A keel is found on most watercraft whether powered by wind or motor. It is commonly understood to be a fixed underwater extension protruding from the bottom of a boat although some versions are movable. This extension provides stability and resists sideways movement or drift. Sideways movements are caused by wind or cross currents and are not only countered by the shape and draft (depth) of a keel structure, but also by its weight (or ballast).
Movable keels are typically only associated with sailing vessels. Most move with regard to their being positioned in or out of the water, though canting keels refer to movement that occurs underwater while the keel is already in place. In these instances, it swings or angles more severely whenever the watercraft heels (tips) to extreme angles.
The adjustment in the angle of the keel will reposition the center point of its weight and allow for more advantageous hydrodynamics in service to speed through the water. One drawback of a canting keel is the resulting reduction of resistance to sideways movement. This is usually compensated for with additional fins or double-rudder systems designed to maintain resistance to sideways forces.
In the case of smaller sailboats, a keel may exist in a retractable form known as either a centerboard or dagger board. The centerboard is usually pivoted and is manually levered into position, whereas the dagger variation is simply thrust into position. Both are operated from within the vessel.
There are various modifications of keels as seen in the many compound uses of the name: dorsal keel, fin keel, winged keel, bulb keel, etc. Usually, these variations relate to its method of deployment or shape; however, there is another definition of the term that precedes the underwater extension definition. It refers to a structural element in traditional boat building and describes the timber traveling from stem (foremost part) to stern (rearmost part), and is the "spine" of the entire vessel. This center beam from which most other parts of a ship’s framework is connected to is properly known as the keel. Its primary function is to provide strength, integrity and centering balance.
The modern word descends from the ancient Anglo-Saxon word ceol, which means ship. The Latin version of the word is carina. Over the years, whenever a ship was hauled over on its side to clean the underlying hull, it was described as being "careened," an obvious variation of the Latin term.