1 The lowest and principal timber of a wooden ship, or the lowest continuous line of plates of a steel or iron ship, which extend the whole length of the vessel and to which the stem, sternpost, and ribs or timbers of the vessel are attached. It could be called the backbone of the ship and is its strongest single member. In the sailing barges of the Thames the keel is a continuous run of oak some 40 centimetres (16 in.) square, an indication of the great strength required in the keel of any ship, though junks were traditionally built without keels.
Yachts have been built with a variety of keels:(a) Centreline Ballast Keel. This is the traditional yacht keel developed early in the 20th century. It had the ballast bolted onto the wood keel above and being fairly long it allowed the yacht to take the ground safely, gave protection to the rudder, and the hull shape allowed tanks to be installed beneath the cabin sole.(b) Bilge Keel. Although the first yacht to be recorded with this configuration was the 18.3-metre (60-ft) Iris, a ketch built in Dublin Bay in 1894, it only became popular during the 1930s as it allowed cheaper, shallow tidal moorings. With the added pressure on mooring in the last few decades it has become even more popular with its ability to stay upright on level ground and requiring no shoring up on slipways or trailers.(c) Fin and Skeg Keel. First used on small racing yachts during the 19th century, this configuration was only introduced for cruising yachts in the 1960s, particularly with the lighter GRP yachts then being produced. It also proved the most successful configuration for racing yachts, but was not so well suited for cruising unless the fin was relatively large.(d) Fin and Bulb Keel. First developed in the 19th century by yacht designers such as Sibbick with his small skimming dish raters. Changes to the rating rules stopped this type, but the recent introduction of the modern high-performance racing yacht, such as the America's Cup class, the open 60, and many others, has led to an enormous development of this type of keel where the fin is machined from solid steel, maybe 6 metres (14.3 ft) deep and perhaps only a metre (3 ft 3 in.) long with a lead bulb at the bottom, weighing perhaps twice as much as the rest of the yacht.(e) Canting Keel. This is a development of the fin and bulb keel where the whole fin and bulb can be swung mechanically out to windward, the pivot being at the top of the keel, so that more righting moment is provided.(f) Swing Keel. The large, cast-iron aerofoil (1) is pivoted in the grounding plate (2) by a pivot pin (3) with a hydraulic ram (4) which retracts the keel into the keel case (5) at the touch of a button. The whole assembly is attached to the hull (6) with keel attachment bolts (7). The substantial cast-iron grounding plate serves as fixed ballast and protects the bottom of the boat when drying out. If the keel hits a submerged object, it will take the full impact of the blow and, by simply swinging from its pivot point and retracting into the hull, will absorb the impact.(g) Wing Keel. Developed initially by Ben Lexcen and Peter van Oossanen for the Australian America's Cup challenger Australia II, it became perhaps unduly credited for her winning the Cup in 1983. Previous work on winglets on aircraft wing tips had improved the efficiency of the wing by reducing tip vertices and induced drag. These keel winglets were also well positioned to allow ballast weight to be lower and so provide a greater righting moment and less heel. The keel was also innovative in that it was smallest at the hull and largest at the bottom, so improving its efficiency and compensating for the added wetted surface of the wings. The plan form and size of the wings depends greatly on the aspect ratio of the keel itself, but in cruising yachts, where draught or the practicability of taking the ground are important, the ballast weight may be in a single wing with a relatively small vertical fin.The Collins keel, with two related fins above a single delta-shaped wing, successfully combines efficiency with reduced draught. Jeremy Lines