B Why do sails on a boat generate lift perpendicular to the sails?

[inuka00123]

assuming wind is not coming perpendicular to the sail, how it generates lift perpendicular to the sail, as I feel It should rotate from the force of the wind is not distributing evenly across the surface of the sail. with direction of the wind being not perpendicular to the sail, creating a pressure difference in the sail, like the wind direction finders I used to build as a child

 

[DaveE]

The keel/centerboard and the rudder prevent the boat from blowing downwind or turning into the wind. Also the sheet (ropes holding the sail) prevent the sail from twisting with the wind. If you let go of the sheets or the rudder the sail will turn into the wind just like your weathervane.

 

[russ_watters]

The keel/centerboard and the rudder prevent the boat from blowing downwind or turning into the wind.

If you sail a boat with a tiller you can test this by letting go of the tiller. In most situations the boat will turn upwind.

 

[inuka00123]

If you sail a boat with a tiller you can test this by letting go of the tiller. In most situations the boat will turn upwind.

I guess I was correct then :)

 

[Patxitxi]

The keel/centerboard and the rudder prevent the boat from blowing downwind or turning into the wind. Also the sheet (ropes holding the sail) prevent the sail from twisting into the wind. If you let go of the sheets or the rudder the sail will turn into the wind just like your weathervane.

Eventually all pressure force (not lift) it's decomposed into 2 forces: one (wasted) pushing the water perpendicular to the plane of the rudder and the other pushing the boat in the direction of its plane.
Skilled navegation consists in maximizing the force used into movement minimizing the one wasted fighting against the water.

 

[tech99]

When wind passes an object it creates forces on it. By convention, a force arising at right angles to the wind direction (not right angles to the sail) is called lift. A force acting in the direction of the wind is called drag. In addition to this one can say that a torque is created by the wind (as you mention). If the boat is sailing to windward, the lift can be resolved into two components: one acting in the desired direction and another acting sideways on the boat. This is resisted by the under water foils/hull shape providing underwater lift. In addition to the torque arising from the sail itself, torque arises from the relative positions of the "air" and "water" centres of pressure. This is opposed by torque created by the lift of the rudder. The underwater foils work the same as the wings and tail plane of an aircraft. When a glider is flying at an angle of sink, that is creating an angle of attack for its wings. And the glide slope is analogous to the leeway of a sailing boat, which in the same way creates an angle of attack for the centre board/keel and provides water lift to oppose the sideways wind forces.

 

[A.T.]

By convention, a force arising at right angles to the wind direction (not right angles to the sail) is called lift. A force acting in the direction of the wind is called drag.

It should be noted that the above convention is based on the wind relative to the boat (apparent wind) not relative to the Earth (true wind).

The underwater foils work the same as the wings and tail plane of an aircraft.

This is true for an efficient sail as well. On fast boats, sail and keel are both preforming complimentary functions, in different media. This symmetry is nicely explained here:
https://www.onemetre.net/design/CourseTheorem/CourseTheorem.htm

 

[Meir Achuz]

If you sail a boat with a tiller you can test this by letting go of the tiller. In most situations the boat will turn upwind.

Not on a boat with a well balanced jib. I can sail upwind with my hands off the tiller

 

[tech99]

The force on the tiller is not necessarily related to the sail balance, but just to the relationship between rudder centre of lateral resistance and the pintles. On the other hand, the position of the tiller when sailing straight is an indication of sail balance. Most sailors prefer the tiller held a little to windward during straight sailing, so that the boat rounds up to the wind if you let go, but for an aircraft it is more stable to have the opposite, where the tail plane is pushing downwards

 

[sophiecentaur]

Not on a boat with a well balanced jib. I can sail upwind with my hands off the tiller

True but you just introduced an 'air rudder'. That sort of exercise can be very satisfying, too and, I suspect, is more efficient use of the wind.