keel has 5 distinct synonyms spread across 1 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a projection or ridge that suggests a keel
the median ridge on the breastbone of birds that fly
one of the main longitudinal beams (or plates) of the hull of a vessel; can extend vertically into the water to provide lateral stability
Synonyms, antonyms and the broader and narrower relations for keel come from Princeton University’s WordNet. Full sources and licences.