The lug sail, or lugsail, is a fore-and-aft, four-cornered sail that is suspended from a spar, called a yard. When raised, the sail area overlaps the mast. For "standing lug" rigs, the sail may remain on the same side of the mast on both the port and starboard tacks. For "dipping lug" rigs, the sail is lowered partially or totally to be brought around to the leeward side of the mast in order to optimize the efficiency of the sail on both tacks.
The lug sail is evolved from the square sail to improve how close the vessel can sail into the wind. Square sails, on the other hand, are symmetrically mounted in front of the mast and are manually angled to catch the wind on opposite tacks. Since it is difficult to orient square sails fore and aft or to tension their leading edges (luffs), they are not as efficient upwind, compared with lug sails. The lug rig differs from the gaff rig, also fore-and-aft, whose sail is instead attached at the luff to the mast and is suspended from a spar (gaff), which is attached to, and raised at an angle from, the mast.
Lug sails are divided into three types: standing lug, balanced lug and dipping lug.
Whereas a standing lug may be tacked conventionally by moving the sail across the vessel, as the wind crosses the bow, a dipping lug must be brought around to the leeward side by a multi-step procedure:
This procedure is also necessary for gybing a dipping lug. Reportedly, this action can be completed expeditiously on a larger boat with four hands. On smaller boats, the sail is simply lowered and the mast unstepped to allow the sail to be moved beneath it to the other side and the mast to be re-stepped and the sail raised.
On larger luggers, like the Fifie, large dipping lug sails were possible only with the introduction of steam-powered capstans to facilitate with dipping.
Block reports that the rig was widely used in Europe from the seventeenth through the nineteenth centuries for small fishing vessels and other coasters because of their good performance to windward. This popularity extended to smugglers and privateers and the French chasse-marée fishing boats.
Currently, lug rigs are used on certain small sailing craft, like the International Twelve Foot Dinghy, a dinghy, the SCAMP, a pocket cruiser, and the Oz Goose 12-foot (3.7 m) sailing dinghy.
There are several lug rigged boat classes of long history that have been raced more or less continuously for a century. One example is the balance lug rigged Lymington Scow that has become highly developed in almost continuous racing since 1905.