fin keel(n = noun.artifact) - a metal plate projecting from the keel of a shallow vessel to give it greater lateral stability;
is a kind of plate
is a part of keel
has particulars: centerboard, centreboard, drop keel, sliding keel

   A projection downward from the keel of a yacht, resembling in shape the fin of a fish, though often with a cigar-shaped bulb of lead at the bottom, and generally made of metal. Its use is to ballast the boat and also to enable her to sail close to the wind and to make the least possible leeway by offering great resistance to lateral motion through the water. [Webster 1913 Suppl.]

bilge keel, caudal fin, dorsal fin, drop keel, fin, fin de si'8acle, fin de siecle, fin whale, heterocercal fin, homocercal fin, keel, keel arch, keel over, pectoral fin, pelvic fin, sliding keel, tail fin, ventral fin, vertical fin