Square Rig Sailing
Bare masts, spars, and rigging of a traditional square rig sailing ship.
Square rig sails refers to the sails and rigging of any sailboat or sailing vessel with horizontal spars to which one or more sails are rigged. In this arrangement, the square sails are perpendicular, or cross-wise, to the boat's length, rather than running fore-and-aft.
A full-rigged ship is a ship with 3 or more square-rigged masts, while a full-rigged brig is a ship with 2 square-rigged masts. But square rig as a generic term refers to any sailboat or ship with at least one square-rigged sail.
Square rig sailing was the earliest type of sailing, and was based on a simple arrangement in which the the sail catches the wind from behind the boat. This type of sailing works well for boats heading downwind, but isn't very effective for ships needing to travel upwind.
Origins of Square Rig Sailing
It is believed that square rig sailing originated in the 800s and 900s with the Vikings of Norway and Sweden, who explored, pillaged, and plundered using single-masted open longboats or longships with a single square sail. (See the History of Square Rig Sailing for more information on the long traditions of this type of sailing.)
Horizontal spars were attached to the mast, and the top and bottom of the sail was attached to the spars. The spars could be turned around the mast to provide some ability to maneuver the boat across the wind or slightly upwind.
Square rig sails also use what's called "loose-footed" rigging, in which the sail is supported by a single top horizontal spar. Upwind performance is improved by attaching a line to the upwind edge of the sail to stabilize it.
For a more in-depth look at how square rig ships and sailboats work, see What Is Square Rig Sailing?