The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]