A sail stalls when it is pulled in too far for the wind angle: the air on the leeward side can no longer follow the curve of the sail and breaks away, so the sail produces less drive and more heel. The cure is to ease the sheet.
A sail produces its drive from air flowing smoothly round both sides of it. On the leeward side the air has to follow the curve of the sail, and it can only do that if the sail meets the wind at a fairly small angle. If the sail is pulled in too far for the direction of the wind, or the boat turns away from the wind without the sail being eased, the angle becomes too large. The air on the leeward side breaks away from the surface, becomes turbulent, and the sail stalls, just as an aircraft wing does when it is tilted up too steeply.
A stalled sail is easy to miss, because it still looks full and firm. The signs are:
- The leeward telltale on the jib droops, hangs limp or spins round.
- The leech telltales on the mainsail disappear round behind the sail instead of streaming aft.
- The boat heels more but goes slower, and may feel heavy on the helm.
The cure is to ease the sheet until the telltales stream again. If you are close-hauled with the sails already where they should be, the cause is usually that you have steered too far from the wind, so head up slightly instead.
Over-trimming is one of the commonest trimming mistakes, partly because pulling a sheet in feels like adding power. The way to avoid it is to find the edge: ease the sail until its luff just starts to lift, then pull it back in only a little.
On a dead run, the sails are working mainly by being pushed, and the airflow over them is stalled in any case. That is normal, and the aim there is to present as much sail to the wind as possible.
More in the book: Chapter 5, Trimming the sails