The mainsail is the principal sail of a sailing boat, set on the aft side of the mainmast with its luff on the mast and, on most boats, its foot along the boom. Beginners learn that the main is the sail you hoist first and drop last, and the one that most affects how the boat steers. The experienced sailor and sailmaker add that the mainsail is the engine of the rig – the sail with more controls than any other, which on a modern fractional rig may carry two-thirds of the working sail area.
Anatomy of a mainsail
A mainsail is a triangle with corners at the head, tack and clew and edges at the luff, leech and foot. The luff runs up the mast in a groove or on slides; the foot is laced into a boom groove or, on a loose-footed main, attached only at tack and clew. The leech is where the sailmaker adds roach, the area outside the straight line from head to clew, supported by battens in pockets across the sail. Reef cringles allow the sail to be shortened, and the leech carries a leech line and telltales. In rating terms the mainsail is defined by P, the luff length between the black bands on the mast, and E, the foot length from the mast to the black band on the boom.
Types of mainsail
- Conventional battened main: short battens support a moderate roach and slab reefing pulls the sail down to reef cringles. The classic cruiser-racer sail.
- Full-batten main: battens run the full width, supporting a large roach and stopping the sail flogging. Popular on cruisers with lazyjacks and on performance boats with square-top heads.
- In-mast furling main: rolls into a hollow mast. Convenient, but the hollow leech loses area and shape; vertical battens recover some of both.
- In-boom furling main: rolls into the boom, keeping full battens and roach at the price of careful boom angle when furling.
- Shelf Foot main: a UK Sailmakers design in which a panel along the foot opens for depth off the wind and flattens under the outhaul upwind – an extra gear for a cruising sail.
Cloth and construction
A cruising mainsail is most often woven polyester, either cross-cut with panels running across the sail so that the low-stretch fill yarns line up with the leech load, or radially cut so that warp-oriented panels fan out from the corners along the load lines. For racing, laminated sails put low-stretch aramid, carbon or Vectran® fibres between Mylar® films and hold shape across a far wider wind range. UK Sailmakers’ Tape-Drive® construction laid load-bearing tapes along computed load lines over a light base laminate, and its successors Titanium™ and X-Drive® refined the idea into sails that hold their designed shape for years. In any material the aim is the same: a luff curve that matches the mast bend, a draft that stays forward as the wind builds, and a leech the controls can open and close.
Trimming the mainsail
The mainsail has more controls than any other sail because it does more jobs. The mainsheet and traveller together set the boom angle and the leech tension; upwind, the traveller sets the angle and the sheet sets the twist. The vang, or kicker, holds the boom down when the sheet is eased on a reach. The outhaul flattens the lower third for a beat and eases for reaching, the halyard and cunningham govern luff tension and draft position, and the backstay bends the mast to flatten the whole sail as the breeze builds. A well-trimmed main upwind shows the top leech telltale streaming most of the time and stalling occasionally, with the top batten roughly parallel to the boom.
Cruising vs racing
A cruising mainsail has to be easy to hoist, reef and stow, forgiving of imperfect trim and durable in sunlight. Sensible choices are slightly heavier cloth than a racer would pick, full battens with lazyjacks or a stackpack, two or three deep reefs and a UV-resistant finish. The racing mainsail is a shape-holding instrument built as light as its wind range allows, with a square-top for area aloft, a flattening reef rather than a working reef and a leech that answers a fingertip on the fine-tune. Both benefit from the same discipline: know what the controls do, look up at the leech often, and reef before the boat asks.
Frequently Asked Questions about the Mainsail
Why is the mainsail hoisted first and lowered last?
With the mainsail up the boat lies head to wind and behaves predictably under sail, and the main provides steering balance while headsails are handled. Dropping it last keeps that control until the end.
How do I know when to reef the mainsail?
When the boat heels past about twenty degrees, the helm loads up and the boat starts rounding up in gusts. Reef early: a reefed main and full headsail is usually as fast and far more comfortable than dragging the rail.
The mainsail is the sail that defines the boat, and trimming it well pays off on every point of sailing. Understand its controls, keep its shape matched to your rig, choose construction that suits how you sail, and the main will reward you with speed, balance and a boat that steers itself.