Either of two lengthwise fins attached along the outside of a ship's bilge; reduces rolling WordNet 3.0 & Open English WordNet

A bilge keel is a nautical device used to reduce a ship's tendency to roll. Wikipedia

instrument Wikidata

A strake, or small keel or blister, running along much of the length of the hull. It is used to reduce the hull's tendency to roll. They are typically fitted one on each side, low down on the side of the hull, so as not to increase the draft of the vessel. In battleships they were often quite large and used as part of the torpedo protection system. Wiktionary