Sail is a wing
In the times of the Age of Discovery boats sailed under square sails. These worked like a barrier for the wind and boats moved due to the high pressure on the windward side of the sails. Boats with such sails can sail only downwind. Modern triangular sails work like plane wings and boats can sail upwind as well, up to 35 degrees to the wind.
In modern sailboats (Bermuda or Marconi rig) the airflow enters the sail on the one side passing along the sail and leaving the sail on the other side. The shape of the sail forces the air flow on the leeward side to take a longer path than on the windward side. Therefore the air has to increase its velocity on the leeward side of the sail resulting in a lower pressure than on windward side. In effect a sailboat may be sucked through the water due to the low pressure on the leeward side. This is a general principle of wing and sail work.
For airflow to pass smoothly along the sail, one should be set at the correct angle to the wind. What happens if it´s not? As you can see in the picture below when the angle is less than necessary, the sail will flap and the low pressure on the leeward side won’t be produced. More difficult to notice is if the sail is overtrimmed and angle is too big. In this case the sail stays full and looks perfect but doesn´t work because the air stream on the leeward side is not smooth and low pressure is not produced as well.
Interesting to notice that boat itself is not painted in these pictures. We can put the boat here in any direction relative to the wind and it will still be correct. It’s only the angle between sail and the wind that is important.
Telltales. Angle of attack.
What is telltales? - Small strings located in the forward side of the jib (forward sail) and on the back side of the main sail are named telltales. They show us how airflow passes along the sail. If it passes smoothly or if the angle of attack is incorrect, so there are vortexes in the flow. If telltales stream horizontally – the angle of attack is correct. If not – we should trim or ease our sail. If only windward is not horizontal – trim, if leeward or both – ease.
But, as always, there is an exclusion. When we are sailing downwind, - sails work in "oldstyle" way, just as a bag for the wind, pushing boat due to high pressure on the windward side. And telltales doesn´t work anymore as airflow don´t pass along the sail. In this case the only way to find a right angle of attack is to ease a sail till it just starts fluttering, and then trim just enough to fill the sail with a wind and thus stop flutter.
So the full algorithm of setting sails is as following.
- If your sail is fluttering, always trim it till it fills with a wind and become stable.
- If your sail is stable but telltales don´t stream horizontally, ease the sail till one of two will happen: telltales will become horizontal or the sail will start to flutter. In the second case trim it back to stop fluttering.