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The principal sail of a sailing vessel. On a square-rigged vessel this is the lowermost (and largest) sail carried on the mainmast, and is usually termed the main course. The earliest known mainsails in European waters, as depicted on Roman pottery and mosaics of the 3rd century ad were set with a sprit. This rig held good in the Netherlands and North Sea ports until about the 15th century, when the sprit was superseded by the gaff rig, while in the eastern Mediterranean it became the lateen. Thames sailing barges, however, are still rigged with sprit mainsails for ease of handling with a crew of two.

In the past the mainsail on gaff-rigged smacks was loose footed while on other types of craft used for commercial fisheries, such as the bawley or Plymouth hooker, it had an extra long gaff and no boom, the mainsheet being led from two or more points on the sail's leech to blocks on the horse on the counter. In Bermuda from the late 18th century a simple form of triangular or jib-headed mainsail was used on local sloop-rigged boats, which was set on a sharply raked mast having a long boom but no gaff. In this early version the boom was almost as long as the luff of the sail. A more sophisticated application of this type of sail is the Bermudan rig, which was adapted by a few small racing yachts in the USA at the end of the 19th century. By the First World War (1914–18) it had become the normal rig for the smaller International Metre Class yachts in Europe, and by the 1930s was almost universal amongst racing yachts of all sizes, including the J-class. By then it had developed into a very tall and narrow sail with a luff-to-foot proportion—or aspect ratio—of, in the case of the J-class, about 3:1. Nowadays many mainsails are fully battened to provide an almost rigid aerofoil which does not flap or shiver, and which is just one step away from a solid aerofoil.

Subjects: Maritime History.

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