The Collaborative International Dictionary of English v.0.48:

Boyer \Boy"er\, n. [D. boeijer; -- so called because these
   vessels were employed for laying the boeijen, or buoys: cf.
   F. boyer. See Buoy.] (Naut.)
   A Flemish sloop with a castle at each end. --Sir W. Raleigh.
   [1913 Webster]