so it is not parallel to the centre line .now the sail is not pulling so much sideways. Increase your lift to maximum.now the sail is creating power. Decrease your drag to minimum. Now the sail is not being slowed down. You only need to change the way look at the problem. Dont imagine the lift as 1 single force. Imagine it as thrust and perpendicular to that is leeway. The heeling moment is not from sail alone. it is from the centre board acting in the opposite direction at a perpendicular distance. This results in an adverse moment. Which is caused by a useful force.

Yes the ac72s were radical. But they used the same rulebook newton used.. they get rid of the heeling moment which allows them to use the lift better. Yes they were winged. What is the name of the argument. I think they are a very extreme example of a sailboat. If we are talking about sailboats I think the more "sailboaty" the sailboat is the better. As we have said before the problem is not the lift. Loosen the sheet and all of a sudden the lift is thrust. So the problem is not lift.
The problem is heel. All the problems start with heel almost all boats try to reduce heel(save for fouling moths). Not lift.
The heel is perpendicular to the centre line of the boat. The effect is adverse. The lift is perpendicular to airflow (not centering or boom) thus is at an angle far closer to being parallel with and in the direction of the centerline. Sure . There is a component of the force which is creating heel. But since the force in the thrust direction is greater you can't say that the entire force is adverse. . At the heeling moment is adverse. But a plane wing is exactly the same in that regard. If you take away one wing the plane will also heel over. That other wing adds a righting moment in the opposite direction. You must counteract the adverse moment or else the system will not work. You can't say sails don't work because I let my boat fall over. Same way a pilot cant say a wing doesn't work because the other one fell off.

The best way to decrease leeway and heel is to get a big centre board and put a big weight on the bottom. Well maybe its not the best way but it's a good way. A better way is to bit the gig weight to windward on a very beamy boat. best way to increase thrust. dont oversheet your sail. But get a big sail with a high lift to drag ratio.. if these 2 concepts are put together you get a trimaran catamaran or proa with a sail with maximum L/D ratio. Its almost like I'm describing a BOR70, ac 72 , or sailrocket 1 or 2.. power is friend . Heel is enemy. They are not equal to one another. They are not part of the same argument. Forces are not moments. Now go squeeze a melon seed to observe balanced forces creating perpendicular motion.

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### GlueandcoffeeJunior Member

You should be.

I am proposing that, in the language of physics

and for the purpose of describing how boats use the wind on sails to work upwind,

Your way of describing boats going upwind is wrong

it is more appropriate to resolve the "total aerodynamic force" (vector) around the centreline

It's not better to resolve around the centerline.

of the boat into "Thrust" and "Leeway/Heel" (scalar)

Leeway is still a vector. Heel is a symptom of bad sailing not bad sail physics

components than it is to to use the aircraft wing analogy which resolves the total aerodynamic force around the chord of the wing into "Lift" and "Drag".

Dont resolve aerodynamic forces around the vehicle. Resolve them around the airflow. Resolve the useful components of the aerodynamic forces around the useful directions on the vehicle.

Let me repeat my thesis:
The wing analogy is wrong because:
• Unlike a wing, a sail does not generate adverse Drag
• It does.
• Unlike a wing, a sail does not generate useful Lift
• It does if you use it in the direction it's meant to be used
• Unlike a wing, a sail does generate useful Thrust.
• Yes it does. Boats work
• Unlike a wing, a sail does generate adverse Leeway and Heel.
• Only when nothing is done about it.

Last edited: Feb 9, 2021
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