Keelson, 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. ].

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.

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OXFORD DICTIONARY

Keelson, n. (also kelson) a line of timber fastening a ship's floor-timbers to its keel.

Etymology

ME kelswayn, perh. f. LG kielswin f. kiel KEEL(1) + (prob.) swin SWINE used as the name of a timber

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