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Keel
In boats and ships,
keel can refer to either of two parts: a structural element, or a hydrodynamic element. These parts overlap.
A structural keel is a large beam around which the
hull of a ship is built. The keel runs in the middle of the ship, from the
bow to the
stern, and serves as the foundation or spine of the structure, providing the major source of structural strength of the hull. The keel is generally 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, is often a momentous event in a ship's construction--so much so that the event is often marked with a ceremony, and the term lay the keel has entered the language as a phrase meaning the beginning of any significant undertaking.
The keel contributes substantially to the longitudinal strength and effectively local loading caused when docking the ship. The most common type of keel is the 'flat plate keel', and this is fitted in the majority of ocean-going ship and other vessels. A form of keel found on smaller vessels is the bar keel. The bar keel may be fitted in trawlers, tugs,etc.. and this is also found in 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 perform the same function.
In sailboats, keels use the forward motion of the boat to generate lift to counter the lateral force from the sails. Sailboats have much larger keels than non sailing hulls. Keels are different from centerboards 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 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.
There are several types of fixed keels including: full keels, fin keels, winged keels, bulb keels, and twin keels or bilge keels among other designs.
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 much more righting moment for a lot less weight, as the keel moves out to the windward-side of the boat. The perpendicular distance from weight to pivot is increased, therefore a larger righting moment is produced.
