Fin keel. (Naut.)
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