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In boats and ships, keel can refer to 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 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 word can also be used as a synecdoche to refer to a complete boat, such as a keelboat. ==Structural keels== A structural keel is a 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 a basic 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. Modern ships are now largely built in a series of pre-fabricated, complete hull sections rather than being built around a single keel, so the start of the shipbuilding process is now considered to be when the first sheet of steel is cut. 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 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. If a ship suffers severe structural stress — classically during a shipwreck when running aground in a heavy sea — it is possible for the keel to break or be strained to the extent that it loses structural integrity. In this case the ship is commonly said to have "broken its back". Such a failure means that the entire structure of the ship and its machinery has been compromised and repairing such damage would require virtually re-building the ship from the ground up. A ship that has broken its back is almost certainly unsalvagable and subsequently written off by its insurers. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「keel」の詳細全文を読む スポンサード リンク
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