The sails on your boat is the engine that drives you .................. The WIND is the fuel ! ................... Set the sails in the wrong position is like having the wrong choke setting and you will be going no where fast.
Wind dynamics is a complicated science. I hope the few drawings and explanations below will be suffice to give you a basic understanding.
Its not about understanding the detail, just enough so you can figure out what works.
What happens in practical terms is that the boat will tend to screw or pivot and alter course to WINDWARD so that it will be sailing at the optimum angle to the wind as it was doing in Fig 1 ........... Which is how you set the sails up when CLOSE HAULED sailing to WINDWARD
Aerofoils on wings and sails are essentially the same thing .… LIFTING SURFACES.
The flow of air ( wind ) over the WINDWARD side of the sail and the LEWARDside of the sail create lift ........... Or the FORCE that drives you forward through the water.
There is an Optimum ANGLE OF ATTACK or angle of incidence between the APPARENT WIND and the Sail where
There is basically laminar flow over both surfaces like what is shown in Figure 4 on the left.
If the wind hits the sail at too great an angle as shown in Figure 5 then the flow over the LEWARD SIDE of the sail detaches ( STALLS ) and causes turberlance and the whole lifting surface looses it effectivemess .........
In other words you loose horsepower ! And you will be going nowhere fast.
This is espcially true when sailing or BEATING going to windward with the sails pulled in tight or REACHING with the wind coming more from the side of the boat.
The only time you will have a stalled sail is when you are going RUNNING DOWNWIND or BEFORE THE WIND ie with the wind behind the boat or comming over its TRANSOME or STERN