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Ship classification based on construction has traditionally divided boats into two major families: shell-built, in which the exterior skin of planking or other material is the primary component, and skeleton-built, in which internal framing is dominant. A large number of vessels, largely flat-bottomed and confined to inland and coastal waters, do not fit neatly into either category. They appear to mix both shell and skeleton concepts, or employ methods that do not seem to be either. These vessels belong in a distinct, third family: bottom-built construction, in which the bottom of the vessel is the primary component.
Most of the excavated, northwestern European vessels of the Roman period, which have been previously identified as members of a native "Celtic" tradition, are bottom-built craft. The distinguishing feature of this tradition is a heavy, flat bottom of straight planks, temporarily fastened together as the first stage of construction. After the bottom has been shaped, the temporary fastenings are replaced by heavy floor timbers and the sides completed.
In the Middle Ages, this tradition produced cogs, the dominant seagoing ships of the twelfth through fourteenth centuries, as well as a variety of related inland craft. Careful examination of an early fifteenth-century cog from the reclaimed land of the Zuiderzee, Almere Wijk 13 reveals details of the construction process and structural thought involved in cog construction. Despite the abandonment of lapstrake planking in favor of caravel in the fifteenth century, Dutch shipwrights continued to build even large seagoing ships in a bottom-based manner and were thus able to take quick advantage of the economic benefits of the new technology. A late sixteenth-century ferry from the Zuiderzee, Oost Flevoland B71, demonstrates the manner in which bottom-based construction was adapted to caravel construction, as well as the limitations it imposed.
A brief look at shipbuilding traditions in the New World shows that bottom-based concepts were transported to the English colonies, and that native bottom-based methods may have been merged with European skeleton methods to produce hybrid vessels.
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