Most sailors learn to pull the mainsheet in and out and stop there. But a mainsail has half a dozen other controls, and each one changes the shape of the sail in a specific way. Learn what the outhaul, cunningham, halyard, boom vang, traveler and backstay do, and you can make the same sail powerful in light air and flat and easy to handle in a breeze. That means more speed, less heel and a boat that is far nicer to steer.

Quick answer: the outhaul controls depth in the bottom of the sail; the cunningham and halyard control where the draft sits (forward or aft); the boom vang controls twist when the sheet is eased; the mainsheet controls twist and leech tension upwind; the traveler controls the boom angle without changing twist; and the backstay bends the mast to flatten the whole sail. In light air make the sail fuller and more twisted; as the wind builds, flatten it, move the draft forward and open the leech.

Why Sail Shape Matters

A sail is a flexible wing. Three shape characteristics determine how much power it makes and how efficiently it does it:

• Depth (camber or draft): how deep the curve of the sail is. A deep sail makes more power and is good in light air and chop; a flat sail makes less drag and heel and points higher in a breeze.
• Draft position: where the deepest part of the curve sits. In a mainsail it is usually around 45 to 50 percent back from the luff. Wind pressure tends to push the draft aft, which increases heel and weather helm.
• Twist: how much the top of the sail opens (falls off to leeward) compared with the bottom. More twist spills power from the top of the sail; less twist (a tight leech) increases power and pointing.

Every control on the main changes one or more of these. If any of the parts are unfamiliar, start with our guide to names of sails on a sailboat and what is sailboat rigging.

Halyard and Cunningham: Draft Position

Both the main halyard and the cunningham tension the luff of the sail.

• Tightening luff tension pulls the draft forward and flattens the front of the sail. Use it as the wind builds, when the draft starts to drift aft.
• Easing luff tension lets the draft move aft and makes the sail fuller. In light air, ease it until small horizontal wrinkles just appear along the luff.
• The cunningham is a line through a cringle a short distance above the tack. It pulls the luff down without moving the head, so you can adjust luff tension quickly while sailing, even after the halyard is fully hoisted. Many cruising boats don’t have one; then you use the halyard.

A simple guide: if you see horizontal wrinkles from the luff, add tension; if you see a vertical crease running up the sail just behind the mast, you have too much.

Outhaul: Depth in the Foot

The outhaul pulls the clew of the sail toward the end of the boom. It mainly affects the lower third of the sail.

• Ease the outhaul in light air, chop and when reaching to make the bottom of the sail deeper and more powerful.
• Tighten the outhaul upwind in moderate to strong winds to flatten the foot, reduce drag and heel and help pointing.
• A good starting point: a hand’s width (roughly 4 to 6 inches, or 10 to 15 cm) between the foot of the sail and the boom at its deepest point in moderate wind. In strong winds, flatten it until the foot is almost along the boom.

On boats with in-mast furling the outhaul also controls the sail while it unrolls and furls, so handle it with care.

Boom Vang: Twist Off the Wind

The boom vang (also called a kicker) pulls the boom down. Its main job is controlling twist when the mainsheet is eased.

• Upwind, the mainsheet is pulling mostly downward, so it controls the leech and the vang is often slack on cruising boats. On dinghies and performance boats the vang is used upwind in strong winds (“vang sheeting”) to bend the lower mast and flatten the sail.
• Reaching and running, the mainsheet pulls mostly outward, so without the vang the boom lifts and the top of the sail twists away, losing power and making the boat roll. Tighten the vang until the top batten is roughly parallel with the boom.
• In gusts off the wind, easing the vang a little lets the top of the sail open and spill power, reducing round-ups.
• Ease the vang before reefing or dropping the main, and before gybing with a rigid vang if the boom needs to lift.

A rigid vang also supports the boom when the sail is lowered, replacing the topping lift. Downwind trim is explained further in downwind sailing techniques.

Mainsheet and Traveler

Mainsheet

Upwind, the mainsheet controls both the boom angle and the leech tension (twist). Sheeting in harder pulls the boom down, closes the leech and adds power and pointing; easing it lets the leech twist open. Check your mainsail leech telltales: upwind, the top telltale should stream most of the time and occasionally stall behind the sail.

Traveler

The traveler moves the bottom of the mainsheet across the boat. It lets you change the boom angle without changing twist, which makes it the main power control upwind.

• Light air: pull the traveler up to windward so the boom is near the centerline, and ease the mainsheet slightly to keep some twist.
• Medium air: traveler near the center, boom roughly on the centerline.
• Strong air and gusts: drop the traveler to leeward in the puffs and pull it back up in the lulls. This is the fastest way to reduce heel and weather helm without losing too much drive.
• Reaching: the traveler goes down to leeward and the vang takes over control of twist.

Boats without a traveler (or with a short one) control power mainly with the mainsheet, and need to reef sooner.

Backstay and Mast Bend

On rigs with an adjustable backstay, tightening it pulls the masthead aft and bends the middle of the mast forward. This has two effects:

• It flattens the mainsail, especially in the upper part, and opens the leech, reducing power and heel.
• It tightens the forestay, reducing forestay sag and flattening the headsail, which helps pointing.

Ease the backstay in light air and chop for power; tighten it as the wind builds. Fractional rigs with bendy masts respond much more than stiff masthead rigs, where the backstay mainly controls forestay tension. Never over-tension a backstay: check the manufacturer’s or rigger’s limits.

Quick Settings by Wind Strength

Control	Light air (under ~8 knots)	Medium (~8 to 15 knots)	Strong (15+ knots)
Halyard / cunningham	Eased; small wrinkles OK	Just remove wrinkles	Tight; draft forward
Outhaul	Eased (deep foot)	Moderate	Tight (flat foot)
Mainsheet	Eased slightly for twist	Firm; top telltale occasionally stalls	Played in gusts
Traveler	Up to windward	Near center	Down in gusts
Boom vang (upwind)	Slack	Light tension	Firm on performance boats; slack enough to twist in gusts on cruisers
Backstay	Eased	Moderate	Tight
Reefing	Not needed	Consider first reef at the top of the range	Reef

Treat these as starting points and adjust for your boat, sails and sea state. When the controls can no longer keep the boat on its feet, it is time to reef the main. If you need to stop the boat to do it, heave to first.

Putting It Together: An Upwind Routine

1. Set the halyard or cunningham so the luff is smooth.
2. Set the outhaul for the wind strength.
3. Trim the mainsheet until the top leech telltale stalls occasionally.
4. Use the traveler to keep the boat at a comfortable heel with light weather helm.
5. Add backstay as the wind increases.
6. When you are easing the traveler all the way down and still overpowered, reef.

These skills are especially useful on the race course; see our top tips for sailboat racing for more, and points of sail explained for how trim changes with wind angle.

Mainsail Controls: Summary

The outhaul controls depth low in the sail, the halyard and cunningham move the draft forward or aft, the mainsheet and vang control twist, the traveler sets the boom angle, and the backstay flattens the whole sail. Make the main full and twisted in light air, flatter with the draft forward and the leech open as the wind builds, and reef when the controls are no longer enough.