The Collaborative International Dictionary of English v.0.48:

Keelson \Keel"son\, n. [Akin to Sw. k["o]lsvin, Dan.
   kj["o]lsviin, G. kielschwein; apparently compounded of the
   words keel and swine; but cf. Norweg. kj["o]lsvill, where
   svill is akin to E. sill, n. ] (Shipbuilding)
   A piece of timber in a ship laid on the middle of the floor
   timbers over the keel, and binding the floor timbers to the
   keel; in iron vessels, a structure of plates, situated like
   the keelson of a timber ship.
   [1913 Webster]

   Cross keelson, a similar structure lying athwart the main
      keelson, to support the engines and boilers.
      [1913 Webster]

WordNet (r) 3.0 (2006):

keelson
    n 1: a longitudinal beam connected to the keel of ship to
         strengthen it