n. [ Akin to Sw. kölsvin, Dan. kjölsviin, G. kielschwein; apparently compounded of the words keel and swine; but cf. Norweg. kjölsvill, 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 ]