Kelson is a term used to refer to a piece of wood fitted lengthwise in the keel of a ship. While it may be a specific term, there are many synonyms that can be used interchangeably to describe this type of boat structure. Some of the most commonly used synonyms include floor, flat, keelson, and keelboard. Other synonyms that are less commonly used include plank, clamp, and stringer. Regardless of the specific term used, the kelson is an important part of any ship's structure, providing stability and support for the vessel as it navigates the open seas.