A topsail is a sail set above another larger sail. However not all such sails are topsails.
On a square rigged vessel, a topsail is a sail normally set perpendicular to the length of the ship. It is rigged above the course sail and below the topgallant sail. A fully rigged ship has upper and lower topsails on each mast, the lower topsail being the second sail above the deck, and the upper topsail being the third.
Although described as a "square" sail, the topsail was always trapezoidal, so as to match the lengths of the yards at top and bottom (upper yards being smaller). The bottom edge (foot) of the topsail was slightly concave, much less so than other sails.
Topsails first came into use sometime in the 15th century. Initially small, and carried only on main and fore masts, they gradually increased in size and importance. By the beginning of the 18th century, they were the principal sails of the ship; though never quite as large as the courses, they were the first sails to be set, and the last to be taken in. It was quite common for a ship to sail with topsails and jibs alone; the position of the topsails well above the sea ensured that they received a steady breeze even if the seas were rough.
The larger topsails were difficult and dangerous to handle in strong winds. Sometime in the 1680s, reef-bands were introduced to tie up part of the sail, with topsails eventually getting four of these, and reefing the sails became a regular occupation of sailors. In the 19th century, topsails were split into separate upper and lower topsails that could be managed separately.
On a gaff rigged sailing boat, a topsail is a triangular sail set between the gaff and the top of the mast. These rigs were once common, particularly on two-masted racing schooners and later on racing sloops but have now been largely superseded by the bermuda rig, which has no topsails. There may be one topsail on each gaff-rigged mast.
On a fully rigged schooner and other rigs having three or more jibs, the highest of these, setting on a topmast forestay, is called the jib topsail.
See also: sailing, sail-plan
Square rig
Gaff rig
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