\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.