The Collaborative International Dictionary of English v.0.48:

Galleass \Gal"le*ass\ (?; 135), n. [F. gal['e]asse, gal['e]ace;
   cf. It. galeazza, Sp. galeaza; LL. galea a galley. See
   Galley.] (Naut.)
   A large galley, having some features of the galleon, as
   broadside guns; esp., such a vessel used by the southern
   nations of Europe in the 16th and 17th centuries. See
   Galleon, and Galley. [Written variously galeas,
   gallias, etc.]
   [1913 Webster]

   Note: "The galleasses . . . were a third larger than the
         ordinary galley, and rowed each by three hundred galley
         slaves. They consisted of an enormous towering
         structure at the stern, a castellated structure almost
         equally massive in front, with seats for the rowers
         amidships." --Motley. Gallegan