# A sail is not a wing

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

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### gonzoSenior Member

Energy gets converted from one form to another. It doesn't get "gathered" in any direction. Please read Newton's laws, they are really short. I don't mean to insult you, but the statement shows some basic misunderstanding of physics. A boat in irons is going somewhere, but nowhere useful. I think the underlying assumption is that the discussion is about boat or planes moving in a useful and controlled direction.

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### GlueandcoffeeJunior Member

Like the very center of eye of a hurricane or the very center of a high pressure system there is often no wind. Because there is no discernable gradiant iin pressure.A boat in 0 wind has is in a position where there is no atmospheric pressure gradient. No available energy in the atmosphere. There is no potential or difference in pressure around one side of the sail and the other. The boat goes nowhere. If a glider is on the ground there is no gravitational gradient. No available energy from gravity. There is no potential for pressure difference on the wing. The glider goes nowhere. . A boat can sail across a pressure gradient to a place of high pressure. This is going upwind. Gaining potential against an opposing force(Atmospheric pressure ). A boat can also sail the other direction to a place of low atmospheric pressure. This is going downwind. Losing potential against the opposing force. (Atmospheric pressure.)

A glider can fly to a place of high altitude. This is the thermaling. Gaining potential against the opposing force gravity. A glider can also fly to a place of low altitude. This is gliding. Losing potential against the opposing force gravity.

The sourse of energy in both is solar to thermal to kinetic to potential. Then while going back downwind or gliding back to earth. The energy goes from potential to kinetic.

The difference in the argument is 90°.

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### gonzoSenior Member

There is energy, but not in the form of wind.
Gravity is a force and potential is accumulated energy. They can't oppose each other.

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### GlueandcoffeeJunior Member

That's ok . I agree "gathering energy"is a misnomer.
I think the underlying assumption is that the discussion is about boat or planes moving in a useful and controlled direction.[/QUOTE]
The discission on the first page was sails going up wind dont work like wings going up.
I agree that we are talking about the use in a useful and controlled direction. A plane wants to use the airflow to be controlled to a higher altitude. A boat wants to use the airflow to be controlled towards where the wind is coming from. A sail cannot sail directly into wind and a wing cant fly directly upwards. There must be some perpendicular component to the vehicles motion .
The difference in the discussion is 90°.

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### GlueandcoffeeJunior Member

In the scenario you've given, what is the "useful" form of energy you are referring to that isn't wind.

Gravity is not a force. Being held against gravity is a force. Force =mass x acceleration. Gravity is an acceleration. Aerodynamic push is a force . wind is the cause of this force. Wind is the effect of the potential between high and low atmospheric pressure.

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### GlueandcoffeeJunior Member

And are gathering and accumulating not the same thing. You said I was wrong and should go read the physics again and then say the same thing I said with a synonym.

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### GlueandcoffeeJunior Member

And again i apologise for awful formatting my posts are taking. I still haven't figured out how to post on the forum. What I'm saying keeps getting mixed in with quotes and quotes keep getting mixed in with what I'm saying.

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### Boat Design Net ModeratorModerator

Keep what you want to quote within the Quote Tags. Note there must be an opening and a closing quote tag for each quote.
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### HowlandwoodworksMember

Are these the forces we are talking about?
Newtonian Gravity late 1800's
Fg=Gx(M1xM2/r2)

Or less relative general speaking and not considered as a force at all.
ΔE=TΔS-W
Energy = heat - work

Sometimes when I look at a cloud of sails, lets say in a Montague Dawson painting or the soft winged sail of the new AC75. I wonder if the mathematics that they were created from was sent here on the wings of angels.

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### GlueandcoffeeJunior Member

Poetic.

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### HowlandwoodworksMember

When I roll and tack my little sunfish "Icarus" in a stiff breeze it seems as if it has wings and the American Magic looked as if it had wings last week.

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### tlouth7Senior Member

This image is actually really useful to the discussion here. It may help to explain some of the terms being used. There is only one force being generated by the action of the air on the sail. This is called "aerodynamic force" in various places in the thread. It is labelled F in the diagram, and can be thought of as the 'real' force being generated.

It is often helpful to split a force up into a pair of components at right angles to each other. This is useful because the force points in an unhelpful direction as determined by the aerodynamic effects. We can choose the directions of the components to be helpful: two sets are given in the diagram.

First there is "driving force" (often called thrust) and "heeling force" (hereafter called 2sideways force). These are the two components of the aerodynamic force as defined to be parallel and perpendicular to the boat's hull. Clearly a big driving force and a small sideways force is desirable, which corresponds to the arrow F pointing as much forwards as possible. Remember these aren't two new forces, just a different way of looking at the force F.

Second there is "lift" and "drag". These are the components of F defined perpendicular and parallel to the free stream (apparent wind). These are the bread and butter of aerodynamicists. You should be able to see that if a sail has a large lift component and a small drag component then the total of these (the F arrow) will be pointing forwards somewhat relative to the sail. Remember that having F pointing forwards as much as possible is a good thing!

A rig with low lift/drag ratio (low aspect ratio perhaps) will have a bigger drag component. This means that at a given AWA the F arrow will be pointing closer to perpendicular to the hull, and so the driving force component will be small. Thus at a given AWA there will be less thrust, or to put it in normal terms, the boat will be less close-winded, needing to sail further from the wind to achieve sufficient driving force to maintain way.

Using lift and drag as defined in aerodynamics is not necessary to design a sailing boat, but it is valid, and can be helpful. It explains why a boat with a tall, narrow rig and deep, narrow keel is likely to be able to sail closer to the wind than one with a dumpy rig and long keel. It also allows us to quantify the benefits of having an endplate on any foil, either a deck sweeping sail or a winged keel. Just read any post on these fora by TSpeer to get a sense of the value of an aerodynamics approach to sailing vessel design.

Last edited: Feb 11, 2021
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### tlouth7Senior Member

I will attempt to argue that both sets of components (lift/drag and thrust/side force) are useful for designing or characterising a sailing rig.

I agree that decomposing aerodynamic force into thrust and side force is helpful in order to work out how fast and with how much leeway a boat will move in any given windspeed and TWA. The problem is that the two components change for any change in sheeting angle.

In contrast the lift and drag of the sail depend only on the windspeed and angle of attack of the sail. This is pretty constant over a wide range of AWA, because we adjust the sheet to maintain sail AoA (even if we don't know that is what we are doing). So these are more general measures which can be used to compare rig designs.

You also argue that lift is not beneficial, and drag is not adverse. Given how these are defined, for any heading between due upwind and a beam reach the lift vector will be somewhat forwards, and the drag vector will be somewhat backwards. Thus if a rig has high lift to drag ratio it will perform better in that AWA range than a rig with lower L/D. For sailing angles with the wind aft of the beam we tend to move to sails with much lower L/D, namely spinnakers of various types. Given the importance of upwind performance it makes sense to use lift and drag to design rigs for this regime. It is trivial to convert these into thrust and heeling force when necessary.

Think of it like this, if you ignored lift and drag, how would you actually
go about optimising a rig? Thrust to sideways force ratio can always be improved by bearing away, which isn't very helpful to the designer. You might say "Aha, but I will optimise thrust/sideways force for a fixed AWA", but if you do that, why not optimise for L/D which will give you the same benefit but for all AWA up to 90 degrees?

Last edited: Feb 11, 2021
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### gonzoSenior Member

Heeling is not a force. It is the moment created by the horizontal component of the forces on the sail perpendicular to the centerline of the boat, and the horizontal component of the hydrodynamic forces on the hull and appendages.

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### tlouth7Senior Member

Quite right, I will replace all mentions of heeling force with sideways force to be more clear.

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