On the mainsail, pull the outhaul, the cunningham and, on most rigs, the backstay tighter. A flatter sail gives less power and less heel, which is what you want as the wind builds; in light winds ease them for a fuller, more powerful sail.
The depth of a sail, how curved it is from front to back, controls how much power it produces. A deep, full sail gives a lot of power, which is right for light winds and choppy water, when the boat needs help to keep moving. A flat sail gives less power and less drag, which is right when it is windy and the boat is heeling too much.
The main controls for flattening the mainsail are:
- The outhaul, which pulls the clew out along the boom. Tighter makes the lower part of the sail flatter.
- The cunningham, which pulls the luff down near the mast. Tighter flattens the sail and moves the deepest part forward, which the wind blowing harder tends to push aft.
- The backstay, on rigs where it is adjustable. Tightening it bends the mast, which pulls the middle of the mainsail flatter. On a masthead rig it mainly tightens the forestay, which flattens the jib.
- The kicker or vang, which holds the boom down. On many rigs pulling it on also bends the mast and flattens the sail, but it mainly controls twist.
On the jib, tightening the halyard moves the depth forward, and moving the jib sheet car aft flattens the lower part of the sail and lets the top twist open.
Flattening is the first step in depowering. As the wind builds further, the next steps are to ease the traveller to leeward in gusts, let the kicker off a little so the top of the mainsail twists open and spills wind, and then reef. A boat that is heeling well past about 20 degrees is not going faster, only harder to steer.
As the wind drops, reverse it all: ease the outhaul, cunningham and backstay to give a fuller, more powerful shape.
More in the book: Chapter 5, Trimming the sails