keel: one of the main longitudinal beams (or plates) of the hull of a vessel; can extend vertically into the water to provide lateral stability
beam: long thick piece of wood or metal or concrete, etc., used in construction
four-stroke engine: an internal-combustion engine in which an explosive mixture is drawn into the cylinder on the first stroke and is compressed and ignited on the second stroke; work is done on the third stroke and the products of combustion are exhausted on the fourth stroke