studding sail

from the GNU version of the Collaborative International Dictionary of English.

• (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun nautical A sail attached to the side of another in a square-rigged ship to increase the speed of the vessel