Hi,

I'm trying to understand how a boat can sail against the wind.
Explanations on the internet usually go like this: "The sail has an
airfoil shape, the air goes faster on one side than on the other,
creating lift by Bernouilli's principle." But I know that for flying
planes, the wing's airfoil shape is not essential (otherwise planes
couldn't fly on their back): it's the angle of attack that matters. So
I'm wondering if the same is true for sailing: would it be possible to
sail against the wind with a sail that's a rigid flat surface, not an
airfoil-shaped piece of cloth?

Thanks,
Axel

Axel Boldt wrote:

Hi,

I'm trying to understand how a boat can sail against the wind.
Explanations on the internet usually go like this: "The sail has an
airfoil shape, the air goes faster on one side than on the other,
creating lift by Bernouilli's principle." But I know that for flying
planes, the wing's airfoil shape is not essential (otherwise planes
couldn't fly on their back): it's the angle of attack that matters. So
I'm wondering if the same is true for sailing: would it be possible to
sail against the wind with a sail that's a rigid flat surface, not an
airfoil-shaped piece of cloth?

Thanks,
Axel

Yes, but slowly - a flat surface is not an efficient aerofoil.

A boat goes to windward by generating lift from its sail, just like an
aircraft's wing. If you understand a wing you are 3/4 of the way there.
Of course it generates drag as well; the resultant force from the sail
will be pointing a bit behind right angles to the boom (the long hard
thing usually made of wood or metal at the bottom of the sail) - in your
"flat surface" case, a bit behind right angles to the surface.

The force is angled sideways and forwards by controlling the amount the
sail is directed away from the centre line of the boat.

The forward vector of the force propels the boat forwards.

The sideways vector is absorbed by the centreboard, keel, or sometimes
just the shape of the hull, which acts as another aerofoil shape but
underwater. (It's technically a hydrofoil, but most people think of a
hydrofoil as something to lift the boat out of the water).

Of course, in absorbing the sideways force the underwater parts of the
boat create some drag.

It's much easier to understand with a boat to look at and wave your hand
over - or at least a piece of paper to scribble on!

HTH

Andy

Yes, but slowly - a flat surface is not an efficient aerofoil.

not necessarily. at certain a of a it is more efficient. an s-shaped airfoil
is more efficient.

If you understand a wing you are 3/4 of the way there.

no, 100%. airfoils don't know whether they are vertical or horizontal.

In article ,
(JAXAshby) wrote:

If you understand a wing you are 3/4 of the way there.

no, 100%. airfoils don't know whether they are vertical or horizontal.

So?

--
Alan Baker
Vancouver, British Columbia
"If you raise the ceiling 4 feet, move the fireplace from that wall
to that wall, you'll still only get the full stereophonic effect
if you sit in the bottom of that cupboard."

Of course it generates drag as well;

just as true of a sail as an aircraft wing. For the same reasons.

So
I'm wondering if the same is true for sailing:

yes.

In article ,
(Axel Boldt) wrote:

Hi,

I'm trying to understand how a boat can sail against the wind.
Explanations on the internet usually go like this: "The sail has an
airfoil shape, the air goes faster on one side than on the other,
creating lift by Bernouilli's principle." But I know that for flying
planes, the wing's airfoil shape is not essential (otherwise planes
couldn't fly on their back): it's the angle of attack that matters. So
I'm wondering if the same is true for sailing: would it be possible to
sail against the wind with a sail that's a rigid flat surface, not an
airfoil-shaped piece of cloth?

Thanks,
Axel

Sure. Just not very well.

--
Alan Baker
Vancouver, British Columbia
"If you raise the ceiling 4 feet, move the fireplace from that wall
to that wall, you'll still only get the full stereophonic effect
if you sit in the bottom of that cupboard."

"Axel Boldt" wrote...

I'm trying to understand how a boat can sail against the wind.
Explanations on the internet usually go like this: "The sail has an
airfoil shape, the air goes faster on one side than on the other,
creating lift by Bernouilli's principle." But I know that for flying
planes, the wing's airfoil shape is not essential (otherwise planes
couldn't fly on their back): it's the angle of attack that matters. So

An airfoil creates lift when the air flows across it. The amount of lift
depends on the speed of the air flow, the shape of the airfoil, and the angle of
attack.

An airplane can fly on its back because the wing airfoil is rigid. It can
create lift in the "wrong" direction if the angle of attack is high enough in
the proper direction. Competitive acrobatic aircraft use airfoils that are
nearly symmetric (same curve on top and bottom) for better performance
up-side-down. Few wings are actually flat on the bottom; they just have less
curve than on the top.

Similarly, a sailboat can tack upwind by positioning the sail so the wind blows
on the opposite surface, while positioning the sail over the hull so there is a
net lift force in the desired direction. The sail then acts like the bottom
surface of the airpalne as it fills on the opposite side. The sail assumes the
same shape on the other side, and behaves like a symmetric airfoil.


I'm wondering if the same is true for sailing: would it be possible to
sail against the wind with a sail that's a rigid flat surface, not an
airfoil-shaped piece of cloth?

An airplane uses the engine to pull the airplane through the air, so lift can be
created by a less efficient surface. A sailboat depends on the wind to provide
the propulsive force, so it is much more difficult. While you might be able to
make some net upwind progress with such a device, it would be slow and
inefficient.

An airplane uses the engine to pull the airplane through the air, so lift can
be
created by a less efficient (sic) surface. A sailboat depends on the wind to
provide
the propulsive force, so it is much more difficult.

An aircraft wing is one hell of a lot more efficient at creating lift as
compared to drag than a sailboat sail is. A sail has a pretty miserable L/D
ratio, but a sail doesn't weigh much, it can be shortened for greater wind
speeds, or more sail put up for lesser wind speeds, can be stored easily, costs
less and a whole bunch of other things practical on a sailboat.