Steffy: An internal longitudinal timber or line of timbers, mounted atop the frames along the centerline of hte keel, that provided additional longitudinal strength to the bottom of the hull; an internal keel.
Most commonly, a single keelson was installed that was no larger than the keel. On very large vessels, however, various combinations of as many as a dozen keelsons were assembled. Where extra molding was required, one or more additional keelsons, called rider keelsons or false keelsons, were bolted to the top of the main keelson. Auxiliary keelsons bolted along-side the main keelson were known as sister, (US), side, auxiliary, or assistant keelsons. However, care should be exercised in interpreting the various keelsons from contracts. For instance, some nineteenth-century American contracts for large schooners refer to the keelson above the main keelson as the sister, and the one above that as the assistant sister keelson. On occasion, large square timbers were placed at the floor head line or near the bilge, usually above the bilge keels. These were called bilge keelsons or, in some British documents, sister keelsons. Secondary keelsons did not necessarily run the full length of the hull, terminating at the ends of the hold, the last square frames, or some other appropriate location.