The most basic control of the sail consists of setting its angle
relative to the wind. The control line that accomplishes this is called a
"sheet." If the sheet is too loose the sail will flap in the wind, an
occurrence that is called "luffing." Optimum sail angle can be
approximated by pulling the sheet in just so far as to make the luffing
stop, or by using of tell-tales - small ribbons or yarn attached each side of the sail that both stream horizontally to indicate a properly trimmed sail. Finer controls adjust the overall shape of the sail.
Two or more sails are frequently combined to maximize the smooth flow of air. The sails are adjusted to create a smooth laminar flow over the sail surfaces. This is called the "slot effect". The combined sails fit into an imaginary aerofoil outline, so that the most forward sails are more in line with the wind, whereas the more aft sails are more in line with the course followed. The combined efficiency of this sail plan is greater than the sum of each sail used in isolation.
More detailed aspects include specific control of the sail's shape, e.g.:
Two or more sails are frequently combined to maximize the smooth flow of air. The sails are adjusted to create a smooth laminar flow over the sail surfaces. This is called the "slot effect". The combined sails fit into an imaginary aerofoil outline, so that the most forward sails are more in line with the wind, whereas the more aft sails are more in line with the course followed. The combined efficiency of this sail plan is greater than the sum of each sail used in isolation.
More detailed aspects include specific control of the sail's shape, e.g.:
- Reefing, or reducing the sail area in stronger wind.
- Altering sail shape to make it flatter in high winds.
- Raking the mast when going upwind (to tilt the sail towards the rear, this being more stable).
- Providing sail twist to account for wind speed differential and to spill excess wind in gusty conditions.
- Gibbing or lowering a sail.