Studding sail

A studding sail, studsail or stunsail (traditionally pronounced stuns'l) is an extra sail on a square rigged vessel or 1950s racing skiffs for use in light winds.

A studding sail is an extra sail hoisted alongside a square-rigged sail on an extension of its yardarm. It is named by appending the word studding to the name of the working sail alongside which it is set (e.g. "fore topsail studdingsail"). These sails provide extra speed in fine weather.

Studding sails have also been used to increase the sail area of a fore-and-aft spanker, again by extending the upper spar. Such a sail extending the leech of a fore-and-aft sail is known as a ringtail. Ringtail sails were used by Flying 18 foot skiffs in Sydney Harbour in the 1950s to enlarge the downwind sailing capacity in light airs. Sails that extend below the boom to deck level or lower are known as watersails.

...
Wikipedia