Keelson definitions

kĕl'-
A timber or girder fastened above and parallel to the keel of a ship or boat for additional strength.
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A longitudinal beam or set of timbers or metal plates fastened inside the hull of a ship along the keel to add structural strength.
noun
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(nautical) A longitudinal beam fastened on top of the keel of a vessel for strength and stiffness.
noun
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Origin of keelson

First attested from 1611. Compare with Dutch kolzwijn, kolsem, Low German, kielswîn, German Kielschwein, Danish kølsvin, kölsvin, all with the same meaning. First part is keel while the second part is uncertain; possibly sill.