1. a longitudinal beam connected to the keel of ship to strengthen it;
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.
Cross keelson, a similar structure lying athwart the main
keelson, to support the engines and boilers.
WordNet (r) 3.0 (2006):
n 1: a longitudinal beam connected to the keel of ship to