On boats and ships, the keel is either of two parts: a structural element that sometimes resembles a fin and protrudes below a boat along the central line, or a hydrodynamic element. These parts overlap. As the laying down of the keel is the initial step in the construction of a ship, in British and American shipbuilding traditions the construction is dated from this event. Only the ship's launching is considered more significant in its creation.
The adjustable centerboard keel traces its roots to the medieval Chinese Song dynasty. Many Song Chinese junk ships had a ballasted and bilge keel that consisted of wooden beams bound together with iron hoops. Maritime technology and the technological know-how allowed Song dynasty ships to be used in naval warfare between the Southern Song Dynasty, the Jin Dynasty, and the Mongols.
A structural keel is the bottom-most structural member around which the hull of a ship is built. The keel runs along the centerline of the ship, from the bow to the stern. The keel is often the first part of a ship's hull to be constructed, and laying the keel, or placing the keel in the cradle in which the ship will be built may mark the start time of its construction. Large, modern ships are now often built in a series of pre-fabricated, complete hull sections rather than being built around a single keel, so shipbuilding process commences with cutting the first sheet of steel.
The most common type of keel is the "flat plate keel", and this is fitted in the majority of ocean-going ships and other vessels. A form of keel found on smaller vessels is the "bar keel", which may be fitted in trawlers, tugs, and smaller ferries. Where grounding is possible, this type of keel is suitable with its massive scantlings, but there is always a problem of the increased draft with no additional cargo capacity. If a double bottom is fitted, the keel is almost inevitably of the flat plate type, bar keels often being associated with open floors, where the plate keel may also be fitted.
Duct keels are provided in the bottom of some vessels. These run from the forward engine room bulkhead to the collision bulkhead and are utilized to carry the double bottom piping. The piping is then accessible when cargo is loaded.
The keel surface on the bottom of the hull gives the ship greater directional control and stability. In non-sailing hulls, the keel helps the hull to move forward, rather than slipping to the side. In traditional boat building, this is provided by the structural keel, which projects from the bottom of the hull along most or all of its length. In modern construction, the bar keel or flat-plate keel performs the same function. There are many types of fixed keels, including full keels, long keels, fin keels, winged keels, bulb keels, and bilge keels among other designs. Deep-draft ships will typically have a flat bottom and employ only bilge keels, both to aid directional control and to damp rolling motions
In sailboats, keels use the forward motion of the boat to generate lift to counteract the leeward force of the wind. The rudimentary purpose of the keel is to convert the sideways motion of the wind when it is abeam into forward motion. A secondary purpose of the keel is to provide ballast. Keels are different from centreboards and other types of foils in that keels are made of heavy materials to provide ballast to stabilize the boat. Keels may be fixed, or non-movable, or they may retract to allow sailing in shallower waters. Retracting keels may pivot (a swing keel) or slide upwards to retract, and are usually retracted with a winch due to the weight of the ballast. Since the keel provides far more stability when lowered than when retracted (due to the greater moment arm involved), the amount of sail carried is generally reduced when sailing with the keel retracted.
Types of non-fixed keels include swing keels and canting keels. Canting keels can be found on racing yachts, such as those competing in the Volvo Ocean Race. They provide considerably more righting moment as the keel moves out to the windward-side of the boat while using less weight. The horizontal distance from the weight to the pivot is increased, which generates a larger righting moment.
The word "keel" comes from Old English cēol, Old Norse kjóll, = "ship" or "keel". It has the distinction of being regarded by some scholars as the very first word in the English language recorded in writing, having been recorded by Gildas in his 6th century Latin work De Excidio et Conquestu Britanniae, under the spelling cyulae (he was referring to the three ships that the Saxons first arrived in).
Carina is the Latin word for "keel" and is the origin of the term careen (to clean a keel and the hull in general, often by rolling the ship on its side). An example of this use is Careening Cove, a suburb of Sydney, Australia, where careening was carried out in early colonial days.