fin keel

from The Century Dictionary.

• noun A projection downward from the bottom of a sail-boat or yacht, in general shape like the ventral fin of a fish. The fin-keel is usually made of metal, and acts as additional ballast, and being low down gives increased stability.

from the GNU version of the Collaborative International Dictionary of English.

• (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun nautical A type of keel common in sailing yachts, its shape resembling a fin of fish.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a metal plate projecting from the keel of a shallow vessel to give it greater lateral stability