keelson

noun

1. A longitudinal beam fastened on top of the keel of a vessel for strength and stiffness.

   “With regard to materials, all the frames should be of oak and so should the stem piece, stern post, upper portion of dead woods, knight heads, apron, beams, shelf clamp, bilge strakes, and keelson; the keel will generally be found to be either English or American elm.”