The Collaborative International Dictionary
Studding sail \Stud"ding sail`\ (Naut.)
A light sail set at the side of a principal or square sail of
a vessel in free winds, to increase her speed. Its head is
bent to a small spar which is called the studding-sail boom.
See Illust. of Sail.
--Toten.
Wiktionary
n. (context nautical English) A sail attached to the side of another in a square-rigged ship to increase the speed of the vessel
Wikipedia
A studding sail, studsail or stunsail (traditionally pronounced stuns'l) is a sail used to increase the sail area of a square rigged vessel or 1950s racing skiffs in light winds.
It 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.
Usage examples of "studding sail".
It was the fished main-yard, splinted between the two spare studding sail booms.
According to Harland, the danger of dipping a [[studding-sail]] boom into the water even applied to a topgallant studding sail.
In the darkness the men scrambled out along the yards, their hands feeling for the gaskets, the canvas strips securing the furled sails, while the studding sail booms were triced up out of the way.
Someone gasped as smoke belched from her quarter, and before the smoke had been thrust aside a ball slammed through Avenger's sails and rigging, ripping holes in the starboard studding sail and main alike.
It was the fished mainyard, splinted between the two spare studding sail booms.